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Here we go, time for my first thread about a new paper, out today on your local arXiv server! #sciencetwitter #physics #hepph #darkmatter #cosmology #earlyuniverse #plasma arxiv.org/pdf/1902.08623…
1/ Backstory: we've been looking for dark matter for a long, long time and we haven't found it. We're pretty sure it's a particle based on the concordance of evidence from a wide variety of astrophysical environments, but we're not sure what kind of particle it is.
2/ Since we've been looking pretty hard in a couple of particle mass ranges (most notably we've been looking for dark matter particles that are heavier than a proton) and since we haven't found it, folks are starting to wonder if other dark matter masses are possible/interesting
3/ One range that's pretty unique is the range where dark matter is lighter than an electron. It's pretty hard to make dark matter in this mass range that isn't in tension with observations of the universe. This mass range corresponds to energies where we're pretty confident...
4) ...that we understand what's going on in the universe. If there are a lot of these dark matter particles floating around with this mass at a time when the thermal energy of the Universe is comparable to the mass, then Big Bang Nucleosynthesis is likely to get messed up
5) (by the way, Big Bang Nucleosynthesis is how helium and other light elements get made in the early universe. It's a delicate balance of different nuclear reactions and if you tweak stuff even a little then things can get badly thrown off... lots of exponents in the math)
6) anyway, what we realized in this paper is that there's actually a way to make dark matter in this mass range without having too much dark matter around messing up nucleosynthesis. And you guys, I think this is REALLY COOL (but I am obviously biased here...)
7) We realized that the early Universe is a dense plasma, and in a plasma photons pick up an in-medium mass. This is sort of similar to the drag you feel while swimming-- your inertial mass is higher because you have to push water out of the way to move, which takes energy
8/ because photons have this inertial mass (let's call massive photons "plasmons"), they can actually **decay** into light particles. At first, this blew my mind, but it's actually standard when thinking about stars (which are also plasmas) and how they lose energy to neutrinos
9/ anyway, if the dark matter is somehow able to interact with photons, then dark matter can be effectively produced by these decaying plasmons. Because of how the math works out, this turns out to be most effective in the dark matter mass range below the electron's mass
10/ this is how we make dark matter out of light! and this dark matter is pretty funky relative to other models, especially with regard to how quickly it's moving and how we would best be able to detect it (hint: maybe not in a lab)
11/ The effective plasmon mass is a function of temperature, and the Universe is cooling pretty quickly at this time, so when working out conservation of energy for this decay, you end up with a really funky distribution of speeds that does not look thermal
12/ even more peculiar is that the dark matter inherits kinematic properties of the hot photons, so on average the dark matter is born going pretty close to the speed of light... because it's going so fast it's going to have a hard time clustering (gravity won't easily trap it)
13/ we can look for the suppression of clustering using cosmological probes to see if we can find dark matter gets born this way. Another way to look for dark matter is using the CMB (the afterglow of the big bang) to see if this hot dark matter was dragging around regular matter
14/ if this dragging (which would happen via similar interactions as the kind of interactions that make the dark matter in the first place) was significant then it would mess up fluctuations we see in the CMB, fuzzing them out in a detectable way
15/ after all this cosmology, if we still don't see this kind of dark matter, we have ways to potentially look for it in the lab. For instance, there are some proposals to use weird materials with interesting properties to look for dark matter in this mass range.
16/ The energy thresholds would have to be super small, but with some R&D this might be possible in a few years. I think it's definitely worth pursuing for science reasons, and maybe we could even find something useful to do with this tech for "everyday" purposes
17/ to summarize, even though dark matter is dark it's future is bright and its past may have been as well! Stay tuned for more work in this space: we have another paper coming out soon that explores the cosmology side a lot more (the paper I linked here is mainly about plasmons)
18/ I'm so grateful to have had an opportunity to learn more about plasmas and non-equilibrium stat mech. At the same time it was a humbling experience and I'm learning new stuff all the time. Our Universe is always keeping me on my toes and it's crucial to keep an open mind!
19/ Thanks for reading!! Schutzie out
wanna say a big THANK YOU to my collaborators Tongyan Lin and @CoraDvorkin
