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Table of contents

0:00
Intro
1:01
Cold Dark Matter
3:03
The WOMB Miracle
7:13
Cold Dark Matter Solutions
10:15
Ordinary Matter Matters
14:46
Conclusion
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00:00:06
Just luminous sprinkling on top of the  vast oceans of dark matter that dominate  
00:00:11
the gravitational universe. Although we  don't know what dark matter actually is,  
00:00:16
for a long time we thought we at least had it's  behavior nailed. The so-called cold dark matter  
00:00:22
model had to be right because it worked so well  in explaining structure in the universe.  
00:00:28
Well that is until it didn't. Distressing disagreements with newer observations sent many physicists  
00:00:35
back to the blackboard. But even more recently, a new generation of sophisticated, high-power computer  
00:00:42
simulations may save our favorite dark matter  model after all. With some unexpected help - it turns out  
00:00:50
that visible matter may be much more important  in shaping the universe than we ever imagined.
00:01:01
We have no idea what dark matter is, other than  it’s some source of gravity that is completely  
00:01:06
invisible but exerts way more pull that all of  the regular matter. More than all of the stars,  
00:01:12
all of the gas, all of the black holes…unless dark  matter is black holes, then black holes are most  
00:01:17
of everything. Dark matter constitutes  80% or so of the mass in the universe,  
00:01:22
which means even our Milky Way galaxy is mostly  a vast ball of dark matter that happens to have  
00:01:28
attracted a relative sprinkling of  baryons—atoms in the form of gas,  
00:01:32
which lit up as starry glitter spinning in the  middle of this invisible gravitational well.
00:01:38
But for all of dark matter’s mysteriousness,  it’s remarkably simple stuff in terms of its  
00:01:44
behavior. Or at least so we have long thought.  The mainstream model for dark matter is called  
00:01:48
cold dark matter, or CDM. This type of dark matter  is described as a fluid of particles that don’t  
00:01:54
interact with each other or anything else except  by gravity, and that have pretty low speeds,  
00:02:00
making them cold. If a universe starts out full  of such a fluid, any tiny lumps in density of that  
00:02:08
fluid tend to attract more dark matter and grow.  We can now do these incredible supercomputer simulations  
00:02:15
of this process, and based on the CDM model,  this type of dark matter seems to lead to exactly  
00:02:22
the types of giant structures—galaxies, galaxy  clusters, etc—that we see in the universe today.
00:02:27
Although we don’t know what this “CDM” might be  made of, there are some seemingly very plausible  
00:02:34
types of particle that could do the job. Broadly,  we have weakly interacting massive particles or  
00:02:39
WIMPs, which are predicted by various extensions  to the standard model of particle physics.  
00:02:45
Or there's another exotic particle, the axion, or even the frozen Planck-scale relics of evaporated  
00:02:52
black holes all behave like cold dark matter,  and we’ve talked about all of these before.
00:02:57
The fact that there are many fairly natural ways  to make cold dark matter is a point in favour of  
00:03:02
the idea. But another one is the so-called  WIMP miracle. Most elementary particles in  
00:03:10
our universe were created soon after the Big  Bang during a short period when the universe  
00:03:15
was hot enough for matter-antimatter  pairs to be created spontaneously from  
00:03:19
the extremely energetic radiation of that  time. That creation process stopped when the  
00:03:25
universe expanded and cooled, and most of this  stuff annihilated with itself. Regular matter,  
00:03:32
which is much more able to interact with itself,  almost completely annihilated so that the matter  
00:03:38
that was left is just due to the 1-in-a billion  overabundance of normal matter over antimatter.
00:03:45
But dark matter particles in the WIMP category wouldn’t have self-annihilated so quickly due to its  
00:03:52
weakly-interacting nature. Instead, it stopped  annihilating only when the universe just got  
00:03:57
too big for dark matter particles to find each  other. It turns out that if you calculate how  
00:04:04
much dark matter should have been left when  that annihilation stops you get pretty much  
00:04:09
the right number for the amount of dark matter  we see in the universe today. That’s the WIMP  
00:04:15
miracle. Now this does require the existence of  some unknown heavy non-standard-model particle  
00:04:21
like those predicted by supersymmetry, but  actually the WIMP miracle is not tied to any particular theory.
00:04:27
Long story short, CDM was looking great.  But then, starting around 20 years ago,  
00:04:33
we began to notice that our rapidly  improving simulations and observations  
00:04:38
of the universe stopped matching up quite so  well. Let’s start on the simulation side.  
00:04:45
Because cold dark matter is cold, it should be  able to form pretty small blobs with relatively  
00:04:51
little gravity holding them together. That’s  because cold means slow-moving particles, which  
00:04:57
have trouble escaping even weak gravitational  fields. So if we try to simulate, say,  
00:05:03
the formation of the Milky Way with this type of  dark matter we get something weird. We get a big  
00:05:09
dark matter halo that attracts the gas that forms  the stars of the main galaxy—that part is fine.  
00:05:15
But within this halo these CDM simulations also  predict a lot of sub-haloes - smaller dents in the  
00:05:24
big gravitational well that themselves should  presumably attract gas and form sort of mini  
00:05:30
galaxies - what we call satellite galaxies.  CDM simulations predict the Milky Way  
00:05:37
and really any big galaxy should be orbited by  thousands of these things. But our observations  
00:05:43
of real galaxies don’t find anywhere near enough  of them. This is the “missing satellites” problem,  
00:05:49
and it was a clue that our CDM model  might not be as miraculous as we once thought.
00:05:55
Another prediction of CDM is about the shape of big galaxies.  
00:06:01
Gas in a room pushes on itself due to the fact that the particles interact creating an outer pressure.
00:06:09
But the weakly interacting CDM particles don’t push each other apart. Add the fact that they are  
00:06:16
moving very slowly, and it becomes possible for these particles to form very dense, tight clusters. CDM  
00:06:22
models predict that inside galaxies, the density  of dark matter should go up and up as you travel  
00:06:29
from the outskirts inwards, and should reach  a very high density cusp near the very center.  
00:06:36
And regular matter should fall towards this  dark matter concentration, resulting in lots  
00:06:41
of stars near the galactic center. But in  real galaxies, the density usually flattens  
00:06:46
out within several thousand light years of the  center, leaving a more rounded core. This is  
00:06:51
known as the “cusp-core” problem, and it’s  the second strike against cold dark matter.
00:06:57
Of course, physics thrives on discrepancy,  
00:07:00
and these problems have inspired many years of  work on alternatives to the standard CDM model.
00:07:06
The most obvious fix to cold dark matter is to  change the temperature—change the “cold” part.  
00:07:13
We can’t heat up dark matter too much  or its particles won’t form halos at  
00:07:18
all—the particles will just wizz randomly through  the universe and be pretty useless. But what if  
00:07:22
we increase temperature just a bit to give us warm dark matter, or WDM? There is a way to do this, sterile neutrinos could give  
00:07:31
us a WDM, and we’ve discussed previously. However this neutrino abundance doesn’t “miraculously” come  
00:07:39
out just right from the particle physics and  the cosmology in the way that WIMPs seem to.
00:07:44
If dark matter is made of a relatively light  particle that has a slightly higher “temperature”,  
00:07:50
they can still form big haloes but have  a harder time forming these subhaloes. That’s  
00:07:56
simply because small haloes don’t have strong  enough gravitational fields to hold these faster  
00:08:01
moving particles. So maybe that's why the  Milky Way has very few satellite galaxies.
00:08:07
This doesn't really help us with the cusp-core problem, but here's a proposal that may. 
00:08:13
Self-interacting dark matter (SIDM) proposes  that dark matter particles actually do interact  
00:08:19
with themselves a bit more than standard CDM  would assume. That means they can repel and  
00:08:24
scatter off each other, which stops them becoming  packed too tightly above a certain density.
00:08:29
Finally, there’s also fuzzy dark matter (FDM),  which we talked about in our recent episode.  
00:08:36
In this model, dark matter is a superfluid of  ultra-light axions with a de Broglie wavelength  
00:08:41
thousands of light years long. It can’t form  structures smaller than this wavelength,  
00:08:46
so may neatly eliminate tiny satellites  and may eliminate very concentrated galactic cores,  
00:08:53
perhaps solving both the “missing  satellites” and “cusp-core” problems.
00:08:56
OK, so we have a few reasonable solutions  to our dark matter woes. But which is right?  
00:09:03
Well, not so fast. While all of this new physics  was being concocted, there was a parallel effort  
00:09:11
to see if the problems with cold dark matter could  be fixed while keeping dark matter cold. Remember,  
00:09:17
CDM is appealing as a model for more reasons than  its behavior as dark matter. Many believe that it  
00:09:24
is the most natural type of dark matter to have  formed in the early universe. That theoretical  
00:09:29
convenience isn’t enough for us to accept  CDM if it doesn’t explain our observations.  
00:09:34
But it turns out that despite hints to the  contrary, CDM may still be our best bet.
00:09:41
The CDM simulations I’ve been talking  about so far are “dark-matter-only”,  
00:09:46
which means that they don’t include ordinary  matter; they’re just simulating how dark matter  
00:09:51
behaves in the absence of gas and stars  and all that visible stuff. Now that sounds  
00:09:55
a bit forgetful on the part of the  simulators. But remember that there’s  
00:10:00
more than 5 times more dark matter than  ordinary matter in the universe. Almost  
00:10:04
all the gravitational oomph in the universe is  from dark matter, so it’s actually not crazy  
00:10:10
for these simulations to ignore the relatively  piddling contribution from atoms. But actually,  
00:10:16
we’re now discovering that there are a couple of  reasons why ordinary matter really does matter.
00:10:24
First, ordinary matter is the stuff we actually  see, so when you’re trying to use a simulation  
00:10:30
to predict what it is we should see, it’s helpful  to have that stuff in your simulation; otherwise,  
00:10:36
how do you predict what you’re going to  see through your telescope? For example,  
00:10:40
we guessed that all of the dark matter  sub-halos around a system like the Milky  
00:10:45
Way should contain visible satellite galaxies.  But that turns out to be wrong. Sub-halos that  
00:10:52
are less than around a billion times the mass  of the sun are simply incapable of capturing  
00:10:58
gas from the intergalactic medium, because  the gas is too hot__its particles moving  
00:11:03
too quickly—to become trapped in the shallow  gravitational wells of these low-mass sub-halos.  
00:11:10
So most of these sub-halos have few to no  stars, and therefore we can’t actually see  
00:11:16
them. If this is right then the satellites  aren’t missing, they’re just invisible.
00:11:22
Also, our telescopes are becoming more and more  sensitive and we’re starting to see a lot of very  
00:11:29
faint galaxies and substructures, so we’re getting  closer to the predictions of the CDM models.
00:11:36
And now the second reason that regular matter  matters. Although so-called “baryonic matter”  
00:11:43
represents only a small fraction of the mass in  the universe, this stuff is way more interesting  
00:11:48
than dark matter because it can form us for example, but it can also influence dark  
00:11:54
matter in unexpected ways. Because it can create  stars, it can also produce supernova explosions  
00:12:01
when the most massive of those stars die. Every  time a supernova goes off, it blows away a huge  
00:12:08
chunk of the gas surrounding it. That gas can  be forced out of the galactic core, or even out  
00:12:14
of the galaxy entirely. And this outgoing gas  also drags a little bit of dark matter with it.  
00:12:20
The dense galactic core of a young galaxy will  experience burst after burst of star formation,  
00:12:26
which leads to multiple waves of supernovae.  Over time, quite a bit of dark matter can get  
00:12:34
dragged gravitationally out of the core of the galaxy. This can ultimately flatten out what started as  
00:12:40
a very high density core, eliminating  the cusps predicted by the CDM model.
00:12:46
But CDM isn’t totally out of the woods yet. In  recent recent years, we’ve found some counter examples  
00:12:52
that seem to defy these needs fixes for CDM’s  problems. For example, there are several galaxies  
00:12:58
that have the typical low density cores but don’t  seem to have had enough star formation in their  
00:13:03
pasts to do that supernova-driven smearing. And  strangely, also some systems with lots of star  
00:13:10
formation that actually have high-density cusps.  Like for some reason the dark matter ignored the  
00:13:16
supernovae in these cases. So what started out as a “cusp-core” problem has actually evolved into  
00:13:23
what’s being called a “density-diversity” problem.  There’s a very wide range of central densities in  
00:13:30
galaxies that doesn’t appear to be correlated with  star formation or the mass of the surrounding dark  
00:13:36
matter halo. Currently it’s hard to see how the  CDM model can accommodate this kind of diversity.
00:13:43
The missing satellites solution also has a new  problem. While it’s true that small satellites  
00:13:49
are going to be invisible due to not holding onto  enough gas, those CDM simulations do predict a  
00:13:57
reasonable number of larger sub-haloes within a  galaxy, with masses more than a billion times that  
00:14:04
of the sun. These things should have no problem  holding onto enough gas to make stars, so we  
00:14:11
should be able to see them. But we don’t really  — at least not in the numbers predicted by CDM  
00:14:16
simulations. So the “missing satellites” problem  has evolved into what’s known as “too-big-to-fail”  
00:14:22
problem: we expected to not see the smaller  subhaloes, but we still seem to be missing  
00:14:29
some of the bigger CDM halos—the ones that should be too massive to have failed to form galaxies and stars.
00:14:36
Both the “density-diversity” and “too-big-to-fail”  problems are challenges to the CDM model because  
00:14:43
they’re difficult to solve by including  regular matter. But they aren’t quite the  
00:14:48
hypothesis-killers for CDM that the cusp-core and missing satellite problems once seemed to  
00:14:54
be. It may be that improved simulations  and observations will solve these too.
00:15:00
As the prospects of CDM recovered, alternative  models like warm or fuzzy dark matter are coming  
00:15:07
under strain. Our improving telescope sensitivity  is finding more and more small-scale structures  
00:15:13
that conflict with the predictions of WDM and  FDM, and remember those were are all about stopping small-scale  
00:15:20
structure. Now there are ways to tweak these models to better match our observations, but that kind  
00:15:26
of defeats their original purpose, so they're  perhaps not as parsimonious as they once were.
00:15:32
The bottom line is that there is still  a lot of uncertainty and debate about  
00:15:37
both the microscopic constituents and the  macroscopic behavior of dark matter. But  
00:15:43
one thing is clear: ordinary matter is not nearly  as insignificant to cosmology as we once thought.  
00:15:50
In order for us to finally solve one of the  greatest mysteries in all physics—in order  
00:15:54
to figure out the nature of dark matter, we’ll  need to continue to improve our understanding  
00:15:59
of the complex ways in which all types of  matter shape the structures in Space Time.

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PBS Member Stations rely on viewers like you. To support your local station, go to: https://www.pbs.org/donate/ Sign Up on Patreon to get access to the Space Time Discord! https://www.patreon.com/pbsspacetime We have no idea what dark matter is, other than it’s some source of gravity that is completely invisible but exerts way more pull that all of the regular matter. More than all of the stars, all of the gas, all of the black holes…unless dark matter is black holes, then black holes are most of everything. Dark matter constitutes 80% or so of the mass in the universe, which means even our Milky Way galaxy is mostly a vast ball of dark matter that happens to have attracted a relative sprinkling of baryons—atoms in the form of gas, which lit up as starry glitter spinning in the middle of this invisible gravitational well. Check out the Space Time Merch Store https://crowdmade.com/collections/pbsspacetime Sign up for the mailing list to get episode notifications and hear special announcements! https://mailchi.mp/1a6eb8f2717d/spacetime Search the Entire Space Time Library Here: https://search.pbsspacetime.com/ Hosted by Matt O'Dowd Written by Taha Dawoodbhoy & Matt O'Dowd Post Production by Leonardo Scholzer, Yago Ballarini, Adriano Leal & Stephanie Faria Directed by Andrew Kornhaber Associate Producer: Bahar Gholipour Executive Producers: Eric Brown & Andrew Kornhaber Executive in Charge for PBS: Maribel Lopez Director of Programming for PBS: Gabrielle Ewing Assistant Director of Programming for PBS: John Campbell Spacetime is produced by Kornhaber Brown for PBS Digital Studios. This program is produced by Kornhaber Brown, which is solely responsible for its content. © 2023 PBS. All rights reserved. End Credits Music by J.R.S. Schattenberg: https://www.youtube.com/user/MultiDroideka Space Time Was Made Possible In Part By: Big Bang Supporters Bryce Fort Peter Barrett David Neumann Sean Maddox Alexander Tamas Morgan Hough Juan Benet Vinnie Falco Fabrice Eap Mark Rosenthal Quasar Supporters Glenn Sugden Alex Kern Ethan Cohen Stephen Wilcox Christina Oegren Mark Heising Hypernova Supporters Stephen Spidle Chris Webb Ivari Tölp Zachary Wilson Kenneth See Gregory Forfa Kirk Honour Joe Moreira Bradley Voorhees Marc Armstrong Scott Gorlick Paul Stehr-Green Ben Delo Scott Gray Антон Кочков Robert Ilardi John R. Slavik Donal Botkin John Pollock Edmund Fokschaner Chuck Zegar Jordan Young Daniel Muzquiz Gamma Ray Burst Robin Bayley Piotr Sarnicki Massimiliano Pala Thomas Nielson Joe Pavlovic Ryan McGaughy Chuck Lukaszewski Edward Hodapp Cole Combs Andrea Galvagni Jerry Thomas Nikhil Sharma Ryan Moser John Anderson David Giltinan Scott Hannum Bradley Ulis Craig Falls Vivaan Vaka Kane Holbrook Ross Story Teng Guo Mason Dillon Matt Langford Harsh Khandhadia Thomas Tarler Susan Albee Frank Walker Matt Quinn Michael Lev Terje Vold James Trimmier Jeremy Soller Andre Stechert Paul Wood Kent Durham Ramon Nogueira Paul Suchy Ellis Hall John H. Austin, Jr. Diana S Poljar Faraz Khan Almog Cohen Daniel Jennings Cameron Sampson Jeremy Reed David Johnston Michael Barton Andrew Mann Isaac Suttell Bleys Goodson Robert Walter Mark Delagasse Mark Daniel Cohen Nickolas Andrew Freeman Shane Calimlim Tybie Fitzhugh Eric Kiebler Craig Stonaha Graydon Goss Frederic Simon Dmitri McGuiness John Robinson Jim Hudson Alex Gan David Barnholdt David Neal John Funai Bradley Jenkins Jiri Borkovec Vlad Shipulin Cody Brumfield Thomas Dougherty King Zeckendorff Dan Warren Patrick Sutton John Griffith Dean Faulk

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