Daniel and Jorge Explain the UniverseDaniel and Jorge talk about the effort to track down all the quarks in the Universe.
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Hey Daniel, I think we've been using too much toilet.
Humor you mean all those obvious dark matter jokes we make.
Yeah, you know, I'm sure it makes all the nine year old to giggle and the audience. But I don't think we want to undercut our educational message.
All right, that's a good point. Let's try that, all right.
Well, so what are we talking about today?
Today? We're talking about hot gas.
Well that didn't last very long. Kai am poorham Mack, cartoonist and the author of Oliver's Great Big Universe.
Hi.
I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm often full of hot air.
Are an all physicists full of hot air?
I'm just talking about the weather here in southern California. I don't know what you mean.
What do you mean the weather is inside of it?
I'm breathing in the atmosphere literally.
I guess if you were breathing out cold there, that would be bad news, because we all know physicists aren't very cool.
I'm trying to make physics hot, is what I'm doing.
But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio 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.
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.
I thought we're avoiding the toilet jokes.
Well, that was in the joke. I mean, there is physics in toilets, isn't there.
That's true. You once challenged our listeners to record their toilet spinning to see if they flush differently in Australia.
Oh did they do it?
I haven't gotten any did yet, so we're still waiting for the results of those experiments. But that is toilet science.
Yeah, there you go.
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?
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?
And where have they been dropping all their trash?
Wait?
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?
I was just wondering, you know, so we could say, hi, not find their keys.
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?
Although if they leave their keys to their spaceship line around, I'm not real going to return that one. No one's staying with me.
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. 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 we're made out of, is still something of a mystery.
Wait what so then, how do we know how much of it there is out there?
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.
I see, we know how much there should be, but we just haven't found it. Is that what you're saying?
That's basically it episode done?
All right, Well, thanks you for joining us. I can go and do something else now.
Well, maybe the aliens have stolen all that missing matter.
WHOA, that's a pretty serious allegation. I mean, you're just, you know, I computing the the goodwill of the aliens and their legality.
Well, maybe instead of making a big mess, they've been a little bit too aggressive about cleaning up after themselves.
Maybe it's the physicist hmmm who stole all the matter on the planet Earth with the wrench.
In the end, it's not about understanding the universe. It's about figuring out who to blame for it.
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 todayend the podcast, we'll be asking the question where 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?
I mean, you're making it sound like an I. Gota Christie novel, like the Case of the Missing Matter, Like we put all this hydroieden over here and we came back and it was gone.
Yeah. Yeah, there was a blackout, the lights went out. There some screams, and suddenly there was a missing matter. We're all trapped on an island with a limited number of suspects.
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.
Now you're saying that this is actually called, or it's called in physics, the missing baryon problem.
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 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 barrier problem.
M sounds very mysterious, and you also kind of make it sound like it's somebody else's problem.
Hey, it's all about pre assignment to blame, right.
Right, Yeah, Like if you say like, yeah, it's a problem, I think you're basically saying it's somebody else's problem.
Mistakes were made, right.
That's right? Yeah, things went missing.
Grand Funding misallocated.
I don't know, 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.
So thanks very much to everybody who participates in this segment of the podcast. We would love to hear your voice among the coors of listeners, so please don't be shy. Write to me too. Questions at Danielandjorge dot com.
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?
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 existed 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.
I don't know what the missing baryon is, but I hope someone finds it.
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.
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's holding something like an atom together.
All right, or interviews here didn't give us a lot of clues.
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 a famous problem in physics, but the missing baryon problem is sort of like its second cousin that doesn't get top building.
It sounds like maybe it's a branding problem, you know, like dark matter. Where's 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.
What they should have called it the dark baryons or something.
M yeah, or some other name, right, shining matter, super matter.
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.
You might think that physicists name things very confusingly.
The Missing Physics name committee.
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 bare matter?
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 on 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 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 cork, and that gives you a color neutral object like a proton or a neutron that has no overall strong force.
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.
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 could put three strange quarks together other or you can make an up and down in 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. 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.
So today we're talking about which kind specifically all of them or mostly protons and neutrons.
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 neutrons, or in helium or in hydrogen. We just want to know how much of our kind of matter, quark based matter, is there, and how much of the other stuff is there, and can we figure out where all the quarks went.
So you're saying baryon matter on which kind of matter it settles in.
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?
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?
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.
HM.
So I think you're saying that we could have just titled the episode where are all the Missing quarks?
Yeah? Where are all the missing quarks? Exactly? But in physics it's called the missing baryon problem, and it makes up the kind 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 stuff made out of quarks. That's the thing we're still trying to understand after all these years.
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?
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 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.
Wait, so there's electrons missing too well.
Electrons are part of the five percent of the universe made out of normal matter, basically quarks and leptons.
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.
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 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 detorium there is in the universe. Both of those are very sensitive to the quark density in the early universe, and so can tell us how many quarks there should be.
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.
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.
Well, maybe break it down for people. How does the ratio of hydrogen and helium tells how many quarts the universe started with?
So in the very early universe, things were super duper dense and hot, right, the basic story of the universe is things were very very hot and dense. We don't know how we got to that state, that's sort of 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 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. 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 the density of quarks. Like you have more quarks floating 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.
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?
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, 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 atoms, and you'll see this little peak of some water that's a little heavier.
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?
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 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 deuterium inside hydrogen.
All right, So then that tells us how much quark matter there should be in the universe, and how much is that amount?
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 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.
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 hat 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.
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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. Pricimately 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.
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. We have other measurements from the cosmic microwave background radiation which come from much later in the universe, like three hundred 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 more precise even than the Big Bang nucleosynthesis. And it agrees it's about five percent of the energy density should be baryons.
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?
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. We would be surprised and confused if those two numbers didn't agree with each other.
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 percent percent of the universe is missing.
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 of 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. Add up 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 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, how much is there? And how does it add up? And together it comes to, you know, about fifteen twenty percent of what we.
Expect, fifteen to twenty percent of the five percent that we think should be there.
Hm, 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 quarks as we expect.
I wonder if then you just need to lure your expectations Daniels, Like, maybe your expectation is wrong. Maybe that's the real problem.
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 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 like go out there and look for these missing burials. We're pretty sure they exist, we just hadn't seen them yet.
Well, well, just so you know, that is an option in life. You can just lower your expectations and then you can take a vacation.
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.
All right, Well, let's keep going then. So, there is a certain amount of quark matter in the universe we think should be there, but we can't seem to account for it like we do some accounting of what we can see and what we think is there, and it's not enough, so where could it be and how are we going to find it?
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, this 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 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 know, 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 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.
Galaxies because they can't be in the galaxies. Because you think you can see everything in a galaxy.
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 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.
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.
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.
Yet because I guess the stuff that does clump together is kind of the fancy stuff that everyone pays attention to, right, the stars and the planets.
Yeah, it's got the most glitter and glam.
Right, So then now is that confirmed? Like if you look for things in between galaxies, do you find all of this missing quark matter?
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 batter 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, 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 into 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 the huge space between galaxies millions and millions of light years, and so seeing these things is tricky. One way that we have seen them though, is using quasars.
What do you mean, how do those help us see the hydrogen between galaxies?
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 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 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 there's 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 and the source of the light.
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 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. And so if you look at the 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. You can use these quasars to sort of like X ray the universe and tell you where the hydrogen is.
WHOA, but how often do we get signals like this? How many quasars are pointing directly at.
Its 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 quoasars were very active. It's a whole other mystery like why did 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's sort of like shine these lights through the universe, and we'd like to see more of them. It's tricky, but there's enough that we could have an estimate for how much hydrogen gas there is in these filaments between galaxies.
And so these quasars basically like illuminate the hidden matter between galaxies.
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 it's just stars and galaxies and all that stuff gives you like fifteen percent. Add in the neutral hydrogen between galaxies and you're up to about fifty that we can account for that, we can account for exactly.
What you're saying. It's not missing. Then that we know where it is.
Well, even this very clever technique only brings us to fifty percent. The other half is still not explained.
Mmm, so only half of that five percent is missing.
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 recently, we've had no explanation for the other fifty percent. That part was still missing.
Could it be some other kinds of gases in between galaxies.
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 hydrogen 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 of protons and a bunch of electrons that are too hot settle down into a hygrogen out. They're like flying around free and they wouldn't interact with the quasars 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 whim M.
Interesting acronym there. So you're saying that light doesn't interact with quark matter unless there's an electron attached to it, and that's because light only interacts with electrons.
Light will interact with any charge 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.
Why doesn't it let us see the protons?
Well, what happens when the light 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.
So we wouldn't see it in the signature from the quasars.
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 doesn't create this like obvious signature in the quasar beam. We need another method to see these protons and electrons.
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 of the universe. But first, let's take another quick break.
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All right, we are slowly peezing together this problem, this missing matter in the universe. Apparently there's a lot of cork matter that we think should be there, but it's not. Although I feel like we've already accounted for fifty percent of it, we started with only being able to count fifteen percent of it. Up to fifty percent of it.
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.
The universe, don't Some of these measurements have like a plus or minus fifty percent uncertainty or error bar on them.
Anyway, I guess that's one way to resolve the mystery. Just be like, well, let's just inflate the aeror and it's no longer a mystery.
There you go.
Yeah, there are big uncertainties on some of these measurements, but they're smaller than the discrepancy. 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.
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?
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 kelvin right ten to the sixth to the 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. 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 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 in 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 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.
So how do these bursts of radio waves tell us about this plasma that might be hiding all of the missing cord matter.
Yeah, so you had the basic idea earlier when you're saying, like, woulden photons interact with this whim? Their 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 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 pulse of 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 the density, the more the difference in the arrival times between the long wavelengths and the short wavelengths.
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.
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 by looking at the difference in the arrival times between the short and the long wavelength photon, because the long waveleonging 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 still, but we suspect that they're being made in a very short amount of time, like a one millisecond part.
But how do you know they weren't made at different times.
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.
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 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 slow down some of that second frequency exactly.
This effect is called dispersion, you know, wavelength dependent effect on the speed of light essentially, and by measuring this dispersion 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 over which they arrive.
You also have to know where that burst came from, don't you.
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 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.
Bursts, at least the part of that quark plasma that's hiding that we can tell using this method.
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 this 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 the wim. Remember we think the WIM is a plasma. It's protons and it's 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.
And so using these measurements what is our estimate of r all this missing quark matter up to.
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 clicks into place. Amazing.
So then we think we found all the missing matter.
Then we have cracked the case of the missing matter in the universe, which is like sort of exciting and also sort of disappointing.
So wait, using these radio burths, we think we seem all of the missing matter.
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.
Are in hot gas in between in the middle of nowhere.
Basically, yeah, the universe is half hot gas.
It's incredible, sort of like the US. I guess, so the population lives in the middle of nowhere.
Yeah, exactly. And so if you want to 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.
Well, it's only toilet humor if you're like, if your head is in the toilet.
Maybe it's gout 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 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 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.
Well, you're disappointed that you solve the problem. You wanted more problems, Yes.
I wanted more problems.
Exactly, wanted more more of a job.
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.
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, and now you're complaining that it doesn't get an uh press.
Well, here we are trying to get some more attention, right, So I'm out here trumpeting the case of the missing matter and its whimsical solution.
Well, I think maybe the other reason is that it's not really a problem anymore, exactly.
Yeah, Unfortunately we've mostly figured it out, unfortunately or unfortunately fortunately because it means our theories of physics are mostly working and our techniques are clever and effective on virtually because it means now we've got to move on to something else.
So maybe you just need to rename it, right, It's no longer the missing baryon problem is just the found barian flash.
Yeah, the once missing baryon. The baryon's formerly known as missing all right.
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.
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.
Well, I kind of wish we had read the last chapter of this mystery. I had to save this a lot of time here.
This was decades of work and lots of careful energy and like lots of people's peachdpcs. You know, we're like taking tiny steps in this direction. So yeah, you can summarize it all in about five seconds, but you know, it was a.
Journey, and also it's kind of a still a work in progress, I imagine. I mean, you have some measurements that you can always refine those or somebody might find something that disproved those measurements.
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. You 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 hope will reveal more discrepancies and more surprises in the future, 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.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from Heart Radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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