Does the Universe need dark matter?

Is it possible that dark matter doesn't exist? Could it just be a misunderstanding of gravity?

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, do you ever wonder if physics might be
like really wrong? Are you talking about me or like
the entire community? Well, I know you're never wrong, Daniel,
that's unthinkable, but you know, like, has physics ever gotten
something about the very nature of the universe? Kind of? Uh?
Not right? You mean, like what's the best snack food?

(00:30):
Or what colored lab coat should we wear? Hey? Yeah,
you know, like can I eat antimatter or not? Maybe
it's delicious. Physics could definitely be wrong about snacks for you. Well,
I mean think bigger, like, is it possible that maybe
things are not quite what they seem? Well, it wouldn't
be the first time physics is wrong, and I hope
it's not the last time. I am organ made cartoonists

(01:05):
and the creator of PhD comments, Hi, I'm Daniel. I'm
a particle physicist, and I've never been wrong about snack foods,
and I've never been wrong about being wrong. So I
think that means that I'm always wrong. I don't know,
but welcome to our podcast. I'm definitely right about that.
This is our podcast, Daniel and Jorge Explain the Universe,
a production of I Heart Radio. That's right. Our podcast

(01:27):
in which we take a mental tour of all the
crazy stuff that's out there in the universe and try
to bring it into your head. We try to wrap
up the entire universe, all those trillions and trillions of
stars and weird blobs of gas and dust and invisible stuff,
and inserted through a little hole in your ear into
your brain. Yeah, all of the amazing and incredible stuff

(01:47):
out there in the universe, and also all of the
stuff that's normal. You know, And sometimes I think when
you examine what's spears to be normal in your everyday lives,
it turns out to have all kinds of wonders and
all kinds of small miracles in it. Are there small
miracles in your snack foods? Is that what you're talking about? Like, Oh,
look this chocolate chips in this trail mix. Every snack
is a miracle, Daniel, especially banana's net. But in this podcast,

(02:11):
we go beyond the miracles of banana based snack foods
and talk about the incredible things that scientists are trying
to figure out, Because, contrary to popular perception, scientists don't
have it all figured out. There are lots of really
big mysteries out there and wondering about the universe is
something that belongs to everybody, including you. Yeah, so they're

(02:32):
one of the biggest mysteries out there in the universe,
and literally sort of like not just like because we
don't know it, but also because it's a huge part
of the universe. Is dark matter? Dark matter is a
little bit crazy, right, It's like twenty of the universe,
but we have no idea what it is. That's right
of the energy budget of the universe. It's a pretty

(02:53):
big slice. It's like a little bit more than a
quarter of all the energy in the universe. Is this
weird stuff and we've never seen it directly. We're pretty
sure it's there, but we don't really know what is it.
Is it made out of particles? Isn't made out of
black holes? Isn't made out of lost socks? Two really
huge mystery in modern physics. In fact, I would say

(03:14):
it's one of the biggest open questions in science. I
think it's just dark socks actually, but didn't that makes sense?
I always lose those. How many dark socks have you
lost for her? I mean we're talking a lot of songs. Well,
to be out of time a cartoonists I don't have
to wear dark socks very often or socks at all,
because I also lived in California. But yeah, dark matter,

(03:34):
I mean, it's a big deal. There's five times more
dark matter than there is regular matter, like planets and
stars and gas and dust and black holes, there's all
that stuff. There's actually it's only of the stuff in
the universe, which means the normal matter, the regular matter,
the stuff that you're familiar with, is not actually regular
or normal. It's the unusual stuff. If you just took

(03:56):
like a survey of stuff in the universe, most of
the stuff is dark matter. The things that make up
stars and planets and galaxies and hamsters and me and
you and bananas and snack foods. That's unusual in the universe.
It's a minority of what's out there in the universe.
And yet we still don't know what this dark matter is.
It's a big mystery and a lot of times, you know,

(04:18):
whenever people talk about dark matter, I feel like a
common question we get, at least and in talks and
appearances is that people ask us like what if dark
matter doesn't exist? Like what if it's just an error
in the equations that you have about physics and the universe,
Like what if we just maybe like misunderstood gravity, or
haven't counted all the stars in the galaxy, or you

(04:39):
know what if there's something else that is maybe normal
but we just haven't thought about. And it's a great
question because most of the evidence we have for dark
matter is a little bit indirect, like because dark matter
is so dark and hard to interact with, we don't
have clear pictures of it, right, we haven't seen what
it's made out of. We've only sort of seen its
effects and sometimes second hand, and so it's tempting to

(05:02):
wonder if it's really there. You know. It's like if
you've only seen the footprints of an animal, are you
really sure it exists? Or could it be something else
spoofing you Until you really capture one or see one
in the wild, you don't really believe it exists. And
dark matter has been eluding our searches for decades and
makes people wonder like, well, maybe you people have it wrong. Yeah,

(05:22):
I mean it's kind of a crazy idea, right to
think that there's that much stuff out there, and conveniently
it's invisible and you can't see it. You know what,
I mean like I would be like, maybe you should
take your math or you know, maybe you should have
another grad student do the calculation. So, which do you
think is the conspiracy theory dark matter like there's so
much of it and you can't see it because it's

(05:44):
so dark and that proves that it exists, or the
anti dark matter conspiracy theory. Wait, anti dark matter? That's
another episode right there? Can you have anti dark matter Danny? Yeah,
that's another great question. We think probably not because then
they would annihilate a dark matter and turn into photons,
which we would see, what if it turns into dark photons? Yeah,

(06:05):
that's actually a thing, dark photon. Really? Yeah, there goes
my double price. You don't get credit for that one.
But you know which idea sounds more bonkers, right, that
the universe has filled with an incredible amount of invisible
matter nobody had detected until recently, or that it's not. Yeah.
So today on the podcast, we'll be asking the question

(06:30):
does the universe need dark matter? Is it an essential
part of the unit, Like could you have a universe
without dark manner? Does that make sense? Or is it
maybe that the universe doesn't need it? And maybe it
is kind of an error that we have in our
calculations and observations about the universe. And this kind of
skepticism is very healthy. When you have a crazy idea

(06:50):
you're trying to accommodate when you see results in your
experiments you don't understand. You need to be flexible about
this sort of theoretical framework with which you come at
the problem. You need to be open to crazy new ideas,
but you also need to be open to the fact
that maybe you got your measurements wrong. There always can
be an alternative explanation. So before you go big and say, wow,

(07:11):
we're going to revolutionize our understanding of the universe, you've
got to rule out all the more prosaic, basic, simpler explanations.
And so it's always a good idea to keep those
ideas in mind. YEA, So how sure are we that
dark matter exists in the universe? And could it be
something else? So I went out there into the internet
and I asked people, can we explain what we see
in the universe without dark matter? Are there good alternative

(07:35):
theories that don't require a new particle or a new
blob of stuff. Before you listen to these answers from
the internet. Think about it for a second. Do you
think dark matter is necessary in the universe or do
you think the universe could ignore it or live without it.
Here's what people had to say. I would think we
would be much more baffled if we didn't have dark

(07:56):
matter to explain the expansion of the universe. Oh man,
And this is tough because I still don't have a
good handle on what dark matter really is. But I
think I think we really don't know what dark matter is,
So I'm going to say, yeah, we could definitely explain
what we see without it, because we're kind of just
making up what it is. Yes, dark matter is just

(08:17):
a made a term for the stuff that's there that
we can't explain. So truly, any theory is under the
umbrella of dark matter. I have a strong suspicion that
the thing could be explained without dark matter, recalled Daniel,
saying that dark matter is just the name that we've

(08:40):
come up with for the phenomenon that we can't explain.
I don't know what we would see without dark matter.
My understanding is that we do know that there's dark
matter because the math doesn't work out. At some point,
we will have to explain the universe without dark matter,
because like the dark in the matter sort of implies

(09:03):
that we don't know what it is. I believe that
dark matter is a theory that we came up with
to help explain why we couldn't account for all of
the gravity that we see in the universe. I think
it's a fairly recent discovery. I don't believe Einstein knew
about it, so he must have had some other way

(09:25):
to account for all of the gravity. All Right, some
pretty good answers there, Yeah. I like the people who
treat dark matters just sort of like as an umbrella
idea for all the things we don't understand. And so
whatever we find out there, we just call that dark matter.
And I think that really touches on the sense people
have that we don't really have a clue what's out there.
We just sort of labeled it dark matter, and we're

(09:46):
talking about it as if it's a thing, but it's
really just a name we applied to our cluelessness. Interesting,
like if you put an S at d end, it
becomes dark matters, and then then it's sort of like
an umbrella term for things that are dark. That's true,
and you know, that is definitely true of dark energy.
Dark energy is another piece of the universe pie. Right,
the universe pie is five percent normal matter, twenty dark matter,

(10:12):
dark energy. Dark energy definitely in the category of just
stuff we don't understand. We gave a fancy sounding name
dark energy, this stuff that's making the universe expand right.
Dark matter, on the other hand, is much better understood,
is much more concrete and idea much more detailed observation.
So they're both called dark both things we don't understand.

(10:33):
But dark energy definitely a label for our cluelessness, while
dark matter is a much better founded, well described theory.
It's less dark. I guess we're less in the dark
about exactly. We are less in the dark. Our minds
are not quite so filled with dark shadows. All right.
Well that's the question for today, And that's the question is,
you know, do we need dark matter, like is it

(10:54):
a concept that is totally necessary for the universe to
make sense or is it just something weird that exists
out there and that maybe we could have a totally
wrong idea about, you know, And are there other theories
that are being worked on in the scientific community that
might explain it without needing to add some new kind
of stuff to the universe, darker matter, the darkest matter.

(11:16):
And you'll find that in science there are always competing voices.
You know, there's often like a mainstream most people think
the answer is X, but there's always somebody out there
who thinks it's why, somebody who thinks it's Z. And
you've got to give these people room because sometimes they're right,
and sometimes their ideas of the ones that turn into
the mainstream. That's how the mainstream became mainstream. It used
to once be lunatic fringe or their alternative physicists, you know,

(11:41):
French physicists. There definitely are there are people out there,
once they get tenure, start working on crazy Bonker's theories
and sometimes for decades that nobody pays attention, nobody really
reads their papers, that people even laugh behind their hands.
But sometimes they're right, you know. Literally, the history of
physics is filled with revolutions that start as crazy ideas,

(12:02):
and so we definitely got to pour water on some
of those seedlings because they could sprout into huge new
intellectual trees, awesome dark physics trees. I'm picturing all right, Well,
step us through um like what's the main argument for
dark matter, Like, what's the main evidence about it, and
what makes this think that, you know, it's something new
and different as opposed to maybe it's just more stuff

(12:24):
out there that we can't see. Yes, So if you're
going to come up with another theory of the universe,
another way to explain the way the universe works, you
have to explain what we do see, right, I mean
that's the whole idea behind making a theory of physics.
So if you're gonna come up with your theory, you
have to understand what are the observations, what are the
experiments reveal that need to be explained that you know,

(12:45):
what's the motivation for creating this idea of dark matter?
And the short version is it's all gravity. Like everything
we see out there in the universe that we need
dark matter to explain our weird gravitational effects that we
can't explain with all the other stuff, just the gas
and the dust and the stars right with the universe
feels dark matter in terms of gravity, like it's they're

(13:08):
affecting the gravity of other things, but you can't see it.
That's the main evidence for it. Yeah, basically there's unexplained gravity,
like we thought we knew where all the stuff was
in the universe from the stars and the gas and
the dust, and from that you can calculate how much
gravity there should be, and we can see the effects
of gravity, and will go through in a list of
how we see the effects of gravity. But there's more

(13:28):
gravity than we expected. So either there's more stuff e
dark matter, or gravity is weird and different, and so
it's all about the gravity then, right, because that's kind
of at the basis of the theory about dark matter,
is that it feels gravity but not anything else exactly,
and that's why it's dark. Because if it felt electromagnetism,
it would reflect light, or it would give off light

(13:49):
like everything else does than the universe, it glows, or
if it felt the strong force, it would bind with
corks and form nucleons and interact with us. So it
doesn't interact with us in any way that we know
of other than gravitational And that's why we call it
matter because we think it's something that has new gravity.
But you know, it could also just be a tweak
to the way we understand gravity. But at its core,

(14:11):
it's really an observation that our theory of gravity doesn't
work either because there's missing mass or the theory is wrong. Right,
So those are two sides of the coin, right, Like,
according to what we know about how gravity works, there's
a lot of gravity missing, or there's too much gravity
in the universe. Almost. Yeah, there's gravity out there and

(14:32):
we don't have mass to explain it, and so we
have to sort of fill in those gaps. And that's
an uncomfortable feeling, right, You're like, well, you can't just
like fill in the gaps and assume that your theory
is right and add extra stuff to make it work out.
You know, that feels uncomfortable, Like there's snack food missing
from my fridge. Surely it was my son who cut

(14:52):
up in the middle of the night to eat it.
But you can't you can't just assume that. You can't
just assume that could have been your daughter. Maybe you
sleepwalked and aided or or And that's when you install
a camera in front of your fridge and you get
those weird videos of yourself at four am stuffing cake
in your face. Experience the hypothetically hypothetically hypathetically right, Alright,

(15:13):
So we think dark matter is there because of gravity.
And there are several ways that we have seen this
kind of missing gravity, right, that's in the galaxy and rotations.
The way galaxies rotade is kind of one of the
first one, maybe even Yeah, one of the most dramatic
and earliest pieces of evidence that there was more gravity
in the universe than we expected was looking at how

(15:35):
galaxies rotate. And we can add up all the mass
of the stars and the stuff in the galaxy and say, okay,
we know how much gravity there should be, and then
we can calculate how fast those galaxies are rotating. And
you know, for a galaxy, when it's rotating, it's trying
to push stuff off. It's trying to throw stuff off
the edge. Like if you put ping pong balls on

(15:55):
a merry go round and spin it, the ping pong
balls fly out. The thing that keeps the galaxy from
tearing itself apart from throwing those ping pong balls out
into inter galactic space, is the gravity. So you can
ask how fast is the galaxy spinning and is there
enough gravity to hold it together, because the faster spins,
the more gravity unied. Right. Yeah, It's kind of like

(16:16):
our solar system, right, like you know, the mass of
the Sun is what keeps all the planets kind of
spinning around it. But what if, like the planets for
spinning faster than what you could explain by the mass
of the Sun. You would need some other explanations, that's right,
You need more force to hold them into their orbits.
And so you say, well, maybe there's extra gravity or
some other force or something. And so we know there's
something else holding the galaxy together. And so the first explanations,

(16:40):
oh well, what if there's more invisible mass. It's just
some stuff in the galaxy that's providing gravity and we
just can't see it. And you can explain these rotation
curves the way the stars move around centers of galaxies.
If you distribute a bunch of mass sort of smoothly,
it's like a big clump in the middle, and then
sort of smoothly outpass the edge of the galaxy into

(17:01):
a big halo that's even actually bigger than the galaxy.
That explains to this as that would give you the
kind of gravity that we are seeing, right, because if
you look at a galaxy doesn't have that enough mass, right,
like there aren't enough stars or planets in it that
you can see. That's right. We count up all the
visible stuff and we say how much mass does it have?
And that just doesn't give us enough gravity to explain

(17:22):
how those galaxies are holding themselves together. And that was
the sort of genesis of it. I mean, for a
long time people were like, well, wow, that's must be
a mistake, you know, because coming to the idea that
there's five times as much mass as you can see
is a really big idea. It's not something people just
came to an afternoon and then accepted, Yeah, it doesn't
seem likely. It does not seem likely. It seems more

(17:44):
likely that you mismeasured something that you got the velocity
is wrong, or you know your grad student is pranking
you or something. It's a big idea, right, It's like
jumping to the conclusion right away that maybe there are
five ghosts in my house eating my snacks. You know,
that's like a big that's a big leap from like, hey,
maybe it's just your u. And so it took other

(18:04):
pieces of evidence before dark matter became mainstream. Right, all right,
let's get into those other ways that we know dark
matter is there, and let's get into whether or not
it is there, or me we just have gravity wrong.
But first, let's take a quick break, all right, Daniel,

(18:31):
we're talking about whether dark matter is necessary in the
universe or you know, maybe it's just the hanger on
and the universe could care less about dark matter. But
we we know it's definitely there because we see it
from the rotation of galaxies and also we've set off
kind of can see it, right, We can see it
in the way that distorts the light from other far

(18:51):
away stars. That's right. These days, we have a good
handful of ways to indirectly detect dark matter, and one
of the coolest is seeing it act like a lens
in the sky because dark matter only has gravitational forces,
but gravity can bend space, right, It's a bending of
space and time. So if you have a big blob

(19:11):
of invisible stuff in the sky, it will curve the
space it's in, so that photon is traveling through it
will get bent as if they're moving through a huge lens. Right,
So if there's a big blob of dark matter between
you and some really far away galaxy, it will distort
that galaxy, creating duplicates of it, stretching it just as
if there was a huge lens in the sky, right,

(19:33):
and then we can see that, like if you look
at pictures as a sky, you see these kind of distortions,
these ripples, these lensing effects. You can sort of see
dark matter almost, you can definitely see it. And there's
sort of two categories. There's strong lensing, like there's a
big dense blob and it's distorting the galaxies and it's
pretty hard to explain that kind of lensing in any
other way. And then there's weak lensing. We just sort

(19:54):
of like look at all the galaxies out there to see, like,
are any of them sort of just a little distorted?
They're just looking a little tweaked. And from that we
can get sort of like a map of where in
the sky we think the dark matter is just by
looking at small distortions. And that's how we've gotten a
pretty good map for where we think this missing mass is.
How we know that it's mostly in the center of

(20:15):
the galaxy and how far out past the edge of
the visible galaxy the dark matter halo might go. So
it's a really powerful technique, right, And so that's kind
of how dark matter entered in kind of our view
of the universe was these first initial ways. But since
then and we sort of have put more nails into
sort of the coffin of whether or not dark matter
is out there, right, I mean, it's now sort of

(20:38):
shows up in pictures of the universe, the early universe,
it kind of shows up in our calculations about all
of the energy in the universe. Right, It's gotten more
and more convincing that there's something there that's right, it
becomes very difficult to explain the universe you see without
dark matter. For example, we see the influence of dark
matter on the formation of structures in the universe. Like

(20:59):
the universe be in as a sort of diffuse cloud,
you know, just of gas, mostly hydrogen, and then it
started clumping together, and that clumping comes from gravity. Right,
So gravity is the thing that draws these things together
and eventually gives you stars and galaxies and planets and
all that cool stuff. If you run a stimulation of
the universe without any dark matter, then you don't get

(21:19):
galaxies in the first ten billion years. It takes like
another ten or twenty billion years. So it's dark matter
that's that's created these like gravitational wells for stuff to
fall into to make the stars in the galaxies and us.
So just like the whole structure of the universe would
look very different without some kind of gravitational stuff out there,

(21:40):
and we should have just called them dark clumps or
dark do it. And even earlier in the universe, like
we've talked on this program about the cosmic microwave background radiation.
Those are photons from the very very early universe, three
hundred thousand years after the universe was created, the first
moment when the universe became Rand's parent to light. Before

(22:01):
that it was thick and soupy and light got reabsorbed,
and after that it was cool enough that light could
travel through the universe without being absorbed. And the shape
of that plasma that gave off that light was affected
by dark matter and how much dark matter there was
and much normal matter there was, and that stuff like
bounced off each other and oscillated and like squeezed and

(22:22):
squished that plasma. So the amount of dark matter in
that moment of the universe affects the shape of that light,
the currents, the sort of patterns we see in that
light in a very very precise way that's hard to
describe in any other way other than there's some other
kind of stuff out there. So that's giving us this graphic. Yeah,

(22:42):
you can see like it's imprint in the light from
the early universe, Like it's visually and like tangibly and like,
you know, you can calculate it the storts that light.
That's right. And so if you just take the universe
and you add to it a new particle, a particle
that doesn't move really fast, we call it cold and
doesn't feel anything but gravity, it explains all of this stuff.

(23:03):
It explains why galaxies rotate the way they do, why
the cosmic microwave background rediation looks the way it does,
why the structure of the universe has this structure at
this time in the universe. And also there are very specific,
awesome experiments that the universe has done to sort of
demonstrate dark matter to us. Yeah, I guess, you know,

(23:24):
I think maybe a question that a lot of people have,
and I'm sure if physicists had at the beginning, was,
you know, why does it need to be something special?
Like why does it need to be a new kind
of particle or matter? Couldn't it also just be something regular?
But that you just can't see, Like, you know, what
if there's a whole bunch of black asteroids out there
that are hard to see, or you know, a lot

(23:46):
of small dust that we can see, or maybe even
like a whole bunch of little black holes kind of
spread around the universe. Yeah, that's a great question, like
why do we know it's a new kind of thing.
Why can't it just be more of the same but
just kind of dark, right? Yeah, yeah, Well we don't
really know anything in our current set of particles that
is that dark. I mean, other than neutrinos. Everything else

(24:08):
has some kind of interaction, like if it's a black
rock or something, well that does reflect light, it does
give off light. So if it's made out of the
normal kind of matter, it's going to have the kind
of interactions we have, and therefore we would be able
to see it, except of course, neutrinos. Neutrinos were candidate
for dark matter for a long time. The problem is
neutrinos move away too fast, they zoom around the universe

(24:31):
because they're so light, so they don't give the same
sort of structure to the universe that dark matter does.
You need this thing to be sort of slow moving
and cold in order to stick around long enough to
give the structure of galaxies. Could you have cold neutrinos
like slow moving neutrinos. Then we talk about that the
other in another episode. Could you have cold neutrinos, Yes,

(24:52):
but we don't think that neutrinos are that cold. We
haven't seen cold neutrinos. Neutrinos are so light they have
almost no mass that they essentially almost always travel near
the speed of light. They're always in a rush, They're
always in a hurry. Yes, neutrinos are hot, as we
call them in particle physics. And it literally and figuratively

(25:12):
is it's it's a pretty big trend right now, that's right,
all right? Well, I guess if it's not maybe something
we know about, could it be that just maybe we
have our theories wrong about how things work, Like, you know,
maybe it's not a new particle or a new kind
of matter, but maybe we just have gravity wrong. Like
maybe gravity doesn't work the way we think it is.
And maybe these larger universe size scales could gravity, you know,

(25:36):
could there be a different theory of gravity that would
maybe account for what we think is dark matter. It's
definitely something to consider, right, because all these observations are
just observations of gravity. And what we're doing is we're saying,
we assume there's a certain amount of mass out there,
we assume we know how gravity works. We estimate how
much gravity there should be based on that mass. Right,

(25:56):
But there are two steps in there. There's figure out
where the mass is and in calculate how much gravity
there is from that mass. If that second step is wrong, right,
if the theory of gravity works differently from what we expected,
then yeah, that could possibly explain it. Because remember, gravity
is very very weak, which makes it very very hard
to test, Like it's difficult to measure the force of

(26:18):
gravity at the scale of one centimeter between two pebbles
because there's force there's almost zero, Like you need to
build a very sensitive instrument to measure the gravitational force
between anything that's smaller than you know, planets and moons. Yeah,
because you know, like our current theory of gravity says that,
you know, gravity changes by one of our squared, Like

(26:40):
the force of gravity kind of depends on the distance
between two things squared. And then you put down in
the denominator and that always seems kind of almost too
simple to me, Like, what are the chances at the
universe would pick such a simple little formula to calculate gravity.
You know, why isn't it like one over our squared
times are to the zero point seven five? You know,

(27:01):
I mean, like it seems so simple. Maybe what if
it's wrong, Like what if gravity isn't one of our
our square but maybe changes over distances in a way
that could maybe explain dark matter. Yeah, that's a totally
realistic thing to think. Although you know, there's a lot
of these patterns, these one of our our squared patterns
in physics and enforces and there is a good reason
for it and a fairly simple way to understand it.
If you imagine like the surface of a sphere surrounding

(27:24):
a point. Think about like all the gravitational energy coming
out of a point, the surface of a sphere surrounding it,
all the gravitational energy passes through that sphere, and then
as the radius of that sphere gets larger, what's the
surface of that sphere? Will it goes like are squared,
and so the power at any point should go like
one over our squared. It sort of makes sense geometrically

(27:46):
but what if geometry is wrong. What if geometry is wrong? Right?
What is wrong? What if podcasting is wrong? What if
one plus one is wrong? Well, it kind of, I mean,
you know, we talk about sometimes about how space is
not you know, this nice and neat, orderly thing, and
that sometimes you can even like measure triangles in real
space that have angles that are bigger than a hundred

(28:09):
and eighty degrees, could maybe, like space be weird in
such a way that it's not really one over our squares? Yeah? Absolutely,
And there are forces that don't go like one of
our our squared, Like the strong force doesn't go like
one of our squared at small distances. It gets even
stronger as you get further away. So you definitely got
to be open to weirdness. And so around the time
when dark matter was sort of coming up in the

(28:31):
world as an idea, when it was based mostly on
galaxy rotations, people thought, well, how can I tweak gravity
to explain what I'm seeing without dark matter? What would
I need to change? How what would you have to
be like one over our to the third or one
of our at one point five or whatever in order
to explain these galaxy rotations without dark matter, and so
they came up with a different theory. It's called mond

(28:54):
m O n D for modified Newtonian dynamics. Know, I
would have just called the dark bath. I wish you
could get in a time machine and go back and
tell them because I hate moms. Would that be a
lot catch here and fun? Dark bath? Well, mon, I
guess if you're French, then it's like, oh, yeah, it

(29:15):
means the world. I suppose it's missing the E and
it commits the terrible acronym crime of taking two letters
from one of the words, you know, modified. Yeah, all right,
so there is a kind of a theory in physics.
It says like maybe we do have gravity wrong, right,
is like an idea you guys take seriously, like maybe
there is no dark matter, it's just dark bath. It's

(29:36):
definitely an idea that physics should take seriously in the
sense that we should think about alternatives. This one theory
in particular doesn't have a lot of supporters, and we'll
get into exactly why, but you know, it's an interesting
idea and it says like maybe gravity works differently at
very very large distances, like you know, we've tested gravity
here on Earth. We know how it works. We've tested

(29:57):
gravity in the Solar System pretty well. But you know,
maybe the first time we're looking at gravity on galaxy scales,
maybe gravity just works differently over like, you know, fifty
thou light years, right, we haven't done that experiment before.
Maybe it gets going, like, maybe it gets stronger in
galaxy size scales. Yeah, And so the idea is actually
even weirder than that. It says, maybe gravity works differently

(30:21):
when you have a very very small acceleration, things are
not being pulled very hard. Maybe gravity works a little
bit different. Oh my god, you just blew my mind.
All right, let's let's get into this crazy idea that
gravity depends on how you're moving maybe, and whether or
not that could work. But first let's take another quick break, right, Daniel,

(30:55):
we're talking about dark math to maybe explain dark matter.
And so there's an idea that maybe our formulation of
gravity or how we think gravity works could be wrong.
Maybe and it could maybe act very differently over larger scales,
like maybe it gets stronger the galaxy scale or when
you're telling me it's actually when there's lower accelerations. And
this is just an idea again, right, this is just

(31:16):
an idea. But hey, everything is just an idea, right,
even lunch lunch is just an idea. Remember, Oh no, Daniel,
lunch is very real to me. Well, the idea is,
you know, somehow you have to get gravity to be
stronger without breaking things we already know. So what if
you know affecting things on the edge of the galaxy,
you could have gravity instead of going like one over
our squared, which you make it very very weak as

(31:38):
our gets large, you can make it go like one
over R. It doesn't fall off as quickly, doesn't get
weak as quickly with large distances. Interesting, So like the
gravity I feel with another planet on the other side
of the Milky Way, maybe it's stronger than I think. Yeah,
maybe it's not like one over our squares, more like
one over R and R is still really really large.

(31:59):
So you know, those planets don't affect you because the
gravity from them is still really tiny. But if you're
adding up lots of planets and you're trying to calculate,
like how a whole galaxy spins, then it really does
make a big difference to go like one of our
are instead of one of are squared. But it seems
a bit hacky as opposed to dark matter. Dark matter
doesn't seem hacky. Well, dark matter, you know, you can

(32:21):
explain a lot of really different things adding just one
simple idea, like hey, what if there's a new particle
that's not hacky, Like we don't know why there's a
certain number of particles in the universe and not more,
or if there are more, so adding one new particle
doesn't feel as hacky. Is like let's have gravity have
a knob on it, or like a distance above which
it starts behaving different rules, or you know, small accelerations

(32:44):
below which it starts behaving differently. Really, what why does
it need to be low accelerations? How does that work? In?
It doesn't really work as I think I heard you
react because like the acceleration with respect to what right,
all of a sudden, that gravity you feel depends on
your reference frame, and it brings all sorts of symmetries.
But there is that one idea that accelerations like one

(33:04):
ten trillions of the gravity on Earth. If you feel
an acceleration less than that, then the gravity on you
behaves differently than it does for larger accelerations. Oh, I see.
It's not you're not saying when you're going at a
low acceleration, it's just you mean the when the effects
of gravity are small, maybe it doesn't behave as one
over our square. Yeah, and the effects of gravity at
very very far distances are really small. Like what is

(33:27):
the acceleration on some star at the edge of the
galaxy due to the black hole the center of the galaxy. Well,
the distance is fifty thou light years, so the acceleration
is very small, also because the masses are large, and
so it's just a way to say, let's have it
have an effect where we see this weirdness and not
have an effect anywhere else. Let's try not to break

(33:48):
what we already know and understand and only have this
theory turn on in special cases. All right, so that's
a possibility. But where did this idea come from? Like
why would we think that maybe gravity is wrong? So
for a long time there wasn't really a good idea
that was just like, well, I don't know, but if
I put in this other formula, I can explain the
galaxy rotation curves without dark matter. Without dark matter. Yeah,

(34:09):
So like all right, so either there's missing masks or
gravity works this other weird way, and then people started thinking, well,
why would gravity work that way? Is there any reason
for it? And that's a totally valid line of inquiry, right,
like what theory do we have to have to explain
the data? And does that theory make sense? Can we
come up with a reason why? Maybe that theory is right?
A theory about a theory underpinnings of the theory, right.

(34:32):
We always in physics want to have a microscopic understanding.
We don't want to just say gravity just has this
number on it. We want to know why does it?
Where does that come from? Why isn't it something else?
Just like you were saying earlier, why is it one
of our square not one over our two point one
or whatever? That's the next version upgraded gravity. And so
recently there is an idea from a guy in Holland

(34:54):
to Eric Verlinda, and he has this crazy idea of
gravity called entropic gravity that i'd be able to explain
why gravity works differently at these distance scales. I feel
like maybe he just grabbed two cool sounding things and
put them together. Tropic answer like dynamic gravity or something.
But it comes from a cool place. You know, it

(35:15):
was Stephen Hawking who first connected sort of thermodynamics and gravity.
He started thinking about the temperature of black holes and thinking,
you know, if black holes have temperatures, that means they
should radiate. Oh wait, and that's Hawking radiation. So in
the last sort of forty years, people have been thinking
about gravity and through our dynamics as a connection. They're

(35:37):
trying to understand like, you know, actually, maybe gravity isn't
a fundamental force, and maybe, you know Einstein's idea that
gravity is just a curvature spacetime, maybe that's also wrong.
Maybe instead, gravity is just like an emergent phenomena of thermodynamics.
Maybe it's just like comes out of the manipulation and

(35:58):
interaction of some tiny little bits of space in a
way that feels like gravity to us. That's a little
bit mind blowing. Yeah, it's a little bit mind blowing.
This whole idea of emergent phenomena can be hard to
get your mind around, but it's actually very familiar. You know,
Like we can talk about wind, right, Wind is an
emergent phenomena. It's not a fundamental force in the universe.

(36:19):
You know, two particles interacting don't feel wind. Wind is
a combination of lots of other things we do understand
on a microscopical level that has a macroscopic effect, or
like economics. You know, there are laws of economics that
come from you know, supply and demand or whatever. It's
not a fundamental force in the universe, but still you
could have an understanding of it. And so they think

(36:41):
maybe gravity is just like a macroscopic effect of something microscopic,
maybe the thermodynamics of space time. Like there there is
no gravity, it's just kind of like how space itself
kind of arranges itself. Yeah, because we know that space
likes to increase entropy, likes to increase disorder, and so
we think entropy always increases in the universe, and so

(37:03):
maybe gravity is just an effect of that. You know.
The argument goes something like when things fall into a
black hole, for example, that increases the entropy of the
black hole, because otherwise it would violate the second law
of thermodynamics. Things can't just disappear into the black hole.
But maybe it's the opposite. Maybe it's not that gravity
pulls things into the black hole and then increases the entropy.

(37:24):
Maybe it's entropy that's pushing things into black holes, and
that's actually what gravity is. That's just like you know,
the way gas diffuses in a box. You put a
blob of gas in the corner of a box and
it spreads out into the box. That's entropy the same way.
Maybe entropy is stuff falling into itself, like you know,
having more stuff near itself dense blobs of mass increases

(37:48):
the temperature, which changes the entropy of the situation. I mean,
it's a complicated argument involving like quantum entangled space time,
but I think that's the gist of it, and all
of that kind of crazy idea is just explain how
maybe gravity could not be one over our square. Yeah,
if you have in tropic gravity, so maybe if gravity
is not a fundamental force in the universe but just

(38:09):
like an emergent property of quantum space time, and you
have dark energy, then Virlindis theory predicts that space curs
in this way, that gravity, the effective force of gravity,
goes the way mind needs it to explain the galactic
rotation curves. So that's kind of cool thing. That's an
alternative to dark matter. Is one alternative to dark matter

(38:31):
to say we have this weird gravitational thermodynamics idea that
changes the way gravity works. That explains why we thought
that was missing stuff. Instead, it's just the gravity works
differently than we expect it. Interesting alright, So then I
guess the big question is could that work? Is that
a valid or plausible theory that maybe explains gravity in

(38:52):
a different way that then explains dark matter and and
so that it's not just another particle or kind of stuff.
Could that war? Well, it's a cool idea, and it
does explain galaxy rotation curves, and it actually explains galaxy
rotation curves better than dark matter. There's some galaxies out
there that we still can't explain using dark matter, Like

(39:12):
they rotate in weird ways and we don't understand it.
And you know, but hey, galaxies are weird. They they
have their own history. No prize, sir. My theory, that's right,
My theory that the universe is just weird explains everything,
oh man, the whites and theory of everything, the whites

(39:32):
and theory of weirdness. But it explains the gaxy rotation
curves really nicely. But there's a big caveat there, which
is it was sort of invented to explain those curves,
like you came up with a theory to fit that data. Really,
a good test of a theory is does it explain
other data? Is that a real general principle about the universe,

(39:54):
something which is really deeply true? Or is it just
a mathematical tweak to this one plot, this one fig
or to make. Because if it only works to explain
one thing, it's it's kind of suspicious, right exactly, And
if it only works to explain the one thing that
motivated it, and then probably it's not a deep truth
of the universe, right a right, So maybe it doesn't work.

(40:14):
Can it explain some of the other things that we
know about dark matter, like the lensing and the cosmic
microwave background and the structure of the universe? A tweak
to our theory of gravity also explain all of these things?
In a word, No, it just doesn't work. No, galax
is are just weird. No galax is just weird. No,
it does not explain the cosmic microwave background, Like, it's

(40:36):
very difficult to explain that using anything else other than
some new kind of particle or promodial black hole or
something some new kind of stuff. You just can't explain
it using deviation and gravity because it was on a
really small scale. This is like, you know, things were
near by each other. This wasn't galactic distances interesting, And
it's very difficult to explain the structure of the universe.

(40:58):
And then there is one really awesome example of dark
matter that's sort of like a smoking gun that makes
it almost impossible to explain using anything but some new
kind of matter stuff, some new kind of stuff. Yeah,
and that's this Bullet cluster. There was this collision millions
and millions of years ago, far far away between two
clusters of galaxy that passed through each other. And we

(41:20):
thought that those clusters of galaxies they had normal matter
like stars and planets, etcetera, and then also clumps of
dark matter. So what happened is they passed through each other,
and the gas and the dust it all collided amid
big collisions and slowed down and all that stuff. But
the dark matter, it doesn't interact with the normal matter,
and it doesn't interact with itself very much at all
either because gravity is so weak, so the dark matter

(41:43):
is sort of passed through. So what we see when
we look up in the skies, we see like a
big blob in the center while the gas and dust interacted,
and then on either side we see the dark matter
that passed through, and we can see that because of
the gravitational lensing. Right, But doesn't it all I know,
we talked about the bullet cluster it before, but doesn't
it still just come down to gravitational lensing, Like what

(42:05):
if gravity can explain gravitational lensing, could that also explain
the bullet cluster that we see. It's very difficult to
explain the bullet cluster in any other way because you
need to have gravitational lensing in exactly those right spots
on opposite sides of this very obvious collision. So it's
a very nice explanation to think, oh, there's some invisible
matter that passed through and it's now sitting there distorting

(42:27):
the background galaxy. See, the bullet cluster is like proved
that whatever it's causing these gravitational things is mobile, like
it can move like stuff. But if it was just
a gravitational theory tweak, that wouldn't like keep going or
move or change from here to there exactly, and it
can be separated from the normal matter. It's not just
a gravitational tweak from the normal matter, right, the normal

(42:49):
matter was left behind in the middle. It's its own
kind of stuff. It has its own gravity. And so
that's sort of like a bullet in the brain of
the you know, non dark matter theories. The bullet cluster
is a bullet suspicious. People really took Moms sort of
seriously until then. And then when the bullet cluster was discovered,
people are like, oh, well, that's it. Dark matter is real,

(43:12):
and Mond is dead and never Mond. And at that
point people really didn't take months seriously. So since then,
Mond has been much more of a fringe theory. I mean,
Virlin's idea is more recent, and it's quote of cool,
but it sort of explains something that nobody really takes
seriously anymore. So it's very fringe in a way. The
bullet cluster shows you that dark matter or whatever it's

(43:35):
causing these gravitational distortions and footprints, is mobile like it's
it can move like stuff. It moves like a stuff
can move. Yeah, And so again it's unsatisfying because we
don't know what it is and and we've been looking
for for a while it's not like we're just like, oh,
it's some invisible stuff, let's move on. We've built dedicated
experiments to look for it. We thought several times that

(43:57):
we would definitely find it and then haven't seen it.
So there's definitely some tension there. It's not like dark
matter is a beautiful theory that's all wrapped up, like
we don't understand why we haven't been able to find
the dark matter yet. There's definitely something weird going on there,
and it's very healthy to think about new ideas, but
mond doesn't quite work. There is no other idea out

(44:18):
there that explains what we see nearly as well as
some new cold particles. I have an idea is it's
snack based. It's called Hey, maybe dark matter is weird.
It's weird, yeah, And that's why people are sort of
digging further and further into the barrel of ideas for
dark matter. People thought for a long time, Okay, dark
matter must just be some weakly interacting massive particle, some

(44:41):
new lava stuff, but we haven't seen it, and so
then people are well, maybe it's axions, or maybe it's
primordial black holes, or and there are some other even
crazier ideas like squirmyons, what like they're uncomfortable and social situations.
What do you mean introvertons? Yeah, no, squirmy on are
this crazy idea that you know how particles are like

(45:03):
excitations of quantum fields are like little bundles of energy
in a quantum field. People think, well, maybe not all
energy and quantum fields are particles. Maybe some of them
are like more distributed or spread out in weird ways.
And they found that you can like tangle up quantum
fields in these weird ways, like make nots of them,
and that's what they call squirmyons. And it's not could

(45:24):
like it could create gravity. Yeah, because any energy density
creates gravity. Well, we'll have to dig into that for
another episode. Sounds pretty for sure. So there's lots of
ideas the whole spectrum, right, dark matter is not just
one idea, the sort of spectrum of ideas all satisfied
the condition that it's some new kind of cold object.

(45:46):
But what that object is. It could be one particle,
could be many kinds of particles. It could be black holes,
it could be squirmyons, it could be something else we
haven't even thought of yet. Whatever, it is. We're pretty
sure it's stuff. It's something that has gravity stuff when
it's there, that's right. But I mean it's really interesting
because I feel like we've known dark matters is there
for a while now, and we just can't seem to
crack it. We can't seem to find it keeps um

(46:08):
eluding us, you know, like it just keeps on hiding
there and not letting us know what it is. That's right,
But we don't know how long the story is. You know,
we were looking for the Higgs boson for fifty years
and then we found it. We were looking for the
top cork for twenty years before we found it. A
lot of those things. People thought, oh, we'll find this
in the next year two and then they were confused
and disappointed to not discover it soon. But you know,

(46:29):
eventually we got there and we we figured it out.
So maybe we're just five years away from discovering dark matter,
or maybe it's going to be another hundred. Maybe we
need a new crazy idea for what dark matter is.
But dark matter, I'm pretty sure is so all those
physicists squirming around relax. Maybe dark matter is in our future.
That's right. I certainly hope it is. It would be fascinating.

(46:50):
And remember that it's most of the stuff out there
in the universe. Like, what a crazy opportunity to learn
about the way the universe works, you know, to know
that of the stuff out there in the universe has
been hidden from us. The day we crack that open
and get to learn about it, like, it could have
incredibly complex structure, it could have interactions and biology and
chemistry and all sorts of crazy stuff. Movest of the

(47:12):
stuff out there in the universe we haven't yet gotten
to play with, and so we're eager, we're desperate to
figure out what this stuff is. Yeah, because if you're
the scientist that discovers what dark matter is, I mean,
that's like a lot of three cred you know. It's
like you could say that you discovered eight eight percent
of everything in the universe. I think they would have
to give you five Nobel Prizes, you know, just for

(47:33):
that one discovery. They have to give you eighty percent
of all the Nobel prizes. I think, Yeah, yeah, there
you go. You get all the Nobel Prizes for the
next four hundred years, just to balance it out. Yeah,
So it's a big question, and who knows, maybe one
of our listeners will be the one who discovers it.
That's right, It could be you out there, it could
be your kids out there. What we definitely need to
do or keep our minds open and come up with

(47:53):
new ideas for what dark matter is really cool? All right, Well,
we hope you enjoyed that view into this story is
dark matter and how we know it's there and how
we know it's not just a weird fluke of gravitational equations.
Thanks for tuning in, See you next time. Thanks for listening,

(48:17):
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio. For more podcast For
my heart Radio, visit the i heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows.