Where is all the missing matter?

Daniel and Jorge talk about the effort to track down all the quarks in the Universe.

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, I think we've been using too much toilet humor.

Speaker 2 (00:11):
You mean, all those obvious dark matter jokes we make.

Speaker 1 (00:14):
Yeah, you know, I'm sure it makes all the nine
year old to google in the audience. But I don't
think we want to undercut our educational message.

Speaker 2 (00:20):
All right, that's a good point. Let's try that. All right.

Speaker 1 (00:23):
Well, so what are we talking about today today?

Speaker 2 (00:24):
We're talking about hot gas.

Speaker 1 (00:26):
Well, that didn't last very long. Hi. I am joeham Mack,
cartoonists and the author of Oliver's Great Big Universe. Hi.

Speaker 2 (00:48):
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I'm often full of hot air.

Speaker 1 (00:55):
Are an all physicists full of hot air?

Speaker 2 (00:57):
I'm just talking about the weather here in southern California.
I don't know what you mean.

Speaker 1 (01:00):
What do you mean the weather is inside of you.

Speaker 2 (01:02):
I'm breathing in the atmosphere literally.

Speaker 1 (01:05):
I guess if you were breathing out cold there, that
would be bad news, because we all know physicists aren't
very cool.

Speaker 2 (01:11):
I'm trying to make physics hot, is what I'm doing.

Speaker 1 (01:13):
But anyways, welcome to our podcast. Daniel and Jorge explain
the Universe, a production of iHeartRadio.

Speaker 2 (01:18):
In which we try to marinate in all of the
wonders and mysteries of the universe. We think that everything
that's out there should make sense to you, can make
sense to you, will make sense to you if you
just think about it, ask enough questions and listen to
this podcast long enough.

Speaker 1 (01:34):
That's why we try to breathe in the universe and
breathe it out and think about all of the hot
and cold stuff out there in the universe, even the
things in toilets.

Speaker 2 (01:44):
I thought we're avoiding the toilet jokes.

Speaker 1 (01:46):
Well that was in the joke. I mean, there is
physics in toilets, isn't there.

Speaker 2 (01:50):
That's true. You once challenged our listeners to record their
toilet spinning to see if they flush differently in Australia.

Speaker 1 (01:56):
Oh did they do it.

Speaker 2 (01:57):
I haven't gotten any didy yet, so we're still waiting
for the rest to those experiments. But that is serious
toilet science.

Speaker 1 (02:03):
Yeah, there you go.

Speaker 2 (02:04):
But in the non toilet realm of the universe, we
are very curious about how everything works out there, and
more specifically, what's out there and where is it all?
Can we figure out what in the end the universe
is made out of and where it's all distributed.

Speaker 1 (02:18):
Yeah, because that is a fundamental human quest to figure
out what's going on out there. What is this universe
we're in, what's in it? Who else is in it?
And what is it made out of?

Speaker 2 (02:27):
And where have they been dropping all their trash?

Speaker 1 (02:30):
Wait?

Speaker 2 (02:30):
What well you mentioned who else is in it? Makes
it sound like, you know, we're trying to figure out
where all their stuff is, Like did they lose their keys?
Where did that box go? This kind of stuff?

Speaker 1 (02:40):
I was just wondering, you know, so we could say, hi,
not find their keys.

Speaker 2 (02:44):
The first thing we want to do when we talk
to the aliens is ask them where they left their stuff.
Is this your trash? Did you leave this over here?
Please pick that up?

Speaker 1 (02:51):
Although if they leave their keys to their spaceship line around,
I'm not really going to return that one. No one's
staying with me.

Speaker 2 (02:58):
Well. On this podcast, we are often talking about one
of the deepest mysteries in modern physics, which is where
the dark matter is. We know that most of the
stuff in the universe is an invisible kind of matter
We've only recently discovered and have very little concrete information
about what it is. So we're used to the concept
of not understanding everything that's out there in the universe.

(03:21):
But it might surprise you to learn that even the
kind of stuff that we're used to, the hydrogen, the helium,
the kind of matter where made out of, is still
something of a mystery.

Speaker 1 (03:30):
Wait what so then, how do we know how much
of it there is out there?

Speaker 2 (03:32):
We have a bunch of really clever ways of figuring
out how much normal matter there should be out there
in the universe, but it's tricky to actually find all
of it.

Speaker 1 (03:41):
I see, we know how much of there should be,
but we just haven't found it.

Speaker 2 (03:45):
Is that what you're saying, that's basically it episode done?

Speaker 1 (03:47):
All right, Well, thank you for joining us. I can
go do something else now.

Speaker 2 (03:53):
Well, maybe the aliens have stolen all that missing matter.

Speaker 1 (03:56):
WHOA, that's a pretty serious allegation or just you know,
impugning the goodwill of the aliens and their legality.

Speaker 2 (04:05):
Well maybe instead of making a big mess, they've been
a little bit too aggressive about cleaning up after themselves.

Speaker 1 (04:11):
Maybe it's the physicist mmm who stole all the matter
on the planet Earth with the wrench.

Speaker 2 (04:17):
In the end, it's not about understanding the universe. It's
about figuring out who to blame for it.

Speaker 1 (04:21):
Or who do thank for it? Right? Also, right, maybe
it's good that we live in this universe. I would
think so. But anyways, it is a big question about
where all the matter in the universe is that we
think should be there, and where it all went. So
today end the podcast, we'll be asking the question where

(04:43):
is all the missing matter? I guess this is kind
of a surprising question because because I didn't know there
was missing matter? Did this happen recently or a long
time ago?

Speaker 2 (04:53):
I mean, you're making it sound like an Agatha Christie novel,
like the Case of the Missing Matter, Like we put
all this hydrogen over here and we came back and
it was gone.

Speaker 1 (05:01):
Yeah. Yeah, there was a blackout, the lights went out,
there were some screens, and suddenly there was a missing
matter and we're all trapped on an island with a
limited number of suspects.

Speaker 2 (05:10):
That's right. No, it's been a long standing mystery. It's
gotten a little bit less play and a less attention
than the grander mystery of dark matter, but it's still
a very important question in understanding how galaxies form and
how the universe looks the way that it does, and
where all this stuff is.

Speaker 1 (05:26):
Now you're saying that this is actually called, or it's
called physics, the missing baryon problem.

Speaker 2 (05:31):
Yeah, that's right, because the kind of matter that we
are made out of is made of protons and neutrons,
and those are things called baryons. A baryon is anything
made out of three quarks, and protons and neutrons are
made out of three quarks. So the kind of matter
that we are made out of, me and you, and
stars and galaxies and all the dust, all the visible
matter that's out there, we call that baryonic matter. And

(05:52):
so scientists have been trying to understand, like, where are
all the baryons in the universe? Are there as many
as we think there should be, And when they couldn't
find them, they call it the missing baryon problem.

Speaker 1 (06:02):
M sounds very mysterious, and you also kind of make
it sound like it's somebody else's problem.

Speaker 2 (06:07):
Hey, it's all about pre assignment to blame, right.

Speaker 1 (06:10):
Right, Yeah, Like if you say like, yeah, it's a problem,
I think you're basically saying it's somebody else's problem.

Speaker 2 (06:15):
Mistakes were made, right.

Speaker 1 (06:17):
That's right, Yeah, things went missing.

Speaker 2 (06:21):
Grand funding misallocated, I don't know.

Speaker 1 (06:24):
So as usual, we were wondering how many people out
there knew or know that there is missing baryonic matter
out there in the universe.

Speaker 2 (06:31):
So thanks very much to everybody who participates in this
segment of the podcast. We would love to hear your
voice among the coorse of listeners, so please don't be
shy write to me to questions at Danielandjorge dot com.

Speaker 1 (06:43):
So think about it for a second. Do you know
where the missing baryonic matter in the universe could be?
What is the missing baryon problem?

Speaker 3 (06:52):
I have never heard of the missing baryon problem, but
it might be something like the way that we had
predicted that the Higgs boson exists and we hadn't experimentally
verified it. So maybe there is a baryon, some form
of Bearyon particle that we mathematically know must exist, but
have it found.

Speaker 1 (07:09):
I don't know what the missing baryon is, but I
hope someone finds it.

Speaker 4 (07:13):
This is the term I've actually heard of before, if
I remember correctly. It has to do with the fact
that there is unexplained difference between the matter that existed
right after the Big Bang and the matter that exists today.

Speaker 5 (07:27):
The baryon sounds like some sort of barrier to a atom,
So I suppose if it's missing, then it would be
some sort of other force that we cannot explain, that
it is holding something like an atom together.

Speaker 1 (07:46):
All right, or interviews here didn't give us a lot
of clues.

Speaker 2 (07:51):
This has not gotten a lot of press compared to
dark matter, out of which they've been like dozens and
dozens of books written, and it's all sorts of podcasts.
Whatever it's name is problem in physics, but the missing
baryon problem is sort of like its second cousin that
doesn't get top building.

Speaker 1 (08:06):
It sounds like maybe it's a branding problem, you know,
like dark matter. Where's the dark matter in the universe?
That sounds mysterious and intriguing. Where's the baryonic matter in
the universe. It's like, I'm not a fan of Barry.

Speaker 2 (08:19):
What they should have called it the dark baryons or something.

Speaker 1 (08:22):
M yeah, or some other name, right, shining matter, super matter.

Speaker 2 (08:28):
Well, you know, dark means a lot of different things.
As you know, dark can mean mysterious, unknown, not yet understood.
It can mean literally dark light does not emit light.
And it's confusing because there are things out there that
are dark and are made of matter, but are not
dark matter, right, Like a lump of charcoal is pretty dark,
but it's not dark matter.

Speaker 1 (08:47):
You might think that physicists name thinks, very confusingly.

Speaker 2 (08:51):
The missing Physics name committee.

Speaker 1 (08:53):
So there's a bunch of matter that's missing that we
think should be there, but it's missing. That's what we'll
be talking about here today. And so let's break it down, Daniel.
What is baryonic matter?

Speaker 2 (09:02):
So baryonic matter is our kind of matter, hydrogen, helium,
all of the elements are built out of baryons, because again,
a baryon is a particle made of three quarks. Number.
Quarks are these little particles that we think are probably fundamental,
maybe fundamental, but they interact with the strong nuclear force.
And the way they form stable objects is either you

(09:23):
get a pair of quarks like quark antiquark that can
make a pion, or you can get three of them
together to cancel out a red quark, a green cork,
and a blue quark, and that gives you a color
neutral object like a proton or a neutron that has
no overall strong force.

Speaker 1 (09:38):
Okay, So a baryotic matter is matter made out of
quarks basically, right, that's the basic definition of it, like
the things that we're made out of, which are protons
and neutrons. But it sounds like there are other things
besides protons and neutrons you can make out of quarks.

Speaker 2 (09:51):
Yeah, you can make all kinds of things out of quarks.
You can make other hadrons. There's other combinations of quarks
that you can use to make other hadrons, like you
put three strange quarks together, or you can make an
up and down and a strange etc. There's lots of
different baryons you can make out of three quarks. You
can also make combinations out of pairs of quarks. It's
a huge zoo of particles made out of quark pairs.

(10:14):
The only stable one is the proton. The proton by
itself we think will last for a long long time,
and the neutron is stable when combined with the proton
inside of nucleus. So that's why protons and neutrons are
the most common kind of baryon out there.

Speaker 1 (10:28):
So today we're talking about which kind specifically all of
them or mostly protons and neutrons.

Speaker 2 (10:34):
Mostly protons and neutrons, because that's what we expect the
baryons out there to be made out of. If you
have other baryons out there, they typically decay down to
protons and neutrons. Really, though, we're trying to account for
all the quarks. In the end, we don't really care
if they're in protons or in neutrons, or in helium
or in hydrogen. We just want to know how much
of our kind of matter, quark based matter, is there,

(10:54):
and how much of the other stuff is there, and
can we figure out where all the quarks.

Speaker 1 (10:58):
Went saying baryon matter, barry on which kind of matter
it settles in.

Speaker 2 (11:06):
Yeah, that's right, And it's a fascinating situation to be
in because we have all these really clever ways of
knowing how many quarks there should be in the universe.
That seems sort of crazy, like, how could you possibly
have an idea of how many quarks they're on the universe.
They're here, they're there, they're everywhere. How could you possibly
count them?

Speaker 1 (11:24):
Well, I mean that's kind of basically what you're asking, right,
is you're asking where are all the quarks in the universe?

Speaker 2 (11:29):
Right, exactly. We are asking that, but we're asking in
two ways. One way is using information from the very
early universe, which tells us how many quarks there should be,
and then another way is more direct, is going out
there and actually looking for them and saying, can we
find all the quarks that our early universe theories predict
are out there? And that's where the discrepancy comes from.

Speaker 1 (11:50):
Hmmm, So I think you're saying that we could have
just titled the episode where are all the Missing quarks?

Speaker 2 (11:55):
Yeah, where are all the missing quarks? Exactly? But in
physics it's called the missing barrier problem, and it makes
up the kind of matter that we're familiar with. Right,
we think that dark matter is not made of quarks,
that's made of something else entirely. So this little sliver
of the universe that we think is about five percent
of all the energy density of the universe, baryonic matter

(12:15):
stuff made out of quarks. That's the thing we're still
trying to understand after all these years.

Speaker 1 (12:21):
Is there an important distinction between asking where all the
baryonic matter is and asking where all the quarks are? Like,
are there quarks that are not in baryonic matter? Or
is it all the same term.

Speaker 2 (12:32):
There are no quarks that are not in some kind
of particle because quarks can't be by themselves, so they
always form either masons, which are quark quark pairs, or baryons,
which are triplets of quarks. Baryonic matter technically probably also
includes the electrons. So if you have, for example, a
hydrogen atom that's a proton and an electron that you
could call baryonic matter because it's based on the baryon

(12:53):
the proton, that technically includes the electron. So baryonic matter
is probably more accurate description because it includes the electrons.
Also they bind with the protons.

Speaker 1 (13:02):
Wait, so what there's electrons missing too?

Speaker 2 (13:05):
Well, electrons are part of the five percent of the
universe made out of normal matter, basically quarks and leptons.

Speaker 1 (13:11):
Okay, so then there's a certain amount of quarks and
electrons in the universe that we think should be there.
And you're saying, we have an idea of how much
there should be there based on our measurements of the
origin of the universe.

Speaker 2 (13:22):
Yeah, we have all these really clever ways of looking
at details from their early universe and using that to
figure out essentially how many quarks there should be today.
In order to build stuff up, we should be able
to predict how much hydrogen and how much helium and
all sorts of stuff there are from our pictures of
the early universe. And there's two totally separate ways to

(13:42):
predict how much baryonic matter there should be left over today.
One of them comes from the cosmic microwave background radiation,
this very early light from about three hundred and eighty
thousand years after the Big Bang, and another comes from
the ratio of the elements, how much hydrogen, how much helium,
how much detery there is in the universe. Both of
those are very sensitive to the quark density in the

(14:05):
early universe and so can tell us how many quarks
there should be.

Speaker 1 (14:09):
Meaning like, we maybe start with a guess and see
if that makes the universe make sense as we see
it today, and then you adjust that until you get
an amount that do you think makes what we see
in the cosmic microwave background and in the amount of
stuff we see makes sense.

Speaker 2 (14:26):
Yeah, I don't know that we have to start with
a guess. It's more like there's information in the cosmic
microwave background radiation that tells us exactly how many baryons
there should be. And also by measuring the ratios of
the elements how much hydrogen, how much helium, we can
use that to make a calculation of how many baryons
there should be, so we don't have to guess. We
can just like extract it directly from these measurements.

Speaker 1 (14:48):
Well, maybe break it down for people. How does the
ratio of hydrogen and helium tells how many quarts the
universe started with?

Speaker 2 (14:55):
So in the very early universe, things were super duper
dense and hot, right, the basic story of the universe,
things were very very hot and dense. We don't know
how we got to that state, that's sort of a
big question mark, but we're very certain that things were
very hot and dense and very compressed. And then the
universe expanded, and as it expands, it cools. So you
start out with like crazy high energy, and then things

(15:15):
cool further and those quarks form protons and neutrons, et cetera.
And then as things cool even further, those protons and
neutrons start to form bonds, so you make for example, deuterium,
which is a combination of protons and neutrons the deuterium
can then fuse into helium. So what's happening is the
universe is cooling and things are sort of like settling
into place. You're like baking bits and pieces of the universe.

(15:37):
After about twenty minutes, things are then too cold to
make any more helium or make any more deuterium, so
you sort of ran out of time to make deterium.
So in the very early universe you had this little
window to make deterium and to make helium, and the
rest of everything is just hydrogen. And the amount of
deuterium and helium you get depends very very sensitively on

(15:57):
the density of quarks, Like you have more we're exploding
around in that window, you get more deterium, you have
fewer quarks, you get less deterium. So if you measure
the hydrogen deterium helium ratios, now you can tell the
quark density back in that first little window in the
first twenty minutes of the universe.

Speaker 1 (16:16):
And how do you measure that ratio right now? Like
we can we go out there into space and gather
hydrogen and helium. How do we determine it?

Speaker 2 (16:24):
Yeah, you can actually just fill up a glass of
water from your tap, because one out of like every
six thousand atoms of hydrogen is actually an isotope of
hydrogen called deuterium, has a little neutron stuck to it,
and that deuterium is pretty stable. So the amount we
made back then is still the amount we make now.
There's like basically no other natural, significant sources of deuterium,

(16:44):
So the universe is kind of like locked into this
deterium ratio. When you fill a glass of water at
the tap, one out of six thousand atoms of those
waters has a hydrogen in it that's actually deuterium. How
do you measure that? You can just put it through
like a mass spectrometer to measure the weight of the
at and you'll see this little peak of some water
that's a little heavier.

Speaker 1 (17:03):
But how do I know that's just not the water
in my town that has that level of deuterium, or
even like in our solar system or even galactic neighborhood.
How do you do you extrapolate my tap water to
the entire universe?

Speaker 2 (17:17):
You're right, You've unraveled this entire science. No, we obviously
don't just base it on the top water in your
house or in anybody else's house. We make measurements all
over the place. We can make measurements in the rest
of the Solar system by looking at like vibrational modes,
because deuterium has slightly different energy levels than normal hydrogen,
so you can see evidence for this all over the universe.
And so we see a pretty well known mixture of

(17:38):
deuterium inside hydrogen.

Speaker 1 (17:40):
All right, So then that tell us how much quark
matter there should be in the universe, and how much
is that amount?

Speaker 2 (17:46):
That's about five percent of the energy density of the universe.
And this is a number that's easy to misunderstand. What
we mean by that is like, take a big chunk
of the universe, like a cubic light year, and out
of all the energy inside of it, all the photons,
all the dark matter, all the normal matter, all the
dark energy, all of that stuff, and the normal matter
should account for five percent of the energy density of

(18:08):
that chunk. So we're not saying anything about the size
of the universe or the total number. We're just saying, like,
what's the ratio five percent of all the energy in
any given chunk of space should be due to buryonic matter.

Speaker 1 (18:19):
According to what we know of the Big Bang and
the cosmic microwave background. But it seems that some of
that matter is missing. Somebody took it or destroyed it,
or I don't know, hate it. And so let's get
into that mystery and who we can blame for that
in more detail. But first let's take a quick break.

(18:52):
All right, we're talking about some missing matter in the universe.
There's a certain amount of quark matter in the universe
that we think should be there. Five percent of the
energy and matter in the universe should be quark matter.
But Daniel, it sounds like that's not what we're seeing.

Speaker 2 (19:06):
Yeah, that's right. We have not yet figured out where
that five percent of matter is. And if you're skeptical
about that five percent calculation, know that we have other
ways to calculate this number that are totally independent. Right.
The description we gave you about the deterium fraction of
the universe, that's called Big Bang nucleosynthesis. It's understanding how
much of various elements were made in the very early universe.

(19:26):
We have other measurements from the cosmic microwave background radiation
which come from much later in the universe, like three
hundred and eighty thousand years that are completely independent, totally
separate measurements. There, we see the early universe plasma sloshing
around in a way that's sensitive to the number of
baryons and the amount of dark matter and the number
of photons. And that's a very very precise measurement, much

(19:48):
more precise even than the Big Bang nucleosynthesis, and it degrees.
It's about five percent of the energy density should be baryons.

Speaker 1 (19:56):
But I wonder are they really that independent? I mean,
don't they both depend on our model of the universe
and or at least our model of the Big Bang.

Speaker 2 (20:04):
Absolutely, yeah, there are a lot of assumptions in common,
but there are independent measurements. Like they have different sources.
You know, one is measuring the fraction of deterium in
the universe. The other one is like looking at these
very cold photons in the night sky. They also come
from a different age in the universe. So they're absolutely
they're not completely independent, but they're very useful cross checks. Right,

(20:24):
we would be surprised and confused if those two numbers
didn't agree with each other.

Speaker 1 (20:28):
Right, all right, So then those measurements are telling us
there's missing matter, how much quark matter in the universe
is missing, so like most of it, five percent of
the universe is missing.

Speaker 2 (20:39):
More like eighty percent of the universe. If you look
around for quark matter, you can find loss of it. Right,
Like I'm made a cork matter, You're made of cork matter, right,
your lunch is made of cork matter. The Earth, the
Sun is made out of quark matter. All this stuff
is pretty easy out of all the galaxies and the
stars and the gas that glows in the universe, and
then add the harder bits. Right, Some of the stuff

(21:00):
that's out there in the universe, like we were talking
about earlier, is matter that is dark, but it's not
dark matter. You know, things like black holes or things
like big massive planets that are not glowing. These things
are harder to spot and harder to account for. But
people have done a sort of census of all of
this stuff. Where is all the stuff that we know about,

(21:21):
how much is there, and how does it add up?
And together it comes to, you know, about fifteen twenty
percent of what we.

Speaker 1 (21:27):
Expect, fifteen to twenty percent of the five percent that
we think should be there.

Speaker 2 (21:31):
Mm hmm exactly. So most of the buryonic matter in
the universe is not in the stars and in the
galaxies and in the gas, or in black holes, or
in planets, or we think in big chunks of rock
floating out there in the universe. And again we're not
talking about dark matter, right. We know dark matter is
out there and it's another mysterious thing. We're just talking
about the missing quarks. We just can't find as many

(21:52):
quarks as we expect.

Speaker 1 (21:54):
I wonder if then you just need to lure your
expectations anyway, like mactation is wrong. Maybe that's the real problem.

Speaker 2 (22:03):
Yeah, but we have these two fairly independent measurements that
tell us that the universe should be five percent. And
this all fits in very nicely with our model of
the universe, how it expands and how structure has formed.
We have all these ideas for how the universe comes
together from the hot gas to forming these very cold
galaxies later on, and all these things are very sensitive

(22:24):
to the dark energy, dark matter, and normal matter fraction
of the universe. So it's the number we feel pretty
confident in five percent, and it gives us enough confidence
that we want to go out there and look for
these missing burials. We're pretty sure they exist, we just
hadn't seen them yet.

Speaker 1 (22:40):
We Well, just so you know, that is an option
in life. You can just lower your expectations and then
you can take a vacationion.

Speaker 2 (22:46):
Well, I want to encourage all of our listeners in
the opposite direction to keep pushing forward until your questions
are answered. Don't give up.

Speaker 1 (22:52):
All right, Well, let's keep going then. So, there is
a certain amount of cork matter in universe we think
should be there, but we can't seem to for it.
Like we do some accounting of what we can see
and what we think is there, and it's not enough
to where could it be and how are we going
to find it?

Speaker 2 (23:07):
So one obvious place to look is between the galaxies.
Like we know there's a lot of quark matter in galaxies.
We can see it. There's gas, this dust, is stars,
is all that stuff. But we also know that there
should be a lot of matter between the galaxies, that
there should be these huge filaments of gas and dark
matter as well between the galaxies. Because remember, the universe

(23:27):
is not just like all these little dots of stars
and dots of galaxies. It's more like a big cosmic web.
Because as the universe cooled down. It was this hot,
dense plasma. You have these little dense spots that gather
together more stuff. The universe is expanding, and then those
dense spots see the formation of structure, right, they see
those galaxies, but they don't become isolated. You still have

(23:50):
these strands between them. And so the place to look,
the place that our simulations predict there should be a
lot of quark matter that's sort of hard to spot
is between the gas.

Speaker 1 (24:00):
Galaxies because they can't be in the galaxies. Because you
think you can see everything in a galaxy.

Speaker 2 (24:06):
We think we know how much matter there is in
a galaxy. Yeah, we can see all the luminous stuff
that's there, all the gas, and all the stars and
the dark matter, and the motion of those stars tells
us a lot about the gravitational profile of the galaxy. Remember,
as the galaxy spins, we can tell how much gravitational
force there is on those stars by looking at the

(24:27):
rotation velocity of the stars. That's how we deduce the
existence of dark matter in the first place. We're pretty
sure we understand the density profiles of galaxies, which is
why outside of galaxies is a good target.

Speaker 1 (24:39):
So you're saying that maybe eighty to eighty five percent
of the missing quark matter in the universe might be
in between galaxies where we can't see them or what.

Speaker 2 (24:48):
Yeah, that's exactly right. Most of the quarks in the
universe are not in galaxies. Like you might imagine that.
You know, matter forms in the Big Bang and then
things cool and clump together and form galaxies, and that's
part of the story. But it turns out it's not
most of the story, that this galaxy formation process is
kind of inefficient, that most of the normal matter in
the universe didn't participate it or hasn't.

Speaker 1 (25:10):
Yet, because I guess the stuff that does come together
is kind of the fancy stuff that everyone pays attention to,
right the stars and the planets.

Speaker 2 (25:19):
Yeah, it's got the most glitter and glam.

Speaker 1 (25:22):
Okay, So then now is that confirmed? Like if you
look for things in between galaxies, do you find all
of this missing quark matter?

Speaker 2 (25:29):
So there's several steps here. The first thing is to
look for hydrogen, so like, are there huge amounts of
hydrogen between the galaxies? And you can imagine the galaxies
is sort of like in these gravitational wells you have
a blob of dark matter which has gathered together the
normal matter to form stars and galaxies. And you can
think about like gravitational filaments like feeding into these wells,

(25:49):
sort of the way rivers feed into a lake, and
gas flowing into these galaxies. And we know that gas
is flowing into these galaxies, we can see like the
impact of gas flowing to these galaxies. Sometimes it even
affects star formation in those galaxies. But this gas can
be tricky to see because it's very, very dilute. Remember,
there huge space between galaxies, millions and millions of light years,

(26:12):
and so seeing these things is tricky. One way that
we have seen them though, is using quasars.

Speaker 1 (26:17):
What do you mean? How do those help us see
the hydrogen between galaxies?

Speaker 2 (26:22):
They basically light it up for us in this really
cool way. Remember, a quasar is like a black hole
at the center of a galaxy that's actively feeding. It's
like gobbling up a lot of stuff and emitting a
huge amount of radiation. Now it's confusing for people sometimes
when you say a black hole is emitting a lot
of radiation, the black hole itself is not emitting the radiation.
But if there's a very intense disk of matter near

(26:44):
the black hole. It's going to be very hot because
of all the gravitational tidal forces glowing, and a lot
of that radiation gets funneled up because of the magnetic
field of the black hole, and you get these extraordinarily
powerful beams of light that sort of like pencil raised
through the universe. Some of them hit the Earth. So
if there's this very powerful beam of light that passes
all the way through the universe, it's also going to

(27:05):
pass through some of these filaments of gas. And when
it does so, it changes the spectrum of light because
that gas likes to absorb some light. So if this
hydrogen there, it's going to absorb the light that likes
to interact with hydrogen, it's sort of deleted from the spectrum.
So by looking at the spectrum of light from these quasars,
we can tell how much hydrogen there is between us

(27:25):
and the source of the light.

Speaker 1 (27:27):
You mean, like all of this quark matter that's floating
out there between galaxies X kind of like a filter.
So you have something bright like a quasar is shining
just directly at us and it filters through this gas.
You can sort of tell how much of the gas there.

Speaker 2 (27:40):
Is exactly, and it's even more detailed and powerful than that,
because the hydrogen between us and this distant quasar is
all going to be moving at different velocities relative to us,
Like the further away it is, the faster it's going
to be moving away from us, it's going to be
red shifted, and that actually changes the frequency of light
that it interacts with. So if you look at the

(28:01):
spectrum of life from a quasar, you don't just see
one dip that tells you how much hydrogen there is.
You see a lot of dips. You see a forest
of these dips, each one corresponding to absorption of hydrogen
at a different red shift. And so not only does
it tell you how much hydrogen there is between you
and the quasar, it's like a one d map that
tells you where that hydrogen was between you and the quasar.

(28:23):
You can use these quasars to sort of like X
ray the universe and tell you where the hydrogen is.

Speaker 1 (28:28):
WHOA, but how often do we get signals like this?
How many quasars are pointing directly at us?

Speaker 2 (28:33):
Yeah, not as many as we'd like, of course, lots
of them, because there's lots of galaxies out there, and
in the early universe, quasars were very active. It's a
whole other mystery like why the quasars mostly get formed
in the early universe and not so much now. But
there are a lot of very distant, very bright quasars
that sort of like shine these lights through the universe,
and we'd like to see more of them. It's tricky,

(28:53):
but there's enough that we could have an estimate for
how much hydrogen gas there is in these filaments between galaxies.

Speaker 1 (28:59):
And these quasars basically like illuminate the hidden matter between galaxies.

Speaker 2 (29:04):
They do. They illuminate the hydrogen. Right. That's when you
have a proton and an electron together, because that's what's
going to interact with these photons. The neutral hydrogen will
do this. So when you look at this information from
the quasars, you can add it all up and you
can guess how much neutral hydrogen gas there is between galaxies,
and that brings you to about half of the five
percent that we expected. So just stars and galaxies and

(29:26):
all that stuff gives you like fifteen percent. Adding the
neutral hydrogen between galaxies and you're up to about fifty percent.

Speaker 1 (29:33):
That we can account for.

Speaker 2 (29:34):
That, we can account for exactly Well.

Speaker 1 (29:36):
You're saying it's not missing, then that we know where
it is.

Speaker 2 (29:38):
Well, even this very clever technique only brings us to
fifty percent. The other half is still not explained.

Speaker 1 (29:44):
Mm so only half of that five percent is missing.

Speaker 2 (29:47):
Then, Yeah, so like fifteen percent of it is stars
and galaxies and black holes and the obvious easy stuff.
Another like thirty five percent turns out to be this
neutral hydrogen between galaxies. Until very re we've had no
explanation for the other fifty percent that part was still missing.

Speaker 1 (30:05):
Could it be some other kinds of gases in between galaxies?

Speaker 2 (30:08):
So the crucial thing is that this quasar method will
tell us about neutral hydrogen, because you know, the photons
passing through these filaments will excite. Neutral hygrogen has these
very particular energy levels. The rest of a popular theory
is that it's a low density plasma that it's ionized.
It's not like a proton or electron hanging out in
a hydrogen atom. It might just be like a bunch

(30:29):
of protons and a bunch of electrons that are too
hot to settle down into a hygrogen atom. They're like
flying around free and they wouldn't interact with equasars in
the same way. And people argue about whether it's warm
or whether it's hot, and so they give this stuff
the name warm hot Intergalactic medium WHIM or.

Speaker 1 (30:48):
WHIM interesting acronym there. So you're saying that light doesn't
interact with cork matter unless there's an electron attached to it,
and that's because light only interacts with electrons.

Speaker 2 (31:02):
Light will interact with any charged particle. But this particular
signature that we can see relies on a feature of
neutral hydrogen. So photons will interact with protons when electrons
and scatter and do all sorts of stuff. But this
particular method only lets us see the neutral hydrogen.

Speaker 1 (31:17):
Why doesn't it let us see the protons.

Speaker 2 (31:20):
Well, what happens when the life from the quasar hits
a proton or hits an electron is it just basically
gives it a boost. It makes it glow a little bit.
But it's hard to know how to interpret that. We
can't see very well the glow from these protons and
these electrons because they're very very hot, so we think
they might emit some X rays or some UV rays
but it's very hard to detect those here on Earth.

Speaker 1 (31:40):
So we wouldn't see it in the signature from the quasars.

Speaker 2 (31:42):
Exactly, because these free protons and these free electrons can
interact with any kind of photon, so they generally would
just like overall, reduce the signature from the quasars neutral
hydrogen because it's a bound state of the proton, and
the electron is very rigid about which photons it will
interact with, and so it makes this very particular measurable
signature on the quasars. A free proton or free electron
can interact with any kind of photon, and so it

(32:04):
doesn't create this like obvious signature in the quasar beam.
We need another method to see these protons and electrons.

Speaker 1 (32:11):
I see the light from the quasar is maybe getting
absorbed by these free quarks floating out there, but it
would just look like it's a little dimmer to us,
which we can tell if it's because of that or
maybe because the quasar is not as bright as we
thought it is. All right, well, let's get into some
of the ways that we maybe could measure this missing
quark matter and what it all means about our understanding

(32:32):
of the universe. But first, let's take another quick break.
All right, we are slowly piecing together this problem, this
missing matter in the universe. Apparently there's a lot of

(32:55):
quark matter that we think should be there, but it's not.
Although I feel like we've already a kount of for
fifty percent of it. We started with only being able
to count fifteen percent of it, but now we're up
to fifty percent of it.

Speaker 2 (33:05):
Yeah, and you know, I guess fifty percent is like
on the edge of a passing grade. So you might
be tempted to call it a day move on, But
you know, some of us are curious. We want to
know where is the other half of all the matter in.

Speaker 1 (33:16):
The universe, don't Some of these measurements have like a
plus or minus fifty percent uncertainty or error bar on them.

Speaker 2 (33:23):
Anyway, I guess that's one way to resolve the mystery.
Just be like, well, let's just inflate the aerror and
it's no longer a mystery.

Speaker 1 (33:29):
There you go.

Speaker 2 (33:31):
Yeah, there are big uncertainties on some of these measurements,
but they're smaller than the discrepancy. And that's how you
know when you have an interesting scientific puzzle that you
think you have measured things well and yet you still
can't explain it. Things are not adding up. The error
is smaller than the size of the effect you're looking for.

Speaker 1 (33:47):
All right. So now we've accounted for fifty percent of
the quark matter in the universe. There's still fifty percent missing.
How are we looking for it?

Speaker 2 (33:53):
So we're using all sorts of clever techniques to look
for this stuff the whim. And this stuff is hard
to see because even though it could be pretty hot,
we're talking about like a million calvin right ten to six,
ten to seven calvin, it's also very very dilute, you know,
it's like one atom per cubic meter. It's like a
billionth of a billionth of the density of our atmosphere.

(34:15):
So this stuff is not very easy to see, especially
if it's very far away. And so we're looking for
a way to excite it. We're looking for something that's
going to pass through it and get interacted with it
in a characteristic way that can tell us about the
density of this plasma. And one really cool way is
to use another cosmic mystery, these things called fast radio bursts.
Something out there in the universe is generating these very

(34:38):
intense pulses of radio waves. Remember, radio waves are just
photons with very very long frequency. We call it radio waves.
If it's in a certain frequency regime, we call them
X rays, and another frequency regime, and visible light in another.
It's all just photons of different energies. But these very
very bright pulses of radio waves are created somewhere out
there in the universe, passing through all the matter between

(35:01):
us and them, And as we study them here on Earth,
we can look at the details of those radio waves
as a way to sort of like X ray, this whim,
this warm, hot intergalactic medium.

Speaker 1 (35:10):
So how do these bursts of radio waves tell us
about this plasma that might be hiding all of the
missing cord matter.

Speaker 2 (35:18):
Yeah, so you had the basic idea earlier when you're saying, like,
wooden photons interact with this whim. The protons, they're electrons,
they're charged particles, and you're absolutely right they do. But
you need the right kind of photon in order to
tell you what you need to know. As light passes
through matter, it slows down like the speed of light
through a vacuum. Is the famous speed that we all know,
but light passing through glass or through air will move

(35:40):
slower than light through a vacuum, and that effect actually
depends on the energy of the photon. So longer wavelengths
of light are slowed more than shorter wavelengths of light.
So if you start with the pulsive light of several
frequencies and then you measure the arrival time of that
light here on Earth, you can actually measure the density
of stuff between you and the pulse, because the higher

(36:03):
the density, the more the difference in the arrival times
between the long wavelengths and the short wavelengths.

Speaker 1 (36:08):
I see, But don't you need to know what that
bursts looked like before it went through the filter of
this plasma between galaxies? How do we know that if
these are of unknown origin?

Speaker 2 (36:18):
You're right, we do need to know something, But essentially
all we need to know is that they're all produced
at the same moment, or very very close to the
same time. We don't need to know something about the
spectrum because we're looking for it's just the difference in
arrival times. If you shoot a long wavelength and a
short wavelength photon at me at the same time, then
I can tell you the density of matter between us

(36:38):
by looking at the difference in the arrival times between
the short and the long wavelength photon, because the long
wave looking photon will be slowed down more by higher
density material. So I don't need to know anything else.
I just need to know that there's like a pulse
created and these two photons were made of basically the
same moment. And that's what these fast radio bursts do.
We don't know what's actually making them. That's a big mystery,

(37:00):
but we suspect that they're being made in a very
short amount of time, like a one millisecond pulse.

Speaker 1 (37:05):
But how do you know they weren't made at different times.

Speaker 2 (37:07):
Yeah, we're not exactly sure. That's an assumption. When they
arrive here on Earth, they're spread out over a few
seconds or sometimes tens of seconds. But because of the
enormous amount of energy overall, we suspect that it was
a very fast event, though we still don't understand it.

Speaker 1 (37:20):
I think I know what you're saying. You're saying like
there's a burst of radio waves, like a bright flash
of light that we see that was made out there
in the universe, and we measure that burst of light
when it gets here on Earth at different frequencies. You're saying, like,
the bursts at one frequency is going to arrive earlier
than the burst from another frequency, and that difference in

(37:40):
the arrival time tells you like, oh, there must have
been some quark matter in plasma form between us and
that burst that absorb or slowed down some of that
second frequency exactly.

Speaker 2 (37:53):
This effect is called dispersion, you know, wavelength dependent effect
on the speed of light essentially, and by measuring this
dip you can infer the density of the plasma between
you and the source. But you're right, we're making some
assumptions about the nature of the source. We're assuming, essentially
that the length of time over which those radio waves
were produced is negligible compared to the length of time

(38:14):
over which they arrive.

Speaker 1 (38:15):
You also have to know where that burst came from,
don't you.

Speaker 2 (38:18):
Yeah, you do. You have to know the direction. And
so we've been seeing these fast radio bursts over the
last few decades. They were discovered sort of accidentally. We
have a whole fun podcast episode about that, but only
recently have we been able to locate them, to tell
where in the sky they come from, and to do
that you need like larger instruments, or you need coordination
between various instruments so you can tell about their arrival

(38:40):
time at various parts on Earth. But in the last
couple of decades they've been able to do that and
gather enough information to estimate the mass of the WHIM
from these fast radio.

Speaker 1 (38:49):
Bursts, at least the part of that quark plasma that's
hiding that we can tell using this method.

Speaker 2 (38:56):
Yeah, exactly. And you always want to have like multiple
ways to measure things, especially if it's very uncertain, and
if you're talking about half of all the stuff in
the universe or the normal matter. So there actually is
a second, completely independent way to measure this WIM to
see where it is and how much stuff there is.
And this one is more sensitive to the electrons in

(39:17):
the wim. Remember we think the WIM is a plasma.
It's protons and its electrons, and those are separated, and
the electrons themselves can get like jazzed up by interacting
with the old cosmic microwave background light in a way
that some people can see and can use that to
estimate where the WIM is and how much there is.

Speaker 1 (39:35):
And so using these measurements what is our estimate of
orre all this missing quark matter up to.

Speaker 2 (39:41):
So it comes out pretty close to one hundred percent.
So the current idea is that this WIM fills in
the gap that when you add in the WHIM and
the neutral hydrogen between galaxies and then all the stuff
inside the galaxies, it all adds up to explain the
amount of baryonic matter we predicted from the CMB and
from Big Bang nucleosynthesis. So it all sort of like

(40:03):
clicks into place amazingly.

Speaker 1 (40:04):
So then we think we found all the missing matter.

Speaker 2 (40:07):
Then we have cracked the case of the missing matter
in the universe, which is like sort of exciting and
also sort of disappointing.

Speaker 1 (40:14):
So wait, using these radio burss, we think we've seen
all of the missing matter.

Speaker 2 (40:18):
Yeah, the current thinking is that this WHIM is that
missing piece, that fifty percent that we couldn't account for
after we figured out the neutral hydrogen component is probably
all the WHIM, which means that like half of all
the quarks in the universe are in the WHIM.

Speaker 1 (40:31):
Are in hot gas in between in the middle of nowhere.

Speaker 2 (40:34):
Basically, yeah, the universe is half hot gas.

Speaker 1 (40:38):
It's incredible sort of like the US. I guess, so
the population lives in the middle of nowhere.

Speaker 2 (40:44):
Yeah, exactly. And so if you want to like make
a ranked list of all the stuff that's out there
in the universe, it's mostly, you know, stuff that's very
susceptible to toilet humor. It's dark matter is a lot
of the universe, and then of the five percent that
makes up our kind of stuff, half of it is
hot gas floating out there in the universe between galaxies.

Speaker 1 (41:02):
Well, it's only toilet humor if you like, if your
head is in the toilet.

Speaker 2 (41:08):
Maybe it's good or humor then. But you know, it's
exciting to have these confirmation to be like, wow, we
do really understand what's going on out there in the universe.
These incredible calculations from the early universe that make these
predictions about how many baryons should be floating out there
billions of years later are kind of accurate. And we've
been able to like X ray and pinpoint the universe
using all these clever techniques to figure out where the

(41:30):
stuff actually is. And it tells us this amazing story
that galaxies are not the most important thing in the universe.
They're not even the most important part of the normal matter.
There are these massive halos of gas surrounding the galaxies
and then between the galaxies, So that's super exciting, but
it's also kind of a letdown because when you do
these kind of calculations, which you're hoping for is some

(41:52):
great new discovery. Right the way we discover dark matter
by finding a discrepancy in our calculations, this could have
been the discovery of something else, totally weird and new.

Speaker 1 (42:02):
Well, you're disappointed that you solve the problem. You wanted
more problems.

Speaker 2 (42:05):
Yes, I wanted more problems exactly.

Speaker 1 (42:07):
You want it more, more of a job.

Speaker 2 (42:11):
It would be fascinating, right, Like finding out that it's
the whim is cool, it makes sense, But it would
have been more exciting if it was some new kind
of matter, something else that we didn't expect, quarks forming
some new kind of stuff that we hadn't anticipated, or
maybe discovering something was wrong in our early universe calculations.
That would have been I think a bigger discovery because
we would have learned more about the universe.

Speaker 1 (42:33):
Well, maybe that's why this problem didn't get a lot
of press, because you guys sold it as like, Eh,
we found it. Whatever, it's not that exciting. Now you're
complaining that it doesn't get any uh.

Speaker 2 (42:43):
Press, Well, here we are trying to get it some
more attention. Right, So I'm out here trumpeting the case
of the missing matter and its whimsical solution.

Speaker 1 (42:50):
Well, I think maybe the other reason is that it's
not really a problem anymore, exactly.

Speaker 2 (42:55):
Yeah, Unfortunately we've mostly figured it out. I unfortunately or
unfortunately fortunately because it means our theories of physics are
mostly working and our techniques are clever and effective. Unfortunately,
because it means now we've got to move on to
something else.

Speaker 1 (43:07):
So maybe you just need to rename it. Right, It's
no longer the missing baryon problem is just the found
barian flat.

Speaker 2 (43:16):
Yeah, the once missing baryon. The Baryon's formerly known as
missing all right.

Speaker 1 (43:20):
Well, another interesting reminder that the universe keeps surprising us,
even in I guess not so surprising ways. It's surprising
that you can sort of make these models and figure
out where everything should be and where it needs to be.

Speaker 2 (43:34):
Yeah, asking questions in several different ways, trying to do
calculations from this and from that, piecing it all together
is a great way to figure out what's actually out
there in the universe, and sometimes actually leads you to
an answer.

Speaker 1 (43:46):
Well, I kind of wish we had read the last
chapter of this mystery. I had to save this a
lot of time here.

Speaker 2 (43:50):
This was decades of work and lots of careful energy,
and like lots of people's peachdtcs. You know, we're like
taking tiny steps in this direction. So you can summarize
it all in about five seconds, but you know, it
was a.

Speaker 1 (44:03):
Journey, and also it's kind of a still a work
in progress, I imagine. I mean, you have some measurements,
but you can always refine those or somebody might find
something that disproved those measurements.

Speaker 2 (44:13):
Right, yeah, precisely. Now we fold these things into our
models of galaxy formation. Because we have a better understanding
of the density and the temperature of this whim. We
can make sure that it describes the kinds of galaxies
that we see, the sizes of galaxies, the rate of
galaxy formation, how often galaxies merge. It all gets folded
into a more precise description of our universe, which we

(44:34):
hope will reveal more discrepancies and more surprises in the future.

Speaker 1 (44:39):
And more toilet humor inevitable. All right, well it's time
to flush. I guess we hope you enjoyed that. Thanks
for joining us, see you next time.

Speaker 2 (44:57):
Thanks for listening, and remember that. Daniel and Jorge Explaining
You Universe is a production of iHeartRadio. For more podcasts
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