What is the “missing baryon problem”? How do we know how many baryons there ought to be? What does the cosmic web have to do with it? What the heck is a baryon, anyway? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Sometimes it feels like we have no idea what's going on, as in zero. Totally nothing. We might as well be back in the stone ages for all the mysteries that remain in the universe, which I suppose isn't the worst thing. As long as there are mysteries of the cosmos, then astronomers can all keep working. You know, a little job security never hurts.
Right? But some days, it gets a little bit too overwhelming, just a little bit too much, especially when it comes to cosmology, the study of the whole entire universe. Okay. Fine. Look.
We we know some things. We know about the big bang. We know how structures form. We know about the cosmic microwave background. We, I'm sure we know some more things.
There are entire books about the subject, but those are the main ones. And perhaps the most frustrating thing of all is that we know what the universe is made of, but by and large, we don't know what those things are. The kind of matter that we know and love, electrons, protons, neutrons, you and me, stars and galaxies, cheese and crackers, etcetera, are such an overwhelmingly tiny portion of all the contents of the universe that if you could flip a switch and erase all of it from existence, yes, even the g's, it would barely register in the march of cosmic history. We, and more importantly, everything like us, simply don't matter. 5%.
That's it. 5%. The kind of matter that we are, which is known as baryonic matter, and baryon means heavy. And, honestly, it's a poor choice of words because heaviness has nothing to do with it. But once again, I'm not in charge of jargon, so we all just have to roll with it.
The kind of matter that we are take up no more than 5% of all the mass and energy in the entire universe. And what's worse, we don't know where most of the baryons are. We don't know where most of the normal matter is. I'm serious. Of all the baryonic or normal matter out there, only a small and we can get into a debate of whether even though it's a small fraction, we still call it normal.
Whatever. There's a human bias going on here, and we're not gonna get in the way of it. We are made of baryons, and so for us, that's pretty normal, and so we'll call it normal matter. Of all the normal or baryonic matter out there, only a small fraction of that lights up in the form of stars and galaxies. And since we have a hard time seeing anything that isn't a star or a galaxy, we have a hard time seeing the matter that doesn't light up as stars and galaxies.
All told, after all our surveys, we've been able to account for about half of all the expected baryonic or normal matter out there. So that's around 2%. Let that sink in for a bit. After all this work, after decades of cosmology, after giant telescopes and orbiting observatories scanning the heavens, making maps of the cosmos, we've been able to map out, locate, and understand 2% of all the matter and energy in the universe. We simply don't know where most of the stuff is.
And, yes, we check the couch cushions. It's not there. But this immediately raises a very important question. If we can't see half of the normal matter, half of the baryons, How do we actually know we're missing any? Maybe we're just getting the numbers wrong.
I mean, did someone forget to carry the two in one of their calculations? And maybe there is no missing matter. Maybe it's just a misunderstanding. It's a good question, so I'm glad I asked. We know how many baryons are in the universe based on two things.
One, there is something called big bang nucleosynthesis. I've done episodes on this before, but I'll give you a quick recap. A long time ago, our universe was a lot smaller, a lot hotter, and a lot denser. At one point, it was so hot and so dense that it was a completely different state of matter. Temperatures were in the millions of degrees Kelvin and even billions of degrees Kelvin.
Densities were I don't have a number in front of me. High. Let's just go with high. Very, very, very high. And the universe was a different place.
All the matter, all the normal matter, but also everything else, but all the normal matter was squished down into a volume so tightly that it was too hot for atoms to form. And in fact, it was too hot for atomic nuclei to form. And in fact, it was too hot for the fundamental protons and neutrons themselves to form. It was just too hot. A proton or a neutron, which is what is inside of a nucleus of an atom, is itself made of smaller things, smaller things that we call quarks.
There are six quarks in the universe or six kinds of quarks in the universe. Two of them, the up and down quarks, we're not gonna get into the names right now, make up protons and neutrons, but they're like bags of quarks. Right? They're they're just like little condensed little globules of quarks. You can imagine if you start slamming protons and neutrons together against each other with great energy, they will rip apart and they will spill out their guts, and it'll just be a bunch of quarks running around freely.
That is, of course, a very cartoon sketch. The strong nuclear force, which governs all this, is exceedingly complex, but that is not today's episode. I'd I've done that before, talking about the strong nuclear force. But you can imagine things get so hot and dense that protons and neutrons are so squished together that they can't be protons and neutrons anymore. They just have to be their guts, their insides.
But then as the universe cools off, the quarks get together and form protons and neutrons, and then the protons and neutrons can go on their merry way. This is called the big bang nucleosynthesis because this is the era in the universe that created the first protons and neutrons and started getting them together. This period happened when the universe was about a dozen years old. Yeah. Between ten and twenty minutes old compared to the 13,800,000,000 years of its present age.
We're talking ten to twenty minutes old. That's when all of the hydrogen in the universe was manufactured. That's when a good chunk of helium in the universe was first manufactured, and then a little bit of lithium was also manufactured. But honestly, who cares about lithium? What does this have to do with baryons and normal matter and the percentage of the universe?
Well, we understand. Believe it or not, we can actually write down with pencil and paper what the universe was like when it was fifteen minutes old. And we know this because we understand nuclear physics. We kinda sorta understand how quarks glue together to form protons and neutrons. We kinda sorta understand how protons and neutrons slam into each other to form new elements.
We kind of know how nuclear power and nuclear bombs work, and we can use that knowledge, the exact same math, because physics is universal. Thanks, Newton, for figuring that out three hundred years ago. We can figure out, we can calculate what the universe was like with just the power of mathematics, which is beyond amazing to even comprehend, but that's that's a real thing. It's just nuclear physics. This is kindergarten stuff.
Alright? And maybe we went to different kindergartens, but this is this is easy. This is easy physics. They were able to calculate this in the nineteen fifties and sixties. They were able to work out this idea called big bang nucleosynthesis.
And what you have in the early moments of the universe when it's only ten to twenty minutes old is an expanding universe filled with really hot stuff that is starting to chill out. At the beginning, it's too hot. It's too dense for protons and neutrons to form. And at the end of the era, things are too spread out for nuclear reactions to take place. The neutrons and protons just don't find each other.
So there's a very limited window of where these reactions can take place. And because there's a very limited window, this tells us what the universe is made of. It can tell us how much hydrogen is out there, how much helium is out there, how much lithium is out there. It can tell us how much no matter there is compared to, say, radiation, which is another important component in this very early universe, but isn't so important nowadays. It can make predictions, and big bang nucleosynthesis makes predictions.
The theory makes predictions that agree with observations. We predict that the universe is about three quarters hydrogen, one quarter helium, and then other. And what do we observe with what we can observe? Three quarters hydrogen, one quarter helium, and a little bit of other. The exact same calculations that make those predictions for the, abundances of elements throughout the universe predict how much total normal matter there has to be in the universe, and it's around 5% of the total mass and energy of the entire universe.
We have a firm number based on mathematics that we trust and based on observations that we've been able to make. So that's one thing. The other thing is the cosmic microwave background. This happened way later in the history of the universe when it was positively ancient, when it was 380,000 years old, Still, you know, fresh out of the womb based on our current reckoning of 13,800,000,000 years. But back then, compared to this big bang nucleosynthesis era that is so distantly far in the future, this cosmic microwave background, that it might as well be just it's incomprehensibly far in the future from the perspective of Big Bang Nucleosynthesis.
But it was, again, a very important transition period for the universe. This is where the first atoms formed. So prior to this, it was small. It was hot. It was dense.
We had nuclear. We had protons and neutrons running around. We also had electrons. We also had a lot of radiation, and things were too hot for things to settle down. They couldn't get their act together.
But finally, after three hundred eighty thousand years, the universe cooled off. Everything was a little bit more easygoing. Electrons were able to join atoms. The radiation could roam free, and life proceeded as normal in a very comfortable, very chilled out retirement. The light that was released when the universe was 380,000 years old persisted.
It just soaked the universe. It's just there hanging out, flying all over the place. Some of it lasted to the present day. Some of it, we capture in our microwave antenna. And some of it, we can analyze and study and take pictures of what the universe was like so long ago when it was only 380,000 years old.
And what we see are a bunch of splotches. It was kind of ugly time for the universe. It wasn't very pretty like it is now. Had a the universe needed a little bit more time to grow into itself. This is a very awkward phase.
This is the puberty of the universe. There's just splotches everywhere. So there are hot spots. There are cold spots. There are more dense spots.
There are less dense spots. But the properties of these blotches, like how big they are, how much they're separated on the sky, depends on how much stuff there is in the universe because this is a time when radiation, when light is interacting with normal matter in a very intense way and in a way that would never be able to do so ever since. This is a period where radiation is knocking into protons and electrons and neutrons and all the other ons. If you change the amount of normal matter in the universe, say it's not 5%, maybe you make it 10%, that's gonna change these reactions. These reactions.
That's gonna change what the cosmic microwave background looks like. That's gonna change when it happens. That's gonna change where the dense spots and the cold spots, how big they are. It's gonna change everything. And so we're able to use the cosmic microwave background, this picture of what the universe looked like when it was only 380,000 years old to give us a census, to map out, to tell us how much normal matter is in the universe.
And like I said, we get 5%, the exact same number we get from nucleosynthesis. So we think we're onto something here, folks. We think we might have figured something out. We may have figured out that about 5% of all the matter and energy in the universe is made up of baryons. We've got this really solid number.
We're feeling really good about this 5%. We feel we feel confident. Our heads are held high, and then we go out looking for them, which is kind of hard. One way to look for baryons is to look for all the stars and galaxies. Just, you know, surveys, map out galaxy after galaxy, measure the total amount of light, compare that to how much light a typical star gives off.
Sounds straightforward. It's, of course, very complicated because there's all sorts of different stars out there and all sorts of different ages and different kinds of light outputs, but we can at least get a relatively firm number on it because we're astronomers, and we've had a few decades to figure this out. So we can just count up all the hot glowy stuff. But a whole bunch of normal matter is dim. I mean, Earth isn't exactly aglow with light that can be seen from another galaxy, So you're gonna miss all the Earths.
You're gonna miss all the planets. You're gonna miss all the dim stuff. There are some gas clouds that are so thin that they don't really light up. You you don't glow enough for us to see it with our surveys. So instead, we can use a different trick.
If we're looking through a gas cloud to say a distant star or a distant galaxy, then some of that light from that distant galaxy as it passes through the gas cloud will get absorbed, and we can measure that absorption. So even if we can't see the gas cloud itself, we can see how it affects distant background light. That's pretty handy. And we can measure the amount of stuff that's in random gas clouds. And then all the really small stuff, like black holes and planets and, you know, dim stars that are barely glowing, we can use gravitational effects.
You know, even these little things will bend the path of light a little bit. So we can look out in the sky and do some survey and stare at a bunch of stars. And if we see little bleeps and bloops and little flicks and flashes in the light, then that's caused by stuff passing in front of our field of view or along the line of sight to these distant stars and galaxies. And so we can use surveys like that to to capture stuff. So we've got a bunch of ways.
We've got a bunch of ways to survey the universe, and it only adds up to half of what we expect to be out there. I should note that this is different than the dark matter issue. The dark matter issue is that there aren't enough baryons total to explain observations like the rotation speed of galaxies. That's its own thing, which I've talked about before. Dark matter is its own thing.
The problem I'm talking about today is that we can't find where all the expected baryons are. We expect there based on really solid observations, around 5% of the universe to be baryons. And yet other observations, the observations of going out and hunting for them, we're only getting, like, two or 3%. We're missing them, and this is called the missing baryon problem. So where are they?
Where do we think they are? They gotta be somewhere. They're laying around, obviously, because we really, really trust our big bang nucleosynthesis, and we really trust our cosmic microwave background. But where the heck did all the baryons go? Did someone eat them?
I don't know. They definitely aren't in galaxies. We checked. K? Because we mapped a bunch of galaxies.
They're they're not there. They're not glowing as stars. But there's a whole lot of universe out there besides the galaxies. There's something called the cosmic web. The cosmic web is the largest structure found in nature.
The cosmic web is made of galaxies. The cosmic web is the formation of galaxies in our universe. Galaxies appear in dense knots, dense clumps with long strings of galaxies between them. It looks like, you know, a spider web, hence the name cosmic web. If you zoom out to the very largest scales, this is what you see.
This is how galaxies are arranged in our universe. We suspect that the missing baryons, wherever they are, simply aren't dense enough to ignite stars. Otherwise, we would have seen them in galaxies. That's where stars like to be after all. So we suspect these missing baryons are just floating around.
And a good candidate for a place for them to be floating around is the filaments. Are these places between the galaxies or these places between the dense knots of clusters of galaxies? If you imagine the cosmic web as a spider web, The spider web is a long a bunch of long, thin filaments, and then they connect together at certain knots. The knots are what we call clusters. These are home to, like, a thousand galaxies or more.
They're really massive and really fun. The empty regions in between the cosmic web or in between a spider web are called the voids. Probably not gonna find a lot of baryons there because, you know, void. All that's left are the filaments. These, like, highways of dust and gas and galaxies stringing between the great cities that are the clusters.
But how do we see them? How do we spot the baryons in these filaments? It seems a little bit challenging because they're not glowing very brightly. Otherwise, we would have seen them by now. Thankfully, we have a technique.
And are you ready for the name of this technique? It's called the Tsarnaev Zeldovich effect. Tsarnaev Zeldovich. Two Russian scientists, two awesome names, one amazing technique. Tsarnaev Zeldovich.
Here's this, Adam. First, you need a background source of light. You need it to be everywhere. You need it to be relatively bright. You need it easy to spot.
You you just need some background light, say, the cosmic microwave background. Well, that that's pretty handy. The cosmic microwave background is behind everything else we see in the universe, so there you go. Next, you need a cloud of warm and or hot gas. Not too cold and dense because we can see the dense clouds because they absorb light really well.
But it can't be too hot and dense because that's what we call a star. It just kinda has to be loose and warmish. Okay? Something like the gas in the filaments. In the spaces between the clusters of galaxies, there's a bunch of gas out there, presumably.
It's really too hot to be dense enough to absorb anything, but it's also too cold to, like, glow on its own. And so it's somewhere in the middle ground, which is the hardest thing to spot. But if we shine that background light through the hot gas, that hot gas will literally smack on the photons of the light, making them more energetic. If that light that is passing through, say this light from the cosmic microwave background, is passing through the hot gas and is relatively low energy, like, say, I don't know microwaves, then that hot gas will hit it. Just pack, smack, smack, smack.
And every little smack, it gets a little bit more energetic. It's like it's like entering a room and getting a bunch of high fives. You just leave in a better mood. You're more interject like, heck yeah. I can do it.
I dare you to be in a bad mood, walk in a room, get a dozen high fives, and not feel better. So the Cosmic Microwave Background Light passes through the filaments, and it feels better. If we could look through this cloud of hot gas at the cosmic microwave background, the cosmic microwave background in that direction will appear a little bit more energetic in that direction. And since we know that original temperature of the cosmic microwave background here's a hint. It's cold.
It's like three Kelvin, three degrees above absolute zero. We can use the difference in temperature to figure out the density and size of the gas cloud that made this that piece of the cosmic microwave background feel a little bit better. We do this all the time with clusters of galaxies because there are these there's clusters of galaxies in addition to galaxies, hence the name, also have a bunch of hot gas inside of them. These light up in front of the cosmic microwave background. We use the Sunnier Zel'dovich effect very well here.
Doesn't really work well for the filaments, so why the heck am I talking about this? It doesn't work very well for the filament filaments, the spaces between the clusters, because the filaments aren't very dense, and they're not incredibly hot. This is a very weak effect. It's hard to spot. But you know the phrase, insanity is doing the same thing over and over again and expecting different results?
That basically sums up all of astronomy. This technique of the Sienier Zolovich effect of looking through the filaments to see how the gas in the filament heats up the cosmic microwave background light doesn't really work on any one filament. But if we take one observations of one filament and then another and add them together and then add a third and a fourth and a fifth and a sixth and a seventh and a hundredth and a thousandth, we might be able to build up enough of a signal that we can actually detect this. Astronomers have done this. The result is that it's possible that the missing baryons do indeed live in the filaments, But we're not exactly 100% sure.
This is a relatively new measurement. It's slightly sketchy. Not exactly a lot of confidence here, but it does seem to point in the right direction at least. The missing baryons are not inside of galaxies. They are not inside of stars.
They are not inside of clusters of galaxies. They're definitely not inside the voids. The only place left is the filaments, these lines, these highways connecting the clusters of galaxies. And based on this Sun Yayev Zel'dovich effect, and I will personally give you a dollar if you say that out loud in public. That's not really true, but I think you should still try to say sunnayev zeldovich effect out loud in public.
Or, like, if you need to make up an excuse at work for, like, how you did something, just say, oh, wow. Wow. How do you how do you do that? And you just say, oh, I applied the Sunny of Zoldovich effect. And just keep on rolling.
Guarantee no one's gonna challenge that. So, anyway, we use the Sunny of Zoldovich effect, and we think we may have at least found some of the missing baryons, but we're not exactly sure. But, hey. Like I said, a little job security never hurts. Thank you so much for listening, and thank you Rachel k for the question that led to today's episode.
And of course, thanks to my top Patreon contributors this month. Oh, I didn't do a Patreon joke in the middle of the episode. Bet you're awaiting all episode for it, and it didn't come. Wow. Wow.
Like I said, a little job scare never hurts. The universe is full of mysteries. Thank you. Patreon.com/pmsudder. Thank you.
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