Transcription
Dark matter supposedly makes up 85% of the matter in the universe. Astrophysicists say it's some mysterious stuff very unlike the matter that we see around us. Probably some sort of new particle or a new force or a parallel universe. I wish this was a joke, but it isn't.
A new paper now argues that dark matter might just be normal matter. And to add insult to injury, the idea isn't even new. It's been around for 40 years. It's just that no one paid much attention. Well, that's quite something. Let's have a look.
Astrophysicists have a lot of observations that tell us that dark matter can't just be normal matter. Or so we thought. The most important clue is that well, it's dark. More precisely, it doesn't interact with electromagnetic radiation at all, neither with light in the visible part of the spectrum, nor with radio waves. This immediately rules out most of the particles around us because everything that has an electric charge interacts with electromagnetic radiation. The only particles that are left then in the standard model are the Higgs and the Z boson, but those are highly unstable and the neutrinos are not heavy enough to properly form structures.
Physicists have gone through these arguments and circles for decades and always concluded it needs to be a new particle. Preferably one with a catchy name that sounds good in grand proposals. But now they're having a second look at an idea that goes back to no other than Ed Witten. In 1984, he suggested that quarks, the particles that make up protons and neutrons, could form bigger clumps. These clumps could be stable and they'd be electrically neutral, so they wouldn't interact with light. They'd be dark. They'd fit the bill. These objects are sometimes called quark nuggets.
Normal matter is made from up and down quarks. These are the two quarks with the lowest masses. But Witten figured out that if you have a lot of up and down quarks, this is not energetically the best state. You get a state that's energetically more favorable if you add some of the next heavier quarks, the strange quarks. And because this combination is energetically more favorable if the quark clump is large enough, it doesn't decay. It's stable. This is a particular type of quark nugget called a stranglet.
The size of the stranglet is important for that to work. Small combinations of quarks with strange quarks are unstable. We know this because we can create those in particle colliders. But if Witten's math is correct, and who really wants to question Witten's math, then large chunks should be stable. We can't produce these large stranglets in particle colliders. They're just insufficiently many quarks in the collisions, but they could have been produced in the early universe. And because they're stable, they just remain there sitting around being dark and heavy.
Indeed, even tiny stranglets are enormously heavy. If you had a stranglet the size of a tiny dust grain with a radius of about 10 micrometers, that would weigh about a ton. Yes, you heard that right. If it had a radius of a centimeter, that would be about a billion tons. You have to admit that quark nugget is an adorable name for something that might weigh a mountain.
The idea first caused some excitement and then it caused some panic because the quark matter with the strange quarks is energetically more favorable than the stuff that we are made of. Physicists worry that if you could make just one stable stranglet in a particle collider, it might start converting all our ordinary nuclear matter into strange matter and eat the entire Earth. It would be a similar problem to chemical polymorphs that the science fiction story "Ice-Nine" is about. Those spheres turned out to be unfounded because we can't produce the stuff in colliders. Even if some of the strange matter came by from outer space, it can't convert the normal matter because that only works at super high densities.
The idea that stranglets make up dark matter fell by the wayside because it relied on a specific assumption about the early universe that turned out to be wrong. You see, in the early universe, we have this soup of quarks and gluons called a quark-gluon plasma. The plasma cools and at some point, it starts to clump into smaller pieces like neutrons and protons. This is a phase transition, much like when water vapor condenses to droplets. Indeed, in the 1980s, physicists thought that the phase transition in the quark-gluon plasma is similar to that of water, what's called a first-order phase transition. And in such a transition, you can get big drops of these strangelets, which then just continue to sit around as dark matter.
It then became clear, though, that the phase transition in the quark-gluon plasma just isn't first-order. It's instead a rather well-behaved crossover, and that doesn't make bubbles to create the stranglets. Now, however, physicists are rethinking this for two reasons. The first is that they found that while a first-order phase transition would do the job, it isn't necessary. Even without bubbles, there can be strong correlations in the plasma that can allow large clusters to form. The second reason is that physicists have found that quark matter can do a peculiar sort of superconductivity that conducts not electric charge, but color charge. It's called color superconductivity, and it enhances the stability of quark matter. Take this together, and it turns out that stranglets can form in the early universe after all. Theoretically, they could weigh anything from nanograms to asteroid scale.
There are other types of quark nuggets that physicists are reconsidering now besides the stranglets. The next most popular one is one that's stabilized by axions. These axions are hypothetical particles that many physicists believe exist. It's somewhat difficult to make up dark matter just of axions. However, combine the axions with quark matter, and it works nicely.
Quark nuggets have some specific experimental signatures. The most obvious ones are that if they hit our planet or maybe the moon, they'd leave traces like little tunnels. If they hit stars or white dwarfs, they could also convert the nuclear matter, leaving behind stars that are oddly compact or white dwarfs with weird spectra. But basically, the signatures are subtle and difficult to find, which fits all too well with our observations, doesn't it? Well, if this idea is right, then the biggest problem in cosmology wasn't missing matter, it was missing attention.
A few years ago, I received some ugly letters from a debt collection company. They threatened to sue me for ordering something online and then not paying it. Turned out that someone had stolen my private data. I managed to get out of this, but since then, I've become very protective of my personal information, which is why I've signed up to Incognit, who've been sponsoring this video. Each time you open a website, it'll try to collect data about who you are, and where you are, and what other websites you've visited. Many firms then make money by selling your private information to data brokers. Most countries have laws against that, and you can ask for your data to be removed, but doing this takes up a lot of time. Incognit automates the process of getting you out of those databases. You sign up, and they'll contact the big sinners, request that your personal details be removed. They'll keep on doing that, and if you want, send you updates about the progress they're making. They now also have a custom removal feature where you can submit specific websites. I'll be using this a lot. Incognit. That sounds like something you could need too. Use my code Zabina or the custom link in the info because the first 100 people to use it will get 60% off. Stay safe. Thanks for watching. See you tomorrow.