Perhaps you’ve been following my ongoing series on dark matter. Perhaps, like many, you’re still skeptical. After all, it’s not like we’ve gone and made it in a lab or discovered it in an experiment. 15 years after David Weinberg composed the Dark Matter Rap, we still don’t know exactly what dark matter is.
But there’s a whole lot that we do know about it just from looking out at the Universe.
You see, there are a whole bunch of scales we can see, from galaxies to clusters to superclusters and the large-scale structure of the Universe as a whole.
And– since the big bang happened just under 14 billion years ago — we can look at what these objects looked like at many different times along the way. We can see galaxies back when the Universe was a few hundred million years old, clusters when it was just 3 billion years old,
and the overall large-scale structure all the way back to when the Universe was just a few hundred thousand years old! (Thanks, WMAP!)
And what do we learn about the properties of dark matter from all of this?
1. It isn’t in dense clumps. Not faint, dark stars, not planets, not basketballs, and not gas and dust. We’ve already gone over that it can’t be normal matter, but it also can’t form structures the same way that normal matter does.
2. It doesn’t interact with itself. Yes, it interacts gravitationally, but that’s not what I mean. It doesn’t feel the strong nuclear force, it doesn’t feel the electromagnetic force, and if it feels the weak force, it feels it even more weakly than neutrinos do! (And you need about a light year of lead to stop your typical neutrino.) As a result of this, dark matter is incredibly fluffy and diffuse, which means even in our Solar System, by the heart of our galaxy, dark matter’s effects are negligibly unobservable.
3. These dark halos form very early. This is very important. Looking back as far as we can see, we find that these diffuse collapsed structures have been around for pretty much the entire history of the Universe. This tells us that the dark matter is cold, or that its kinetic energy has pretty much always been much, much less than its mass. This rules out every single standard model particle that we know of from being dark matter, since the stable ones are relatively light and are all produced hot.
So what is it? Unfortunately, this is really all we know about dark matter. We know how much there is, we know what its gravitational effects are, we know how it doesn’t interact, and we know that it’s always been cold and relatively slow-moving. Can you invent something that meets these requirements? Then congratulations, you have just invented a dark matter candidate!
All of the viable candidates fall into three main categories:
- The Weakly Interacting Massive Particle (WIMP): you were once hot and produced during the big bang, but you stopped interacting with the rest of the Universe when it was very young. You cooled and collapsed, and you were heavy enough that by time we see structure, you’re cold. The lower limit on how massive you are is somewhere around 100 times greater than the mass of a proton.
- The Reptile: because you were born cold. You could have a large mass or a teeny-tiny mass, or be anywhere in between. But however you were made, you were born with very little energy, and were therefore made (or at least given a mass) sometime well after the big bang. The axion is a great example of this, and I just invented the moniker “reptile”, so be prepared to explain yourself to your physicist friends.
- The exotic: because you’re not like anything else we know in the Universe. Maybe you’re a bunch of unstable particles held together by some bizarre property that renders you undetectable. Maybe you’re a fundamental fluid with a miniscule amount of viscosity, the only one we’ve ever encountered in the Universe. And maybe you’re something else entirely, but you’re neither WIMP nor a reptile. That’s okay, we’ve got a home for you. “Exotic.”
And that’s it. There are searches going on for all three types, there are indirect tests trying to distinguish the three types, and so far, all we’ve learned is that dark matter isn’t any of the things we would have been able to find so far.
So there’s still a very, very long way to go to discover what it is. But although I may not be able to tell you that it’s an elephant, I know it has tusks, a trunk, and smells terrible!
For myself, I’ve concluded that dark matter very, very likely exists, that these three places are the right places to be looking, and that the more astrophysical evidence we can acquire (to tell us where the dark matter is, what its gravity is doing, etc.), the better we’ll know this unseen elephant.
This concludes my official series on Dark Matter, but I’m always happy to talk more about it and answer good questions!