Thread by @Astropartigirl

Most of you know I study particle dark matter. I find this stuff so interesting that I wanted to write a thread, all about this weird stuff, to tell you why it's so cool, and what other ideas exist to explain this unknown sector of our Universe!

Dark matter is something we do not yet understand (dark energy is even *less* understood, and I don't see a resolution to dark energy in my lifetime, but I'd happily be proven incorrect if someone figures this stuff out! My money is on dark matter being figured out, first).

Dark matter is something we detect by its interactions with gravity *alone*. We've yet to detect the particle, and do not know its properties. We have ideas, various models for the particle, various paths to solving this problem. None of which have been confirmed, to date.

Let's all take a moment to thank Vera Rubin, the woman who discovered this stuff in the rotation curve of our nearest galactic neighbor, Andromeda. She's my favorite astronomer in the world, and you should learn all about her. Here's a good place to start nature.com/articles/54203…

We see its interactions w/gravity by how stars orbit galaxies: they're moving *way too fast*! Gravity, as we know it, can't support these velocities with just the visible matter without galaxies flying apart! There has to be other stuff that holds galaxies together: dark matter.

Further, Einstein's Theory of General Relativity, tells us that mass curves spacetime. We can see this effect via gravitational lensing: mass bends spacetime so that photons, or light, behind a mass, move along a bent trajectory!

So, if there's a galaxy behind a large mass (ex, large elliptical galaxy), you'll see warped images of that galaxy around that mass. And the amount that the image is warped tells us how much mass there is in the foreground galaxy! (Image LRG 3-757, cred NASA/ESA/Hubble)

And the amount by which the galaxy is warped tells us how much stuff there is in the foreground mass, and we always find that there's way more mass than the visible matter can amount for: another piece of evidence for dark matter!

Yet another dark matter tale is encoded in a whisper of the Big Bang, the Cosmic Microwave Background. It plays a tune that tells us what the Universe is composed of, and how much is dark matter. You can read about it in my previous blog post, here astropartigirl.com/2018/01/dark-m…

So, we know something weird is going on. Everything points to the being easy more stuff than we can see in the Universe (as in, 80% of stuff is dark, and only 20% is stuff that makes up you, me, your smartphone, laptop, chair). But we have *no idea* what it is, yet.

Cold Dark Matter (CDM) is the paradigm that's the most widely accepted, and extremely successful cosmologically. But the are other possibilities, like sterile neutrinos, Kaluza Klein dark matter, and (yay!) Self-Interacting Dark Matter (my favorite because this is what I work on)

I've written all about these and a few more, like WIMPs and Axions, and you can read about them here: astropartigirl.com/2018/01/10/dar…

CDM had been successful in almost every aspect on large scales; one problem, however, is that on small scales (example: dwarf galaxies), it predicts that these galaxies have way too much stuff near their centers, while observations tell us this isn't true.

Self-interacting Dark Matter (SIDM) resolves this bc in SIDM, dark matter particles scatter off of one another! This causes particles to exchange energy, so the particles kinda spread out a bit. This happens from the center out, so that you don't get super dense centers anymore.

The larger the galaxy, the smaller the effect, because
1) larger galaxies/clusters are younger than dwarfs, so there's less time for energy exchange to occur
2) larger galaxies are *huge*, so the thermalizing (exchange of energy) happens on smaller scale relative to the galaxy.

3) (most important of *all* these points) larger systems have larger velocities bc their gravitational potential wells are larger bc more massive. And looks like SIDM has a velocity-dependent cross-section, so that larger velocities mean smaller interaction cross-section.

The converse is true for smaller galaxies: smaller gravitational potential well -> smaller mass -> smaller velocities -> *larger* cross-section.

Roughly speaking, think of an interaction cross-section as the area of two balls moving toward each other. The larger the area, the more likely they'll collide. Now imagine the faster these balls are thrown, the less likely they are to hit each other, and vice-versa. That's SIDM.

SIDM has addressed the issue of CDM on small scales, and works on large scales. My current work (final stages, paper coming soon) works to fit intermediate scales (intermediate means galaxy groups, like our local group of galaxies including our Milky Way, Andromeda, and dwarfs.)

(Further work is something I plan to work on for my PhD; stay tuned)

Other ideas that address small scales is Modified Newtonian Gravity (MOND). It has its problems (check out my blog post), but it's a work in progress. It's one method some physicists are looking into to solve this longstanding problem that is dark matter.

What will dark matter turn out to be? I don't know. What I do know is that it'll have to work on small scales as well as cosmological scales to be a solution. These are exciting times, and I'm proud to be part of this effort to figure out what 25% of our Universe is made of.

And that concludes the dark matter chronicles from me! 🙂

Ah, one thing I should mention, and it's probably the coolest part about SIDM: if dark matter is SIDM, then there is... a new dark force!!

The current forces we know aren't of the scale of SIDM interactions, so they can't be the forces that mediate SIDM particle interactions. So, it's gotta be a new one, making the dark sector even more interesting!

So, if it wasn't obvious why SIDM is cool af, I'm sure it is now 😎

And as always, thread -> Moment =)
⚡️ “What is Dark Matter? We don't know, but here's what we *do* know”

twitter.com/i/moments/1023…