July 4, 2023 • 48 mins
Daniel and Jorge talk about whether galaxies have to have black holes at their hearts.
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Speaker 1 (00:08):
Hey, Daniel, have you started planning for your retirement retirement?
Speaker 2 (00:13):
Why would I ever retire?
Speaker 1 (00:14):
I mean, you're gonna be a physicist forever to you're
dying breath.
Speaker 2 (00:18):
I don't see myself burning out of questions.
Speaker 1 (00:20):
Well, I guess eventually the Sun is gonna expand blow
up and burn the Earth, right, so at some point you.
Speaker 2 (00:26):
Have to stop. I guess so, And eventually the galaxy
will collapse into a black hole. But that doesn't mean
we have to stop.
Speaker 1 (00:33):
You're gonna be doing physics inside of the black hole.
Speaker 2 (00:37):
Absolutely, I'll be writing papers about the singularity that nobody
can ever read.
Speaker 1 (00:41):
Oh, because you're inside the black hole, right, unless I'm
like closer to the center, in which case you could
still technically send me the paper.
Speaker 2 (00:49):
Yeah, and I could never get your comments back. Hey,
that sounds pretty good.
Speaker 1 (00:52):
Actually, it's a win win situation or a lose lose situation.
It's a wind lose wind loose situation.
Speaker 2 (00:58):
I'll be waiting for pure you literally forever.
Speaker 3 (01:17):
Hi.
Speaker 1 (01:17):
I'm Jorge, cartoonist and the creator of PhD comics.
Speaker 2 (01:20):
Hi. I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I do plan on doing this.
Speaker 1 (01:25):
Forever, forever, even after your death.
Speaker 2 (01:30):
I'm hoping my ideas live on.
Speaker 1 (01:32):
Are you going to program an AI to do research
for you and make your children feel guilty after you're gone.
Speaker 2 (01:40):
I'm going to upload my intelligence into the cloud so
I can continue to live as some sort of weird
mixture between biological and mechanical intelligence. But even that eventually
will fall into the galactic center and be gobbled up
by the black hole.
Speaker 1 (01:53):
M But I guess why wait? I mean we could
technically generate this podcast with AI right now.
Speaker 2 (01:58):
Can How do you know we haven't already? Who do
you think he's writing these outlines? Oh? My god?
Speaker 1 (02:03):
Am I talking to an AI right now?
Speaker 2 (02:06):
Maybe I'm talking to an AI right now? Maybe AI
are listening to this podcast.
Speaker 1 (02:10):
He did seem smarter than usual.
Speaker 2 (02:12):
That's the real Daniel.
Speaker 1 (02:15):
And the real Danger. But anyways, welcome to our podcast
Daniel and Jorge Explain the Universe, a production of our
Heart Radio.
Speaker 2 (02:22):
In which we use are still limited biological intelligence to
try to understand how the universe works. We want to
know everything that's out there. We want to know how
it works on the microscopic scale, to weave itself together
into reality that we experience and that we want to understand.
We cast our minds out into the farthest reaches of
(02:42):
space and hope to understand what they might encounter out there.
Speaker 1 (02:46):
That's right, because it is a very perplexing universe full
of interesting mysteries for us to go out there explore
and try to uncover, and try to understand and try
to explain to others.
Speaker 2 (02:56):
Some mysteries have lasted for decades or centuries or millennia,
but others have succumbed to the onslaught of human intelligence.
We have sometimes actually managed to crack puzzles of the
universe and understand what's going on at the atomic scale
or the subatomic scale, or how space bends to form
black holes. So there are times when our limited intelligence
(03:17):
is enough.
Speaker 1 (03:18):
Why I make it sound like we're masters of the universe?
How far along are we are? We up to like
one percent of the universe by now.
Speaker 2 (03:25):
I think that's probably a pretty generous estimate. But it's promising.
You know, we are making progress and we are understanding things,
and the understanding is coming faster and faster. So far,
there's no evidence that we're going to hit some wall
of understanding.
Speaker 1 (03:39):
Especially with the AIS. Could we train AIS or just
ask AIS to do research for us?
Speaker 2 (03:44):
I use AIS in my research all the time. We
have AIS answering questions. The big challenge is understanding what
they're doing. Are they doing physics for themselves or are
they doing it for us?
Speaker 1 (03:54):
What do you mean for themselves? Like for their own curiosity?
Oh my god, are yet that already?
Speaker 2 (04:00):
Well? We have AIS generating YouTube videos that are mostly
watched by other AIS to generate response videos, and now
we have AIS generating physics research, and at some point
AIS are going to be reading that physics research and
summarizing it for themselves.
Speaker 1 (04:15):
Huh. So I wonder if the AI science community is
going to have as much politics and drama as the
real physics community, or maybe even more right, more drama.
But it's all artificial drama.
Speaker 2 (04:26):
So all drama is artificial, and.
Speaker 1 (04:31):
It happens faster, I guess, which is good.
Speaker 2 (04:33):
Though there might be some real drama going on at
the center of our galaxy, as the black hole there
keeps sucking things in that's swirling around it.
Speaker 1 (04:42):
Yes, we discussed on this podcast. There's a lot going
on at the center of galaxies, including some very big mysteries.
Speaker 2 (04:48):
That's right, as we peer towards the center of our
own galaxy, we notice a very very large black hole
sitting there, gobbling up anything that comes near it, and
while we are safely swirling around it at tens of
thousands of light years away, eventually we think that everything
might fall into that black hole, flush down, the gravitational.
Speaker 1 (05:09):
Drain down physics toilet? Is that really the case? Eventually? Like,
there's no prediction where we don't get sucked into that
black hole.
Speaker 2 (05:16):
There are some variations. If the universe keeps expanding at
a faster and faster rate and something called phantom energy
takes over and creates enough space between us and the
central black hole to distance us from it. But if
dark energy stays at the level that it is, the
prediction is that the universe ends up with a bunch
of isolated galaxies, each of which collapse into their own
(05:37):
black holes.
Speaker 1 (05:38):
Wait wait, wait, did you just say phantom energy.
Speaker 2 (05:40):
Phantom energy exactly like.
Speaker 4 (05:44):
The Phantom Menace named after that movie exactly. Not a coincidence,
m I guess we'll have to dig into that in
another episode. But it is interesting how as you said
that every galaxy has a black hole.
Speaker 2 (05:56):
It is pretty cool to have a black hole the
center of our galaxy, and it makes us wonder if
that required for every galaxy. Is that a necessary product
of a galaxy? Do you have to have a black
hole to form a galaxy? Do galaxies form black holes?
Do black holes create galaxies?
Speaker 1 (06:11):
It's a deep question, and so today on the podcast
we'll be asking the question, does every galaxy have a
black hole at its center? You mean, like like a
tasty snack or something like are there any defective galaxies
that they didn't get a black hole in the middle?
Speaker 2 (06:32):
You mean, like you'd be disappointed to bite into a
galaxy and be like, oh, I didn't get a black
hole in the middle.
Speaker 1 (06:37):
I know, that's the tastiest part. It's got the richest flavor.
Speaker 2 (06:41):
So deep. That's interesting because black holes have mass, but
I wonder if they actually have calories. Like if you
eat a black hole, do you lose weight?
Speaker 1 (06:47):
Or is the secret to a new diet?
Speaker 2 (06:50):
I guess really the black hole eats you and it
gains weight, so you become part of the black hole.
Speaker 1 (06:55):
Something gets eaten and it's you, which I guess is
a diet in a way you lose a lot more
than weight.
Speaker 2 (07:02):
Though, definitely do not take health advice from either of us.
Speaker 1 (07:06):
That's right, from either physicist or cartoonists, not the healthiest
people in the world or the universe.
Speaker 2 (07:12):
We are totally unqualified to dispense health advice. I know
that doesn't stop most podcasters, but it does give us pause.
Speaker 1 (07:18):
So, yeah, this is an interesting question. Does every galaxy
have a black hole at its center? I guess a
lot of galaxies have black holes at their centers.
Speaker 2 (07:26):
We certainly have seen a black hole in the center
of our galaxy and a few others. But it's a
really interesting question both about how black holes get formed
and how galaxies get formed. It seems like there's a
fascinating dance between the two.
Speaker 1 (07:39):
I guess. Kind of the deeper question is like, do
you need to have a black hole at the center
to have a galaxy?
Speaker 2 (07:45):
Yeah? Exactly does it count as a galaxy if it's
just a bunch of stars?
Speaker 1 (07:49):
So, as usual, we were wondering how many people have
asked this question, whether every galaxy has a black hole?
So Daniel went out there into the wilds of the
internet to ask folks, does every galaxy a black hole
at its center.
Speaker 2 (08:01):
Thanks very much, my enduring gratitude to everybody who participates
in this segment. But remember it's open to everyone. If
you've been listening to pod and learning for years and
feel ready to answer a random questions, please don't be shy.
Write to me two questions at Danielandjorge dot com. We
want to hear from you.
Speaker 1 (08:19):
So think about it for a second. You think every
galaxy has a black hole. Here's what people had to say.
Speaker 5 (08:26):
Man, who knows if black holes even exist? Like, what
if they're just all dark stars like you guys talked
about earlier on the show. There could be wormholes or
something that we haven't even thought of yet. So no,
not all galaxies.
Speaker 6 (08:40):
A galaxy is a cluster of a bunch of objects
in space swirling around a massive object in the middle.
And the only massive object you can have is a
black hole or a store I guess, and you can't
have such a big star. So I think every galaxy
has to have a black hole in the middle.
Speaker 3 (08:58):
Probably there's got to be something like holding it all together.
But sence space is really big. Maybe there's some galaxies
that don't and they're just like really dense packed like
stars spinning around each other.
Speaker 2 (09:15):
I would guess that just because you're asking the question,
the answer is no, But I don't know. I would
for you guys to explain it to me.
Speaker 1 (09:24):
All Right, some people we have some conspiracy theories, apparently
as listeners, some galactic conspiracy theorists.
Speaker 2 (09:31):
Well, we are guilty of promoting those black hole conspiracy
theories right when we talk on the podcast about how
black holes might be dark stars or fuzzballs or something else.
Even weirder, are you seen?
Speaker 1 (09:41):
We're ground Sero or the black hole ism movement.
Speaker 2 (09:45):
Black hole truthers. Yeah, this is the dark Horse podcast
for black holes exactly.
Speaker 1 (09:50):
Maybe if we switched our podcast name to like universal
conspiracies or conspiracies about everything, maybe our ranking will go up.
Speaker 2 (09:59):
It might go up, we might get a bigger audience.
But I don't think we'd get a better audience.
Speaker 1 (10:04):
I mean, isn't physics, after all, just like the Hunt
for the Conspiracy.
Speaker 2 (10:08):
Of the Universe.
Speaker 1 (10:10):
You're like, there's something going on here, There's some sort
of plan or structure. We got to find out what
it is. We got to find out the truth. Man,
Isn't that what physics is all about.
Speaker 2 (10:20):
Yeah, but conspiracies are usually built on wild speculation and
a lack of evidence.
Speaker 1 (10:25):
Isn't that every other episode we discussed on the podcast.
Speaker 2 (10:29):
Yeah, but we don't trust strong conclusions based on our
lack of knowledge. We talk about what we don't know
and how we might actually figure it out. We're trying
to build evidence. We're looking for data when we're happy
to disprove our ideas if the universe shows us we're wrong.
Speaker 1 (10:43):
It sounds like something someone involved in a conspiracy would say.
Speaker 2 (10:46):
And you've just proven that you can make a conspiracy
about anything with no information.
Speaker 1 (10:50):
But anyways, this is an interesting question. Does every galaxy
have a black hole at its center? And, as you said,
there is one at the center of our galaxy. The
last podcast were recorded talked about how there's a big
one at the center of our galaxy, along with a
whole bunch of other stuff going on down there.
Speaker 2 (11:04):
We certainly think that there is a black hole at
the center of our galaxy.
Speaker 1 (11:08):
What do you say, we think?
Speaker 2 (11:09):
Yeah, we don't really ever know for sure. I mean,
our galactic center is the one that's closest to us,
so it's easiest to study. It's also shrouded in gas
and dust, so it's complicated to study, and in the end,
all of our evidence for black holes is always a
little bit indirect. Usually the observations tell us that there's
something there that's very massive, something that's very small and
(11:31):
also has a lot of gravity, but we've never actually
observed an event horizon directly. It's always a little bit indirect.
Speaker 1 (11:39):
I guess it's kind of hard to see a black
hole because they don't emit light themselves.
Speaker 2 (11:42):
They might emit light. There might be gentle hawking radiation
from black holes, which would be very powerful direct evidence
for black holes, but that's not something we've ever seen.
The story of black holes for the last few decades
is an increasing belief that black holes probably are real.
As we identify these very massive objects and we put
limits on how big they can be, we see things
approaching closer and closer to the black hole the center
(12:05):
of our galaxy, which tells us more and more about
how small it has to be. And there are only
a few things out there that could satisfy all of
those constraints that are that massive and that compact. Black
holes of course the most classic example. But now we
have a few other candidates, dark stars, fuzzballs, et cetera,
et cetera.
Speaker 1 (12:23):
Yeah, it's still a big mystery talking about the ones
that are at the center of galaxies. Let's maybe break
it down for folks and talk about black holes a
little bit in general, and the different kinds of black
holes that are out there and that could be at
the center of different galaxies.
Speaker 2 (12:36):
When we hear about black holes, we're often thinking about
stellar black holes. Like a star burns for billions of
years until eventually it's fusion peters out and it loses
the battle with gravity, so the gravity collapses the star
into a black hole, meaning that there's a region of
space where there's so much mass and energy that space
becomes so dramatically curved that every path within the event
(12:59):
horizon leads towards the center. That any particle that passes
into that event horizon will eventually find its way to
this center of that region. There are no paths out.
Space within the event horizon is so bent that every
timelike path for a particle leads towards the center. So
that's a stellar black hole. And those are awesome, which
is why we call them stellar, but also because they
(13:21):
collapse from individual stars, and they can be quite big,
like ten times the mass of our Sun, but they're
small compared to the kinds of black holes we're going
to talk about today.
Speaker 1 (13:30):
Yeah, those are called super massive black holes.
Speaker 2 (13:33):
Yeah, there's basically two categories of black holes. The stellar
mass black holes that are like a few tens of masses,
and then we jump up to really really really big
black holes, things like ten thousand or one hundred thousand
times the mass of our Sun in one huge black hole.
But they get much much bigger than that as well.
There are black holes that are millions or even billions
(13:55):
of times the mass of our Sun. And these are
the ones found at the hearts of galaxies.
Speaker 1 (14:01):
Yeah, you can sort of see them even in distant,
faraway galaxies, right, Like when you look at little Fuzzz
out there in the night sky with special telescopes, you
can actually kind of see the black hole in the middle. Right.
Speaker 2 (14:11):
Yeah, that's a really interesting and complicated question, like how
do we see these black holes in the milky way?
We have actually a lot of really interesting and cool
ways to look at the black hole because it's so close.
Like some of the best evidence we have is a
picture of the black hole released by the event Horizon telescope,
which is actually a picture of the accretion disk around
the black hole, the hot gas that's swirling very very
(14:33):
close to it and radiating light because it's so hot,
so we can see that. We can also see stars
orbiting very very close to the black hole the center
of our galaxy. There's this one star in particular S two,
with zooms super close to the black hole and whips
around it. We've actually seen an entire orbit of that
star all the way around the black hole, which won
(14:54):
a Nobel prize very recently. So the black hole the
center of our galaxy we've observed very closely. Black Holes
in other galaxies are harder to spot because those galaxies
are further away now.
Speaker 1 (15:04):
For the one at the center of our galaxy. Even
that one, we're not one hundred percent sure it is
a black hole, right, It could still be something else
super massive there, or are we pretty sure it's a
super mass black hole.
Speaker 2 (15:13):
I think both of those things are true. We don't
have direct evidence that it's a black hole. We know
that there's a lot of mass there because we see
the influence of that object on the nearby stars and
all the gas and dust. We know that it's not
emitting any light itself, so it's compact, it's dark, and
it's massive. So most physicists, I think, are pretty convinced
that it's a black hole. But that's just sort of
(15:35):
like the best idea we have. There are these alternative
theories that it could be a very rapidly collapsing region
of space which is going to bounce back and turn
into a white hole, for example. So there are other
theories there, but I think the mainstream physics community is
pretty convinced that it's a black.
Speaker 1 (15:49):
Hole, all right, So then how do we see them
in other galaxies?
Speaker 2 (15:53):
There's sort of two ways to see black holes in
other galaxies. One is when they're feeding. When black holes
are eating stuff, when they have like a lot of
gas swirling very close to them and falling in, it
tends to get hot and that gas radiates and they
can generate very very powerful beams of light. These are
things we call quasars, and we can see these from
(16:13):
like across the universe. There's some objects that are super
duper far away but incredibly bright, and when they were
discovered decades ago, people couldn't believe that they were actually
that far away because they look super bright already here
on Earth, and if they're also very very distant, that
means that at their source they're incredibly bright. And that's
exactly what's happening. The gas around in the black hole
(16:35):
is getting very very heated up, and as it falls in,
it radiates, and it radiation gets channeled by the magnetic
field of the black hole, and you get these very
powerful beams emitted from above and below the black hole,
and we can see those from very very far away.
So those are quasars. But we can only see that
for black holes that are like actively feeding and growing,
and that tends to happen sort of early in the
(16:56):
life cycle of the universe. Quasars peaked about ten billion
years ago, so that's good for seeing like young super
massive black holes very very far away.
Speaker 1 (17:05):
So not every super massive black hole is a quasar, right,
There are some that could be out there just sitting
there being big.
Speaker 2 (17:12):
Exactly most of the super massive black holes we think
are not quasars. We think that a lot of them
stop sort of quazing billions of years ago and are
now more like dark relics, huge black holes sitting at
the centers of galaxies but not being quasars anymore. But
we can still spot those by looking at the motion
of stars around them. So pick them nearby galaxy and
(17:33):
look at the stars that are near the center of it.
You can't resolve them individually. We don't have telescopes that
can say here's a star, there's a star, there's a star.
But we can look at stellar populations at the hearts
of other galaxies, and we can measure their velocities, we
can measure their brightness. It's really complicated and very computationally expensive,
but we can build a model of how fast those
(17:54):
stars are moving around the center of that galaxy, and
from that we can infer the mass of the black hole.
How heavy does the thing have to be at the
center of the galaxy to support this super fast radial
motion by those stars.
Speaker 1 (18:07):
All right, well, let's get into whether every galaxy has
a black hole and what would it mean for a
galaxy to not have a black hole at its center.
But first let's take a quick break. All right, we're
(18:30):
asking the question does every galaxy have a black hole
at its center? Like, does every galaxy have a dark
soul or something?
Speaker 2 (18:37):
I'll leave you the philosophical and moral implications of it.
But from a physics point of view, it's a trend
that we're noticing that galaxies have these black holes, and
so we wonder if it's a rule. You know, it's
basically our strategy in physics is like, look out of
the universe, find patterns, wonder if those patterns reflecting like
deep rules of the universe. Figure out what those rules
have to be. So, you know, we're still in like
step two here.
Speaker 1 (18:57):
Hmmm, Now, I guess what is it to say, Like,
you know, we can see traillions of galaxies out there
in space, can we tell what percentage of them have
black holes? Or does every galaxy we've seen have a
black hole in it that we can tell?
Speaker 2 (19:10):
Yeah, that's a great point. You're right that we can
see zillions and zillions of galaxies out there, like the
Hubble deep field or the James Web deep field. Whenever
it focuses on some random patch of space and resolves it,
you can see evidence for so many galaxies in every
tiny little corner of space. So we know there are
lots and lots of galaxies out there. Very few of those, however,
(19:31):
have we identified a super massive black hole, in a
few very distant ones that happen to be koasars where like, okay,
that's definitely a super massive black hole, and some nearby
ones where we can watch the stellar populations and infer
that there must be something very very heavy at the core.
But most of those we can't do either because they're
either too far away or too quiet. But every galaxy
(19:53):
that's close enough for us to watch the stars, we
have seen a super massive black hole at its center.
So there's lots of galaxies out there, only very small
number can we check for a black hole, And every
single time we've checked, we've found one.
Speaker 5 (20:06):
Mmmm.
Speaker 1 (20:07):
So, like Andromeda, isn't it? Andromeda the closest galaxy to us.
Speaker 2 (20:10):
Andromeda is the closest galaxy to us, and we think
it has a huge black hole at its center, and
it's close enough that we can pretty well model the
velocities of the stars at the center of that galaxy,
and so we're pretty sure there has to be something
super massive and very dense at the heart of Andromeda,
very likely a black hole.
Speaker 1 (20:30):
All right, So then the data kind of suggests that,
like if you look at the galaxies that we can't
look at we do see a black hole in the
middle there, or something that is probably a black hole,
but why else would we extrapolate that to every galaxy
in the universe.
Speaker 2 (20:43):
Yeah, that's a good question. And to underscore sort of
our lack of knowledge here, we really identified black holes
in an approximately like one hundred to two hundred galaxies.
So of the zillions of galaxies that we've seen, only
a very small number have we been able to do
this check. And so you're tentative when you're extrapolating from
like a few hundred examples two trillions of objects out there.
(21:05):
On the other hand, it's every single one that we've
seen so far, right, so we wonder if it really
is a theory.
Speaker 1 (21:10):
So there's no black hole nearby, then we can check
that they didn't have a black hole in it, or
would we know or I guess what are you saying.
Speaker 2 (21:17):
I'm saying there's no galaxy nearby that we can check
that hasn't had a black hole in it, and that
includes really large galaxies like Andrameda, and also even dwarf
galaxies like there's a dwarf galaxy called RGG one one
eight and they recently found what they call a teeny
tiny super massive black hole in it.
Speaker 1 (21:35):
Where is that the actual scientific name? Teeny tiny supermassive?
Speaker 2 (21:40):
That was the title of the press release. I don't
know if that ended up in the paper or not.
Speaker 1 (21:44):
Doesn't that just average to a small black hole.
Speaker 2 (21:46):
It's only fifty thousand times the mass of the Sun,
So for a super massive black hole, it's pretty teeny
tiny compared to the Sun. It's pretty much a monster.
It depends on your perspective. But there's something else going
on here that makes us suspect that maybe every single
galaxy has a black hole in it. It's not just
that every single one we've seen has a black hole
in it, but we see this very tight pattern. We
(22:09):
notice this very close connection between the mass of the
super massive black hole and the mass of the galaxy.
And that might not be a surprise. You figure, like, look,
more stuff means a bigger galaxy, means more stars means
bigger black hole, right, And that's true. But if you
run those simulations, you get kind of a scatter like
you get some bigger black holes and some smaller black holes.
(22:30):
But what we notice when we plot, like the mass
of the black hole versus the mass of the galaxy
is a much tighter correlation than you would expect. Just
from like more stuff means bigger black hole, you get
this very very compact line that suggests that there's like
some feedback between the black hole and the galaxy.
Speaker 1 (22:48):
So you mean, like when we look at a black
hole in another galaxy, we can tell its size first
of all, And if you compare the size of the
black hole to the size with the galaxy, it's like
it's like almost one to one.
Speaker 2 (22:59):
Kind and if you're just made bigger and smaller galaxies
and guess how big the black holes would be, then
the same size galaxy shouldn't always give you the same
size black hole. Depends a little bit on like where
the stuff is and how much falls in, etc. But
what we notice is a very very close connection, as
you say, like one to one, that the mass of
the galaxy and the mass of the black hole track
very very closely.
Speaker 1 (23:20):
Meaning that there's no small galaxy with a big black hole,
and there's no big galaxy with a small black.
Speaker 2 (23:25):
Hole exactly, and two galaxies with the same size that
basically exactly the same size black hole at their heart.
There's almost no variation there, and that tells us that
there must be some sort of connection, that there's something
about how the black hole is forming and how the
galaxy is forming that connects these two things. I mean,
even from a spatial point of view, it's kind of
weird that like this dot at the very very center
(23:46):
of the galaxy. I remember, these things are very massive,
but they're also very very small, So it's weird that
this dot at the center of the galaxy is influenced
by like the mass of the whole huge galaxy that's
like one hundred thousand light years across. So there must
be some sort of connection between them, some information passing
back and forth, some process that's controlling both of them.
(24:07):
And if that's the case, and that makes us think, oh,
there must be a connection between the two. And so
probably every galaxy does have one of these.
Speaker 1 (24:14):
Things, because, like you're saying, if it's sort of inevitable
for a galaxy to get a black hole in the
middle when it forms, then there probably isn't any galaxies
without black.
Speaker 2 (24:26):
Holes, Yeah, exactly. And if there's some feedback mechanisms, something
which is controlling both of these things, then probably generated
both at the same time. It controls the stars that
form in the galaxy and also controls the mass of
the black hole. There must be some process tying these
things together, and being weirdly vague about that process because
we don't know what it is. There's a bunch of
(24:47):
theories about how the black hole might form and grow
and then the radiation from the black hole stops it
from growing, and the same process might control how stars
are formed and how they collapse from blobs of gas
and dust into stars. So there's a bunch of different
ideas out there, but they're all very vague and nobody
can really agree about it, or at the point where
we're just like, it seems like there's something going on here,
(25:08):
but we don't know what it is.
Speaker 1 (25:11):
Although this relationship between the black hole and the galaxy
size comes from data of galaxies that we found that
have black holes, right, Like, maybe there's still the possibility
that won and in a thousand galaxies doesn't have a
black hole.
Speaker 2 (25:24):
Yeah, it's certainly possible that that's the case. There's a
small number of galaxies here. Also, people might remember there
was recently this crazy idea that super massive black holes
or the hearts of galaxies are connected to the cosmic acceleration,
that they're really like bubbles of dark energy. And a
crucial thing that people notice that fueled that idea was
that there's a connection between the expansion of the universe
(25:46):
and the size of these black holes. That the black
holes seem to be more closely connected to the cosmic
expansion than their own galaxies. So that might seem like
it's in contradiction of what we're saying here today. Today
we're saying, oh, the black holes of the hearts of
galaxies are very closely connected to the size of their galaxies,
and a few weeks ago we said, no, they're not.
They're more closely connected to the universe. And the way
(26:07):
to untangle it is to remember that there's two different
kinds of black holes that we're seeing. We're talking about
black holes we see very very close to us, and
in those the black holes very tightly connected to the
mass of the galaxy, versus black holes that are very
far away, very old black holes those are from quasars,
So those are the ones that seem to be connected
to the cosmic expansion. So, long story short, there's a
(26:29):
lot we still don't understand about super massive black holes.
Speaker 1 (26:32):
Even how they form, right, Like, that's still a big mystery.
Speaker 2 (26:36):
Absolutely, we have no idea how these things even got
to exist. If you just start from like a big
blob of stuff and watch it form a galaxy, like
in simulation, you get a black hole at the center,
that's not a mystery, but it's not this big. Like
we look back in time by looking at old light
and looking at really early galaxies, and we notice that
they have huge black holes at their hearts, like already
(26:57):
billions of times the mass of the Sun, like the
first billion years of the universe. These are again the
very distant, very old black holes we see from quasars,
and we can't explain how that happens. In our simulations.
That just doesn't happen so quickly. It takes much longer
for these black holes to get so big. So there's
something else going on to form these black holes that
(27:18):
we don't understand.
Speaker 1 (27:19):
So we really have no idea what could be going on.
I mean, it kind of seems like maybe there's a
simple explanation in there somehow, Like you know, if you
start with a big cloud of gas, and yeah, the
galaxy is going to be bigger, and the black hole
in the middle is going to be bigger, Like what's
the big mystery there?
Speaker 2 (27:34):
Yeah, and that would give you a correlation that would
say that in general, masses of galaxies should be connected
to the masses of the black hole. And we see that.
But again we see a much tighter connection than you
would expect just from that simple explanation. We see that
galaxies with the same mass have basically exactly the same
mass black hole. There's like no variation there. So the
(27:55):
connection is just tighter than what you would expect from
that simple argument. So there must be something else going on.
Is it true that we have no idea? I mean,
people definitely have ideas, and I read like ten papers
about ten different ideas for what could be controlling it
complicated theories about how the gas gets blown in or out,
or gets heated up or cool down. We have lots
of ideas, we just don't know which one might represent reality.
Speaker 1 (28:17):
Interesting And now, is it possible, I guess, for a
galaxy to not have a black hole? Like why couldn't
that happen?
Speaker 2 (28:23):
Yeah, that's a great question. And there's sort of two
questions there, right, Like one is could you make a
galaxy without a black hole. Is it possible to pull
all those stars and all that dark matter together without
making a black hole? And we think the answer to
that is no, That every time you get enough stuff together,
whatever is pulling that together is going to form a
black hole at its center. That's just inevitable.
Speaker 1 (28:45):
Why do we think it's inevitable?
Speaker 2 (28:46):
Well, I guess for a couple of reasons. To summarize,
One is every galaxy we've seen so far has a
black hole at it's heart, right, We've never seen a
galaxy where we're able to check whether there's a black
hole and haven't found one, though there's an asterisk there
which will get to in just a minute. And number
two is this connection between the sizes of them that
tells us that there's probably some mechanism that's controlling both
(29:07):
the mass of the galaxy and the mass of the
black hole together. So back to the asterisk, there actually
are a couple of galaxies nearby that we've looked at
that don't have a black hole at the center. And
that points to another potentially fascinating story, which is whether
it's possible for a galaxy to form a black hole
and then eject it like can a galaxy form, make
its own black hole and then lose that black hole.
Speaker 1 (29:30):
Wait, wait, wait, wait, we're seeing that there are galaxies
we've seen that don't have a black hole in them
in the middle.
Speaker 2 (29:35):
There are a small number of galaxies we've seen that
have a black hole, but it's no longer at the
middle of the galaxy.
Speaker 1 (29:41):
Wait, what where is it? Like at the edge or
really far away from it? What do you mean?
Speaker 2 (29:46):
So there's a few variations. In one case, we've seen
a super massive black hole that's like displaced from the
center and has a pretty high speed away from the center.
So there's this possibility that galaxies could form make a
black hole the center, and then through some dynamical process,
some like interaction with other galaxies, their black hole could
get like kicked out of the center. And we've actually
(30:08):
seen this in a few galaxies.
Speaker 1 (30:09):
What are the other ones like that don't have a
black hole in the middle.
Speaker 2 (30:12):
So there's the one example called CID forty two, which
is about four billion light years away. It has a
super massive black hole, but it's near the center, but
it's sort of displaced from the center. And then there's
another observation just a few weeks ago where they see
a streak of light shooting out of the galaxy. The
streak of light is like two hundred thousand light years long,
and at the end of it there's a black hole.
(30:34):
And so it looks sort of like the black hole
was ejected from this galaxy. It's like a runaway black
hole and left this streak of stars in hot gas
in its wake.
Speaker 1 (30:44):
Well, it can actually see like the skid marks of it.
Speaker 2 (30:47):
Yeah, exactly, And so we think that this might happen
sometimes when galaxies merge. We know that galaxies merge, that
that's a very normal thing. We think the Milky Way
is formed by a bunch of galactic mergers. And Dromeda
is so big because because it's a combination of a
bunch of baby galaxies that all got merged together. And
the normal thing to happen when galaxies merge is that
the black holes also merge. I mean, you have two
(31:09):
clouds of stars, each with a black hole at their center.
They're all going to orbit each other. Eventually, the black
holes that their hearts are going to orbit each other,
and then because of friction and gravitational radiation, they'll eventually
collapse into a single black hole. That's like the normal
thing to happen, but there's some variations there. When two
black holes collapse into one, they also emit a lot
(31:30):
of radiation. Like the mass of the two black holes
doesn't one hundred percent go into the mass of the
final black hole. It loses some mass and it generates
a bunch of gravitational waves. That's how we see these
black hole mergers. We talked about Logo and Virga and
all these observations that see these ripples in space time
generated by these black hole collisions. You get those ripples
(31:51):
because the black holes are accelerating as they orbit each other,
and that radiates a way energy, so it loses some mass. Now,
sometimes that radiation is in every direct like it just
sprays gravitational waves everywhere, but sometimes in special circumstances that
gravitational radiation tends to be in one direction rather than another,
and then it acts sort of like a recoil. It's
like shooting a gravitational wave gun in one direction and
(32:15):
the black holes get pushed back in the other direction
by conservation of momentum.
Speaker 1 (32:20):
So you're saying you can have a galaxy without a
black hole, but the ones we've seen so far that
are like that, there's evidence that it had a black
hole in the middle at some point.
Speaker 2 (32:29):
Exactly, so it might be possible to have a galaxy
without a black hole by building a galaxy with a
black hole and then like ejecting it getting rid of
the black hole.
Speaker 1 (32:39):
I guess one thing that maybe important to understand is
that even though a galaxy can have a super massive
black hole in the middle with the massive millions of
our suns, it's not like the black hole is anchoring
the galaxy, right, like to a galaxy with tondreds of
millions of stars, like one little black hole in the
middle is not super important in the same way that
(33:00):
the Sun is important in our Solar system.
Speaker 2 (33:02):
Right, Yeah, you're absolutely right. The Sun is like ninety
nine percent of the mass of our solar system. But
the black holes the hearts of these galaxies are a
tiny fraction, much less than one percent of the mass
of the galaxy. Like in the Milky Way, we have
billions of stars and so billions of solar masses, and
our black hole is only five million solar masses. So
it's a tiny fraction of the mass of our galaxy.
(33:24):
Even though we're talking about really big objects. Galaxies themselves
are much much bigger. So it's more like if our
Solar system lost Jupiter, right, Jupiter is a big planet,
that'd be kind of a big deal personally, but it
wouldn't affect the dynamics of the Solar System the way
it would if we lost the Sun, Right, that would
be much more dramatic.
Speaker 1 (33:42):
Right, These super massive black holes are really more like
little pimples kind of in the middle of galaxies. Right,
It's not like the galaxies there because of the mass
super massive black hole. It's more like maybe just a
feature that pops up when you're making a galaxy.
Speaker 2 (33:55):
Yeah, I think we don't really understand that. I mean,
I think the causal relationship is probably complicated. It seems
like it's probably a necessary outcome when you form a
galaxy that you get a super massive black hole. So
in that sense, it kind of is necessary to have it.
But you're right, you can get rid of the black
hole and the galaxy can still hold itself together, so
in that sense, you don't need it anymore.
Speaker 1 (34:17):
Like maybe the black hole needed the galaxy to form,
but maybe the galaxy didn't need the black hole to form.
Speaker 2 (34:23):
Yeah, But then the black hole stays a black hole.
When it's been ejected, it's still a black hole out
there wandering an intergalactic space. WHOA. So one way to
get rid of your black holes to generate a bunch
of gravitational radiation in one direction to kick the black hole.
For that to happen, you typically have to have sort
of like a low mass black hole in order to
get enough acceleration to like get out of the galaxy.
(34:44):
Another scenario is to have sort of like a three
galaxy dance. Remember that when three objects interact, it's much
more chaotic, Like, the three body problem is not something
we know how to solve, whereas the two body problem
is simple. So if you have three galaxies that are
merging at the same time, then one of those black
holes can get kicked out instead of merging with the
(35:04):
other two. So that's another scenario that can create a
runaway black hole, essentially kicking a black hole out of
a galaxy.
Speaker 1 (35:11):
Like if three solar systems similar to ours came together,
it would get so chaotic and so scrambled that it
could actually like shoot off the Sun or Jupiter out
into space.
Speaker 2 (35:21):
Right, yeah, exactly. And because of conservation momentum. If you're
going to kick one black hole out in one direction,
then the other two are going to get kicked in
the other direction, and so the whole system might end
up without a black hole at its center.
Speaker 1 (35:35):
All right, let's take a little bit more into what
this all means and maybe how dark matter plays into it.
But first let's take another quick break. All right, we
(35:57):
are asking the question does every galaxy have a black
hole at its center? And I guess we've figured out
the answers. No, right, some galaxies have a black hole
near its edge if it gets kicked out.
Speaker 2 (36:09):
Yeah, we think that probably every galaxy has a black
hole at its center during its formation or at some
point in its history, because we think probably the process
that makes these galaxies, it gathers together all this mass
and funnels it into this gravitational well that remember, in
the end, is made by dark matter. Right, The reason
these galaxies exist is because there's a density of dark
(36:32):
matter there which is pulling in all this other matter
to make this dense stuff that we can make a galaxy.
Probably that process inevitably makes a black hole at the
same time as it makes a galaxy. But then you
can lose that black hole. During the life of these galaxies,
as they merge, as they come together, as they dance
around each other, those black holes can get ejected from
(36:53):
the hearts of those galaxies. So yeah, you could end
up with a galaxy without a black hole at its center.
Speaker 1 (37:00):
Like we said earlier in the episode, that every GALLESSI
we've seen has the black hole at its center, But
it sounds like that's not really true. That there are
galaxies out there we've seen so we can measure it
that don't have one in the middle at all. Have
we seen any with no black hole at all.
Speaker 2 (37:15):
We have not found a galaxy without any kind of
black hole. Either they have a black hole at their center,
or we've identified a black hole that's been ejected from
the center. That's a very small number of cases so far,
or they're too far away for us to tell what's
going on at the center. Then there's one particular galaxy
which is a bit of a mystery. Its called Able
two two six one. It is a really big galaxy
(37:37):
and we have not yet found the super massive black
hole there. We looked at it in the X ray
and not seen the sign of it. It's still sort
of an open question whether people are going to find
evidence of the black hole or not, but it's one
that has a question mark next to it. Remember, this
is like a recent and active field of research. People
are developing new techniques to try to study these galaxies
to try to see if they are black holes in them.
(37:59):
It's definitely a question a lot of people are working on,
so it's evolving rapidly. But either we've seen a black
hole at center, or we've seen a black hole like
leaving its center, or there's this one question mark galaxy
able two two six 's one, or we're just not
able to see it, because remember, the galaxies have to
be close enough for us to like study their centers
in order to be able to see these black holes,
(38:21):
or they have to be active enough for us to
see them emitting. Like another way to see these black
holes is to watch them like burp as they eat something.
Speaker 1 (38:30):
But then you're only seeing the rude black hole.
Speaker 2 (38:33):
That's right. But occasionally, even a quiet black hole, even
when it's not surrounded by a big disc of gas
and dust and emitting like constant beams of light, occasionally
a star might wander close to it and get eaten
and the tidal disruption there will give you a very
bright flare. So we've seen those kinds of things as well,
like otherwise quiet galactic centers that suddenly emit a big,
(38:55):
bright burst of light. We've even seen that from our
own galactic center. Sometimes the way you can observe supermassive black.
Speaker 1 (39:02):
Holes, it's like the final scream of a star before
it gets eaten wo.
Speaker 2 (39:08):
Exactly. Another thing people are doing is trying to identify
these super massive black holes out in intergalactic space. Like
people are curious how often do super massive black holes
get kicked out into space? Remember we talked about like
rogue planets before, planets that used to be orbiting a
solar system but then due to the gravitational chaos of
(39:28):
their system, get thrown out into space. And it turns
out there's a huge number of them out there. People
have discovered rogue planets by micro lensing, looking for moments
when these planets pass in front of a star in
the background and that star's light gets distorted by the
gravity of the planet, And by doing that we can
spot a bunch of these things and then extrapolate to
how many there are. The same way we can look
(39:50):
for super massive black holes in intergalactic space by looking
at for these micro lensing events.
Speaker 1 (39:57):
M sounds kind of scary, the idea that there are
super massive black holes out there roaming space, possibly in
our direction.
Speaker 2 (40:05):
Yeah, it's certainly possible that they're out there. We've seen
some of them getting ejected from their galaxies, but we
think that might be kind of rare, and when we're
talking about like a one percent level event based on
our recent observation, so of course with a big uncertainty.
And there are many fewer galaxies than there are stars
of course, so the number of rogues super massive black
holes out there could be big. But space is of
(40:26):
course really really vast, so these things would be pretty
hard to spot.
Speaker 1 (40:30):
I guess it's hard for even a super massive black
hole to leave a galaxy, right Like, as we said before,
a galaxies huge, got billions and trillions of stars, it's
really massive. It's probably pretty hard for a black hole
to get the escape velocity needed to leave a galaxy
and come towards us exactly.
Speaker 2 (40:47):
That's why it tends to happen mostly for lower mass
super massive black holes, not necessarily even teeny tiny ones,
just ones on the lower edge, because a lot of
them are going to have this asymmetric radiation from gravitational
waves or some sort of chaotic merger event that it's
not totally symmetric. But most of them are massive enough
that they get pulled back to the center of the
(41:07):
galaxy and eventually settle down. But the lower mass ones
can get going pretty fast, like this CID forty two galaxy.
They estimated the velocity of the galaxy to be half
a percent of the speed of light, which is like
very very fast moving black hole.
Speaker 1 (41:23):
It's in a hurry.
Speaker 2 (41:25):
To get out of there, really wants to go somewhere exactly.
Speaker 1 (41:29):
I guess the question is how does this all tie
into dark matter? Because I know dark matter is very
important in the formation of galaxies. Right, It's almost like,
I know we've mentioned this before, the galaxies form around
where dark matter is. Does that effect sort of how
black holes might get formed.
Speaker 2 (41:45):
It definitely affects where galaxies and black holes get formed. Right.
Galaxies basically trace out where the dark matter is in
the universe. If you look at the large scale structure
of the universe, you see these filaments of galaxies and
these sheets of galaxies and that's because that where the
dark matter is. We can't see the dark matter directly,
but we can see that it's gathered together all this
matter and made all of those galaxies. And so every
(42:08):
galaxy has a huge dark matter halo around it, Like
the Milky Way is about one hundred thousand light years across,
but there's a big blob of dark matter that's like
two hundred thousand light years across that the Milky Way
is embedded in. Now, if we shot our black hole
out of our galaxy, then it would pass through that
dark matter halo, and it would gobble up a lot
of dark matter along the way. And so as these
(42:30):
supermassive black holes leave the galaxies, they can increase their
dark matter fraction.
Speaker 1 (42:36):
They can get even darker than black Yeah.
Speaker 2 (42:40):
We think that these black holes already have some dark
matter in them because dark matter is everywhere. But dark
matter also isn't sticky, and so it's very easy for
dark matter to just like rotate in orbit forever around
the galaxy and not fall in. We think will probably
fall into the center of the galaxy because eventually it's
like dynamical friction. Our star will get pushed by others
stars on. All that jostling ends up kicking somebody towards
(43:03):
the center of the galaxy. But dark matter doesn't do that,
right because dark matter just passes right through itself. So
normally dark matter can swirl around the center of the
galaxy not getting eaten by the black hole. But if
the black hole like runs free, then it's basically like
plowing through a buffet of dark matter.
Speaker 1 (43:19):
But maybe when as the galaxy was forming, maybe the
black hole did eat a lot of dark matter. Or
maybe I wonder if the black hole in the middle
of galaxies is maybe mostly made out of dark matter.
Speaker 2 (43:30):
It's hard to know, right, And what does it mean
to be made of dark matter? Because once it goes
past the event horizon, who knows what happens? Everything's dark matter,
everything's something else, some black hole state of matter. Right,
we don't know if dark matter gets annihilated it turns
into something else, or if it retains something of its nature.
We just don't even know because we don't know the
particle properties of it. So, like, what are the conservation
(43:53):
laws for dark matter? The universe might like keep track
of how much dark matter there is and not allow
things to convert. We just don't know so clueless about it.
Speaker 1 (44:01):
So some black holes might be darker than others.
Speaker 2 (44:03):
Absolutely, some might be darker than others. We know that
some galaxies are darker than others, some galaxies have a
larger dark matter fraction than others. That's for sure, all right.
Speaker 1 (44:12):
Well, what does it mean for our understanding of galaxies
and how they form?
Speaker 2 (44:16):
It means that we're still the very beginning of the
journey of understanding how galaxies come together, which is sort
of shocking because we've been setting galaxies for so long,
but every few decades we'd learned something new and surprising
about what's going on with these galaxies, how they come together,
what their history is, and what their future fate is. Right,
and we know that the galaxies and sort of our
(44:37):
local group are gravitationally bound together and so eventually going
to end up falling together and probably forming one super
galaxy while they're being separated from the rest of the
universe by dark energy. And that super galaxy is going
to have a super duper massive black hole at its
center unless that gets kicked out and shot into intergalactic space.
(44:58):
So it tells us a lot about our our potential
future because as we orbit our galaxy and merge with
those other galaxies, we also eventually fall towards the center
of this blob. And if there's a giant black hole
waiting for us there, this is really only one way
for the story to end.
Speaker 1 (45:12):
But even if we kick out that black hole, eventually
the galaxy is going to collapse anyways, right, and maybe
it will collapse into a new black hole.
Speaker 2 (45:19):
Yeah, exactly, it could definitely form a new black hole
even if you kick out the original one. There's no
noncompete clause in galactic formation.
Speaker 1 (45:26):
And when is this supposed to happen, like tomorrow or
trillions of years?
Speaker 2 (45:32):
Oh, we're talking about billions and billions of years for sure,
So we are much more a threat of the Sun
expiring before our system collapses into the center of a
black hole. But if we want to continue on for
billions and billions of years, we definitely need to plan
deep into the future. And also, we're just curious about
how black holes work and how galaxies work, and we
want to understand it because it could be that there's
(45:52):
some other deep insight into the way the universe works
waiting for us. Every time. In science, we're like, don't
really understand how something works, and we deeper, we discover
something fascinating underneath that we didn't even expect to find,
so satisfying our curiosity and like trying to understand this
in great detail is good path to like opening up
some surprising new doors. So we're doing our best to
(46:14):
improve our future prospects for understanding these things.
Speaker 1 (46:17):
Mmm, sounds like a problem for the AIS. You could
just kick back, let to figure it out.
Speaker 2 (46:23):
Well. One of my favorite ways to study these things
in the future is with our space based gravitational Observatory
so LIGO is this gravitational wave observatory that has two
arms that are kilometers long filled with lasers, and they're
planning to build one in space called Lisa LSA mean
much much bigger, and so it's going to be able
to observe gravitational waves at lower frequencies, which is what's
(46:46):
generated by the collisions of super massive black holes. So
we might be able to observe gravitational waves from galactic
mergers and understand this process in more detail. What happens
when two super massive black holes really do come together?
Speaker 1 (47:01):
WHOA, which is what happens when two galaxies collide right.
Speaker 2 (47:04):
Exactly, yeah, yeah, and so we could see the signature
of black holes getting injected from the hearts of their.
Speaker 1 (47:10):
Galaxies, ejected or smushed.
Speaker 2 (47:12):
Smushed or ejected. We don't know both outcomes are possible.
Speaker 1 (47:16):
But will we hear the black hole get injected?
Speaker 2 (47:19):
Well, black hole injection comes with a very powerful, very
directional gravitational wave signature, and so they think they might
be able to distinguish that from just like a normal
combination of two black holes into one. But it all depends
on this crazy set of satellites in space shooting lasers
in each other, measuring tiny changes in their relative distances.
Speaker 1 (47:41):
Well, it sounds like the future is all black hole.
So we bidters start to understand them and get used
to them and figure out how they form.
Speaker 2 (47:50):
Right, absolutely something we'd like to understand, not just because
we're curious about the universe, but because they might control
a fate of our galaxy.
Speaker 1 (47:57):
Yep. Or it might be just something for our AI
replacements to figure out. We might be long gone.
Speaker 2 (48:06):
I'll read about it from my lounge chair at the
center of the black hole.
Speaker 1 (48:09):
All right, we hope you enjoyed that. Thanks for joining us.
See you next time.
Speaker 2 (48:21):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hey, Daniel, have you started planning for your retirement retirement?
Speaker 2 (00:13):
Why would I ever retire?
Speaker 1 (00:14):
I mean, you're gonna be a physicist forever to you're
dying breath.
Speaker 2 (00:18):
I don't see myself burning out of questions.
Speaker 1 (00:20):
Well, I guess eventually the Sun is gonna expand blow
up and burn the Earth, right, so at some point you.
Speaker 2 (00:26):
Have to stop. I guess so, And eventually the galaxy
will collapse into a black hole. But that doesn't mean
we have to stop.
Speaker 1 (00:33):
You're gonna be doing physics inside of the black hole.
Speaker 2 (00:37):
Absolutely, I'll be writing papers about the singularity that nobody
can ever read.
Speaker 1 (00:41):
Oh, because you're inside the black hole, right, unless I'm
like closer to the center, in which case you could
still technically send me the paper.
Speaker 2 (00:49):
Yeah, and I could never get your comments back. Hey,
that sounds pretty good.
Speaker 1 (00:52):
Actually, it's a win win situation or a lose lose situation.
It's a wind lose wind loose situation.
Speaker 2 (00:58):
I'll be waiting for pure you literally forever.
Speaker 3 (01:17):
Hi.
Speaker 1 (01:17):
I'm Jorge, cartoonist and the creator of PhD comics.
Speaker 2 (01:20):
Hi. I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I do plan on doing this.
Speaker 1 (01:25):
Forever, forever, even after your death.
Speaker 2 (01:30):
I'm hoping my ideas live on.
Speaker 1 (01:32):
Are you going to program an AI to do research
for you and make your children feel guilty after you're gone.
Speaker 2 (01:40):
I'm going to upload my intelligence into the cloud so
I can continue to live as some sort of weird
mixture between biological and mechanical intelligence. But even that eventually
will fall into the galactic center and be gobbled up
by the black hole.
Speaker 1 (01:53):
M But I guess why wait? I mean we could
technically generate this podcast with AI right now.
Speaker 2 (01:58):
Can How do you know we haven't already? Who do
you think he's writing these outlines? Oh? My god?
Speaker 1 (02:03):
Am I talking to an AI right now?
Speaker 2 (02:06):
Maybe I'm talking to an AI right now? Maybe AI
are listening to this podcast.
Speaker 1 (02:10):
He did seem smarter than usual.
Speaker 2 (02:12):
That's the real Daniel.
Speaker 1 (02:15):
And the real Danger. But anyways, welcome to our podcast
Daniel and Jorge Explain the Universe, a production of our
Heart Radio.
Speaker 2 (02:22):
In which we use are still limited biological intelligence to
try to understand how the universe works. We want to
know everything that's out there. We want to know how
it works on the microscopic scale, to weave itself together
into reality that we experience and that we want to understand.
We cast our minds out into the farthest reaches of
(02:42):
space and hope to understand what they might encounter out there.
Speaker 1 (02:46):
That's right, because it is a very perplexing universe full
of interesting mysteries for us to go out there explore
and try to uncover, and try to understand and try
to explain to others.
Speaker 2 (02:56):
Some mysteries have lasted for decades or centuries or millennia,
but others have succumbed to the onslaught of human intelligence.
We have sometimes actually managed to crack puzzles of the
universe and understand what's going on at the atomic scale
or the subatomic scale, or how space bends to form
black holes. So there are times when our limited intelligence
(03:17):
is enough.
Speaker 1 (03:18):
Why I make it sound like we're masters of the universe?
How far along are we are? We up to like
one percent of the universe by now.
Speaker 2 (03:25):
I think that's probably a pretty generous estimate. But it's promising.
You know, we are making progress and we are understanding things,
and the understanding is coming faster and faster. So far,
there's no evidence that we're going to hit some wall
of understanding.
Speaker 1 (03:39):
Especially with the AIS. Could we train AIS or just
ask AIS to do research for us?
Speaker 2 (03:44):
I use AIS in my research all the time. We
have AIS answering questions. The big challenge is understanding what
they're doing. Are they doing physics for themselves or are
they doing it for us?
Speaker 1 (03:54):
What do you mean for themselves? Like for their own curiosity?
Oh my god, are yet that already?
Speaker 2 (04:00):
Well? We have AIS generating YouTube videos that are mostly
watched by other AIS to generate response videos, and now
we have AIS generating physics research, and at some point
AIS are going to be reading that physics research and
summarizing it for themselves.
Speaker 1 (04:15):
Huh. So I wonder if the AI science community is
going to have as much politics and drama as the
real physics community, or maybe even more right, more drama.
But it's all artificial drama.
Speaker 2 (04:26):
So all drama is artificial, and.
Speaker 1 (04:31):
It happens faster, I guess, which is good.
Speaker 2 (04:33):
Though there might be some real drama going on at
the center of our galaxy, as the black hole there
keeps sucking things in that's swirling around it.
Speaker 1 (04:42):
Yes, we discussed on this podcast. There's a lot going
on at the center of galaxies, including some very big mysteries.
Speaker 2 (04:48):
That's right, as we peer towards the center of our
own galaxy, we notice a very very large black hole
sitting there, gobbling up anything that comes near it, and
while we are safely swirling around it at tens of
thousands of light years away, eventually we think that everything
might fall into that black hole, flush down, the gravitational.
Speaker 1 (05:09):
Drain down physics toilet? Is that really the case? Eventually? Like,
there's no prediction where we don't get sucked into that
black hole.
Speaker 2 (05:16):
There are some variations. If the universe keeps expanding at
a faster and faster rate and something called phantom energy
takes over and creates enough space between us and the
central black hole to distance us from it. But if
dark energy stays at the level that it is, the
prediction is that the universe ends up with a bunch
of isolated galaxies, each of which collapse into their own
(05:37):
black holes.
Speaker 1 (05:38):
Wait wait, wait, did you just say phantom energy.
Speaker 2 (05:40):
Phantom energy exactly like.
Speaker 4 (05:44):
The Phantom Menace named after that movie exactly. Not a coincidence,
m I guess we'll have to dig into that in
another episode. But it is interesting how as you said
that every galaxy has a black hole.
Speaker 2 (05:56):
It is pretty cool to have a black hole the
center of our galaxy, and it makes us wonder if
that required for every galaxy. Is that a necessary product
of a galaxy? Do you have to have a black
hole to form a galaxy? Do galaxies form black holes?
Do black holes create galaxies?
Speaker 1 (06:11):
It's a deep question, and so today on the podcast
we'll be asking the question, does every galaxy have a
black hole at its center? You mean, like like a
tasty snack or something like are there any defective galaxies
that they didn't get a black hole in the middle?
Speaker 2 (06:32):
You mean, like you'd be disappointed to bite into a
galaxy and be like, oh, I didn't get a black
hole in the middle.
Speaker 1 (06:37):
I know, that's the tastiest part. It's got the richest flavor.
Speaker 2 (06:41):
So deep. That's interesting because black holes have mass, but
I wonder if they actually have calories. Like if you
eat a black hole, do you lose weight?
Speaker 1 (06:47):
Or is the secret to a new diet?
Speaker 2 (06:50):
I guess really the black hole eats you and it
gains weight, so you become part of the black hole.
Speaker 1 (06:55):
Something gets eaten and it's you, which I guess is
a diet in a way you lose a lot more
than weight.
Speaker 2 (07:02):
Though, definitely do not take health advice from either of us.
Speaker 1 (07:06):
That's right, from either physicist or cartoonists, not the healthiest
people in the world or the universe.
Speaker 2 (07:12):
We are totally unqualified to dispense health advice. I know
that doesn't stop most podcasters, but it does give us pause.
Speaker 1 (07:18):
So, yeah, this is an interesting question. Does every galaxy
have a black hole at its center? I guess a
lot of galaxies have black holes at their centers.
Speaker 2 (07:26):
We certainly have seen a black hole in the center
of our galaxy and a few others. But it's a
really interesting question both about how black holes get formed
and how galaxies get formed. It seems like there's a
fascinating dance between the two.
Speaker 1 (07:39):
I guess. Kind of the deeper question is like, do
you need to have a black hole at the center
to have a galaxy?
Speaker 2 (07:45):
Yeah? Exactly does it count as a galaxy if it's
just a bunch of stars?
Speaker 1 (07:49):
So, as usual, we were wondering how many people have
asked this question, whether every galaxy has a black hole?
So Daniel went out there into the wilds of the
internet to ask folks, does every galaxy a black hole
at its center.
Speaker 2 (08:01):
Thanks very much, my enduring gratitude to everybody who participates
in this segment. But remember it's open to everyone. If
you've been listening to pod and learning for years and
feel ready to answer a random questions, please don't be shy.
Write to me two questions at Danielandjorge dot com. We
want to hear from you.
Speaker 1 (08:19):
So think about it for a second. You think every
galaxy has a black hole. Here's what people had to say.
Speaker 5 (08:26):
Man, who knows if black holes even exist? Like, what
if they're just all dark stars like you guys talked
about earlier on the show. There could be wormholes or
something that we haven't even thought of yet. So no,
not all galaxies.
Speaker 6 (08:40):
A galaxy is a cluster of a bunch of objects
in space swirling around a massive object in the middle.
And the only massive object you can have is a
black hole or a store I guess, and you can't
have such a big star. So I think every galaxy
has to have a black hole in the middle.
Speaker 3 (08:58):
Probably there's got to be something like holding it all together.
But sence space is really big. Maybe there's some galaxies
that don't and they're just like really dense packed like
stars spinning around each other.
Speaker 2 (09:15):
I would guess that just because you're asking the question,
the answer is no, But I don't know. I would
for you guys to explain it to me.
Speaker 1 (09:24):
All Right, some people we have some conspiracy theories, apparently
as listeners, some galactic conspiracy theorists.
Speaker 2 (09:31):
Well, we are guilty of promoting those black hole conspiracy
theories right when we talk on the podcast about how
black holes might be dark stars or fuzzballs or something else.
Even weirder, are you seen?
Speaker 1 (09:41):
We're ground Sero or the black hole ism movement.
Speaker 2 (09:45):
Black hole truthers. Yeah, this is the dark Horse podcast
for black holes exactly.
Speaker 1 (09:50):
Maybe if we switched our podcast name to like universal
conspiracies or conspiracies about everything, maybe our ranking will go up.
Speaker 2 (09:59):
It might go up, we might get a bigger audience.
But I don't think we'd get a better audience.
Speaker 1 (10:04):
I mean, isn't physics, after all, just like the Hunt
for the Conspiracy.
Speaker 2 (10:08):
Of the Universe.
Speaker 1 (10:10):
You're like, there's something going on here, There's some sort
of plan or structure. We got to find out what
it is. We got to find out the truth. Man,
Isn't that what physics is all about.
Speaker 2 (10:20):
Yeah, but conspiracies are usually built on wild speculation and
a lack of evidence.
Speaker 1 (10:25):
Isn't that every other episode we discussed on the podcast.
Speaker 2 (10:29):
Yeah, but we don't trust strong conclusions based on our
lack of knowledge. We talk about what we don't know
and how we might actually figure it out. We're trying
to build evidence. We're looking for data when we're happy
to disprove our ideas if the universe shows us we're wrong.
Speaker 1 (10:43):
It sounds like something someone involved in a conspiracy would say.
Speaker 2 (10:46):
And you've just proven that you can make a conspiracy
about anything with no information.
Speaker 1 (10:50):
But anyways, this is an interesting question. Does every galaxy
have a black hole at its center? And, as you said,
there is one at the center of our galaxy. The
last podcast were recorded talked about how there's a big
one at the center of our galaxy, along with a
whole bunch of other stuff going on down there.
Speaker 2 (11:04):
We certainly think that there is a black hole at
the center of our galaxy.
Speaker 1 (11:08):
What do you say, we think?
Speaker 2 (11:09):
Yeah, we don't really ever know for sure. I mean,
our galactic center is the one that's closest to us,
so it's easiest to study. It's also shrouded in gas
and dust, so it's complicated to study, and in the end,
all of our evidence for black holes is always a
little bit indirect. Usually the observations tell us that there's
something there that's very massive, something that's very small and
(11:31):
also has a lot of gravity, but we've never actually
observed an event horizon directly. It's always a little bit indirect.
Speaker 1 (11:39):
I guess it's kind of hard to see a black
hole because they don't emit light themselves.
Speaker 2 (11:42):
They might emit light. There might be gentle hawking radiation
from black holes, which would be very powerful direct evidence
for black holes, but that's not something we've ever seen.
The story of black holes for the last few decades
is an increasing belief that black holes probably are real.
As we identify these very massive objects and we put
limits on how big they can be, we see things
approaching closer and closer to the black hole the center
(12:05):
of our galaxy, which tells us more and more about
how small it has to be. And there are only
a few things out there that could satisfy all of
those constraints that are that massive and that compact. Black
holes of course the most classic example. But now we
have a few other candidates, dark stars, fuzzballs, et cetera,
et cetera.
Speaker 1 (12:23):
Yeah, it's still a big mystery talking about the ones
that are at the center of galaxies. Let's maybe break
it down for folks and talk about black holes a
little bit in general, and the different kinds of black
holes that are out there and that could be at
the center of different galaxies.
Speaker 2 (12:36):
When we hear about black holes, we're often thinking about
stellar black holes. Like a star burns for billions of
years until eventually it's fusion peters out and it loses
the battle with gravity, so the gravity collapses the star
into a black hole, meaning that there's a region of
space where there's so much mass and energy that space
becomes so dramatically curved that every path within the event
(12:59):
horizon leads towards the center. That any particle that passes
into that event horizon will eventually find its way to
this center of that region. There are no paths out.
Space within the event horizon is so bent that every
timelike path for a particle leads towards the center. So
that's a stellar black hole. And those are awesome, which
is why we call them stellar, but also because they
(13:21):
collapse from individual stars, and they can be quite big,
like ten times the mass of our Sun, but they're
small compared to the kinds of black holes we're going
to talk about today.
Speaker 1 (13:30):
Yeah, those are called super massive black holes.
Speaker 2 (13:33):
Yeah, there's basically two categories of black holes. The stellar
mass black holes that are like a few tens of masses,
and then we jump up to really really really big
black holes, things like ten thousand or one hundred thousand
times the mass of our Sun in one huge black hole.
But they get much much bigger than that as well.
There are black holes that are millions or even billions
(13:55):
of times the mass of our Sun. And these are
the ones found at the hearts of galaxies.
Speaker 1 (14:01):
Yeah, you can sort of see them even in distant,
faraway galaxies, right, Like when you look at little Fuzzz
out there in the night sky with special telescopes, you
can actually kind of see the black hole in the middle. Right.
Speaker 2 (14:11):
Yeah, that's a really interesting and complicated question, like how
do we see these black holes in the milky way?
We have actually a lot of really interesting and cool
ways to look at the black hole because it's so close.
Like some of the best evidence we have is a
picture of the black hole released by the event Horizon telescope,
which is actually a picture of the accretion disk around
the black hole, the hot gas that's swirling very very
(14:33):
close to it and radiating light because it's so hot,
so we can see that. We can also see stars
orbiting very very close to the black hole the center
of our galaxy. There's this one star in particular S two,
with zooms super close to the black hole and whips
around it. We've actually seen an entire orbit of that
star all the way around the black hole, which won
(14:54):
a Nobel prize very recently. So the black hole the
center of our galaxy we've observed very closely. Black Holes
in other galaxies are harder to spot because those galaxies
are further away now.
Speaker 1 (15:04):
For the one at the center of our galaxy. Even
that one, we're not one hundred percent sure it is
a black hole, right, It could still be something else
super massive there, or are we pretty sure it's a
super mass black hole.
Speaker 2 (15:13):
I think both of those things are true. We don't
have direct evidence that it's a black hole. We know
that there's a lot of mass there because we see
the influence of that object on the nearby stars and
all the gas and dust. We know that it's not
emitting any light itself, so it's compact, it's dark, and
it's massive. So most physicists, I think, are pretty convinced
that it's a black hole. But that's just sort of
(15:35):
like the best idea we have. There are these alternative
theories that it could be a very rapidly collapsing region
of space which is going to bounce back and turn
into a white hole, for example. So there are other
theories there, but I think the mainstream physics community is
pretty convinced that it's a black.
Speaker 1 (15:49):
Hole, all right, So then how do we see them
in other galaxies?
Speaker 2 (15:53):
There's sort of two ways to see black holes in
other galaxies. One is when they're feeding. When black holes
are eating stuff, when they have like a lot of
gas swirling very close to them and falling in, it
tends to get hot and that gas radiates and they
can generate very very powerful beams of light. These are
things we call quasars, and we can see these from
(16:13):
like across the universe. There's some objects that are super
duper far away but incredibly bright, and when they were
discovered decades ago, people couldn't believe that they were actually
that far away because they look super bright already here
on Earth, and if they're also very very distant, that
means that at their source they're incredibly bright. And that's
exactly what's happening. The gas around in the black hole
(16:35):
is getting very very heated up, and as it falls in,
it radiates, and it radiation gets channeled by the magnetic
field of the black hole, and you get these very
powerful beams emitted from above and below the black hole,
and we can see those from very very far away.
So those are quasars. But we can only see that
for black holes that are like actively feeding and growing,
and that tends to happen sort of early in the
(16:56):
life cycle of the universe. Quasars peaked about ten billion
years ago, so that's good for seeing like young super
massive black holes very very far away.
Speaker 1 (17:05):
So not every super massive black hole is a quasar, right,
There are some that could be out there just sitting
there being big.
Speaker 2 (17:12):
Exactly most of the super massive black holes we think
are not quasars. We think that a lot of them
stop sort of quazing billions of years ago and are
now more like dark relics, huge black holes sitting at
the centers of galaxies but not being quasars anymore. But
we can still spot those by looking at the motion
of stars around them. So pick them nearby galaxy and
(17:33):
look at the stars that are near the center of it.
You can't resolve them individually. We don't have telescopes that
can say here's a star, there's a star, there's a star.
But we can look at stellar populations at the hearts
of other galaxies, and we can measure their velocities, we
can measure their brightness. It's really complicated and very computationally expensive,
but we can build a model of how fast those
(17:54):
stars are moving around the center of that galaxy, and
from that we can infer the mass of the black hole.
How heavy does the thing have to be at the
center of the galaxy to support this super fast radial
motion by those stars.
Speaker 1 (18:07):
All right, well, let's get into whether every galaxy has
a black hole and what would it mean for a
galaxy to not have a black hole at its center.
But first let's take a quick break. All right, we're
(18:30):
asking the question does every galaxy have a black hole
at its center? Like, does every galaxy have a dark
soul or something?
Speaker 2 (18:37):
I'll leave you the philosophical and moral implications of it.
But from a physics point of view, it's a trend
that we're noticing that galaxies have these black holes, and
so we wonder if it's a rule. You know, it's
basically our strategy in physics is like, look out of
the universe, find patterns, wonder if those patterns reflecting like
deep rules of the universe. Figure out what those rules
have to be. So, you know, we're still in like
step two here.
Speaker 1 (18:57):
Hmmm, Now, I guess what is it to say, Like,
you know, we can see traillions of galaxies out there
in space, can we tell what percentage of them have
black holes? Or does every galaxy we've seen have a
black hole in it that we can tell?
Speaker 2 (19:10):
Yeah, that's a great point. You're right that we can
see zillions and zillions of galaxies out there, like the
Hubble deep field or the James Web deep field. Whenever
it focuses on some random patch of space and resolves it,
you can see evidence for so many galaxies in every
tiny little corner of space. So we know there are
lots and lots of galaxies out there. Very few of those, however,
(19:31):
have we identified a super massive black hole, in a
few very distant ones that happen to be koasars where like, okay,
that's definitely a super massive black hole, and some nearby
ones where we can watch the stellar populations and infer
that there must be something very very heavy at the core.
But most of those we can't do either because they're
either too far away or too quiet. But every galaxy
(19:53):
that's close enough for us to watch the stars, we
have seen a super massive black hole at its center.
So there's lots of galaxies out there, only very small
number can we check for a black hole, And every
single time we've checked, we've found one.
Speaker 5 (20:06):
Mmmm.
Speaker 1 (20:07):
So, like Andromeda, isn't it? Andromeda the closest galaxy to us.
Speaker 2 (20:10):
Andromeda is the closest galaxy to us, and we think
it has a huge black hole at its center, and
it's close enough that we can pretty well model the
velocities of the stars at the center of that galaxy,
and so we're pretty sure there has to be something
super massive and very dense at the heart of Andromeda,
very likely a black hole.
Speaker 1 (20:30):
All right, So then the data kind of suggests that,
like if you look at the galaxies that we can't
look at we do see a black hole in the
middle there, or something that is probably a black hole,
but why else would we extrapolate that to every galaxy
in the universe.
Speaker 2 (20:43):
Yeah, that's a good question. And to underscore sort of
our lack of knowledge here, we really identified black holes
in an approximately like one hundred to two hundred galaxies.
So of the zillions of galaxies that we've seen, only
a very small number have we been able to do
this check. And so you're tentative when you're extrapolating from
like a few hundred examples two trillions of objects out there.
(21:05):
On the other hand, it's every single one that we've
seen so far, right, so we wonder if it really
is a theory.
Speaker 1 (21:10):
So there's no black hole nearby, then we can check
that they didn't have a black hole in it, or
would we know or I guess what are you saying.
Speaker 2 (21:17):
I'm saying there's no galaxy nearby that we can check
that hasn't had a black hole in it, and that
includes really large galaxies like Andrameda, and also even dwarf
galaxies like there's a dwarf galaxy called RGG one one
eight and they recently found what they call a teeny
tiny super massive black hole in it.
Speaker 1 (21:35):
Where is that the actual scientific name? Teeny tiny supermassive?
Speaker 2 (21:40):
That was the title of the press release. I don't
know if that ended up in the paper or not.
Speaker 1 (21:44):
Doesn't that just average to a small black hole.
Speaker 2 (21:46):
It's only fifty thousand times the mass of the Sun,
So for a super massive black hole, it's pretty teeny
tiny compared to the Sun. It's pretty much a monster.
It depends on your perspective. But there's something else going
on here that makes us suspect that maybe every single
galaxy has a black hole in it. It's not just
that every single one we've seen has a black hole
in it, but we see this very tight pattern. We
(22:09):
notice this very close connection between the mass of the
super massive black hole and the mass of the galaxy.
And that might not be a surprise. You figure, like, look,
more stuff means a bigger galaxy, means more stars means
bigger black hole, right, And that's true. But if you
run those simulations, you get kind of a scatter like
you get some bigger black holes and some smaller black holes.
(22:30):
But what we notice when we plot, like the mass
of the black hole versus the mass of the galaxy
is a much tighter correlation than you would expect. Just
from like more stuff means bigger black hole, you get
this very very compact line that suggests that there's like
some feedback between the black hole and the galaxy.
Speaker 1 (22:48):
So you mean, like when we look at a black
hole in another galaxy, we can tell its size first
of all, And if you compare the size of the
black hole to the size with the galaxy, it's like
it's like almost one to one.
Speaker 2 (22:59):
Kind and if you're just made bigger and smaller galaxies
and guess how big the black holes would be, then
the same size galaxy shouldn't always give you the same
size black hole. Depends a little bit on like where
the stuff is and how much falls in, etc. But
what we notice is a very very close connection, as
you say, like one to one, that the mass of
the galaxy and the mass of the black hole track
very very closely.
Speaker 1 (23:20):
Meaning that there's no small galaxy with a big black hole,
and there's no big galaxy with a small black.
Speaker 2 (23:25):
Hole exactly, and two galaxies with the same size that
basically exactly the same size black hole at their heart.
There's almost no variation there, and that tells us that
there must be some sort of connection, that there's something
about how the black hole is forming and how the
galaxy is forming that connects these two things. I mean,
even from a spatial point of view, it's kind of
weird that like this dot at the very very center
(23:46):
of the galaxy. I remember, these things are very massive,
but they're also very very small, So it's weird that
this dot at the center of the galaxy is influenced
by like the mass of the whole huge galaxy that's
like one hundred thousand light years across. So there must
be some sort of connection between them, some information passing
back and forth, some process that's controlling both of them.
(24:07):
And if that's the case, and that makes us think, oh,
there must be a connection between the two. And so
probably every galaxy does have one of these.
Speaker 1 (24:14):
Things, because, like you're saying, if it's sort of inevitable
for a galaxy to get a black hole in the
middle when it forms, then there probably isn't any galaxies
without black.
Speaker 2 (24:26):
Holes, Yeah, exactly. And if there's some feedback mechanisms, something
which is controlling both of these things, then probably generated
both at the same time. It controls the stars that
form in the galaxy and also controls the mass of
the black hole. There must be some process tying these
things together, and being weirdly vague about that process because
we don't know what it is. There's a bunch of
(24:47):
theories about how the black hole might form and grow
and then the radiation from the black hole stops it
from growing, and the same process might control how stars
are formed and how they collapse from blobs of gas
and dust into stars. So there's a bunch of different
ideas out there, but they're all very vague and nobody
can really agree about it, or at the point where
we're just like, it seems like there's something going on here,
(25:08):
but we don't know what it is.
Speaker 1 (25:11):
Although this relationship between the black hole and the galaxy
size comes from data of galaxies that we found that
have black holes, right, Like, maybe there's still the possibility
that won and in a thousand galaxies doesn't have a
black hole.
Speaker 2 (25:24):
Yeah, it's certainly possible that that's the case. There's a
small number of galaxies here. Also, people might remember there
was recently this crazy idea that super massive black holes
or the hearts of galaxies are connected to the cosmic acceleration,
that they're really like bubbles of dark energy. And a
crucial thing that people notice that fueled that idea was
that there's a connection between the expansion of the universe
(25:46):
and the size of these black holes. That the black
holes seem to be more closely connected to the cosmic
expansion than their own galaxies. So that might seem like
it's in contradiction of what we're saying here today. Today
we're saying, oh, the black holes of the hearts of
galaxies are very closely connected to the size of their galaxies,
and a few weeks ago we said, no, they're not.
They're more closely connected to the universe. And the way
(26:07):
to untangle it is to remember that there's two different
kinds of black holes that we're seeing. We're talking about
black holes we see very very close to us, and
in those the black holes very tightly connected to the
mass of the galaxy, versus black holes that are very
far away, very old black holes those are from quasars,
So those are the ones that seem to be connected
to the cosmic expansion. So, long story short, there's a
(26:29):
lot we still don't understand about super massive black holes.
Speaker 1 (26:32):
Even how they form, right, Like, that's still a big mystery.
Speaker 2 (26:36):
Absolutely, we have no idea how these things even got
to exist. If you just start from like a big
blob of stuff and watch it form a galaxy, like
in simulation, you get a black hole at the center,
that's not a mystery, but it's not this big. Like
we look back in time by looking at old light
and looking at really early galaxies, and we notice that
they have huge black holes at their hearts, like already
(26:57):
billions of times the mass of the Sun, like the
first billion years of the universe. These are again the
very distant, very old black holes we see from quasars,
and we can't explain how that happens. In our simulations.
That just doesn't happen so quickly. It takes much longer
for these black holes to get so big. So there's
something else going on to form these black holes that
(27:18):
we don't understand.
Speaker 1 (27:19):
So we really have no idea what could be going on.
I mean, it kind of seems like maybe there's a
simple explanation in there somehow, Like you know, if you
start with a big cloud of gas, and yeah, the
galaxy is going to be bigger, and the black hole
in the middle is going to be bigger, Like what's
the big mystery there?
Speaker 2 (27:34):
Yeah, and that would give you a correlation that would
say that in general, masses of galaxies should be connected
to the masses of the black hole. And we see that.
But again we see a much tighter connection than you
would expect just from that simple explanation. We see that
galaxies with the same mass have basically exactly the same
mass black hole. There's like no variation there. So the
(27:55):
connection is just tighter than what you would expect from
that simple argument. So there must be something else going on.
Is it true that we have no idea? I mean,
people definitely have ideas, and I read like ten papers
about ten different ideas for what could be controlling it
complicated theories about how the gas gets blown in or out,
or gets heated up or cool down. We have lots
of ideas, we just don't know which one might represent reality.
Speaker 1 (28:17):
Interesting And now, is it possible, I guess, for a
galaxy to not have a black hole? Like why couldn't
that happen?
Speaker 2 (28:23):
Yeah, that's a great question. And there's sort of two
questions there, right, Like one is could you make a
galaxy without a black hole. Is it possible to pull
all those stars and all that dark matter together without
making a black hole? And we think the answer to
that is no, That every time you get enough stuff together,
whatever is pulling that together is going to form a
black hole at its center. That's just inevitable.
Speaker 1 (28:45):
Why do we think it's inevitable?
Speaker 2 (28:46):
Well, I guess for a couple of reasons. To summarize,
One is every galaxy we've seen so far has a
black hole at it's heart, right, We've never seen a
galaxy where we're able to check whether there's a black
hole and haven't found one, though there's an asterisk there
which will get to in just a minute. And number
two is this connection between the sizes of them that
tells us that there's probably some mechanism that's controlling both
(29:07):
the mass of the galaxy and the mass of the
black hole together. So back to the asterisk, there actually
are a couple of galaxies nearby that we've looked at
that don't have a black hole at the center. And
that points to another potentially fascinating story, which is whether
it's possible for a galaxy to form a black hole
and then eject it like can a galaxy form, make
its own black hole and then lose that black hole.
Speaker 1 (29:30):
Wait, wait, wait, wait, we're seeing that there are galaxies
we've seen that don't have a black hole in them
in the middle.
Speaker 2 (29:35):
There are a small number of galaxies we've seen that
have a black hole, but it's no longer at the
middle of the galaxy.
Speaker 1 (29:41):
Wait, what where is it? Like at the edge or
really far away from it? What do you mean?
Speaker 2 (29:46):
So there's a few variations. In one case, we've seen
a super massive black hole that's like displaced from the
center and has a pretty high speed away from the center.
So there's this possibility that galaxies could form make a
black hole the center, and then through some dynamical process,
some like interaction with other galaxies, their black hole could
get like kicked out of the center. And we've actually
(30:08):
seen this in a few galaxies.
Speaker 1 (30:09):
What are the other ones like that don't have a
black hole in the middle.
Speaker 2 (30:12):
So there's the one example called CID forty two, which
is about four billion light years away. It has a
super massive black hole, but it's near the center, but
it's sort of displaced from the center. And then there's
another observation just a few weeks ago where they see
a streak of light shooting out of the galaxy. The
streak of light is like two hundred thousand light years long,
and at the end of it there's a black hole.
(30:34):
And so it looks sort of like the black hole
was ejected from this galaxy. It's like a runaway black
hole and left this streak of stars in hot gas
in its wake.
Speaker 1 (30:44):
Well, it can actually see like the skid marks of it.
Speaker 2 (30:47):
Yeah, exactly, And so we think that this might happen
sometimes when galaxies merge. We know that galaxies merge, that
that's a very normal thing. We think the Milky Way
is formed by a bunch of galactic mergers. And Dromeda
is so big because because it's a combination of a
bunch of baby galaxies that all got merged together. And
the normal thing to happen when galaxies merge is that
the black holes also merge. I mean, you have two
(31:09):
clouds of stars, each with a black hole at their center.
They're all going to orbit each other. Eventually, the black
holes that their hearts are going to orbit each other,
and then because of friction and gravitational radiation, they'll eventually
collapse into a single black hole. That's like the normal
thing to happen, but there's some variations there. When two
black holes collapse into one, they also emit a lot
(31:30):
of radiation. Like the mass of the two black holes
doesn't one hundred percent go into the mass of the
final black hole. It loses some mass and it generates
a bunch of gravitational waves. That's how we see these
black hole mergers. We talked about Logo and Virga and
all these observations that see these ripples in space time
generated by these black hole collisions. You get those ripples
(31:51):
because the black holes are accelerating as they orbit each other,
and that radiates a way energy, so it loses some mass. Now,
sometimes that radiation is in every direct like it just
sprays gravitational waves everywhere, but sometimes in special circumstances that
gravitational radiation tends to be in one direction rather than another,
and then it acts sort of like a recoil. It's
like shooting a gravitational wave gun in one direction and
(32:15):
the black holes get pushed back in the other direction
by conservation of momentum.
Speaker 1 (32:20):
So you're saying you can have a galaxy without a
black hole, but the ones we've seen so far that
are like that, there's evidence that it had a black
hole in the middle at some point.
Speaker 2 (32:29):
Exactly, so it might be possible to have a galaxy
without a black hole by building a galaxy with a
black hole and then like ejecting it getting rid of
the black hole.
Speaker 1 (32:39):
I guess one thing that maybe important to understand is
that even though a galaxy can have a super massive
black hole in the middle with the massive millions of
our suns, it's not like the black hole is anchoring
the galaxy, right, like to a galaxy with tondreds of
millions of stars, like one little black hole in the
middle is not super important in the same way that
(33:00):
the Sun is important in our Solar system.
Speaker 2 (33:02):
Right, Yeah, you're absolutely right. The Sun is like ninety
nine percent of the mass of our solar system. But
the black holes the hearts of these galaxies are a
tiny fraction, much less than one percent of the mass
of the galaxy. Like in the Milky Way, we have
billions of stars and so billions of solar masses, and
our black hole is only five million solar masses. So
it's a tiny fraction of the mass of our galaxy.
(33:24):
Even though we're talking about really big objects. Galaxies themselves
are much much bigger. So it's more like if our
Solar system lost Jupiter, right, Jupiter is a big planet,
that'd be kind of a big deal personally, but it
wouldn't affect the dynamics of the Solar System the way
it would if we lost the Sun, Right, that would
be much more dramatic.
Speaker 1 (33:42):
Right, These super massive black holes are really more like
little pimples kind of in the middle of galaxies. Right,
It's not like the galaxies there because of the mass
super massive black hole. It's more like maybe just a
feature that pops up when you're making a galaxy.
Speaker 2 (33:55):
Yeah, I think we don't really understand that. I mean,
I think the causal relationship is probably complicated. It seems
like it's probably a necessary outcome when you form a
galaxy that you get a super massive black hole. So
in that sense, it kind of is necessary to have it.
But you're right, you can get rid of the black
hole and the galaxy can still hold itself together, so
in that sense, you don't need it anymore.
Speaker 1 (34:17):
Like maybe the black hole needed the galaxy to form,
but maybe the galaxy didn't need the black hole to form.
Speaker 2 (34:23):
Yeah, But then the black hole stays a black hole.
When it's been ejected, it's still a black hole out
there wandering an intergalactic space. WHOA. So one way to
get rid of your black holes to generate a bunch
of gravitational radiation in one direction to kick the black hole.
For that to happen, you typically have to have sort
of like a low mass black hole in order to
get enough acceleration to like get out of the galaxy.
(34:44):
Another scenario is to have sort of like a three
galaxy dance. Remember that when three objects interact, it's much
more chaotic, Like, the three body problem is not something
we know how to solve, whereas the two body problem
is simple. So if you have three galaxies that are
merging at the same time, then one of those black
holes can get kicked out instead of merging with the
(35:04):
other two. So that's another scenario that can create a
runaway black hole, essentially kicking a black hole out of
a galaxy.
Speaker 1 (35:11):
Like if three solar systems similar to ours came together,
it would get so chaotic and so scrambled that it
could actually like shoot off the Sun or Jupiter out
into space.
Speaker 2 (35:21):
Right, yeah, exactly. And because of conservation momentum. If you're
going to kick one black hole out in one direction,
then the other two are going to get kicked in
the other direction, and so the whole system might end
up without a black hole at its center.
Speaker 1 (35:35):
All right, let's take a little bit more into what
this all means and maybe how dark matter plays into it.
But first let's take another quick break. All right, we
(35:57):
are asking the question does every galaxy have a black
hole at its center? And I guess we've figured out
the answers. No, right, some galaxies have a black hole
near its edge if it gets kicked out.
Speaker 2 (36:09):
Yeah, we think that probably every galaxy has a black
hole at its center during its formation or at some
point in its history, because we think probably the process
that makes these galaxies, it gathers together all this mass
and funnels it into this gravitational well that remember, in
the end, is made by dark matter. Right, The reason
these galaxies exist is because there's a density of dark
(36:32):
matter there which is pulling in all this other matter
to make this dense stuff that we can make a galaxy.
Probably that process inevitably makes a black hole at the
same time as it makes a galaxy. But then you
can lose that black hole. During the life of these galaxies,
as they merge, as they come together, as they dance
around each other, those black holes can get ejected from
(36:53):
the hearts of those galaxies. So yeah, you could end
up with a galaxy without a black hole at its center.
Speaker 1 (37:00):
Like we said earlier in the episode, that every GALLESSI
we've seen has the black hole at its center, But
it sounds like that's not really true. That there are
galaxies out there we've seen so we can measure it
that don't have one in the middle at all. Have
we seen any with no black hole at all.
Speaker 2 (37:15):
We have not found a galaxy without any kind of
black hole. Either they have a black hole at their center,
or we've identified a black hole that's been ejected from
the center. That's a very small number of cases so far,
or they're too far away for us to tell what's
going on at the center. Then there's one particular galaxy
which is a bit of a mystery. Its called Able
two two six one. It is a really big galaxy
(37:37):
and we have not yet found the super massive black
hole there. We looked at it in the X ray
and not seen the sign of it. It's still sort
of an open question whether people are going to find
evidence of the black hole or not, but it's one
that has a question mark next to it. Remember, this
is like a recent and active field of research. People
are developing new techniques to try to study these galaxies
to try to see if they are black holes in them.
(37:59):
It's definitely a question a lot of people are working on,
so it's evolving rapidly. But either we've seen a black
hole at center, or we've seen a black hole like
leaving its center, or there's this one question mark galaxy
able two two six 's one, or we're just not
able to see it, because remember, the galaxies have to
be close enough for us to like study their centers
in order to be able to see these black holes,
(38:21):
or they have to be active enough for us to
see them emitting. Like another way to see these black
holes is to watch them like burp as they eat something.
Speaker 1 (38:30):
But then you're only seeing the rude black hole.
Speaker 2 (38:33):
That's right. But occasionally, even a quiet black hole, even
when it's not surrounded by a big disc of gas
and dust and emitting like constant beams of light, occasionally
a star might wander close to it and get eaten
and the tidal disruption there will give you a very
bright flare. So we've seen those kinds of things as well,
like otherwise quiet galactic centers that suddenly emit a big,
(38:55):
bright burst of light. We've even seen that from our
own galactic center. Sometimes the way you can observe supermassive black.
Speaker 1 (39:02):
Holes, it's like the final scream of a star before
it gets eaten wo.
Speaker 2 (39:08):
Exactly. Another thing people are doing is trying to identify
these super massive black holes out in intergalactic space. Like
people are curious how often do super massive black holes
get kicked out into space? Remember we talked about like
rogue planets before, planets that used to be orbiting a
solar system but then due to the gravitational chaos of
(39:28):
their system, get thrown out into space. And it turns
out there's a huge number of them out there. People
have discovered rogue planets by micro lensing, looking for moments
when these planets pass in front of a star in
the background and that star's light gets distorted by the
gravity of the planet, And by doing that we can
spot a bunch of these things and then extrapolate to
how many there are. The same way we can look
(39:50):
for super massive black holes in intergalactic space by looking
at for these micro lensing events.
Speaker 1 (39:57):
M sounds kind of scary, the idea that there are
super massive black holes out there roaming space, possibly in
our direction.
Speaker 2 (40:05):
Yeah, it's certainly possible that they're out there. We've seen
some of them getting ejected from their galaxies, but we
think that might be kind of rare, and when we're
talking about like a one percent level event based on
our recent observation, so of course with a big uncertainty.
And there are many fewer galaxies than there are stars
of course, so the number of rogues super massive black
holes out there could be big. But space is of
(40:26):
course really really vast, so these things would be pretty
hard to spot.
Speaker 1 (40:30):
I guess it's hard for even a super massive black
hole to leave a galaxy, right Like, as we said before,
a galaxies huge, got billions and trillions of stars, it's
really massive. It's probably pretty hard for a black hole
to get the escape velocity needed to leave a galaxy
and come towards us exactly.
Speaker 2 (40:47):
That's why it tends to happen mostly for lower mass
super massive black holes, not necessarily even teeny tiny ones,
just ones on the lower edge, because a lot of
them are going to have this asymmetric radiation from gravitational
waves or some sort of chaotic merger event that it's
not totally symmetric. But most of them are massive enough
that they get pulled back to the center of the
(41:07):
galaxy and eventually settle down. But the lower mass ones
can get going pretty fast, like this CID forty two galaxy.
They estimated the velocity of the galaxy to be half
a percent of the speed of light, which is like
very very fast moving black hole.
Speaker 1 (41:23):
It's in a hurry.
Speaker 2 (41:25):
To get out of there, really wants to go somewhere exactly.
Speaker 1 (41:29):
I guess the question is how does this all tie
into dark matter? Because I know dark matter is very
important in the formation of galaxies. Right, It's almost like,
I know we've mentioned this before, the galaxies form around
where dark matter is. Does that effect sort of how
black holes might get formed.
Speaker 2 (41:45):
It definitely affects where galaxies and black holes get formed. Right.
Galaxies basically trace out where the dark matter is in
the universe. If you look at the large scale structure
of the universe, you see these filaments of galaxies and
these sheets of galaxies and that's because that where the
dark matter is. We can't see the dark matter directly,
but we can see that it's gathered together all this
matter and made all of those galaxies. And so every
(42:08):
galaxy has a huge dark matter halo around it, Like
the Milky Way is about one hundred thousand light years across,
but there's a big blob of dark matter that's like
two hundred thousand light years across that the Milky Way
is embedded in. Now, if we shot our black hole
out of our galaxy, then it would pass through that
dark matter halo, and it would gobble up a lot
of dark matter along the way. And so as these
(42:30):
supermassive black holes leave the galaxies, they can increase their
dark matter fraction.
Speaker 1 (42:36):
They can get even darker than black Yeah.
Speaker 2 (42:40):
We think that these black holes already have some dark
matter in them because dark matter is everywhere. But dark
matter also isn't sticky, and so it's very easy for
dark matter to just like rotate in orbit forever around
the galaxy and not fall in. We think will probably
fall into the center of the galaxy because eventually it's
like dynamical friction. Our star will get pushed by others
stars on. All that jostling ends up kicking somebody towards
(43:03):
the center of the galaxy. But dark matter doesn't do that,
right because dark matter just passes right through itself. So
normally dark matter can swirl around the center of the
galaxy not getting eaten by the black hole. But if
the black hole like runs free, then it's basically like
plowing through a buffet of dark matter.
Speaker 1 (43:19):
But maybe when as the galaxy was forming, maybe the
black hole did eat a lot of dark matter. Or
maybe I wonder if the black hole in the middle
of galaxies is maybe mostly made out of dark matter.
Speaker 2 (43:30):
It's hard to know, right, And what does it mean
to be made of dark matter? Because once it goes
past the event horizon, who knows what happens? Everything's dark matter,
everything's something else, some black hole state of matter. Right,
we don't know if dark matter gets annihilated it turns
into something else, or if it retains something of its nature.
We just don't even know because we don't know the
particle properties of it. So, like, what are the conservation
(43:53):
laws for dark matter? The universe might like keep track
of how much dark matter there is and not allow
things to convert. We just don't know so clueless about it.
Speaker 1 (44:01):
So some black holes might be darker than others.
Speaker 2 (44:03):
Absolutely, some might be darker than others. We know that
some galaxies are darker than others, some galaxies have a
larger dark matter fraction than others. That's for sure, all right.
Speaker 1 (44:12):
Well, what does it mean for our understanding of galaxies
and how they form?
Speaker 2 (44:16):
It means that we're still the very beginning of the
journey of understanding how galaxies come together, which is sort
of shocking because we've been setting galaxies for so long,
but every few decades we'd learned something new and surprising
about what's going on with these galaxies, how they come together,
what their history is, and what their future fate is. Right,
and we know that the galaxies and sort of our
(44:37):
local group are gravitationally bound together and so eventually going
to end up falling together and probably forming one super
galaxy while they're being separated from the rest of the
universe by dark energy. And that super galaxy is going
to have a super duper massive black hole at its
center unless that gets kicked out and shot into intergalactic space.
(44:58):
So it tells us a lot about our our potential
future because as we orbit our galaxy and merge with
those other galaxies, we also eventually fall towards the center
of this blob. And if there's a giant black hole
waiting for us there, this is really only one way
for the story to end.
Speaker 1 (45:12):
But even if we kick out that black hole, eventually
the galaxy is going to collapse anyways, right, and maybe
it will collapse into a new black hole.
Speaker 2 (45:19):
Yeah, exactly, it could definitely form a new black hole
even if you kick out the original one. There's no
noncompete clause in galactic formation.
Speaker 1 (45:26):
And when is this supposed to happen, like tomorrow or
trillions of years?
Speaker 2 (45:32):
Oh, we're talking about billions and billions of years for sure,
So we are much more a threat of the Sun
expiring before our system collapses into the center of a
black hole. But if we want to continue on for
billions and billions of years, we definitely need to plan
deep into the future. And also, we're just curious about
how black holes work and how galaxies work, and we
want to understand it because it could be that there's
(45:52):
some other deep insight into the way the universe works
waiting for us. Every time. In science, we're like, don't
really understand how something works, and we deeper, we discover
something fascinating underneath that we didn't even expect to find,
so satisfying our curiosity and like trying to understand this
in great detail is good path to like opening up
some surprising new doors. So we're doing our best to
(46:14):
improve our future prospects for understanding these things.
Speaker 1 (46:17):
Mmm, sounds like a problem for the AIS. You could
just kick back, let to figure it out.
Speaker 2 (46:23):
Well. One of my favorite ways to study these things
in the future is with our space based gravitational Observatory
so LIGO is this gravitational wave observatory that has two
arms that are kilometers long filled with lasers, and they're
planning to build one in space called Lisa LSA mean
much much bigger, and so it's going to be able
to observe gravitational waves at lower frequencies, which is what's
(46:46):
generated by the collisions of super massive black holes. So
we might be able to observe gravitational waves from galactic
mergers and understand this process in more detail. What happens
when two super massive black holes really do come together?
Speaker 1 (47:01):
WHOA, which is what happens when two galaxies collide right.
Speaker 2 (47:04):
Exactly, yeah, yeah, and so we could see the signature
of black holes getting injected from the hearts of their.
Speaker 1 (47:10):
Galaxies, ejected or smushed.
Speaker 2 (47:12):
Smushed or ejected. We don't know both outcomes are possible.
Speaker 1 (47:16):
But will we hear the black hole get injected?
Speaker 2 (47:19):
Well, black hole injection comes with a very powerful, very
directional gravitational wave signature, and so they think they might
be able to distinguish that from just like a normal
combination of two black holes into one. But it all depends
on this crazy set of satellites in space shooting lasers
in each other, measuring tiny changes in their relative distances.
Speaker 1 (47:41):
Well, it sounds like the future is all black hole.
So we bidters start to understand them and get used
to them and figure out how they form.
Speaker 2 (47:50):
Right, absolutely something we'd like to understand, not just because
we're curious about the universe, but because they might control
a fate of our galaxy.
Speaker 1 (47:57):
Yep. Or it might be just something for our AI
replacements to figure out. We might be long gone.
Speaker 2 (48:06):
I'll read about it from my lounge chair at the
center of the black hole.
Speaker 1 (48:09):
All right, we hope you enjoyed that. Thanks for joining us.
See you next time.
Speaker 2 (48:21):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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