Where are all the medium-sized black holes?

Daniel and Jorge search for the missing black holes that are not too small or too big, but right in the middle!

Learn more about your ad-choices at https://www.iheartpodcastnetwork.com

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, how do you like your coffee in my mouth?
I guess usually or maybe sometimes in ice cream. I mean, like,
do you like to drink the tiny, strong Italian coffees
or the big week American style coffees? Hmmm, I guess
I like the sort of medium sized coffee, you know,
strong enough to be for grown ups, but not like

(00:28):
a punge in the face. I see. And does that
apply to particles too? Do you prefer tiny particles or
massive ones? I don't know. I like them heavy enough
to be rare and worthy of winning a Nobel Prize,
but light enough that we can discover them at Collider's.
And you're so middle of the road. Sometimes the middle
of the road is exactly where the mysteries lie. Hi

(01:05):
am Jorge, handmade cartoonists and the creator of PhD comics. Hi.
I'm Daniel. I'm a particle physicist, and I usually eat
a medium size portion of cookies for breakfast, for lunch,
or all of the above. Every time you start eating cookies,
you can't just have one. You try to avoid having
the whole box, so I end up somewhere in the middle,

(01:25):
somewhere like half of a box of cookies. Depends are
you're doing arithmetic half or geometric half, you know, it
depends on the day. Then you can do zenos paradox
that can eat half the box, and then you eat
the other half, and then the half the remaining half,
and pretty you ate everything but a crumb. Yeah, well,
that's a great way to avoid eating the whole box, right,
Wisdom from the ancient Greeks. It's the Greek diet, it's

(01:47):
the new Mediterranean diet. Only eat half of the remaining
box of cookies. But welcome to a podcast. Daniel and
Jorge Explained the Universe, a production of I Heart Radio,
in which we take a bite of the entire box
of universe cookies. We chew on the bits to talk
about black holes, We think about all the little chocolate
chips that are particles. We dig down into the crumbs
and try to understand everything that's going out there in space,

(02:10):
in the universe, in history, and in the future. We
very humbly try to explain the entire universe to you.
That's right. There are amazing mysteries all over the universe,
and those mysteries range from gigantic size galaxy cluster billions
of light years wide mysteries and really really tiny mysteries
in the particles and the little quarks and tiny quantum

(02:33):
fluctuations that make up who we are. Are you saying
I'm just a quantum fluctuation that I could just disappear tomorrow.
Well we can all disappear tomorrow, Daniel, depends on what
you guys are working on at the large handring collider there. Well,
if we make a black hole, will make a little
itty bitty when I promise good, I'm glady that's an option.
I'm so glad you're not tempted at all to just

(02:53):
dial it up. We'll have a knob here in front
of me, you know, So tell me what what size
black hole should we be making here the late see?
How about a zero sized black hole? That's no fun?
And how are we going to learn the secrets of
the universe? Keep you inside the tiny black holes? I'm
okay where they are right now. But black holes are amazing.
They are fascinating. They do contain amazing secrets of the

(03:16):
universe inside them. But there are also really fascinating questions
about how they get made and why they are the
size they are or the size they aren't. Yeah, they
seem to be everyone's favorite space mystery. Do you think
maybe it was all in the name, Like if you
had called them, I don't know, vacuum holes or gravity pits,
do you think people would still be as interested in

(03:37):
the I think they would. I think it's their fundamentally
alien nature that makes themselves fascinating. We know they are
really deeply, weirdly different from anything we have seen before,
and I think that's what sparks our curiosity. You know,
we want to go there and see it and explore
it because we want to understand what's possible in the universe.
As we say often on the podcast, it's the extreme

(03:57):
situations that teach us what the rules really are because
they show us the limits. Yeah, and as you say that,
you can find them in all kinds of sizes out
there in space. There are black holes that are humongous
and warm the center of galaxies and help keep galaxies
kind of together, and you can make them tiny little
ones that evaporate right away. Yeah, that's right. There are
these funny, two different populations of black holes, the ones

(04:19):
that seem to come from the collapse of stars and
the huge galaxy gobblers at the center of many galaxies,
and the weak thing is that there don't seem to
be any in between. Really, there's no like Goldilocks black holes.
There's only two options, like VENTI and X are large.
That's right. You either don't open the box of cookies
or you eat the whole thing. The universe never goes

(04:40):
for half a box. Yeah, so there are mostly two
sizes of black holes in the universe. One you're saying
it are the ones that come from the collapse of stars.
Those are I mean, they're pretty big. There's still like
the size of you know, many suns. Yes, these are small,
only on the sort of like cosmic galactic scale, right,
smaller than a whole galaxy. Still a normal is still
more massive than our entire solar system. But yeah, tiny

(05:04):
compared to other things. And then you have the huge ones,
the ones that are how many, like millions of times
the mass of the Sun up to millions or even
billions of solar masses, so many stars compacted into a
tiny little space at the center of some of these
galaxies really just mind bogglingly massive and incredibly powerful objects. Yeah,

(05:25):
and so black holes can be any size really in between,
and you know, from tiny, two huge, But apparently you
don't see a lot of ones in the middle out
there in the cosmos. Most of the ones that you
see out there are either kind of supernova size or
gigantic galaxy center size. Yeah, it's even more dramatic than that.
We have never seen any in between. There's not an example,

(05:47):
a single example of a black hole in between those two.
So we don't actually know if it's possible to have
an intermediate size black hole. Maybe the laws of physics
just don't allow it. So that's the question will be
tackling today. So to be on the program, we'll be
asking the question where are all the intermediate mass black holes?

(06:10):
You've gotta made it easier for me, Daniel, and just
medium mass. Well, that was going to be my next
question is what would you have called these things? Because
intermediate size black holes or intermediate mass black holes is
quite a mouthful. Yeah, I don't know, just go with
like the Starbucks rude and call them large black holes,
which are really the medium sized black holes. That's true,
but everything in the universe is enormous, right, so maybe

(06:31):
the Starbucks naming scheme makes sense. Yeah, large, extra large,
super large, and galectically large, cosmic large. That's right. It's
like ordering fries and McDonald's there are no more small fries,
Is that true? I think so. I think you can
only order large or extra large. I will admit not
having been to McDonald since I worked there when I
was six. And yeah, you seem to know a lot
about their menu, Danniel. I'm a member of society. I

(06:53):
participate in society. I read memes on the internet to
educate me about humanity. You turn your your nose up
at fast food change, sure, like everyone else in society.
But yeah, maybe I would call medium sized black holes.
You know that kind of makes sense. Yeah, you know,
as a middle child, I'm sort of sensitive to this naming.
You know, middle seems to like be defined by the others.
There's nothing like unique and identifying about being in the middle.

(07:16):
You're like defining yourself by what you're not instead of
by what you are. We just opened up a whole
black hole of psychological trauma. There issues there and Daniel,
your middle children out there. You know you know that well,
you have like ninety seven brothers and sisters, right, so
you you've got to understand and is this why you
started a podcast just to err on your sibling rivalry issues. Yes,

(07:37):
it's been a three hundred episode plan to get to
this moment exactly. All right, tell me, Daniel, tell me
about your childhood. I feel defined by my more massive
and less massive brothers. I see now you're making swipes
at their body weight there, man, I meant intellectual gravitas,
of course, yes, of course, of course, but yeah, I'll

(07:59):
rate that. Are talking about black holes, Daniel, and how
there aren't any real middle of the ones out there
in space. You mostly see big ones or smaller ones
the size of a few sons, And so that's the question,
where are all the intermediate mass black holes? So it's
usually we were wondering how many people out there knew
that these black holes are are missing from the universe
and where they could be. So thanks to everybody who

(08:20):
volunteered to answer my random questions with no preparation or
reference materials. If you'd like to participate in future episodes,
please write to me two questions at Daniel and Jorge
dot com. We love your contributions and I promise you
it's fun. So think about it for a second. Where
do you think all the medium or intermediate mass black
holes in the universe are. Here's what people had to say.

(08:44):
That's a good question. I think that there's no proof
that they even exist, but that they do. Maybe they
are lurking somewhere in the intergalactic space where there's just
no stuff for them to eat and growth. They don't
don't form those Christian disks, And basically the only way
that we can spot a black hole is by looking
at the Christian disk and figuring out indirectly that there

(09:05):
must be a black hole in the middle of it.
But if there is no a Christian disk, that we
may have a black hole that is somewhere out there,
but we just have no chance of ever spotting it.
So I would bet for intergalactic space. I know at
the center of a galaxy you get really big black holes.
I think they're called supermassive black holes. So they won't

(09:25):
be there, but maybe be an intergalactic space. But I'm
not sure. All right, not a lot of clarity here
from our listeners. I like the idea that maybe they're
floating out there in space and we just can't see them.
This one president said, made they're lurking in intergalactic space. Yeah,

(09:48):
it's a cool idea. If you're a black hole, can
you do anything else in space except lurk? Oh? Come on,
you could be very dramatic. You could swoop into us
always system and gobble it up and destroy everything. Right,
that's the opposite of lurking. That's like leaping. Yeah, but
then afterwards you go back to lurking. Well, I like
this idea because it suggests that you know, there are
things out there and intergalactic space that we don't know

(10:08):
about that we can't see. That it might surprise us
that basically you could hide anything out there as long
as it's dark enough. And you know, that's a reasonable point.
And I guess the real mysteries that you know there
there could be intermediate mass black holes out there, Like,
there's nothing that we know about so far that would
would prevent them. Theoretically they're possible, but you just don't

(10:29):
see them. Yeah. Well, that's the problem with black holes
is that you can't see them directly, right because they
are black. All you can do is see their effect
on stuff behind them or around them. And so if
you had a black hole deep in the middle of
space with nothing around it, how would you detect it, it
it would be undetectable, it would be very effectively lurking.

(10:50):
And so this is actually a good point. The question
that needs to know how did it get made? How
did you form a black hole out there in the
middle of space with nothing around you. Usually black holes
are made from stuff, and then there's other stuff around
that didn't get black holeified, and you can use that
to detect it. That's the mystery. And so I guess maybe, Daniel,
let's start at the beginning and let's talk about how

(11:10):
you would even define an intermediate black hole, like, what
is there a technical range of masses that we qualify
as intermediate Yeah, there actually is. Unfortunately for the psychology
of intermediate mass black holes, they are defined by what
they are, not they're defined by their bigger and larger siblings. Right,
So it's a black hole that's between a solar mass

(11:32):
black hole like one that comes from a collapsed star,
and we can talk about it, but there's definitely an
upper limit on how big that can be, because there's
an upper limit of how big a star can be.
So it has to be bigger than the black hole
that could come from a star, but then smaller than
black holes we see the center of galaxies, which is
about ten thousand or fifty thousand times the mass of

(11:52):
the Sun. So there's a huge range there between a
hundred and like ten thousand solar masses. We call the
intermedi a mass range for black holes. Oh, I see
there's a limit due to the sizes of stars. Exactly
what kind of black hole do you imagine when you
think about black holes? Probably are the ones that come
from when a star collapses, Right, you know that stars
are formed from gases that are collected by gravity, and

(12:15):
then you get fusion burning, which prevents further collapse. Right,
gravity would just go directly to a black hole if
the star didn't ignite fusion inside of it, which was
pushing out with radiation to prevent the collapse. That happens
for you know, several billion years or so until the
fuel runs out and turns into heavy ash, which accelerates
the gravitational collapse instead of preventing it, and then the

(12:36):
whole thing collapses into a black hole. But there's a range.
They're like, it can't collapse into a black hole if
it doesn't have enough mass. So the lower limit on
a black hole that comes from the Sun is like
ten solar masses. Smaller than that, and it collapses into
like a neutron star or something else because it can
still resist the gravitational pressure. But what about larger than that, Like,

(12:58):
can bigger star collapse into a bigger black hole? It can,
but you can only get up to about eighty or
ninety times the mass of the Sun. And the reason
is that you can't have a star that's much bigger
than about three hundred times the mass of the Sun.
You try to make a star bigger than that, Remember
we talked about this on an episode about the biggest
stars in the universe. Then those stars aren't stable. They

(13:19):
blow themselves apart because as the star gets bigger and
the inside of it gets higher pressure, and that higher
pressure means the fusion burns hotter, and so it produces
more radiation. It's very nonlinear, and so a little bit
more stuff means a much higher temperature inside, which means
radiation blowing out. So then the star literally tears itself apart.
So the biggest star you can have is about three

(13:42):
hundred times the mass of our Sun, which means that
there's a limit on how big a black hole you
can get that comes from a star, I see, But
the limit is smaller than the biggest star. So the
biggest stars can be three hundred times the mass of
our sign, but the ones that turn into black holes
are only the ones that go up to about eighty
times the mass of our Sun. No, no, it's because

(14:03):
not all the stuff in the star ends up in
the black hole, right it's tearing itself apart, and not
all of it collapses in the black hole. You still
get a huge amount of stuff outside the black hole
that it's blown out. You know. Sometimes you get like
a supernova before you get a black hole, and that
shoots a huge amount of energy and mass out into
the universe. So the process to go from star to
black holes not ad efficient, which is why three hundred

(14:26):
solar mass stars turned into about eighty or ninety solar
mass black holes. You're talking about the kind of the
limit of the size of the black hole that can
come from a star. But the stars usually started off
with three hundred times the mass of the Sun, and
then they collapse into eight times the mass of the Sun,
and those are very rare. Like stars that massive are
very very rare because they're very unstable and short lived.

(14:48):
Most stars are much much smaller than that. So most
of the stellar black holes you see out there are
in the lower end of the ten to eighty range.
Eight is like really extreme and crazy and rare. So
then that's like one kind of limit, like the biggest
stars kind of defines what the biggest black holes that
can come from the Sun with their masses exactly nicee.

(15:08):
And we see a lot of those out in the space, right. Yeah,
those are all over the galaxy. Every time you have
a star that's that big, its path will lead it
to become a black hole, And so those black holes
are appearing all the time. They're not rare, they're not weird,
that not unusual, and we see lots of those black
holes around the universe. That's the kind of lower range
of black holes, and then there's like the upper range

(15:29):
of black holes. Yeah, the black holes that we see
at the centers of galaxies are like completely different beasts.
I mean, they are similar from the general relativity point
of view in that they are very compact objects that
are very dense that have incredible gravity, but they seem
like monsters compared to these stellar black holes because they
tend to have masses like a million or a billion

(15:50):
times the mass of one son, and so these are
just really enormous and they really like drive the gravity
of a galaxy. They sit at the center of the
galaxy and they are you know, like one one hundreds
of the mass of the whole galaxy is in that
single black hole and it's pulling on the whole galaxy
and it's slurping everything around and it's gobbling stars at

(16:11):
the middle. We have one like this at the center
of our galaxy. They're pretty common, I guess, Like I
think most galaxies have a supermassive black hole at its center.
Like when you look out into space and you see,
you know, millions and millions of galaxies, most of them
have one of these giant black holes in the middle.
We don't actually know the fraction of these galaxies that
have one, And that's one of these questions, Like we

(16:31):
know that black holes in the center of galaxies tend
to be proportional to the mass of the galaxy. So
bigger galaxy, bigger black hole the center. Smaller galaxy, smaller
black hole the center. We don't know if they all
have black holes, like we've seen a lot without them.
We don't know the fraction of galaxies that have black holes, actually,
but it's not rare. I guess that's what I mean.
You see them often. Yeah, it's not uncommon. Absolutely, They

(16:53):
kind of all over the place, yes, And so what's
the range of size as we've seen for those? So
up to billion, right, like ten billion solar masses is
the biggest black hole we've ever seen. And as far
as we can tell, there's no upper limit to how
big a black hole can get. Like, you keep feeding
that thing, it'll just keep getting bigger. The limit really
is just like cann be around enough stuff and have

(17:15):
enough time to gobble it. On the lower end of
super massive black holes, the smallest one we've seen is
about fifty thousand times the mass of the Sun. This
is in a little mini galaxy we call these dwarf galaxies,
about three hundred forty million light years from here. The
galaxy is called r g G one eighteen. And that's
the smallest black hole we've ever seen inside a galaxy.

(17:37):
It's fifty thousand times the mass of the Sun. That's huge.
Imagine fifty of our sons, Yeah, and then collapsing into
a black hole like the gravity of fifty thousand suns
is nothing to be sneezed at, or if you sneezed
at it, your sneeze will get sucked up by that
black hole right away. This is not we just gets
slurped out of your nose. Probably we should call that
black hole is in tight Hey, yeah, then will be

(17:58):
a green black hole depending on the size of years
these we'll see. All right, Well, it seems like there
are these huge populations of black holes out there, kind
of the the star sized ones and the gassy size ones,
but maybe not so many in the middle. So let's
get into whether or not they actually exist or whether
or not we're just not seeing them. But first let's

(18:19):
take a quick break. All right, Daniel, we're talking about
intermediate mass black holes. Now. Is that the official names
are an acronym for that, That is the official name.

(18:42):
You'll see that in the literature if you search for them.
And you know, I call them I m b H.
But it sort of reminds me of I MDB, so
I think maybe we need a better name for them.
M I see. Well, they do sort of collect stars
in a way, right, I'm sure there are films of
gas clouds I'm really stretching in here. Yeah, well, we
should go look this up on the Internet movie black
Hole Database. I'm sure there is one, isn't There are

(19:03):
dots and physicists kind of keep track of everything and
polish it online. I don't know, but after this episode,
I'm gonna go type in i mbhdb dot com and
see if somebody owns that already. Oh man, maybe you
should have checked that before sponsoring it on the podcast.
Some family with kids are gonna look that up and
then they're gonna be like what. Alright, we're talking about
whether or not intermediate mass black holes exist because we

(19:28):
don't really see them out there in space. We see
the kind that are about the size of suns, a
few hundred suns that came from suns. And we talked
about black holes that are at the center of galaxies,
which are huge tens of thousands or maybe millions or
billions of times the mass of our sun out there
in the middle of galaxies. But you don't see a

(19:48):
lot of black holes in between out there. Yeah, there
seems to be this weird gap there, and it makes
us wonder, like, are there those black holes out there?
But we just can't see them yet, or is there
some reason why they don't even exist, like they're unstable,
or they accelerate really quickly to becoming supermassive, or they
fall apart in some way. Like anytime there's a puzzle

(20:08):
like that, something we don't understand, where we see something
we don't expect, or we don't see something we do expect,
that's a clue. That's an opportunity to figure out something.
It's a moment where we can learn something about the
universe because we're ready to be surprised by the data. Right. Yeah,
it would be like, you know, having a population of
people and then seeing only two sizes of people, like
not seeing you know, sort of a continuum and like

(20:31):
a smooth range of sizes of people. Yeah, and it
would make you wonder like how did all those tall
people get so tall? At some point they must have
been medium height, right, so where are all the future
tall people? Why are there no medium height people who
are growing into those tall people. That's basically the question
we're asking about these intermediate mass black holes. I mean,

(20:51):
for example, you wonder about these supermassive black holes. How
did they get supermassive? They must have started from something.
If they started from something very small, then you know,
by simple arguments, they must have at some point been
in that intermediate range. Right, Well, I guess that's one
of the mysteries, right, Like, we don't actually know where
these supermassive ones in the center of galaxies came from

(21:13):
or how they came to be so large. That's one mystery, right,
that's exactly the mystery. Like the argument had just made
that supermassive black holes may have started from small things
and then grown to supermassive, we don't actually know if
that's true, and we see things about supermassive black holes
that we can't explain using that idea. For example, we
look really far into the edge of the universe to

(21:34):
see old old galaxies, to see galaxies when they were
very very young, right like just a billion years after
the beginning of the universe, there were already galaxies, and
in those galaxies we already see super massive black holes,
like black holes that have two billion solar masses. So like,
only a billion years have passed and already you've made

(21:55):
stars and galaxies and supermassive black holes inside them. We
don't know how it's possible to go from nothing to
supermassive black hole in just a billion years. So it's
a mystery. How did those black holes get started. We
don't know. I see a billion years from the Big Bang,
you're saying, we already had black holes that are billions
of times the massive US Sun. Yeah, exactly, And we
can see them because those super massive black holes have

(22:18):
really big accretion disks around them, like this disc of
gas and dust and other stuff that's getting swirled in
it gets really hot and it glows. It sends off
a huge amount of radiation. Those are called quasars. So
we see these quasars in the very very distant universe
from the very early universe, and if you model the
formation of galaxies, it's really hard to get a black

(22:39):
hole that big that fast. We talked about this in
our episode about supermassive black holes, so you can dig
into that if you want more details. But very briefly,
there's sort of like a limit at how fast a
black hole can grow. It's called Eddington limit. And as
you pump more stuff near a black hole than actually
the radiation from the stuff near it pushes stuff away
from the black hole. So it's hard to feed a

(22:59):
black back hole fast enough to have a grow to
be that big that quickly. So it's a mystery. We
don't know if these black holes actually started from something
small and then passed through this intermediate mass black hole
region and then became supermassive black hole, or if they
somehow skipped it. Could they have just you know, drunken
a lot of strong coffee or Italian style and black espresso,

(23:20):
or maybe American style boxes of cookies. You know, there
are other ideas to explain the supermassive black holes, like,
for example, maybe there were black holes that begin in
the very beginning of the universe, before there were even particles,
before they were stars, before there was even really matter.
Maybe there were these primordial black holes right because the

(23:40):
Big Band was pretty crazy, I imagine, or at least
how the universe was before the Big Bang, and you know,
things were pretty chaotic and crazy, and so maybe why not,
why couldn't you just have black holes form and that
kind of primordial high energy soup. Yeah, you could have.
And if you did that, then you could have gotten
a bunch of really tiny black holes and intermediate mass

(24:01):
black holes and maybe even supermassive black holes, or big
enough at least to become a seed which later turned
into these early supermassive black holes. And so it might
be possible that the supermassive black holes were seeing were
never intermediate mass, that they formed like suddenly during the
Big Bang, already fifty thousand solar masses and then just

(24:21):
grew to a billion masses. So the point there is
just because you have really big black holes doesn't mean
they were once medium sized. Right. It's it's kind of
like having a birthmark, like a mall you're born with,
not a mall of that shows up later. Yeah, and
there's a lot of really fun stuff. We have a
whole episode about primordial black holes, which is a really
fascinating topic because they might actually be real and out there,

(24:44):
and some people still think they might account for the
dark matter. So it's a really fun topic. The problem
with primordial black holes is that if you make them,
you expect to make them out all sizes, like really
big ones and then also really little ones and the
smaller ones. We should see those, and we should see
them evaporating, because from small black holes evaporate and they
give off radiation and as they give off radiation, they

(25:05):
get smaller, and as they get smaller, they give off
more radiation, so they would evaporate very brightly and we
should see that, and we haven't. So there's some skepticism
about whether primordial black holes are a thing. You're saying, like,
at the beating of the universe, we don't see evidence
for those little tiny black holes for me. Yeah, we
don't see evans for it. It doesn't mean that they
didn't happen, but we don't see evidence for it. So

(25:25):
if primordial black holes are the explanation for super massive
black holes, you need some reason why they basically only
got made on the larger size and they didn't make
littler ones that we would also be seen. But that's
not the only problem with that theory, right that Also
the theory is that even if you had started with
tiny black holes from the Big Bang, there's no way
they could have grown that big to what we see today. Yeah, well,

(25:47):
you need to make primordial black holes to be pretty big,
so you need to start from pretty large primordial black
holes so that they could be big enough to seed
the supermassive black holes we see today. You'd make them
at all ales in the Big Bang from tiny to
really massive. And I see. And the problem is that
we don't see evidence for those large seeds or what well,
that would explain the supermassive black holes. But we would

(26:09):
also expect to see the little ones, and we don't
see the little ones anywhere. Like the little ones. We
should see those evaporating all over the galaxy like they
should last, you know, a few billion years, and they
should be evaporating in these bright flashes of light as
they disappear. But we don't see that. So that means
that there aren't little primordial black holes. Doesn't mean there
weren't really big primordial black holes, but it means there

(26:29):
weren't really little primorial black holes. And that means that
it's a little bit more complicated to explain. Oh, I
see right, because we can see back in time. Right
when we look at into space, we can sort of
see back in time towards what was there close to
the Big Bang. Exactly, we do literally see back in
time because light takes time to get here from Earth.
We're seeing now what happened a million or a billion,

(26:51):
or five or ten billion years ago. As we look
deeper into the universe. It's really pretty awesome that we
can look backwards in time. But there are also other
ways that you could form these sue for massive black
holes without going through the intermediate mass black hole route right.
You could, for example, even without forming them in the
Big Bang, you could have this scenario where super massive
black holes formed directly from clouds of gas and dust.

(27:14):
It's called direct collapse. They never stopped and become a
star and burn for billions of years. First they just
go straight to black hole. Really, you can do that.
You can just form a black hole without igniting and
with fusion and everything. It's a theory. We've never proven
it and we've never seen one, but it's a theory
that people can't rule out. If you have enough stuff,
like a really big blob of stuff, then it collapses

(27:36):
fast enough the fusion doesn't have a chance to slow
it down and repel it, and it just goes straight
to black hole. And you can sort of simulate it,
and that can actually happen, like if you like the
clouds somehow takes up enough speed compressing that it starts
to night in the middle, but it's too late. There's
more stuff falling in and then poof, it becomes a
black hole. Yeah, grand gravity wins. And again it's still

(27:57):
just a theory, but the calculations check out out. We've
never seen when we don't have direct evidence for it.
But it's a way to potentially explain how these supermassive
black holes got started and then had a chance to grow.
And it would be another way to have really big
black holes without ever going through this middle phase, this
awkward teenage years of the black hole, when you head
between a hundred and ten thousand solar masses. They could

(28:21):
have directly collapsed to something really really big fifty tho
solar masses and then just kept growing from there. Isn't
there another theory that these supermassive black holes come from
smaller black holes colliding and merging into bigger and bigger ones.
For sure, But that takes a long time, right. If
you think that these big black holes came from really
tiny black holes, then they spend a long time in

(28:43):
the tiny stage, right because the gravity is proportional to
the size of it. And so if you start from
just like one stellar black hole and then at another
and at another, there's not enough time in the universe
to get up to billions of solar masses. That's the problem, right,
because I guess you know it thinks in space on
just like collapse head on. Right, they have to circle
each other for a while, you have to date, you know,

(29:06):
you have to go grab some coffee, eat some cookies,
and then eventually they collapse into each other. Right, that's
kind of the idea, Like things just don't running to
each other out there in space. Yeah, it's not like
some huge cosmic kid out there is trying to build,
you know, a supermassive black hole by sticking the black
hole legos together. It's got to happen. And for that
to happen, the things have to be in the right
arrangement and they have to collapse into each other. And

(29:26):
it gets easier as your black hole gets bitter because
it's more powerful and it can gobble stuff, so like
the sphere of its influence grows and where it can
eat from grows, so then it grows more quickly. But
that's why a black hole that starts from a really
small object spends most of its time small before it
accelerates its growth near the later phase. So we would
see those and they would take too long to get

(29:48):
to the supermassive black hole stage, so they can't explain
all the really old, huge black holes that we see, right,
and so that's the big mysteries that we know where
the smaller size black holes come from. They come from stars,
and then we see a bunch of the huge black
holes in the center of galaxies, but we don't see
the ones in between, and we don't know how the

(30:10):
big ones got as big as they did, Yeah, exactly.
We don't know those big ones were once intermediate size,
and we can't see any examples of the intermediate sized ones,
which seems like a head scratcher, right. Yeah, it's kind
of like a double mystery, like we don't know how
they got so big, and also we don't see any
of the middle ones. Yeah, and we'd love to unravel
that mystery. We'd love to understand how these supermassive black

(30:30):
holes form. And so finding one like when it was
a child might really give us a clue to how
they formed, or if we could prove that they never
were children, they were like fully born as supermassive black holes,
then one fell swoop that would also be fascinating. That
is a possibility, is that maybe they were made super
big in the big bank as primordial black holes or
through direct collapse. There are some mechanisms to make these

(30:54):
things and skip that intermediate phase. Right, That would make sense, right,
you know, to have their own proper origin story that
doesn't involve growing. Yeah, it does make sense. But you know,
galaxies come in all sizes. There are really big galaxies
and there are smaller galaxies, and it goes down to
pretty small like galaxies can be down to just a

(31:14):
few thousand stars. And so if black holes inside galaxies
are proportional to the mass of the black hole, and
again we don't know that, we've just seen that relationship
for larger galaxies, then it stands to reason that these
smaller galaxies should have smaller black holes inside them. Well
that's the mystery, and so let's talk about now how
we could find them or are people looking for these

(31:36):
medium sized black holes and is it theoretically possible to
see them or that they can exist. So let's get
into that, but first let's take another quick break. Alright,

(31:57):
we're talking about the ignored middle child, i'll black holes
of the universe, And I know this is a personal
topic for you. Daniel I'm sticking up for these black holes.
You're just trying to make them the cool ones. I think, Yeah,
we're like, who cares about? The little ones are the
big ones, They get all the attention. The cool ones
are the ones that you know, sit in the back
and plot their revenge with a podcast years later, decades,

(32:21):
decades into making This is My Revenge. No, set the
record straight. I love my brothers. They're wonderful and they're supportive,
and we're all very good friends. Right, Yes, did that
not sound sincere I meant it to. That sounded a
little tacked on at the end, But hey, you know,

(32:42):
I know how it goes your lawyer or a therapist.
Is there supposed to be a difference? Really? All right, well,
let's get into this mystery of the medium sized black holes.
We can't see them anywhere, now, Daniel. Do you think
we don't see them because we can't see them or
because they don't exist? That's the question. We don't know
the answer too, because frustratingly, smaller black holes are harder

(33:04):
to see. Like, one thing we're pretty good at is
figuring out what we're good at, and we're good at
seeing really big black holes because we can see them
affect how stars move in the center of galaxies, or
sometimes they have like huge quasars. We also know that
we're not very good at seeing smaller black holes, and
so we haven't seen any. But we also know that

(33:25):
we're not great at spotting them, so that leaves the
questions sort of up in the air. Right. Yeah, even
the ones that come from giants sons, they're kind of
hard to spot in space, right, It's not like they glow,
they're transparent invisible, and space is pretty dark, and maybe
they're not heavy enough to kind of like have a
big impact in the stars around them. Yeah, you have
to infer them. You have to be lucky. So you

(33:45):
have to see for example, gravitational lensing where the light
that passes near them gets bent as if there was
a huge invisible mass of stuff there, and you know,
you have to know that it's really compact and dense,
so it's not just like some big diffuse cloud or
dark matter, or you have to see their impact on
other nearby stars with nothing else to explain it. So
it's usually just like a process of elimination. We see

(34:07):
something happening in space and we can't explain it in
any other way other than a black hole. It's pretty rare.
You get very direct evidence of a black hole though sometimes,
and that's usually in the center of a galaxy, when
they're really big and there's like quasar emissions and other
stars moving very close to them. Have we seen the
smaller black holes, like, is there a catalog of them
or are they still kind of theoretical that they exist? Now,

(34:29):
we have definitely observed stellar mass black holes. Absolutely, we've
seen their gravitational lensing, we have seen their effect on
other bodies. In fact, we did a whole fun podcast
episode about the history of how people became convinced that
was not just a mathematical concept, but they were real
and they were out there, and the first one was
at the center of our galaxy is supermassive black hole.
But then later we observed a few candidates whereas a

(34:51):
stellar mass black hole m I see. And so we
see those, and we sort of see them in our
neighborhood kind of in our galaxy at least, but we
don't see the ones that are bigger, which you would
think we'd see, right because they are more massive and
so they would have more more of an impact on
the motion of stars around them, but we don't see them.
We see them if they're big enough, right, A million

(35:11):
mass black hole has a big impact on what's going
on in the center of galaxy. And so we can
look at those galaxies and we can study the motion
of stars in their center, and we can infer the
presence of a black hole, just like we do for
our galaxy. The problem is that smaller galaxies have fewer
stars and smaller black holes. So if you're looking at
a smaller galaxy like a dwarf galaxy, and you're trying

(35:32):
to understand does the motion of the stars tell me
there's some invisible sorts of gravity there? Then these are
small galaxies. It's sort of hard to just like get
the angular resolution you need to tell the motion of
the stars in these far away small galaxies. Oh, I see.
So you're saying that maybe they are there in smaller galaxies,
but we just can't tell. We know that we can't

(35:53):
see them. We know that if they were there, we
could not see them today. We just do not have
the resolution in current tell lescopes to see these stars
moving well enough to answer the question is there an
intermediate mass black hole there, Like, we just don't have
the eyeballs that we need. So they could be out there,
then it could be out there. We know that we
couldn't see them if they were there, so they could

(36:14):
be out there. And we have big plans like the
thirty meter telescope or the extremely large telescope. These are
things that are being built now and will come online
in tennish years. These will have much better resolution. They'll
be able to like get crisper pictures of these small
galaxies that are far away, to get a sense for
the motion of the stars near their center and tell

(36:34):
us if there's an intermediate black hole there or not.
We just can't see them today, and we haven't been
able to see them, right, Like, we know that they've
been kind of hiding from us if they exist, which
we don't know if they exist. Yeah, we know they've
been hiding from us. But that's just one technique to
see intermediate mass black holes, right is to look for
the motion of the stars near their center. There are

(36:55):
other techniques that people have been trying as well. What
are those techniques? Well, another one is not to look
at galaxies at all, but to look for other kinds
of things. Is this stuff out there in the universe
called globular clusters. We had a whole fun podcast EFFS
just about what are globular clusters and how much fun
it is to say that word. But these are weird
spherical collections of stars, and because they're much smaller than galaxies,

(37:18):
people think they might have intermediate mass black holes in them.
The upside is that they're much closer than other galaxies.
They tend to be like orbiting our galaxy. Like we
have several globular clusters orbiting our galaxy, so they're nearby,
so we can like take pretty good pictures and see
the motions of the stars inside them and get a
sense for whether or not there's an intermediate mass black

(37:39):
hole inside. Oh, I see these clusters. They're busier basically, right,
They have more stuff to them, more stars, and so
you would be able to maybe see the effects of
a medium black hole in them. Yeah, and they're much
closer than these other dwarf galaxies which tend to be
pretty distant, and so we would have a shot. The
problem is that these globular clusters aren't as tightly packed
sometimes as a dwarf galaxy, and so the speeds of

(38:01):
the stars are slower, so it's hard to tell if
there's an intermediate mass black hole. There are not. You
need to take pictures over like decades to see the
motions of those stars and infer it. And there's a
few globular clusters out there where people think maybe there's
a black hole there based on the measurements of velocities
of the stars, but none of them really hold a
to scrutiny. Like somebody announces, wow, look we found an

(38:22):
intermediate mass black hole, and then another group looks at
the same measurements and they say, no, we can explain
this without a black hole, So it doesn't really hold up.
You see, it's tricky. It's tricky because you have to
kind of like see the footprint of the black hole,
and it's not easy to do. And another technique is
to look not for the motion of stars, but for
the radiation from the accretion disc Right, we know that

(38:44):
black holes are not on their own, and especially the
massive ones in the centers of galaxies are surrounded by
gas and dust that's emitting a lot of radiation, so
these are called quasars. And we think that intermediate mass
black holes should also have intermediate mass accretion disks and
be intermediate as quasars, So we're looking for these as well.
Really we've only seen like supermassive quasars or stellar size quasars. Yes,

(39:08):
we've only seen supermassive quasars. Stellar mass black holes don't
tend to have quasars because they don't have the mass
to get their creation disk like up to that energy,
so they tend to only be from the centers of galaxies.
And the problem here is that we're looking for like
really old galaxies before they got really really big, so
like intermediate size galaxies, and these tend to be pretty

(39:29):
far away. And if you're looking for the black hole,
because it's a quasar, it tends to like drown out
the rest of the galaxy, so you can't like really
see what's going on in the galaxy. So we have
these quasars we've identified as maybe candidates for intermediate mass
black holes, but they're so far away we can't like
see the stars and really identify whether there's a black
hole there. Oh, I see, because there could just be

(39:50):
like a bunch of stars, yeah, clustered together. So but
that's one thing people are doing. But I think the
most exciting is this brand new way we have looking
at the universe first seeing things that's not based on
light but based on gravitational waves, right, like the ripples
and off space. Yes, out there in the universe, the
ripples in space itself, because when anything accelerates in the universe,

(40:12):
it creates a change in the gravitational field, and that
change ripples through space. And if the thing is really
really big, really really massive, and moving really really fast,
then you can actually measure these gravitational waves billions and
billions of light years away, and we have and the
things that have generated the gravitational waves that we've been
able to see here on Earth in observatories like Lego

(40:35):
are the mergers. For example, of two black holes falling
into each other, they accelerate around each other in this
crazy death spiral, emitting a huge amount of energy as
gravitational waves, and that we can use to figure out
how massive were the two black holes that fell in
and how massive is the resulting black hole, right because
you're seeing like the actual ripples and gravity from these

(40:56):
black holes, So just the ripple itself kind of tell
how big the things are that are crashing into each
other it's a very very detailed fingerprint because the ripples
tell you how fast things were going, and from that
you can back out things like how massive they must
have been, how strong was the gravity, and so yeah,
you can figure out how massive the individual black holes
were before they collapsed, and the mass of the final

(41:17):
black hole, which is not just the two black holes
added together. Some of the energy from the original two
black holes get lost in gravitational radiation, these gravitational waves,
and so the idea is that you know, we can
listen to when these black holes crashing into each other,
and so you're saying, like, maybe one day we'll see
or hear are the tag two intermediate black holes crashing

(41:38):
into each other, and then we'll be able to say, hey,
there's an intermediate one. Yes, absolutely, that's a possibility, because
we can use this as a way to weigh black
holes and we cannot otherwise directly see right, because remember,
gravitational waves can pass through anything. No amount of gas
or dust or radiation or whatever can stop them. So
it's just another very powerful orthogonal way of looking at
the universe, so we can get glimpses. Also, there's another

(42:01):
angle there, which is maybe we can see two really
really big stellar mass black holes merge. Like what happens
when an eighties solar mass black hole and a seventies
solar mass black hole get together, Well, they must form
a black hole that's heavier, right, that maybe even over
that threshold into the intermediate mass range. Here, it's an

(42:21):
X rated event there that Yeah, and this has actually happened.
And in two thousand nineteen they did see a merger
that resulted in a black hole that weighed a hundred
and forty two solar masses. And so this is really cool.
You can look this up on Lego. They call this
the black hole graveyard, and they showed like the masses

(42:42):
of the black holes that formed the new black holes.
And so we're now seeing more and more of these things,
and they're sort of pushing up that limit. Like it's
true that you can't make a black hole from a
single star that's bigger than eighty but as you said earlier,
as you start to combine these, they can creep up
a little bit and get over that hundred stellar mass threshold. Right,
And we've seen this, right, you're saying, you can go
online and look at the data of a hundred and

(43:03):
forty two solar mass black hole basically being born. Yes,
you can see it. It happened billions of years ago,
but just recently seeing the evidence here on Earth, so
that's exciting. It sort of cracks that number. It doesn't
really answer the mystery, right because it doesn't explain why
we don't see them. At A thousand and five thousand
and ten thousand, it's like just over the threshold of
what you might consider intermediate mass black hole. But we're

(43:26):
planning to build future crazy gravitational observatories that might be
able to see even bigger ones from further away. I
don't know if you've heard about the Lisa experiment. No,
what is it. The Lisa experiment is a version of Ligo.
This experiment we have mirrors dangling underground in Louisiana and
in Washington and in Italy to observe the ripples of space.

(43:47):
This is a version of it, but using three satellites
in space placed like really really far apart, but precisely
aligned relative to each other. Wow, that's pretty cool in space.
In space, I know, it's names bonkers, right that you
could have like two satellites and have them maintain an
exact distance relative to each other. It's going to be
very complicated and very expensive. But if they build it

(44:11):
this facility, Lisa will be very powerful at seeing gravitational
waves and you could detect them from intermediate mass black
hole murders Lisa in space. It sounds like a song.
He ripped that off from the Beatles, Lisa and the
Sky black Holes. All right, Well, I think you convince me, Daniel.
I think the middle children are pretty cool or they

(44:32):
don't exist one of the two. Sometimes I feel that way.
They're definitely lurking, They're big. Middle children are big lurkers.
For sure. We are psychological lurkers. Yes, So it's a
really fun question like why don't we see these things?
How did the supermassive black holes get so big without
going through this stage? Or maybe we don't see them
just because we don't have the telescopes yet, And in
twenty or thirty years we'll see a bunch of them

(44:53):
and they'll answer a lot of our questions about how
black holes get made because they are, I guess, in general,
just a cool family. You know, the youngest siblings and
the oldest siblings. They're all pretty cool in their own
way and pretty impactful in the universe. Right, everybody contribute
to the family. We love every part of it. All right,
I guess we'll leave that question dankling. What happened to
the intermediate black holes? Nobody knows. It's a black hole

(45:16):
in itself of information, but we'll figure it out. Stay
tuned for more updates from science. You might have to
wait another three episodes until Daniel figures it out his therapist,
but we'll get there, okay, all right. We hope you
enjoyed that. Thanks for joining us, See you next time.

(45:39):
Thanks for listening, and remember that Daniel and Jorge explained.
The Universe is a production of I Heart Radio. For
more podcast from my Heart Radio, visit the i heart
Radio app, Apple Podcasts, or wherever you listen to your
favorite shows.