Half the universe was missing... until now

This episode was sponsored by KiwiCo More about them at the end of the show Until recently,

half

the

universe

was

missing

or hidden or just... undetected. And no, I'm not talking about dark matter or dark energy, which make up 27 and 68 percent of our

universe

, respectively. No, I'm talking about ordinary, normal matter that makes you and me, planets, stars and nebulae, and basically everything you can see. And since most of this matter is made of protons and neutrons, which are forms of baryons, this is known as the

missing

baryon problem. We expect the universe to be made up of 5 percent baryonic matter.

But when we searched, we only found 2.5 percent. Now, the first question you're probably asking yourself is: "Why should we expect the universe to have 5 percent ordinary baryonic matter in the first place?" The answer is because, with that density, we can explain the relative abundances of different elements that we observe in the universe. Specifically, the ratio of deuterium, hydrogen and helium. In the beginning, right after the Big Bang, there were all these neutrons and protons buzzing around. It was incredibly hot and there was tons of radiation. The universe was dominated by radiation. But, as the universe expanded, it cooled to the point where protons and neutrons could begin to fuse.

A particularly stable nucleus to form would be helium-4, made up of two neutrons and two protons. The problem was that, to form helium-4, you first had to form deuterium, a proton and a neutron. And this is a less stable core. And as quickly as it formed, it would shatter. But, about 10 seconds after the big bang, the universe had cooled enough that deuterium could form. And as soon as it did, it would quickly fuse into helium. The speed at which this happened depended on the density of matter in the early universe. The higher the density, the faster this fusion could occur.

Then, 20 minutes after the Big Bang, the temperature had dropped enough that fusion could no longer occur. So at this point, the elemental abundances were locked in. Like a snapshot of this moment. It was 75% hydrogen and 25% helium, by mass. Which is basically what we observe in the universe today. Of the hydrogen nuclei, 26 out of every million were deuterium. The surprising thing about deuterium is that it is stable: it does not decay. And there are no known processes that can produce it in significant quantities since the Big Bang. And that means that virtually all of the deuterium in the current universe, including one in every 6,000 hydrogen atoms in tap water, was not created in the stars but in the first 20 minutes after the Big Bang.

When we look deep into space, the oldest light we can see is Cosmic Microwave Background Radiation. The glow of the Big Bang that travels freely through the universe from about 400,000 years after the Big Bang. And then we can literally count those photons and calculate the radiation density immediately after the Big Bang. And using the value of 26 deuterium nuclei per million hydrogen nuclei, well, we can calculate the ratio of baryonic matter to photons, and that's how we calculate that there should be about five percent baryonic matter in the universe. So in the late 1990s scientists started looking for all this baryonic matter.

It was a kind of census. They added up all the planets, stars, black holes, galaxies, dust clouds, gas, basically everything that can be seen or inferred to exist with a telescope. And what they discovered is that everything that I normally think of as the real matter in our universe only represents a mere 20 percent of all the baryonic matter. So where is the rest? Well, not all ordinary matter shines brightly or is illuminated by nearby stars. It is not dark matter, but ordinary matter that is simply found in the dark. So if you want to find those baryons, well, one way is to use a backlight, a bright light source very far away, and that also means in the early universe.

And quasars are the perfect backlight. Its luminosity can be thousands of times greater than that of entire galaxies. The light comes from the accretion disk of a supermassive black hole at the center of a primitive galaxy as it gobbles up all this matter. And because it is so distant, the light we receive from quasars is strongly redshifted. For example, the light emitted when a hydrogen atom transitions from its first excited state to its ground state, the Lyman-alpha transition, produces ultraviolet light of about 121.6 nanometers in a laboratory. But from a quasar it can be observed as a peak in its spectrum at more than 560 nanometers: that is, yellow light.

The fascinating thing is that if you look to the left of this peak you will see many small drops. These are absorption lines created by neutral hydrogen atoms that lie along our line of sight to the quasar. When the light from the quasar reaches the neutral hydrogen, photons are absorbed that can excite the electrons from the ground state to the first excited state. This is the same Lyman-alpha transition, but because these patches of hydrogen gas are closer to us, they are less redshifted, so the notches they make in the spectrum have shorter and shorter wavelengths the closer you are. the gas from us.

This has been described as the Lyman-alpha forest. It is like a one-dimensional map that shows us where and how much neutral hydrogen gas is found along the line that connects us to the quasar. Adding all that neutral hydrogen gas to our baryon budget brings us to almost 50 percent. So where is the other

half

of the baryons? Well, computer simulations of the entire universe suggested that they are out there right between the galaxies in these sheets or filaments and they are very spread out: only one to ten particles per cubic meter. Additionally, these particles are ionized so they do not absorb light like neutral hydrogen gas.

And they are in a temperature range between about 100,000 and 10 million Kelvin, a range that astronomers like to refer to as warm-hot, so this is known as the warm-hot intergalactic medium, or WHIM for short. But finding WHIMs has been a real challenge because they are ionized, due to their temperature, they only emit or absorb high-energy UV rays or low-energy X-rays. Now, some people have used very sophisticated techniques to try to find the WHIM, but recently a natural physical phenomenon allowed us to find all the missing baryons. Let's find out how. First we need to talk about lightning and I promise this is related.

Well, did you know that it is possible to detect lightning from the other side of the earth? This is because lightning produces a flash of electromagnetic radiation in all parts of the spectrum. I mean, we see white light, but broad spectrum radio waves are also released and if you were close, you could detect them as a pulse. But very low frequency radio waves can actually travel up and out of the atmosphere and are guided along the Earth's magnetic field lines to several radii from the Earth and then return back down where they can be detected. in the other hemisphere.

Except that if they are detected there, they do not appear as a single pulse, but spread out like a hiss. Now, if you play these radio waves through a speaker, we can actually hear them, so listen to this. Do you hear that descending tone that sounds like a sci-fi laser gun? Yes, that is lightning to the other side of the earth. So what is going on here? Well, as radio waves travel through the Earth's magnetosphere they encounter free electrons, which slows them down and even more so for lower frequency waves: this is dispersion. Just as a prism separates white light into its component colors, the plasma in the magnetosphere separates radio waves into its component frequencies: low frequencies slow down more than high frequencies, so what started as a pulse ends as a hiss.

And the amount of scattering indicates how many free electrons that radio wave had to pass through to reach the detector. Now let's imagine that we could do something very similar to find all the ionized baryons in the universe. All we would need is a bright flash of radio waves somewhere in the distant universe, and as if on cue, in 2007 astronomers found the first fast radio burst, which is exactly what it sounds like: a very short pulse. duration of intense radio waves. And it came from the deep universe, from other galaxies. Now, these pulses can be incredibly powerful, I'm talking billions or trillions of times more powerful than the sun, but they last on the order of a millisecond.

We don't really know what creates them, although some people suspect they are magnetars or neutron stars or some kind of collision between these very powerful massive objects like black holes and neutron stars. But for our purposes all we need to know is that these flashes exist and that we can use them to observe their scattering and determine how many ionized baryons are between us and the source. And this is exactly what a recent paper in Nature did: they plotted the dispersion measurement of several of these fast radio bursts versus the redshift of their host galaxy. And what they discovered was, sure enough, the further away these fast radio bursts were, the more scattered their signal was when it reached Earth.

And in fact, using their measurements, they were able to estimate the total baryon matter that exists, and that includes all the ionized particles in the WHIM, and they found that it was five percent. They found the missing baryons. About 50 percent of them are in that warm, hot intergalactic medium, validating what we'd been thinking all along. You know, what surprised me in making this video was realizing how little ordinary matter from the Big Bang ended up in things like stars and galaxies, which I normally think of as the stuff of the universe. No, that's only 10 or 20 percent of all baryonic matter.

So it turns out that the formation of these interesting structures is a really inefficient process. But this discovery is another triumph for science. Those computer simulations performed decades ago turned out to be largely correct, so everyone involved should be congratulated. But this also highlights for me the difference between scientists and non-scientists. I feel like non-scientists like to be right, they like when things turn out the way they expected, but scientists, on the other hand, want things to not turn out the way they expected because that's how we get clues about what's new. The physics is still to be discovered.

I guess for now we'll have to be content with being right. Hey, this video was sponsored by kiwiko, which I'm really excited about because I've been using their monthly boxes since the beginning of the year with my four year old son, he received it as a Christmas gift and I have to say that they have been a great resource for learning at home. Each box has cool hands-on projects that are fun to do and expose kids to STEAM concepts: that's science, technology, engineering, art, and math. Now each box includes all the necessary supplies, meaning there are no extra trips to the store and Kiwico offers eight subscription lines. for different age groups and themes, they sent me some of the older age group boxes to try and I had a lot of fun doing this... this is a planetarium and something you may not know is that kiwico is not just a subscription service .

You can browse their store and purchase individual projects or value packs for different ages, no subscription required, so let's talk about why Kiwico is so useful right now. I mean, anyone who's been home with kids over the past few months knows how important it is to get them engaged in something other than a screen—the summer brain drain happens every year, but it's especially challenging right now. , since disruptions in school and online learning are not as good as they know in practical in-person learning, now what I know. From my own experience, my son loves doing kiwico projects with me, he has so much fun and doesn't even realize that he is learning.

Do you see? He is a star! You know one day we'll talk about the missing baryons, but for now these projects are a great way to connect and learn together, so if you want to try it out, you can get 20% off everything in the kiwico store using the code: veritasium or visit kiwico.com/veritasium. I'll put that link in the description so I really want to thank kiwiko for supporting me and I want to thank you guys for watching.