the dark matter

80% of the matter in the Universe is from a yet-discovered particle(s). Using astrophysics and cosmological observations to disentangle the nature of the dark matter is a topic that has recently become a focus area in my research group.

Early structure formation constraints on ultralight axion models in the post-inflation scenario.

2020-2022: Axions!!!! ...in the post-inflation scenario

Axions are perhaps the second most motivated candidate particle for the dark matter after the WIMP. The University of Washington is in some respects the center of the axion universe, as the particle theory group has a strong axion influence, the Institute for Nuclear Theory is here (the QCD axion solves a major problem in nuclear theory), and the largest axion detection experiment in the world, ADMX, is also on site. My group has felt the gravity of these efforts.

If the symmetry breaking that results in axion dark matter happens after inflation, then this results in white-noise density fluctuations that make axion models distinguishable from other dark matter models . We got interested in this possibility, leading to three separate studies:

We studied the spectrum of dark matter halos these white noise fluctuations grow into. We showed that tweaked semi-analytic models developed for the standard cosmology still work for the very-blue spectrum of perturbations in these models. We also tested simple models for the concentration of axion halos (these halos are much more concentrated than their vanilla CDM counterparts). We also studied the survival of QCD axion minihalos.
The lighter the axion, the larger the amplitude of white noise density fluctuations. The higher the amplitude, the more enhanced the abundance of small halos. We thus investigated the constraint on the axion mass set by the agreement of observations of cosmic structures with models that have no white power. In particular, we used observations of the Lyman-alpha forest and high-redshift galaxies to set a lower bound on the axion mass.
The symmetry breaking that establishes an axion-like particle could also result in cosmic strings that would be stable today if this particle does not acquire a significant mass. ( There are even more well-motivated models for cosmic strings where a gauge symmetry is broken, to which our study also applies.) We realized that future radio telescopes targeting fast radio bursts will likely put the tightest constraints on the existence of strings, ultimately improving over limits achievable with galaxy survey and CMB constraints.

Figure from our work, showing the constraints on `millicharged' dark matter, as well as the possible constraints owing to plasma instabilities in the Bullet Cluster.

2018 and 2022: Charged dark matter with a massless force carrier (which is maybe even the photon)

We have been interested in what the implications are for the dark matter having a charge with a massless (or very light) force carrier.

In 2022, graduate student Akaxia Cruz worked out how plasma instabilities, which results in momentum coupling of counter streaming dark matter, likely rules out a large range of electromagnetic charges. Even extremely minute Standard Model charges are excluded (see figure on the left). In particular, for parameter space where such instabilities can occur, systems like the Bullet cluster in which one cluster is streaming through another just could not occur. We find that it is likely only very small `millicharges' are allowed. Even stronger constraints are placed on models with dark charges.

In 2018, with graduate student Akshay Ghalsasi we worked on the implications of a then popular model for a dark matter component that has a dark electromagnetic sector (with a massless dark photon) and so can cool and condense like the baryons -- making it have interesting astrophysical implications. The calculations were quite fun as we reconstructed how many critical numbers/cross sections/etc depend on the dark fine structure constant as well as the mass of the light and heavier darkly charged particle. We also borrowed from semi-analytic galaxy formation models to create a semi-analytic picture of the dark universe. We showed that the end result of a typical `baryonic’ dark matter model would likely not be disks -- and instead be a mess of spheroids or halo-scale fragments. This result is easy to understand. In our Universe, feedback processes from, e.g., supernovae make galaxy formation inefficient until ~10^12 Msun halos. This allows a lot of stars to form in disks that have not yet had time to be disrupted -- most galactic disks reside in ~10^12 Msun halos. However, larger systems (galaxy clusters), which are constructed from hundreds of ~10^12 Msun, have their cooled baryonic mass in the spheroidal central galaxy or as smaller galaxies that are on the halo scale. This would happen on much smaller mass scales than clusters in `baryonic’ dark matter models (assuming no dark feedback processes -- probably the most likely scenario as even for standard baryons, changing any force even just a little would likely shut off supernovae feedback). We also estimated the size of the smallest dark fragments (dark white dwarfs or black holes) and showed that over much of the otherwise viable dark baryon parameter space, the dark baryons are ruled out by observations of stellar clusters in dwarf galaxies, rotation curve measurements of the Milky Way, and stellar microlensing.

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