December 5, 2023 • 42 mins
Daniel and Jorge discuss how dark matter shapes our Universe and whether it would be possible to have a galaxy without it.
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Speaker 1 (00:08):
Hey, Daniel, most of the stuff in the universe is
dark matter, isn't that right?
Speaker 2 (00:12):
Yeah, it's about eighty percent of the matter in the universe.
Speaker 1 (00:16):
And it's all around us, like right here with us
immersed in dark matter.
Speaker 2 (00:20):
Yeah, we are swimming in it.
Speaker 1 (00:22):
And it's also inside of us, is it?
Speaker 2 (00:24):
Yeah? It passes through us, it doesn't bounce off your skin.
Speaker 1 (00:27):
Does that mean that we're partly made out of dark matter? Oh?
Speaker 2 (00:30):
I guess. So that's kind of dark to think about.
Speaker 1 (00:33):
So if I discover myself, I'm discovering dark matter.
Speaker 2 (00:36):
Know thyself. Win a Nobel Prize.
Speaker 1 (00:38):
Or at least a dark Nobel Prize. Technically all Nobel
prizes have dark matter inside of them too.
Speaker 2 (00:43):
Maybe they'll give you the cash in dark money.
Speaker 1 (00:45):
Ooh, that got dark quick.
Speaker 3 (01:02):
Hi.
Speaker 1 (01:02):
I'm Hoeham and cartoonists and the author of Oliver's Great
Big Universe.
Speaker 2 (01:06):
Hi. I'm Daniel. I'm a particle physicist at CERN and
a professor at UC Irvine. And I'll take your money
dark or light.
Speaker 1 (01:14):
Really, you'll take dark money. Isn't that dangerous for a
tenured professor?
Speaker 2 (01:17):
No, that's what ten years for man. I can take money.
Speaker 1 (01:19):
From us visuals. I guess technically, the university takes the
money and you get a cut of it.
Speaker 2 (01:27):
I never really had to worry about that. Like, nobody
who got wealthy with dubious techniques ever offered me a
slush fund, So I've never had to really grapple with
that question.
Speaker 1 (01:37):
Wait, how do you know? I mean, you've taken money.
Have you done your due diligence? How deep did you go?
Speaker 2 (01:44):
You know, almost all my research money comes from the government,
and we all know the various crimes that the government
has committed.
Speaker 1 (01:50):
Exactly all government money is dark money.
Speaker 2 (01:53):
M do you believe taxes are theft? Is that where
this is going?
Speaker 1 (01:57):
Well, technically, or funding comes from taxpayers and there must
be some sketchy tax payers out there, so your work
is painted.
Speaker 2 (02:04):
So you're saying drug dealers who are paying taxes and
indirectly funding my research have made me complicit in their crimes.
Speaker 1 (02:11):
That's right. Yes, assuming drug dealers pay taxes, which I
guess they do. If they're laundering money, I don't know.
I feel like we need a whole podcast episode just
to cover dark money.
Speaker 2 (02:21):
Yeah, and then I suppose everybody's guilty.
Speaker 1 (02:23):
That's right, We're all guilty of paying taxes, I guess.
But anyways, Welcome to our podcast Daniel and Jorge Explain
the Universe, a production of iHeartRadio.
Speaker 2 (02:32):
In which we try not to overtax your brain while
explaining all of the mysteries of the universe. We want
to reveal the light universe and the dark universe, the
visible and the invisible. We want to show you how
the universe is not just what you see around you
and experience day to day, but so much more, so
much deeper, so many more mysteries waiting to be solved.
Speaker 1 (02:54):
That's why we explore the universe, the dark corners of it,
the light corners, and the sketchy corners of it, because
our understanding of the universe is still a little bit sketchy, like.
Speaker 2 (03:04):
The Mafia corners of the universe.
Speaker 1 (03:06):
No, like, not clearly drawn.
Speaker 2 (03:08):
Oh I see, I thought we're gonna be talking about
like the physics of New Jersey or something.
Speaker 1 (03:12):
Yeah, there's the only some dark matters going on out there.
Speaker 2 (03:17):
How do you eat all those rich cookies and not
gain weight? Really, it's amazing.
Speaker 1 (03:20):
Yeah, Well, I think the secret is dark chocolate. It's
lower and fat.
Speaker 2 (03:25):
Isn't it?
Speaker 1 (03:26):
Is it?
Speaker 2 (03:26):
Really? I don't know.
Speaker 1 (03:27):
You're like, what, what, Why aren't I eating more dark chocolate.
Speaker 2 (03:33):
I'm not really stopping myself from eating dark chocolate. But
that wasn't the reason I don't see dark chocolate is
like diet chocolate.
Speaker 1 (03:40):
It's chocolate light. Oh no, wait, it's chocolate dark.
Speaker 2 (03:43):
It's dark chocolate exactly. But there are lots of fun
questions out there in the universe, not just about how
waste management organizations in New Jersey are getting their money
and the calories that Daniel's eating. We're wondering about the
biggest questions in the universe, like where is all the stuff?
What is most of the universe made out of? But
how has it shaped the night sky that we see
(04:03):
today in the galaxy that we live in.
Speaker 1 (04:05):
That's right, because what the universe is made out of
is maybe one of the biggest questions we can ask
about the universe. What is this whole place made out of?
What are we made out of? Are we made out
of dark matter? Can we exist without dark matter?
Speaker 2 (04:18):
And we talk on this podcast a lot about dark matter.
We have lots of episodes about what it is and
where it is and how it works. And one thing
we often stress is that dark matter is part of
our galaxy, that most of our galaxy is actually dark
matter that has played a big role in the formation
of our galaxy. We wouldn't be here without it.
Speaker 1 (04:35):
Yeah, we do talk a lot about dark matter. Daniel,
What percentage of our episodes would you say we talk
about dark matter? Is it representative of the amount of
dark matter in the universe? Are we ignoring dark matter
in a way?
Speaker 2 (04:48):
Is dark matter underrepresented matter on podcasts? That's a good question.
Speaker 1 (04:52):
Yeah, well, especially our podcast. If we're trying to explain
the universe and the universe is twenty seven percent dark matter.
Speaker 2 (04:58):
You know, we might reach twenty seven percent of our
podcast being about dark matter. It's been a lot of them,
and I'm pretty sure we don't have two thirds of
them about dark energy. Maybe we should.
Speaker 1 (05:07):
Yeah, yeah, that's what everyone is asking for more dark
energy episodes. But it is a pretty important part of
the universe. It's a pretty important part of our existence
because without dark matter, maybe galaxies would not have formed
the same way that they formed. The Milky Way would
might not be the same way it is now.
Speaker 2 (05:23):
But we're always tempted to over general life to say
that the way we live in our certain situation over
here is the way the whole universe works. It's important
to take a step back and to ask whether our
way of life and our way of galaxying is the
only way that it can be.
Speaker 1 (05:38):
And so today on the podcast, we'll be asking the question,
are there galaxies without dark matter? Would you call these
light galaxies then? Or galaxies light diet galaxies?
Speaker 2 (05:54):
I call them tragic galaxies because they're probably galaxies where
everybody only eats white chocolate.
Speaker 1 (05:59):
Or milk chocolate, some chocolate snobs. I also call that
a tragedy.
Speaker 2 (06:04):
Also known as crimes against chocolate.
Speaker 1 (06:06):
Are you saying Hershey's is dark money as well?
Speaker 2 (06:10):
Her She's his garbage man?
Speaker 1 (06:12):
Oh my gosh. And there goes our sponsorship deal with Hershe's.
We were so close to funding this thing for the
next twenty years, and you have to go and insult them.
Speaker 2 (06:25):
Maybe you could hear my hesitation there. I'm being torn
between being honest and truthful on a hard science podcast
and pandering to our sponsors, and I just had to
be honest about it. I passed on hershe'es. I'd rather
have no chocolate than Hershe's.
Speaker 1 (06:38):
That was pretty harsh to garbage. Oh my gosh. Well,
here here's the thing, Like when you eat it. Let's
say you eat like a chocolate cake at a restaurant.
How do you know they didn't use Hershey's chocolate.
Speaker 2 (06:48):
You can taste it. Man, that's sour affront to chocolate.
It's turned so many people off of chocolate when chocolate
is this wonderful, amazing thing. Even milk chocolate can be
high quality, can be amazing. What they may in Pennsylvania. O, man,
it's a crime.
Speaker 1 (07:03):
Now you're insulting the whole state of Pennsylvania.
Speaker 2 (07:07):
Let's keep going. See how many people I can offend.
Speaker 1 (07:09):
Yeah, yeah, let's moves and let's insult the whole universe.
Speaker 2 (07:13):
Why don't you No, Fortunately, most of the universe is
dark matter and therefore appreciates dark chocolate, and so we're good.
We have the majority firmly on our side.
Speaker 1 (07:21):
But doesn't Hershey's own some fancy brands like Scharfenberger or
something like that.
Speaker 2 (07:25):
I don't know, I hope not.
Speaker 1 (07:26):
Well, maybe you've been eating Hershey' chocolates all this time.
A plot twist. Anyways, we're talking about dark matter and
galaxies and could there exist galaxies without dark matter. These
would be like galaxies that don't have any dark matter
in them or around them.
Speaker 2 (07:43):
Yeah, exactly, just stars and gas and dust and black holes,
no dark matter, all.
Speaker 1 (07:49):
Right, Well, whether they are tragic or not. We were
wondering how many people out there had wondered about this
question and if they have any ideas about the answer.
Speaker 2 (07:56):
Thanks very much to everybody out there who answers these questions,
they or not there supporters of Hershey's Crimes against Chocolate
would really appreciate everything you do. If you would like
to join this group, just write to me two questions
at Danielandjorge dot com.
Speaker 1 (08:10):
I feel like maybe there's a Hurshi's employee out there
who listens to our podcast and it's now very very sad.
Speaker 2 (08:16):
There's an easy fix for that, find a new job
or a new podcast. No, you've got to go to
the root of the problem.
Speaker 1 (08:22):
That's right anyway. So think about it for a second.
Do you think there can be galaxies without dark matter?
Here's what people had to say.
Speaker 4 (08:30):
I feel like there probably aren't, only because I know
that dark matter, as far as I know, is distributed
pretty much evenly throughout the universe. I think it tends
to be clustered in galaxies, but I think it tends
to be pretty uniform. So I would be surprised if
there were galaxies without dark matter.
Speaker 5 (08:44):
I'm going to say why not, because, like going back
to an episode about uranium on Uranus, there could be
a tiny bit here and there in a galaxy. So
I'm going to say I don't know why there wouldn't
be dark matter in a galaxy. I'm also going to
say I don't know why there would.
Speaker 3 (09:03):
I don't believe there are galaxies without dark matter because
dark matter is, in my understanding, a general term for
unknown matter, which is this and makes up the overwhelming
majority of the universe. Therefore, I think it is not
possible for galaxies not to have at least some dark
matter in them.
Speaker 6 (09:23):
I'm not sure that we've observed any galaxies without dark matter,
but I suppose anything could be possible in this crazy universe.
It's also possible that all galaxies are without dark matter
and we just don't understand gravity all right.
Speaker 1 (09:33):
Most people are skeptical about this question.
Speaker 2 (09:35):
Yeah, people have the idea that dark matter is everywhere.
Speaker 1 (09:38):
It's inescapable, you can't get away from it. It seems
in our question, although one person has kind of said,
why not, that's a good attitude to have.
Speaker 2 (09:45):
Yeah, that's the whole attitude about physics, Like, well, maybe
everything is different from what we've boughter. Maybe there's something
really weird out there that could teach us something new
about the universe.
Speaker 1 (09:54):
I do feel like that is a guiding question in
theoretical physics at least. Why not? Yes? Why not?
Speaker 2 (10:00):
Sure? Maybe everything is just tiny cats at the quantum scale.
Speaker 1 (10:04):
That's right, Maybe everything's just made out of Hersy's charga.
Speaker 2 (10:06):
No, I got to hop somewhere else in the multiverse.
Speaker 1 (10:11):
If that's the case, all right, Well let's get going
before this podcast gets too dark. Daniel, give us the basics.
What is dark matter? For those of us who haven't
listened to the twenty seven percent of our podcast.
Speaker 2 (10:22):
Episodes, dark matter is fascinating because we simultaneously know a
lot about it and very little. Like we know that
there's a lot of dark matter in the universe, and
we know that it's matter. We know there's something out
there that's creating gravity or curvature of space time, but
that it's invisible. It doesn't glow, it doesn't give off light.
It doesn't reflect light. We sense it only because we
(10:45):
see its gravitational effects on stuff. It's curving space, which
changes how things are moving through that space. We see
galaxies rotating much faster than their gravity would be able
to hold them together if there wasn't also dark matter
in them holding them together. On the other hand, we
don't know what stuff it is. We know that it
has gravity, so it's matter, but we don't know it's
(11:06):
some weird new kind of neutrino or a totally different
kind of particle we've never seen before, or a thousand
new kinds of particles, or something that's not even a particle.
So we know a lot about it on the sort
of cosmological scale, but very very little or almost nothing
about it at the particle level. Right.
Speaker 1 (11:22):
Dark matter is this kind of mysterious stuff in the
universe that we kind of feel its presence. We can
see its presence through gravity, but as you said, you
can't see it because it doesn't interact with elechormagnetic light
or maybe any of the other forces in the universe,
and so you can't see it. And that's why you
call it dark exactly.
Speaker 2 (11:38):
And a bunch of listeners write in with the idea
that maybe dark matter is matter in another universe that's
somehow leaking in to ours. Remember that dark matter is
creating gravity in our universe, or changing the curvature of
space in our universe, which means that it's in that space. Right,
it shares that space with us, which kind of means
that it's in our universe. We really do know that
(12:00):
dark matter is something in our universe that's changing the
shape of space. We can only see sort of indirectly
through gravity, which is really frustrating because gravity is the
worst way to see things. It's so weak that it
makes it very, very difficult.
Speaker 1 (12:14):
But wait, couldn't it bend our space and not be
part of our space.
Speaker 2 (12:18):
That's possible if you overthrow general relativity in our entire
understanding of space time. General relativity says that matter tells
space how to bend, and space tells matter how to move,
and that means matter in our space. Like in general relativity,
the curvature or space comes from the energy density in
that space. So if you have some like parallel space
overlaid on top of it, which can also bend that space,
(12:42):
then it seems to me like it would be part
of our space, you know, sort of like the by definition,
but yet you could augment or throw out general relativity
replace it with something totally different. But the simpler idea
is that it's just some kind of mass we can't
see that explains almost everything we see out there in
the universe. So it's sort of the best going explanation.
You can always make more or baroque complicated explanations if
you like.
Speaker 1 (13:02):
Well, it kind of might as well be in another universe, right,
because if it doesn't feel a lot of the same
force as we don't, it's just kind of like ghostly
matter that's kind of living on top of us. There
might be beings made out of dark matter, right.
Speaker 2 (13:14):
Physicists call the different sectors of the universe. If you
have like two different sets of particles that don't interact
at all except for through gravity, then we call those
like the lights of the visible sector and the dark
sector of the universe. And that's totally possible that you
could have a whole complicated physics happening in the dark
sector that we can't see. Now. Mostly we know that
dark matter can't interact with itself. If it did interact
(13:36):
with itself, it would form all sorts of complicated structure
and do all sorts of interesting things. We think that
dark matter is pretty spread out. However, there could be
a little component dark dark matter. Dark matter may be
lots of different kinds of things, and one little component
of it might be more complicated and do complicated things
like form life or ice cream, cones or cats or
(13:56):
good chocolate without violating what we've seen dark matter do,
which is mostly spread out smoothly.
Speaker 1 (14:01):
All right, Well, the question here today is can there
be galaxies without dark better? Why is this even a question? Like,
are most of the galaxies that we see out there?
Do they all have dark matter?
Speaker 2 (14:12):
Yeah, the galaxies that you see out there in the
sky are like tracers. They're basically telling you where the
dark matter is in the universe. Remember that dark matter
is not something we can see, but it also dominates
the universe. Our estimates are that four fifths of the
matter in the universe is dark matter. So if you
like spin the wheel and pick a random object in
the universe, most likely you're going to get dark matter.
(14:34):
It's like overwhelmingly dark matter. So when the universe is
forming its structure and the gravity that determines like where
things are going to be. It's mostly the gravity of
dark matter that decides where things are going to clump
together and where things are not going to clump together. Remember,
the very early universe is mostly smooth, with a few
little blobs that are denser than others. The gravity of
(14:56):
those over dense pieces pull things together to form structure,
and that's where you get like galaxies in one part
of space and not galaxies in another part of space.
Where you have galaxies is where you had more dark
matter to pull that stuff together to form those galaxies.
Speaker 1 (15:10):
Yeah, that's pretty wild to think that something we can't see,
that is invisible to us basically kind of dictates the
entire structure of the universe, right at least at the
galaxy level. Does dark matter also dictate things like superclusters
and beyond exactly?
Speaker 2 (15:26):
And so you can imagine like these invisible wells, like
dark matter's curving space with Shepherd's the other kind of
matter together. So every time you look up at the
night sky and you see a galaxy, you should imagine
there's an invisible blob of dark matter surrounding that galaxy.
There's a whole halo that's created the conditions to form
that galaxy. We run simulations, for example, of a universe
(15:46):
without dark matter, and it doesn't form galaxies after fourteen
billion years. So quite literally, we would not be alive
without dark matter.
Speaker 1 (15:54):
And now is the same true for things like superclusters
and those giant bubbles of galaxies they're in the universe.
Is that dictated by dark matter as well? Or is
that more of the quantum fluctuations of the universe.
Speaker 2 (16:07):
Well, both, because there's this cosmic web that tells us
where dark matter will be denser and where dark matter
will not be dense. These filaments in some places they
overlap and you get these wells where things pool together.
And so the whole cosmic web is dictated by dark
matter and the light matter. The normal matter, baryons and
quarks and electrons just follows that. And so it's not
(16:27):
like a supercluster has a super halo. A supercluster is
made of galaxies connected together by these filaments.
Speaker 1 (16:34):
So it also has filaments of dark matter.
Speaker 2 (16:37):
Absolutely, yes, there are filaments of dark matter connecting these
halos of around each galaxy.
Speaker 3 (16:43):
Hmmm.
Speaker 1 (16:43):
Interesting. All right, Well, let's dig into the question of
whether a galaxy can be clean of dark matter, or
whether it's kind of a requirement for a galaxy to form.
So let's dig into that, but first let's take a
quick break. All Right, we're asking the question, can you
(17:10):
have a galaxy without dark matter? Could you maybe have
a scripy little galaxy out there? That was like, no,
I don't care about dark matter. I'm just going to
gather all these all this gas and dust and on
my own without any help. That's kind of what we're asking.
Speaker 2 (17:24):
Today, right, Yeah, exactly.
Speaker 1 (17:26):
And so you talked about how most of the galaxies
that we see out there probably have dark matter, right,
we think they have dark matter, right, because they couldn't
be holding together without dark matter.
Speaker 2 (17:37):
That's right. And it's even more than just most galaxies
have dark matter. It turns out galaxies are sort of
like extra rich in dark matter, Like most galaxies have
more dark matter than the average dark matter density in
the universe.
Speaker 1 (17:50):
Wait, what what do you mean? Like, what are some numbers?
Speaker 2 (17:52):
So if you average over the whole universe, like what
fraction of matter in the universe is dark matter? That's
eighty four percent by counting. We don't know how many
dark matter particles there are that's by mass, Like, what
fraction of the mass of stuff in the universe is
dark matter? That's about eighty four percent. But when you
look at galaxies and you ask, like, what fraction of
the mass in a galaxy is dark matter, that's more
(18:15):
like ninety one percent. So galaxies have like about half
as much normal matter as the average normal matter density
in the universe. Galaxies are like concentrated blobs of dark matter.
Speaker 1 (18:27):
And we get these numbers by measuring how fast the
galaxies are rotating and kind of guessing how much dark
matter you need to hold it all together.
Speaker 2 (18:36):
Yeah, not so much guessing, measuring right, But you're right.
It's looking at how the galaxy rotates. We can measure
the speed of those stars in the galaxy as they
whizz around the center by looking at their light and
seeing how it's red shifted or blue shifted. You're looking
at a galaxy, some of the stars will be moving
away from you and some moving towards you, so to
be red or blue shifted their light from the Doppler shift,
(18:57):
so you can measure their velocities. So you can look
at the velocity of stars as they get further and
further away from the center, and in order to hold
a star at a certain velocity a certain radius, you
need a gravitational force there, so you can calculate exactly
how much gravity is needed to hold a star at
a certain radius. They have all these stars at different
distances from the center, telling you exactly how much gravity
(19:19):
you need to keep those stars going at that speed,
and then you can add up how much you can see,
like count all the visible stars and the rest you
suppose is dark matter. I guess that's what you mean
by guessing.
Speaker 1 (19:31):
Yeah, inferring guessing.
Speaker 2 (19:34):
There's a whole field of statistical inference that we should
just call guessing.
Speaker 1 (19:37):
Yeah, Well, I mean you don't actually know how many
stars there are in that galaxy right, so far away
you can't see the individual stars, so you're also sort
of inferring how many stars there are there. You're guessing
a little bit, aren't you.
Speaker 2 (19:51):
There's always uncertainty in these measurements. Absolutely, and you're right
that we cannot resolve individual stars, especially near the center
where things get very dense. But we can see this streams, right,
we can see streams of stars. We have models for
how these galaxies work. But absolutely there's always uncertainty, but
the uncertainty in these calculations is tiny compared to the
size of dark matter. So there's no uncertainty that there's
(20:12):
a lot of dark matter in these galaxies because remember
the fractions we're talking about here, like ninety percent, which
means you're looking at a star, you're measuring its velocity.
You figure out how much gravity is needed to hold
it there so it doesn't fly out into intergalactic space.
And when you add up all the stars, you get
like ten percent of the gravity you need. So there's
a huge missing chunk.
Speaker 1 (20:34):
And you're sure it's not just you know, a lot
of asteroids or rocks that don't close.
Speaker 2 (20:38):
Yeah, that's a great question. Could dark matter just be
normal matter that we're not seeing right, just like dark
chunks of matter. So people have looked for that directly.
Those are called MACHOs massive compact halo objects, and we
think we would see those occasionally, like they would pass
in front of stars if there was a lot of them.
If they were really big, we would have spotted them.
(20:58):
So people have looked for that kind of stuff and
not seen it. Plus we know something about how much
normal matter there was in the very early universe because
it dictates the fraction of elements that were produced, like
the hydrogen and helium and lithium very sensitive to the
density of quarks and electrons in the early universe. We
talked about that once Big Bang nucleosynthesis. So we have
(21:20):
a pretty good handle on how much normal matter there
was around, and we can explain where most of that
is now and the rest of it's got to be
dark matter.
Speaker 1 (21:28):
And so basically every galaxy out there that we've seen,
we see that it's spinning faster than it should, or
it's holding together more than it should, so we think
it has dark matter. And now is that true for
every galaxy we've seen out there?
Speaker 2 (21:40):
It turns out there's a pretty wide variety. Like when
you look at galaxies out there, some of them have
a lot of dark matter, and some of them have
a huge amount of dark matter. There is a variety.
It turns out that smaller galaxies tend to have more
dark matter than really massive galaxies.
Speaker 1 (21:57):
Not by absolute amounts, but just relative town these stars
they have.
Speaker 2 (22:00):
Yeah, exactly higher dark matter fractions, I should say, And
that's because galaxies are better at holding onto their dark
matter than they are their normal matter. Galaxies are crazy places.
There's winds from all the stars, right every star is
a fusion furnace and pushing out protons and electrons. These
cellar winds are pushing gas out of the galaxies. Then
(22:21):
there are supernovas going off all the time, blowing things
up and pushing things out. There's radiation, really intense radiation
from the center of the galaxy that's pushing gas out.
So galaxies are basically exploding and they're pushing a lot
of their matter out. And so the smaller galaxy is,
the less it's capable of holding onto its normal matter,
the less it's capable of resisting these forces that push
(22:44):
gas out of the galaxies.
Speaker 1 (22:46):
Because the bigger galaxies have more gravity.
Speaker 2 (22:48):
Basically right exactly, the bigger galaxies are still doing this,
but they have more gravity so they can hold onto
their normal matter. So smaller galaxies, which have weaker gravity,
lose more of this normal matter. So you look out
there at dwarf galaxies, really tiny ones. They can be
like ninety nine percent dark matter.
Speaker 1 (23:03):
But wouldn't the larger galaxies also be better at holding
onto their dark matter, Like wouldn't smaller galaxies lose some
dark matter eventually, like it might evaporate or something.
Speaker 2 (23:12):
Yeah, that depends on what dark matter does. And in
this theory, dark matter does nothing but gravity, and so
you can't really lose your dark matter. Like to lose
your dark matter, you need some force that's pushing out
on it. But gravity is just attractive. So all these
forces like the solar winds and the radiation and the
supernova basically have no impact on the dark matter. Dark
matter just like brushes it right off, Like supernova could
(23:33):
happen right next to you, and a dark matter particle
would be like whatever, dude.
Speaker 1 (23:37):
And so we haven't seen any galaxies without dark matter,
So then why are we asking the question are there
galaxies without dark matter? Is it more of like, is
it possible to have a galaxy without dark matter? Or
are we asking like could there be galaxies where we
haven't noticed it doesn't have dark matter?
Speaker 2 (23:51):
Yeah, so great question. We're curious about this for lots
of reasons, Like number one, we have a theory about
how the structure of the universe came to be and
how it made galaxy and this nice story we told
you about over densities clumping together to form galaxies et cetera,
et cetera. But we'd like to test that. We'd like
to make sure that's correct. We're often surprised when we
look out in the universe and see how things actually work,
(24:14):
and so what we'd like to do is check our
predictions about like the dark matter fractions of galaxies against
reality and see is this really the way things work. Also,
this really helps us understand what dark matter is, because
seeing how dark matter varies across the universe can tell
us something about the nature of dark matter and help
us test various alternative theories about what dark matter might
(24:35):
or might not be. But it's not exactly true that
we've never seen a galaxy without dark matter. People are
out there looking for these and they found some pretty
weird cases.
Speaker 1 (24:43):
Interesting, all right, what are some of these cases?
Speaker 2 (24:46):
So this galaxy group kind of nearby on cosmic scales,
that's sixty three million light years away. It's called in
GC one zero five two, and basically it's an elliptical
galaxy in the Cetus constellation. We've known about if like
two hundred and fifty years or so, but there's actually
a little group of galaxies. It's like a major galaxy
with a bunch of little galaxies nearby. They call these
(25:08):
dwarf galaxies, so it's a whole group, so they call
the group of galaxies. And these little galaxies are actually
ultra diffuse galaxies. That means they're galaxies that are not
very bright, they have very few stars in them. And
these ultra diffuse galaxies near this NGC group they think
might have no dark matter in them at all.
Speaker 1 (25:27):
Why do they think that?
Speaker 2 (25:28):
So they look at the rotations of these galaxies and
they do that calculation and they estimate zero dark matter.
Like every time you're doing this, you're not assuming the
dark matter. You're measuring it. And sometimes it comes out
ninety percent, sometimes eighty four percent, sometimes ninety nine percent.
In this case, it comes out close to zero or
consistent with zero. So they think these are little galaxies
(25:48):
that have no dark matter in them at all.
Speaker 1 (25:50):
So that's pretty wild. That means that you can have
a galaxy without dark matter.
Speaker 2 (25:53):
Yeah, it's fascinating because remember, our theory of galaxy formation
is that basically every big galaxy is a merger of
a bunch of small galaxies. Big galaxies don't like form
all at once in a single collapse. You have a
bunch of baby galaxies then merge to make bigger and
bigger galaxies, so like a bottoms up approach. And so
if your big galaxy ends up with a lot of
dark matter in it, that means that the little galaxies
(26:16):
that made it should each have their own dark matter.
And we look out a dwarf galaxies and we mostly
see them having dark matter. In fact, some of them
have a lot. So it is really weird to see
these little galaxies without any dark matter at all. And
the question is like, did they form this way or
did something happen to strip them of their dark matter.
Speaker 1 (26:32):
Or maybe they formed later in the universe.
Speaker 2 (26:34):
Yeah, exactly, And so that's a fascinating question. And so
there's a group that's done a study of these and
they have a theory about how these little diffuse galaxies
ended up without any dark matter in them.
Speaker 1 (26:44):
What's a theory.
Speaker 2 (26:45):
So the theory is basically a mini version of the
Bullet cluster. You remember. The Bullet cluster is this famous
example that really convinced a lot of people that dark
matter was a real thing. It was a cluster of
galaxies that collided with another cluster of galaxies and we
saw that what happened to them the gas and the
dust and the dark matter was very different. So the
gas and the dust interacted and created collisions. The dark
(27:06):
matter passed right through because it doesn't they direct at
that level, gravity's not strong enough, so basically separated the
dark matter from the normal matter. So the bullet cluster,
and now you have a blob in the middle with
a bunch of normal matter in it, and then you
have dark matter on both sides, so we can see
through gravitational lensing. So they think that might be similar
to what happened in this case, that maybe there was
(27:26):
a big collision between two other objects, and these two
things that we're seeing now they call them DF two
and DF four are basically the results of that, like
chunks of stars and gas and dust that got stripped
of their dark matter and a collision and then tossed aside.
Speaker 1 (27:42):
Well, why wouldn't some little bit of dark matter go
with them.
Speaker 2 (27:45):
Because the dark matter and the normal man have very
different experiences in a collision, Like dark matter basically passes
the right through. There's not really a collision when it
comes to dark matter. It's like two ghosts just phasing
through each other whereas two people bumping into a hallway
are going to change their direction. So imagine you have
like a ghost inside you and somebody else is a
ghost inside them, and you have a collision in the hallway.
(28:06):
The ghosts just keep on going and the living people
bounce off each other. Now you know you're separated from
your ghost.
Speaker 1 (28:12):
But there's so many that they discovered. Did they all
get that way from the collision?
Speaker 2 (28:15):
So they've only found these two, and they have this
reconstruction of the collision that suggests that these two things
happen somewhere near NGC and created its collision, and it
should have also created a bunch of other ultra diiffuse
galaxies that they should be able to spot. That there
should be like five or six of these that came
out of the collision that also have no dark matter.
(28:36):
So they're going to go and look for those.
Speaker 1 (28:39):
So these are you said, these are dwarf galaxies.
Speaker 2 (28:41):
Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so
they're small and they're not.
Speaker 1 (28:46):
Very bright, so in a way they kind of got
made later or not? Are they as old as the universe?
Speaker 2 (28:50):
Well, they think this collision happened about eight billion years ago,
and so how you age these things, I guess depends,
like the way they are now started about billion years ago. Now,
of course, have some progenitor or something that they came from. Right,
there was a larger object they were a part of
which definitely had dark matter in it, and their dark
matter is now sprayed in some other direction. So they
(29:12):
become separated from their dark matter. And this must have
been a pretty mammoth event. I mean, they reconstruct this
thing and it's like a collision at three hundred kilometers
per second of these huge cosmic objects.
Speaker 1 (29:24):
Well, so these are galaxies that had dark matter, but
then they got stripped away of their dark matter, and
so that shows that, hey, you can have a galaxy
without dark matter. But I guess maybe the larger question
is can you form a galaxy without dark matter?
Speaker 2 (29:38):
Right? And so as you say, that's the deep question
about the nature of the formation of structure in the universe,
and so far the answer to that is no, we
do not think it's possible to form a galaxy without
dark matter. We think you need that dark matter around
to gather enough gas and dust to make stars and
to make a galaxy that without dark matter, normal matter
doesn't have enough gravity to have formed galaxies this early
(29:59):
in the UNI. If you had a universe without dark matter,
or big section of it without dark matter and just
normal matter, it would form galaxies eventually, but it would
take a lot longer to do so.
Speaker 1 (30:09):
But you know, as I understand it, and during the
Big Bang, things were really hot and dense, and there
were pockets of things, and there were quantum fluctuations which
maybe created pockets of extra densities here and there. Couldn't
there have been a pocket of extra density of normal
stuff but not dark matter. That then when the universe
blew up, it became a galaxy without dark matter, like
(30:30):
during the Big Bang. Why does the normal matter have
to follow the dark matter?
Speaker 2 (30:34):
I guess it does in general because dark matter dominates
because it's just so much more of it. So it
basically like sets the scene for everything. But you're right,
it's theoretically possible to have a downward fluctuation in the
dark matter and an upward fluctuation in the normal matter.
So you get some region of space where you have
like extra super dense normal matter and almost no dark matter.
(30:55):
That's possible. Yeah, and so in principle that could happen,
and if it had enough matter, then it would form
a galaxy on its own. So in principle that's not impossible,
but we've never seen that, and I don't know what
the chances are of that happening theoretically.
Speaker 1 (31:08):
Like, how many galaxies have we've done this calculation to
make sure that it has dark matter in it.
Speaker 2 (31:12):
Yeah, that's a great question. We've measured the rotation velocity
of thousands and thousands of galaxies, but that's a tiny
fraction of the number of galaxies that are out there.
In the number of galaxies we can see, most of
the galaxies we can see, we can't measure their rotation
velocity because you're looking at like one pixel or two pixels.
You need to be able to sort of resolve the
whole galaxies you can see like from one side versus
(31:33):
light from the other side. So it's tricky. But yeah,
we haven't looked at that many galaxies as possible. There
are galaxies out there that did really form without dark matter.
Speaker 1 (31:42):
All right, Well, it seems like we kind of answered
the question of the episode, which is can you have
a galaxy without dark matter? The answer is yes, you
can have maybe galaxies that had dark matter, but then
they lose it or they get left behind by the
dark matter and so they're dark matter less. Or maybe
they could have forded at the beginning of the universe
in theory, but we haven't seen one yet. All right, Well,
(32:04):
let's dig into what this all means about our understanding
of dark matter and also gravity and whether or not
it needs to be overhauled. But first, let's take another
quick break. Orright, we're asking the question can there be
(32:30):
a galaxy with no dark chocolate? And the answer is probably,
But that's not a universe Daniel wants to live in.
Speaker 2 (32:38):
No, that's right, Transport me somewhere else in the multiverse asap.
Speaker 1 (32:42):
What if you end up in the universe where there's
only milk or white chocolate?
Speaker 2 (32:46):
Just keep smashing that button until I get somewhere good.
Speaker 1 (32:48):
No, No, you only get one trim.
Speaker 2 (32:49):
Would you take the risk? I'm pretty happy with our universe,
you know.
Speaker 1 (32:54):
It's a pretty good one. We're talking about whether galaxies
can exist without dark matter, and the answers yes, they
can be stripped away of their dark matter, or theoretically
they could form in the early universe, but we haven't
seen one yet, and so maybe probably not. What do
you think happened? Why haven't we seen any? If they
can form without dark matter, why haven't we seen any.
Speaker 2 (33:13):
I think that'd be really unlikely. I mean, the kind
of fluctuations we're talking about are very, very large. In
the early universe, you had just sort of like energy,
and then it decays into matter. As the universe expands
in cools, and every kind of matter is sort of
made uniformly, so you get more dark matter made and
less normal matter. But in order to have no dark
matter made, you'd need a really big fluctuation. You expect
(33:35):
to get like eighty five percent and you get zero.
It's like flipping a coin one hundred times in a
row and getting only heads instead of half heads and
half tails. It's pretty unlikely. Now the universe is really big,
of course, so that means that eventually it's going to happen,
especially if the universe is infinite. But it's so unlikely
that it's not going to be the first kind of
galaxy we see out there, or even in the first tranch.
(33:56):
Eventually we might spot one.
Speaker 1 (33:57):
Well, as you said, it is possible to have a
galaxy without dark matter. We've seen it in some ultra
diffuse galaxies and they've done a measurement on these galaxies, right,
They've measured how fast it's spinning, and they're pretty sure
there's no dark matter in them.
Speaker 2 (34:10):
Yeah, there were a series of papers where people said, oh,
there's no dark matter, and another group did a different
measurement said no, there is some dark matter. And then
there were follow up papers arguing, and now they're pretty
sure there's no dark matter in these but there's always
somebody out there who disagrees. I mean, it's astronomy, after all.
Speaker 1 (34:25):
That's right, they're all just guessing.
Speaker 2 (34:28):
They're all just doing their best statistical inference.
Speaker 1 (34:33):
That's right. That's a great word for guessing. I'm just kidding.
A best guessing. How about that best guessing. Nobody has
a better guess.
Speaker 2 (34:42):
You know, those whole departments of people who do nothing
but statistics for a living. I'm trashing Hershey's, but you're
trashing statistics. Man.
Speaker 1 (34:50):
No, there's nothing wrong with guessing wrong.
Speaker 2 (34:55):
How do you know the universe is the way it is.
We're just guessing.
Speaker 1 (34:58):
A best guess doesn't mean that you're making thing. So randomly.
You're just using the best information you have to make
a best estimate or inference, right.
Speaker 2 (35:05):
I suppose so. I think if it's very well informed,
it's not really a guess, you know.
Speaker 1 (35:09):
But you're one hundred percent sure it's also not a fact.
Speaker 2 (35:13):
Yeah, that's true. That's why we use statistics to describe
our uncertainties. Anyway. One of the things we are uncertain
about is the nature of dark matter. Like, a lot
of the stuff we talk about for dark matter is
kind of unsatisfyingly indirect, and a lot of people out
there treat dark matter like it's some placeholder, not a
real theory of the universe because we never see it directly.
(35:35):
We don't can't really grapple with it and grasp.
Speaker 1 (35:37):
It directly, right right. Well, I think it's interesting that
we have or that astronomers have found galaxies without dark matter,
because it almost gives you kind of like a test
case to confirm that the other galaxies that we have
seen with dark matter actually have dark matter and it's
not just some weird, you know, fluke or mistake in
our theory of gravity.
Speaker 2 (35:56):
Mm hmm. Yeah, it's a cool test case. It's like
a control, right, would you see galaxies without dark matter
if they were there. So it's nice to have some
verification that we're seeing that. It's also, as you say,
a great test bed for comparing various theories of dark matter,
which make different predictions about what would happen in these scenarios.
Speaker 3 (36:14):
Mm.
Speaker 1 (36:15):
Yeah, different guesses about dark.
Speaker 2 (36:17):
Matter, different ideas, different theoretical.
Speaker 1 (36:22):
All right, well, how do these galaxies help us decide
what dark matter is made out of.
Speaker 2 (36:27):
Well, one of the most popular alternatives to dark matter
as a theory of matter, some kind of stuff in
the universe is an alternative theory of gravity to say, well,
there's no other stuff in the universe. We're seeing everything
there is. It's just that gravity works differently from what
we expected. Because remember, the argument for dark matter is like,
we understand gravity, and there's a lot more gravity than
(36:47):
we can explain with the visible stuff, So there must
be more stuff. There must be invisible stuff creating that gravity.
But what if instead we just don't understand how gravity
works and it can be explained by all the visible
stuff if you tweak your theory of gravity.
Speaker 1 (37:01):
Meaning like what if gravity just gets stronger, The bigger
the distances that might account for why galaxies are holding
on together without needing dark matter. That's kind of the idea, right.
Speaker 2 (37:12):
That's kind of the idea. More specifically, there's this theory
called mond modified Newtonian dynamics that suggests that gravity's mostly
like Newton described, but there are some tweaks. It depends
on the acceleration of these objects, and for some accelerations,
gravity gets stronger or weaker, and you know, it's a
little baroquely like added these terms and these tweaks basically
(37:33):
to explain these rotation curves, to say like, oh, these
stars are accelerating more than those stars. So if we
change the way gravity works, can we describe the rotation
curves that we see. The answer is yes, you can
devise a theory to describe the rotation curves that explain
how these galaxies are rotating without needing dark matter. If
you tweak gravity, right, you have to tweak something, either
(37:55):
change the amount of matter that's there, or you change
the way gravity works.
Speaker 1 (37:58):
Well, first of all, you just saw of the whole
period of human history, the Baroque period.
Speaker 2 (38:03):
I meant that in a positive way, right right.
Speaker 1 (38:07):
The second of lie, I know that we've talked about before.
How you know, maybe Mond modified Newtonian dynamics, it could
replace dark matter. But we confirm dark matter in other
ways right exactly.
Speaker 2 (38:17):
So Mond is a success in describing the rotations of
galaxies by making these beautiful baroque extensions to Newton's theory.
But there's lots of other ways we've seen dark matter,
like in the ripples of the cosmic microwave background from
the very early universe. We can see how that early
universe plasma sloshing around, and that depends very sensitively on
(38:38):
the amount of dark matter, which slashes differently in that
plasma than normal matter did, and Mond cannot explain that. Also,
the Bullet cluster shows us that dark matter can be
separated from normal matter. It's not just a different way
that gravity works for normal matter. It really is something
else with its own gravity. So Mond really struggles to
explain everything that the theory of dark matter can explain,
(39:01):
but it's still a popular alternative, and this is another way.
These dark matter free galaxies are another way to draw
contrast between what Mond predicts and what dark matter predict
because according to Mond, there is no dark matter and
gravity only depends on the visible matter. And so these
altered diffuse galaxies with no dark matter should behave the
same way all the other galaxies do because there's no
dark matter in any of them. But we do see
(39:22):
a difference. We see that these guys are rotating more slowly, right,
and so Mond struggles to explain how slowly rotating these
galaxies are without any dark matter, whereas dark matter can
explain all of it. It's like, well, this has more
dark matter, that has less dark matter. So because dark
matter can be variable in the universe, some galaxies have
more and some have less. Whereas the rules of gravity
(39:44):
have to be the same, Mind is sort of hamstrung
and can't really explain the variation of all these galaxies.
Speaker 1 (39:51):
Well, I feel like Mond was already kind of dead
in the water for all these other reasons for a while.
But it is kind of interesting that seeing a galaxy
without dark matter almost kind of helps prove that it exists.
Speaker 2 (40:01):
Yeah, that is really interesting, and I agree with you
that Mond is not a theory we should take terribly seriously.
In dark matter is overwhelming the better guess for what's
going on in the universe.
Speaker 1 (40:11):
Yeah, there you go. See, I brought you on board.
Speaker 2 (40:15):
I'm loving that word.
Speaker 1 (40:16):
Now you're like, I guess, I guess so.
Speaker 2 (40:18):
But you know, full caveats. There are some things that
dark matter can't explain. There are a few galaxies out
there that don't make any sense that no dark matter
can really explain. Some people think that some hybrid like
mostly dark matter with a little bit of mind is
what we need to explain everything that's out there in
the universe, and so it's best to keep an open mind.
It's also always nice to find a new way to
test our understanding of dark matter and gravity in general,
(40:40):
and so these galaxies without dark matter are a nice
test bed.
Speaker 1 (40:43):
For that interesting you could call the new theory darkmond
all right, well, another interesting example of how the universe
just always has surprises. Like you think that maybe you
need dark matter to have a galaxy, but only one
day you find galaxies without dark matter, and it makes
you think, and it actually maybe helps you confirm the
existence of something as mysterious as dark.
Speaker 2 (41:05):
Matter, and it goes to show you that the universe
does all these experiments for us. We can just look
up in the night sky and find the examples of
galaxies smashing into other galaxies or black holes colliding. All
these things are wonderful experiments that help reveal the nature
of the universe, the rules that it follows, and how
it all works.
Speaker 1 (41:24):
Yeah, I guess.
Speaker 2 (41:27):
Or you guess in the end, aren't we all just guessing? Man?
Speaker 1 (41:32):
All right, Well, we hope you enjoyed that. Thanks for
joining us, See you next time.
Speaker 2 (41:42):
For more science and curiosity, come find us on social media,
where we answer questions and post video. We're on Twitter
at this word instant and now TikTok. And remember that
Daniel and Jorge Explain the Universe is a production of iHeartRadio.
More podcasts from iHeart Radio visit the iHeartRadio you Apple
Apple Podcasts, or wherever you listen to your favorite shows.
(42:09):
M HM.
Hey, Daniel, most of the stuff in the universe is
dark matter, isn't that right?
Speaker 2 (00:12):
Yeah, it's about eighty percent of the matter in the universe.
Speaker 1 (00:16):
And it's all around us, like right here with us
immersed in dark matter.
Speaker 2 (00:20):
Yeah, we are swimming in it.
Speaker 1 (00:22):
And it's also inside of us, is it?
Speaker 2 (00:24):
Yeah? It passes through us, it doesn't bounce off your skin.
Speaker 1 (00:27):
Does that mean that we're partly made out of dark matter? Oh?
Speaker 2 (00:30):
I guess. So that's kind of dark to think about.
Speaker 1 (00:33):
So if I discover myself, I'm discovering dark matter.
Speaker 2 (00:36):
Know thyself. Win a Nobel Prize.
Speaker 1 (00:38):
Or at least a dark Nobel Prize. Technically all Nobel
prizes have dark matter inside of them too.
Speaker 2 (00:43):
Maybe they'll give you the cash in dark money.
Speaker 1 (00:45):
Ooh, that got dark quick.
Speaker 3 (01:02):
Hi.
Speaker 1 (01:02):
I'm Hoeham and cartoonists and the author of Oliver's Great
Big Universe.
Speaker 2 (01:06):
Hi. I'm Daniel. I'm a particle physicist at CERN and
a professor at UC Irvine. And I'll take your money
dark or light.
Speaker 1 (01:14):
Really, you'll take dark money. Isn't that dangerous for a
tenured professor?
Speaker 2 (01:17):
No, that's what ten years for man. I can take money.
Speaker 1 (01:19):
From us visuals. I guess technically, the university takes the
money and you get a cut of it.
Speaker 2 (01:27):
I never really had to worry about that. Like, nobody
who got wealthy with dubious techniques ever offered me a
slush fund, So I've never had to really grapple with
that question.
Speaker 1 (01:37):
Wait, how do you know? I mean, you've taken money.
Have you done your due diligence? How deep did you go?
Speaker 2 (01:44):
You know, almost all my research money comes from the government,
and we all know the various crimes that the government
has committed.
Speaker 1 (01:50):
Exactly all government money is dark money.
Speaker 2 (01:53):
M do you believe taxes are theft? Is that where
this is going?
Speaker 1 (01:57):
Well, technically, or funding comes from taxpayers and there must
be some sketchy tax payers out there, so your work
is painted.
Speaker 2 (02:04):
So you're saying drug dealers who are paying taxes and
indirectly funding my research have made me complicit in their crimes.
Speaker 1 (02:11):
That's right. Yes, assuming drug dealers pay taxes, which I
guess they do. If they're laundering money, I don't know.
I feel like we need a whole podcast episode just
to cover dark money.
Speaker 2 (02:21):
Yeah, and then I suppose everybody's guilty.
Speaker 1 (02:23):
That's right, We're all guilty of paying taxes, I guess.
But anyways, Welcome to our podcast Daniel and Jorge Explain
the Universe, a production of iHeartRadio.
Speaker 2 (02:32):
In which we try not to overtax your brain while
explaining all of the mysteries of the universe. We want
to reveal the light universe and the dark universe, the
visible and the invisible. We want to show you how
the universe is not just what you see around you
and experience day to day, but so much more, so
much deeper, so many more mysteries waiting to be solved.
Speaker 1 (02:54):
That's why we explore the universe, the dark corners of it,
the light corners, and the sketchy corners of it, because
our understanding of the universe is still a little bit sketchy, like.
Speaker 2 (03:04):
The Mafia corners of the universe.
Speaker 1 (03:06):
No, like, not clearly drawn.
Speaker 2 (03:08):
Oh I see, I thought we're gonna be talking about
like the physics of New Jersey or something.
Speaker 1 (03:12):
Yeah, there's the only some dark matters going on out there.
Speaker 2 (03:17):
How do you eat all those rich cookies and not
gain weight? Really, it's amazing.
Speaker 1 (03:20):
Yeah, Well, I think the secret is dark chocolate. It's
lower and fat.
Speaker 2 (03:25):
Isn't it?
Speaker 1 (03:26):
Is it?
Speaker 2 (03:26):
Really? I don't know.
Speaker 1 (03:27):
You're like, what, what, Why aren't I eating more dark chocolate.
Speaker 2 (03:33):
I'm not really stopping myself from eating dark chocolate. But
that wasn't the reason I don't see dark chocolate is
like diet chocolate.
Speaker 1 (03:40):
It's chocolate light. Oh no, wait, it's chocolate dark.
Speaker 2 (03:43):
It's dark chocolate exactly. But there are lots of fun
questions out there in the universe, not just about how
waste management organizations in New Jersey are getting their money
and the calories that Daniel's eating. We're wondering about the
biggest questions in the universe, like where is all the stuff?
What is most of the universe made out of? But
how has it shaped the night sky that we see
(04:03):
today in the galaxy that we live in.
Speaker 1 (04:05):
That's right, because what the universe is made out of
is maybe one of the biggest questions we can ask
about the universe. What is this whole place made out of?
What are we made out of? Are we made out
of dark matter? Can we exist without dark matter?
Speaker 2 (04:18):
And we talk on this podcast a lot about dark matter.
We have lots of episodes about what it is and
where it is and how it works. And one thing
we often stress is that dark matter is part of
our galaxy, that most of our galaxy is actually dark
matter that has played a big role in the formation
of our galaxy. We wouldn't be here without it.
Speaker 1 (04:35):
Yeah, we do talk a lot about dark matter. Daniel,
What percentage of our episodes would you say we talk
about dark matter? Is it representative of the amount of
dark matter in the universe? Are we ignoring dark matter
in a way?
Speaker 2 (04:48):
Is dark matter underrepresented matter on podcasts? That's a good question.
Speaker 1 (04:52):
Yeah, well, especially our podcast. If we're trying to explain
the universe and the universe is twenty seven percent dark matter.
Speaker 2 (04:58):
You know, we might reach twenty seven percent of our
podcast being about dark matter. It's been a lot of them,
and I'm pretty sure we don't have two thirds of
them about dark energy. Maybe we should.
Speaker 1 (05:07):
Yeah, yeah, that's what everyone is asking for more dark
energy episodes. But it is a pretty important part of
the universe. It's a pretty important part of our existence
because without dark matter, maybe galaxies would not have formed
the same way that they formed. The Milky Way would
might not be the same way it is now.
Speaker 2 (05:23):
But we're always tempted to over general life to say
that the way we live in our certain situation over
here is the way the whole universe works. It's important
to take a step back and to ask whether our
way of life and our way of galaxying is the
only way that it can be.
Speaker 1 (05:38):
And so today on the podcast, we'll be asking the question,
are there galaxies without dark matter? Would you call these
light galaxies then? Or galaxies light diet galaxies?
Speaker 2 (05:54):
I call them tragic galaxies because they're probably galaxies where
everybody only eats white chocolate.
Speaker 1 (05:59):
Or milk chocolate, some chocolate snobs. I also call that
a tragedy.
Speaker 2 (06:04):
Also known as crimes against chocolate.
Speaker 1 (06:06):
Are you saying Hershey's is dark money as well?
Speaker 2 (06:10):
Her She's his garbage man?
Speaker 1 (06:12):
Oh my gosh. And there goes our sponsorship deal with Hershe's.
We were so close to funding this thing for the
next twenty years, and you have to go and insult them.
Speaker 2 (06:25):
Maybe you could hear my hesitation there. I'm being torn
between being honest and truthful on a hard science podcast
and pandering to our sponsors, and I just had to
be honest about it. I passed on hershe'es. I'd rather
have no chocolate than Hershe's.
Speaker 1 (06:38):
That was pretty harsh to garbage. Oh my gosh. Well,
here here's the thing, Like when you eat it. Let's
say you eat like a chocolate cake at a restaurant.
How do you know they didn't use Hershey's chocolate.
Speaker 2 (06:48):
You can taste it. Man, that's sour affront to chocolate.
It's turned so many people off of chocolate when chocolate
is this wonderful, amazing thing. Even milk chocolate can be
high quality, can be amazing. What they may in Pennsylvania. O, man,
it's a crime.
Speaker 1 (07:03):
Now you're insulting the whole state of Pennsylvania.
Speaker 2 (07:07):
Let's keep going. See how many people I can offend.
Speaker 1 (07:09):
Yeah, yeah, let's moves and let's insult the whole universe.
Speaker 2 (07:13):
Why don't you No, Fortunately, most of the universe is
dark matter and therefore appreciates dark chocolate, and so we're good.
We have the majority firmly on our side.
Speaker 1 (07:21):
But doesn't Hershey's own some fancy brands like Scharfenberger or
something like that.
Speaker 2 (07:25):
I don't know, I hope not.
Speaker 1 (07:26):
Well, maybe you've been eating Hershey' chocolates all this time.
A plot twist. Anyways, we're talking about dark matter and
galaxies and could there exist galaxies without dark matter. These
would be like galaxies that don't have any dark matter
in them or around them.
Speaker 2 (07:43):
Yeah, exactly, just stars and gas and dust and black holes,
no dark matter, all.
Speaker 1 (07:49):
Right, Well, whether they are tragic or not. We were
wondering how many people out there had wondered about this
question and if they have any ideas about the answer.
Speaker 2 (07:56):
Thanks very much to everybody out there who answers these questions,
they or not there supporters of Hershey's Crimes against Chocolate
would really appreciate everything you do. If you would like
to join this group, just write to me two questions
at Danielandjorge dot com.
Speaker 1 (08:10):
I feel like maybe there's a Hurshi's employee out there
who listens to our podcast and it's now very very sad.
Speaker 2 (08:16):
There's an easy fix for that, find a new job
or a new podcast. No, you've got to go to
the root of the problem.
Speaker 1 (08:22):
That's right anyway. So think about it for a second.
Do you think there can be galaxies without dark matter?
Here's what people had to say.
Speaker 4 (08:30):
I feel like there probably aren't, only because I know
that dark matter, as far as I know, is distributed
pretty much evenly throughout the universe. I think it tends
to be clustered in galaxies, but I think it tends
to be pretty uniform. So I would be surprised if
there were galaxies without dark matter.
Speaker 5 (08:44):
I'm going to say why not, because, like going back
to an episode about uranium on Uranus, there could be
a tiny bit here and there in a galaxy. So
I'm going to say I don't know why there wouldn't
be dark matter in a galaxy. I'm also going to
say I don't know why there would.
Speaker 3 (09:03):
I don't believe there are galaxies without dark matter because
dark matter is, in my understanding, a general term for
unknown matter, which is this and makes up the overwhelming
majority of the universe. Therefore, I think it is not
possible for galaxies not to have at least some dark
matter in them.
Speaker 6 (09:23):
I'm not sure that we've observed any galaxies without dark matter,
but I suppose anything could be possible in this crazy universe.
It's also possible that all galaxies are without dark matter
and we just don't understand gravity all right.
Speaker 1 (09:33):
Most people are skeptical about this question.
Speaker 2 (09:35):
Yeah, people have the idea that dark matter is everywhere.
Speaker 1 (09:38):
It's inescapable, you can't get away from it. It seems
in our question, although one person has kind of said,
why not, that's a good attitude to have.
Speaker 2 (09:45):
Yeah, that's the whole attitude about physics, Like, well, maybe
everything is different from what we've boughter. Maybe there's something
really weird out there that could teach us something new
about the universe.
Speaker 1 (09:54):
I do feel like that is a guiding question in
theoretical physics at least. Why not? Yes? Why not?
Speaker 2 (10:00):
Sure? Maybe everything is just tiny cats at the quantum scale.
Speaker 1 (10:04):
That's right, Maybe everything's just made out of Hersy's charga.
Speaker 2 (10:06):
No, I got to hop somewhere else in the multiverse.
Speaker 1 (10:11):
If that's the case, all right, Well let's get going
before this podcast gets too dark. Daniel, give us the basics.
What is dark matter? For those of us who haven't
listened to the twenty seven percent of our podcast.
Speaker 2 (10:22):
Episodes, dark matter is fascinating because we simultaneously know a
lot about it and very little. Like we know that
there's a lot of dark matter in the universe, and
we know that it's matter. We know there's something out
there that's creating gravity or curvature of space time, but
that it's invisible. It doesn't glow, it doesn't give off light.
It doesn't reflect light. We sense it only because we
(10:45):
see its gravitational effects on stuff. It's curving space, which
changes how things are moving through that space. We see
galaxies rotating much faster than their gravity would be able
to hold them together if there wasn't also dark matter
in them holding them together. On the other hand, we
don't know what stuff it is. We know that it
has gravity, so it's matter, but we don't know it's
(11:06):
some weird new kind of neutrino or a totally different
kind of particle we've never seen before, or a thousand
new kinds of particles, or something that's not even a particle.
So we know a lot about it on the sort
of cosmological scale, but very very little or almost nothing
about it at the particle level. Right.
Speaker 1 (11:22):
Dark matter is this kind of mysterious stuff in the
universe that we kind of feel its presence. We can
see its presence through gravity, but as you said, you
can't see it because it doesn't interact with elechormagnetic light
or maybe any of the other forces in the universe,
and so you can't see it. And that's why you
call it dark exactly.
Speaker 2 (11:38):
And a bunch of listeners write in with the idea
that maybe dark matter is matter in another universe that's
somehow leaking in to ours. Remember that dark matter is
creating gravity in our universe, or changing the curvature of
space in our universe, which means that it's in that space. Right,
it shares that space with us, which kind of means
that it's in our universe. We really do know that
(12:00):
dark matter is something in our universe that's changing the
shape of space. We can only see sort of indirectly
through gravity, which is really frustrating because gravity is the
worst way to see things. It's so weak that it
makes it very, very difficult.
Speaker 1 (12:14):
But wait, couldn't it bend our space and not be
part of our space.
Speaker 2 (12:18):
That's possible if you overthrow general relativity in our entire
understanding of space time. General relativity says that matter tells
space how to bend, and space tells matter how to move,
and that means matter in our space. Like in general relativity,
the curvature or space comes from the energy density in
that space. So if you have some like parallel space
overlaid on top of it, which can also bend that space,
(12:42):
then it seems to me like it would be part
of our space, you know, sort of like the by definition,
but yet you could augment or throw out general relativity
replace it with something totally different. But the simpler idea
is that it's just some kind of mass we can't
see that explains almost everything we see out there in
the universe. So it's sort of the best going explanation.
You can always make more or baroque complicated explanations if
you like.
Speaker 1 (13:02):
Well, it kind of might as well be in another universe, right,
because if it doesn't feel a lot of the same
force as we don't, it's just kind of like ghostly
matter that's kind of living on top of us. There
might be beings made out of dark matter, right.
Speaker 2 (13:14):
Physicists call the different sectors of the universe. If you
have like two different sets of particles that don't interact
at all except for through gravity, then we call those
like the lights of the visible sector and the dark
sector of the universe. And that's totally possible that you
could have a whole complicated physics happening in the dark
sector that we can't see. Now. Mostly we know that
dark matter can't interact with itself. If it did interact
(13:36):
with itself, it would form all sorts of complicated structure
and do all sorts of interesting things. We think that
dark matter is pretty spread out. However, there could be
a little component dark dark matter. Dark matter may be
lots of different kinds of things, and one little component
of it might be more complicated and do complicated things
like form life or ice cream, cones or cats or
(13:56):
good chocolate without violating what we've seen dark matter do,
which is mostly spread out smoothly.
Speaker 1 (14:01):
All right, Well, the question here today is can there
be galaxies without dark better? Why is this even a question? Like,
are most of the galaxies that we see out there?
Do they all have dark matter?
Speaker 2 (14:12):
Yeah, the galaxies that you see out there in the
sky are like tracers. They're basically telling you where the
dark matter is in the universe. Remember that dark matter
is not something we can see, but it also dominates
the universe. Our estimates are that four fifths of the
matter in the universe is dark matter. So if you
like spin the wheel and pick a random object in
the universe, most likely you're going to get dark matter.
(14:34):
It's like overwhelmingly dark matter. So when the universe is
forming its structure and the gravity that determines like where
things are going to be. It's mostly the gravity of
dark matter that decides where things are going to clump
together and where things are not going to clump together. Remember,
the very early universe is mostly smooth, with a few
little blobs that are denser than others. The gravity of
(14:56):
those over dense pieces pull things together to form structure,
and that's where you get like galaxies in one part
of space and not galaxies in another part of space.
Where you have galaxies is where you had more dark
matter to pull that stuff together to form those galaxies.
Speaker 1 (15:10):
Yeah, that's pretty wild to think that something we can't see,
that is invisible to us basically kind of dictates the
entire structure of the universe, right at least at the
galaxy level. Does dark matter also dictate things like superclusters
and beyond exactly?
Speaker 2 (15:26):
And so you can imagine like these invisible wells, like
dark matter's curving space with Shepherd's the other kind of
matter together. So every time you look up at the
night sky and you see a galaxy, you should imagine
there's an invisible blob of dark matter surrounding that galaxy.
There's a whole halo that's created the conditions to form
that galaxy. We run simulations, for example, of a universe
(15:46):
without dark matter, and it doesn't form galaxies after fourteen
billion years. So quite literally, we would not be alive
without dark matter.
Speaker 1 (15:54):
And now is the same true for things like superclusters
and those giant bubbles of galaxies they're in the universe.
Is that dictated by dark matter as well? Or is
that more of the quantum fluctuations of the universe.
Speaker 2 (16:07):
Well, both, because there's this cosmic web that tells us
where dark matter will be denser and where dark matter
will not be dense. These filaments in some places they
overlap and you get these wells where things pool together.
And so the whole cosmic web is dictated by dark
matter and the light matter. The normal matter, baryons and
quarks and electrons just follows that. And so it's not
(16:27):
like a supercluster has a super halo. A supercluster is
made of galaxies connected together by these filaments.
Speaker 1 (16:34):
So it also has filaments of dark matter.
Speaker 2 (16:37):
Absolutely, yes, there are filaments of dark matter connecting these
halos of around each galaxy.
Speaker 3 (16:43):
Hmmm.
Speaker 1 (16:43):
Interesting. All right, Well, let's dig into the question of
whether a galaxy can be clean of dark matter, or
whether it's kind of a requirement for a galaxy to form.
So let's dig into that, but first let's take a
quick break. All Right, we're asking the question, can you
(17:10):
have a galaxy without dark matter? Could you maybe have
a scripy little galaxy out there? That was like, no,
I don't care about dark matter. I'm just going to
gather all these all this gas and dust and on
my own without any help. That's kind of what we're asking.
Speaker 2 (17:24):
Today, right, Yeah, exactly.
Speaker 1 (17:26):
And so you talked about how most of the galaxies
that we see out there probably have dark matter, right,
we think they have dark matter, right, because they couldn't
be holding together without dark matter.
Speaker 2 (17:37):
That's right. And it's even more than just most galaxies
have dark matter. It turns out galaxies are sort of
like extra rich in dark matter, Like most galaxies have
more dark matter than the average dark matter density in
the universe.
Speaker 1 (17:50):
Wait, what what do you mean? Like, what are some numbers?
Speaker 2 (17:52):
So if you average over the whole universe, like what
fraction of matter in the universe is dark matter? That's
eighty four percent by counting. We don't know how many
dark matter particles there are that's by mass, Like, what
fraction of the mass of stuff in the universe is
dark matter? That's about eighty four percent. But when you
look at galaxies and you ask, like, what fraction of
the mass in a galaxy is dark matter, that's more
(18:15):
like ninety one percent. So galaxies have like about half
as much normal matter as the average normal matter density
in the universe. Galaxies are like concentrated blobs of dark matter.
Speaker 1 (18:27):
And we get these numbers by measuring how fast the
galaxies are rotating and kind of guessing how much dark
matter you need to hold it all together.
Speaker 2 (18:36):
Yeah, not so much guessing, measuring right, But you're right.
It's looking at how the galaxy rotates. We can measure
the speed of those stars in the galaxy as they
whizz around the center by looking at their light and
seeing how it's red shifted or blue shifted. You're looking
at a galaxy, some of the stars will be moving
away from you and some moving towards you, so to
be red or blue shifted their light from the Doppler shift,
(18:57):
so you can measure their velocities. So you can look
at the velocity of stars as they get further and
further away from the center, and in order to hold
a star at a certain velocity a certain radius, you
need a gravitational force there, so you can calculate exactly
how much gravity is needed to hold a star at
a certain radius. They have all these stars at different
distances from the center, telling you exactly how much gravity
(19:19):
you need to keep those stars going at that speed,
and then you can add up how much you can see,
like count all the visible stars and the rest you
suppose is dark matter. I guess that's what you mean
by guessing.
Speaker 1 (19:31):
Yeah, inferring guessing.
Speaker 2 (19:34):
There's a whole field of statistical inference that we should
just call guessing.
Speaker 1 (19:37):
Yeah, Well, I mean you don't actually know how many
stars there are in that galaxy right, so far away
you can't see the individual stars, so you're also sort
of inferring how many stars there are there. You're guessing
a little bit, aren't you.
Speaker 2 (19:51):
There's always uncertainty in these measurements. Absolutely, and you're right
that we cannot resolve individual stars, especially near the center
where things get very dense. But we can see this streams, right,
we can see streams of stars. We have models for
how these galaxies work. But absolutely there's always uncertainty, but
the uncertainty in these calculations is tiny compared to the
size of dark matter. So there's no uncertainty that there's
(20:12):
a lot of dark matter in these galaxies because remember
the fractions we're talking about here, like ninety percent, which
means you're looking at a star, you're measuring its velocity.
You figure out how much gravity is needed to hold
it there so it doesn't fly out into intergalactic space.
And when you add up all the stars, you get
like ten percent of the gravity you need. So there's
a huge missing chunk.
Speaker 1 (20:34):
And you're sure it's not just you know, a lot
of asteroids or rocks that don't close.
Speaker 2 (20:38):
Yeah, that's a great question. Could dark matter just be
normal matter that we're not seeing right, just like dark
chunks of matter. So people have looked for that directly.
Those are called MACHOs massive compact halo objects, and we
think we would see those occasionally, like they would pass
in front of stars if there was a lot of them.
If they were really big, we would have spotted them.
(20:58):
So people have looked for that kind of stuff and
not seen it. Plus we know something about how much
normal matter there was in the very early universe because
it dictates the fraction of elements that were produced, like
the hydrogen and helium and lithium very sensitive to the
density of quarks and electrons in the early universe. We
talked about that once Big Bang nucleosynthesis. So we have
(21:20):
a pretty good handle on how much normal matter there
was around, and we can explain where most of that
is now and the rest of it's got to be
dark matter.
Speaker 1 (21:28):
And so basically every galaxy out there that we've seen,
we see that it's spinning faster than it should, or
it's holding together more than it should, so we think
it has dark matter. And now is that true for
every galaxy we've seen out there?
Speaker 2 (21:40):
It turns out there's a pretty wide variety. Like when
you look at galaxies out there, some of them have
a lot of dark matter, and some of them have
a huge amount of dark matter. There is a variety.
It turns out that smaller galaxies tend to have more
dark matter than really massive galaxies.
Speaker 1 (21:57):
Not by absolute amounts, but just relative town these stars
they have.
Speaker 2 (22:00):
Yeah, exactly higher dark matter fractions, I should say, And
that's because galaxies are better at holding onto their dark
matter than they are their normal matter. Galaxies are crazy places.
There's winds from all the stars, right every star is
a fusion furnace and pushing out protons and electrons. These
cellar winds are pushing gas out of the galaxies. Then
(22:21):
there are supernovas going off all the time, blowing things
up and pushing things out. There's radiation, really intense radiation
from the center of the galaxy that's pushing gas out.
So galaxies are basically exploding and they're pushing a lot
of their matter out. And so the smaller galaxy is,
the less it's capable of holding onto its normal matter,
the less it's capable of resisting these forces that push
(22:44):
gas out of the galaxies.
Speaker 1 (22:46):
Because the bigger galaxies have more gravity.
Speaker 2 (22:48):
Basically right exactly, the bigger galaxies are still doing this,
but they have more gravity so they can hold onto
their normal matter. So smaller galaxies, which have weaker gravity,
lose more of this normal matter. So you look out
there at dwarf galaxies, really tiny ones. They can be
like ninety nine percent dark matter.
Speaker 1 (23:03):
But wouldn't the larger galaxies also be better at holding
onto their dark matter, Like wouldn't smaller galaxies lose some
dark matter eventually, like it might evaporate or something.
Speaker 2 (23:12):
Yeah, that depends on what dark matter does. And in
this theory, dark matter does nothing but gravity, and so
you can't really lose your dark matter. Like to lose
your dark matter, you need some force that's pushing out
on it. But gravity is just attractive. So all these
forces like the solar winds and the radiation and the
supernova basically have no impact on the dark matter. Dark
matter just like brushes it right off, Like supernova could
(23:33):
happen right next to you, and a dark matter particle
would be like whatever, dude.
Speaker 1 (23:37):
And so we haven't seen any galaxies without dark matter,
So then why are we asking the question are there
galaxies without dark matter? Is it more of like, is
it possible to have a galaxy without dark matter? Or
are we asking like could there be galaxies where we
haven't noticed it doesn't have dark matter?
Speaker 2 (23:51):
Yeah, so great question. We're curious about this for lots
of reasons, Like number one, we have a theory about
how the structure of the universe came to be and
how it made galaxy and this nice story we told
you about over densities clumping together to form galaxies et cetera,
et cetera. But we'd like to test that. We'd like
to make sure that's correct. We're often surprised when we
look out in the universe and see how things actually work,
(24:14):
and so what we'd like to do is check our
predictions about like the dark matter fractions of galaxies against
reality and see is this really the way things work. Also,
this really helps us understand what dark matter is, because
seeing how dark matter varies across the universe can tell
us something about the nature of dark matter and help
us test various alternative theories about what dark matter might
(24:35):
or might not be. But it's not exactly true that
we've never seen a galaxy without dark matter. People are
out there looking for these and they found some pretty
weird cases.
Speaker 1 (24:43):
Interesting, all right, what are some of these cases?
Speaker 2 (24:46):
So this galaxy group kind of nearby on cosmic scales,
that's sixty three million light years away. It's called in
GC one zero five two, and basically it's an elliptical
galaxy in the Cetus constellation. We've known about if like
two hundred and fifty years or so, but there's actually
a little group of galaxies. It's like a major galaxy
with a bunch of little galaxies nearby. They call these
(25:08):
dwarf galaxies, so it's a whole group, so they call
the group of galaxies. And these little galaxies are actually
ultra diffuse galaxies. That means they're galaxies that are not
very bright, they have very few stars in them. And
these ultra diffuse galaxies near this NGC group they think
might have no dark matter in them at all.
Speaker 1 (25:27):
Why do they think that?
Speaker 2 (25:28):
So they look at the rotations of these galaxies and
they do that calculation and they estimate zero dark matter.
Like every time you're doing this, you're not assuming the
dark matter. You're measuring it. And sometimes it comes out
ninety percent, sometimes eighty four percent, sometimes ninety nine percent.
In this case, it comes out close to zero or
consistent with zero. So they think these are little galaxies
(25:48):
that have no dark matter in them at all.
Speaker 1 (25:50):
So that's pretty wild. That means that you can have
a galaxy without dark matter.
Speaker 2 (25:53):
Yeah, it's fascinating because remember, our theory of galaxy formation
is that basically every big galaxy is a merger of
a bunch of small galaxies. Big galaxies don't like form
all at once in a single collapse. You have a
bunch of baby galaxies then merge to make bigger and
bigger galaxies, so like a bottoms up approach. And so
if your big galaxy ends up with a lot of
dark matter in it, that means that the little galaxies
(26:16):
that made it should each have their own dark matter.
And we look out a dwarf galaxies and we mostly
see them having dark matter. In fact, some of them
have a lot. So it is really weird to see
these little galaxies without any dark matter at all. And
the question is like, did they form this way or
did something happen to strip them of their dark matter.
Speaker 1 (26:32):
Or maybe they formed later in the universe.
Speaker 2 (26:34):
Yeah, exactly, And so that's a fascinating question. And so
there's a group that's done a study of these and
they have a theory about how these little diffuse galaxies
ended up without any dark matter in them.
Speaker 1 (26:44):
What's a theory.
Speaker 2 (26:45):
So the theory is basically a mini version of the
Bullet cluster. You remember. The Bullet cluster is this famous
example that really convinced a lot of people that dark
matter was a real thing. It was a cluster of
galaxies that collided with another cluster of galaxies and we
saw that what happened to them the gas and the
dust and the dark matter was very different. So the
gas and the dust interacted and created collisions. The dark
(27:06):
matter passed right through because it doesn't they direct at
that level, gravity's not strong enough, so basically separated the
dark matter from the normal matter. So the bullet cluster,
and now you have a blob in the middle with
a bunch of normal matter in it, and then you
have dark matter on both sides, so we can see
through gravitational lensing. So they think that might be similar
to what happened in this case, that maybe there was
(27:26):
a big collision between two other objects, and these two
things that we're seeing now they call them DF two
and DF four are basically the results of that, like
chunks of stars and gas and dust that got stripped
of their dark matter and a collision and then tossed aside.
Speaker 1 (27:42):
Well, why wouldn't some little bit of dark matter go
with them.
Speaker 2 (27:45):
Because the dark matter and the normal man have very
different experiences in a collision, Like dark matter basically passes
the right through. There's not really a collision when it
comes to dark matter. It's like two ghosts just phasing
through each other whereas two people bumping into a hallway
are going to change their direction. So imagine you have
like a ghost inside you and somebody else is a
ghost inside them, and you have a collision in the hallway.
(28:06):
The ghosts just keep on going and the living people
bounce off each other. Now you know you're separated from
your ghost.
Speaker 1 (28:12):
But there's so many that they discovered. Did they all
get that way from the collision?
Speaker 2 (28:15):
So they've only found these two, and they have this
reconstruction of the collision that suggests that these two things
happen somewhere near NGC and created its collision, and it
should have also created a bunch of other ultra diiffuse
galaxies that they should be able to spot. That there
should be like five or six of these that came
out of the collision that also have no dark matter.
(28:36):
So they're going to go and look for those.
Speaker 1 (28:39):
So these are you said, these are dwarf galaxies.
Speaker 2 (28:41):
Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so
they're small and they're not.
Speaker 1 (28:46):
Very bright, so in a way they kind of got
made later or not? Are they as old as the universe?
Speaker 2 (28:50):
Well, they think this collision happened about eight billion years ago,
and so how you age these things, I guess depends,
like the way they are now started about billion years ago. Now,
of course, have some progenitor or something that they came from. Right,
there was a larger object they were a part of
which definitely had dark matter in it, and their dark
matter is now sprayed in some other direction. So they
(29:12):
become separated from their dark matter. And this must have
been a pretty mammoth event. I mean, they reconstruct this
thing and it's like a collision at three hundred kilometers
per second of these huge cosmic objects.
Speaker 1 (29:24):
Well, so these are galaxies that had dark matter, but
then they got stripped away of their dark matter, and
so that shows that, hey, you can have a galaxy
without dark matter. But I guess maybe the larger question
is can you form a galaxy without dark matter?
Speaker 2 (29:38):
Right? And so as you say, that's the deep question
about the nature of the formation of structure in the universe,
and so far the answer to that is no, we
do not think it's possible to form a galaxy without
dark matter. We think you need that dark matter around
to gather enough gas and dust to make stars and
to make a galaxy that without dark matter, normal matter
doesn't have enough gravity to have formed galaxies this early
(29:59):
in the UNI. If you had a universe without dark matter,
or big section of it without dark matter and just
normal matter, it would form galaxies eventually, but it would
take a lot longer to do so.
Speaker 1 (30:09):
But you know, as I understand it, and during the
Big Bang, things were really hot and dense, and there
were pockets of things, and there were quantum fluctuations which
maybe created pockets of extra densities here and there. Couldn't
there have been a pocket of extra density of normal
stuff but not dark matter. That then when the universe
blew up, it became a galaxy without dark matter, like
(30:30):
during the Big Bang. Why does the normal matter have
to follow the dark matter?
Speaker 2 (30:34):
I guess it does in general because dark matter dominates
because it's just so much more of it. So it
basically like sets the scene for everything. But you're right,
it's theoretically possible to have a downward fluctuation in the
dark matter and an upward fluctuation in the normal matter.
So you get some region of space where you have
like extra super dense normal matter and almost no dark matter.
(30:55):
That's possible. Yeah, and so in principle that could happen,
and if it had enough matter, then it would form
a galaxy on its own. So in principle that's not impossible,
but we've never seen that, and I don't know what
the chances are of that happening theoretically.
Speaker 1 (31:08):
Like, how many galaxies have we've done this calculation to
make sure that it has dark matter in it.
Speaker 2 (31:12):
Yeah, that's a great question. We've measured the rotation velocity
of thousands and thousands of galaxies, but that's a tiny
fraction of the number of galaxies that are out there.
In the number of galaxies we can see, most of
the galaxies we can see, we can't measure their rotation
velocity because you're looking at like one pixel or two pixels.
You need to be able to sort of resolve the
whole galaxies you can see like from one side versus
(31:33):
light from the other side. So it's tricky. But yeah,
we haven't looked at that many galaxies as possible. There
are galaxies out there that did really form without dark matter.
Speaker 1 (31:42):
All right, Well, it seems like we kind of answered
the question of the episode, which is can you have
a galaxy without dark matter? The answer is yes, you
can have maybe galaxies that had dark matter, but then
they lose it or they get left behind by the
dark matter and so they're dark matter less. Or maybe
they could have forded at the beginning of the universe
in theory, but we haven't seen one yet. All right, Well,
(32:04):
let's dig into what this all means about our understanding
of dark matter and also gravity and whether or not
it needs to be overhauled. But first, let's take another
quick break. Orright, we're asking the question can there be
(32:30):
a galaxy with no dark chocolate? And the answer is probably,
But that's not a universe Daniel wants to live in.
Speaker 2 (32:38):
No, that's right, Transport me somewhere else in the multiverse asap.
Speaker 1 (32:42):
What if you end up in the universe where there's
only milk or white chocolate?
Speaker 2 (32:46):
Just keep smashing that button until I get somewhere good.
Speaker 1 (32:48):
No, No, you only get one trim.
Speaker 2 (32:49):
Would you take the risk? I'm pretty happy with our universe,
you know.
Speaker 1 (32:54):
It's a pretty good one. We're talking about whether galaxies
can exist without dark matter, and the answers yes, they
can be stripped away of their dark matter, or theoretically
they could form in the early universe, but we haven't
seen one yet, and so maybe probably not. What do
you think happened? Why haven't we seen any? If they
can form without dark matter, why haven't we seen any.
Speaker 2 (33:13):
I think that'd be really unlikely. I mean, the kind
of fluctuations we're talking about are very, very large. In
the early universe, you had just sort of like energy,
and then it decays into matter. As the universe expands
in cools, and every kind of matter is sort of
made uniformly, so you get more dark matter made and
less normal matter. But in order to have no dark
matter made, you'd need a really big fluctuation. You expect
(33:35):
to get like eighty five percent and you get zero.
It's like flipping a coin one hundred times in a
row and getting only heads instead of half heads and
half tails. It's pretty unlikely. Now the universe is really big,
of course, so that means that eventually it's going to happen,
especially if the universe is infinite. But it's so unlikely
that it's not going to be the first kind of
galaxy we see out there, or even in the first tranch.
(33:56):
Eventually we might spot one.
Speaker 1 (33:57):
Well, as you said, it is possible to have a
galaxy without dark matter. We've seen it in some ultra
diffuse galaxies and they've done a measurement on these galaxies, right,
They've measured how fast it's spinning, and they're pretty sure
there's no dark matter in them.
Speaker 2 (34:10):
Yeah, there were a series of papers where people said, oh,
there's no dark matter, and another group did a different
measurement said no, there is some dark matter. And then
there were follow up papers arguing, and now they're pretty
sure there's no dark matter in these but there's always
somebody out there who disagrees. I mean, it's astronomy, after all.
Speaker 1 (34:25):
That's right, they're all just guessing.
Speaker 2 (34:28):
They're all just doing their best statistical inference.
Speaker 1 (34:33):
That's right. That's a great word for guessing. I'm just kidding.
A best guessing. How about that best guessing. Nobody has
a better guess.
Speaker 2 (34:42):
You know, those whole departments of people who do nothing
but statistics for a living. I'm trashing Hershey's, but you're
trashing statistics. Man.
Speaker 1 (34:50):
No, there's nothing wrong with guessing wrong.
Speaker 2 (34:55):
How do you know the universe is the way it is.
We're just guessing.
Speaker 1 (34:58):
A best guess doesn't mean that you're making thing. So randomly.
You're just using the best information you have to make
a best estimate or inference, right.
Speaker 2 (35:05):
I suppose so. I think if it's very well informed,
it's not really a guess, you know.
Speaker 1 (35:09):
But you're one hundred percent sure it's also not a fact.
Speaker 2 (35:13):
Yeah, that's true. That's why we use statistics to describe
our uncertainties. Anyway. One of the things we are uncertain
about is the nature of dark matter. Like, a lot
of the stuff we talk about for dark matter is
kind of unsatisfyingly indirect, and a lot of people out
there treat dark matter like it's some placeholder, not a
real theory of the universe because we never see it directly.
(35:35):
We don't can't really grapple with it and grasp.
Speaker 1 (35:37):
It directly, right right. Well, I think it's interesting that
we have or that astronomers have found galaxies without dark matter,
because it almost gives you kind of like a test
case to confirm that the other galaxies that we have
seen with dark matter actually have dark matter and it's
not just some weird, you know, fluke or mistake in
our theory of gravity.
Speaker 2 (35:56):
Mm hmm. Yeah, it's a cool test case. It's like
a control, right, would you see galaxies without dark matter
if they were there. So it's nice to have some
verification that we're seeing that. It's also, as you say,
a great test bed for comparing various theories of dark matter,
which make different predictions about what would happen in these scenarios.
Speaker 3 (36:14):
Mm.
Speaker 1 (36:15):
Yeah, different guesses about dark.
Speaker 2 (36:17):
Matter, different ideas, different theoretical.
Speaker 1 (36:22):
All right, well, how do these galaxies help us decide
what dark matter is made out of.
Speaker 2 (36:27):
Well, one of the most popular alternatives to dark matter
as a theory of matter, some kind of stuff in
the universe is an alternative theory of gravity to say, well,
there's no other stuff in the universe. We're seeing everything
there is. It's just that gravity works differently from what
we expected. Because remember, the argument for dark matter is like,
we understand gravity, and there's a lot more gravity than
(36:47):
we can explain with the visible stuff, So there must
be more stuff. There must be invisible stuff creating that gravity.
But what if instead we just don't understand how gravity
works and it can be explained by all the visible
stuff if you tweak your theory of gravity.
Speaker 1 (37:01):
Meaning like what if gravity just gets stronger, The bigger
the distances that might account for why galaxies are holding
on together without needing dark matter. That's kind of the idea, right.
Speaker 2 (37:12):
That's kind of the idea. More specifically, there's this theory
called mond modified Newtonian dynamics that suggests that gravity's mostly
like Newton described, but there are some tweaks. It depends
on the acceleration of these objects, and for some accelerations,
gravity gets stronger or weaker, and you know, it's a
little baroquely like added these terms and these tweaks basically
(37:33):
to explain these rotation curves, to say like, oh, these
stars are accelerating more than those stars. So if we
change the way gravity works, can we describe the rotation
curves that we see. The answer is yes, you can
devise a theory to describe the rotation curves that explain
how these galaxies are rotating without needing dark matter. If
you tweak gravity, right, you have to tweak something, either
(37:55):
change the amount of matter that's there, or you change
the way gravity works.
Speaker 1 (37:58):
Well, first of all, you just saw of the whole
period of human history, the Baroque period.
Speaker 2 (38:03):
I meant that in a positive way, right right.
Speaker 1 (38:07):
The second of lie, I know that we've talked about before.
How you know, maybe Mond modified Newtonian dynamics, it could
replace dark matter. But we confirm dark matter in other
ways right exactly.
Speaker 2 (38:17):
So Mond is a success in describing the rotations of
galaxies by making these beautiful baroque extensions to Newton's theory.
But there's lots of other ways we've seen dark matter,
like in the ripples of the cosmic microwave background from
the very early universe. We can see how that early
universe plasma sloshing around, and that depends very sensitively on
(38:38):
the amount of dark matter, which slashes differently in that
plasma than normal matter did, and Mond cannot explain that. Also,
the Bullet cluster shows us that dark matter can be
separated from normal matter. It's not just a different way
that gravity works for normal matter. It really is something
else with its own gravity. So Mond really struggles to
explain everything that the theory of dark matter can explain,
(39:01):
but it's still a popular alternative, and this is another way.
These dark matter free galaxies are another way to draw
contrast between what Mond predicts and what dark matter predict
because according to Mond, there is no dark matter and
gravity only depends on the visible matter. And so these
altered diffuse galaxies with no dark matter should behave the
same way all the other galaxies do because there's no
dark matter in any of them. But we do see
(39:22):
a difference. We see that these guys are rotating more slowly, right,
and so Mond struggles to explain how slowly rotating these
galaxies are without any dark matter, whereas dark matter can
explain all of it. It's like, well, this has more
dark matter, that has less dark matter. So because dark
matter can be variable in the universe, some galaxies have
more and some have less. Whereas the rules of gravity
(39:44):
have to be the same, Mind is sort of hamstrung
and can't really explain the variation of all these galaxies.
Speaker 1 (39:51):
Well, I feel like Mond was already kind of dead
in the water for all these other reasons for a while.
But it is kind of interesting that seeing a galaxy
without dark matter almost kind of helps prove that it exists.
Speaker 2 (40:01):
Yeah, that is really interesting, and I agree with you
that Mond is not a theory we should take terribly seriously.
In dark matter is overwhelming the better guess for what's
going on in the universe.
Speaker 1 (40:11):
Yeah, there you go. See, I brought you on board.
Speaker 2 (40:15):
I'm loving that word.
Speaker 1 (40:16):
Now you're like, I guess, I guess so.
Speaker 2 (40:18):
But you know, full caveats. There are some things that
dark matter can't explain. There are a few galaxies out
there that don't make any sense that no dark matter
can really explain. Some people think that some hybrid like
mostly dark matter with a little bit of mind is
what we need to explain everything that's out there in
the universe, and so it's best to keep an open mind.
It's also always nice to find a new way to
test our understanding of dark matter and gravity in general,
(40:40):
and so these galaxies without dark matter are a nice
test bed.
Speaker 1 (40:43):
For that interesting you could call the new theory darkmond
all right, well, another interesting example of how the universe
just always has surprises. Like you think that maybe you
need dark matter to have a galaxy, but only one
day you find galaxies without dark matter, and it makes
you think, and it actually maybe helps you confirm the
existence of something as mysterious as dark.
Speaker 2 (41:05):
Matter, and it goes to show you that the universe
does all these experiments for us. We can just look
up in the night sky and find the examples of
galaxies smashing into other galaxies or black holes colliding. All
these things are wonderful experiments that help reveal the nature
of the universe, the rules that it follows, and how
it all works.
Speaker 1 (41:24):
Yeah, I guess.
Speaker 2 (41:27):
Or you guess in the end, aren't we all just guessing? Man?
Speaker 1 (41:32):
All right, Well, we hope you enjoyed that. Thanks for
joining us, See you next time.
Speaker 2 (41:42):
For more science and curiosity, come find us on social media,
where we answer questions and post video. We're on Twitter
at this word instant and now TikTok. And remember that
Daniel and Jorge Explain the Universe is a production of iHeartRadio.
More podcasts from iHeart Radio visit the iHeartRadio you Apple
Apple Podcasts, or wherever you listen to your favorite shows.
(42:09):
M HM.
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Daniel Whiteson
Kelly Weinersmith
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