Ultralight Dark Matter: Undergraduate Physics in Modern Cosmology

Abstract

Ultralight dark matter is a hypothetical class of particle with a number of interesting theoretical and experimental properties, many of which are best understood by direct analogy with or application of undergraduate physics. We present a series of exercises and discussions which may inspire the reader to bring contemporary research on ultralight dark matter into the undergraduate classroom.

Figures (1)

• Figure 1: (Color online) Dark matter candidates organized by particle mass (log scale), in units of eV (per $c^2$), kg, and Solar masses ($M_\odot$). Only a few of the hundreds of dark matter models are considered here to convey a sense of scale for the parameter space. The Planck mass roughly separates elementary particle candidates from "macroscopic" candidates, such as black holes PBH, hidden populations of cold brown dwarf stars brown_dwarfs, or other massive compact halo objects (MACHOs) MACHO:2000qbb. Among particle DM candidates, the Tremaine--Gunn bound provides a lower limit on the mass of fermions, but not on bosons. The Tremaine--Gunn bound and the upper and lower limits on the ultralight DM mass range (hatched region) are explored further in the Exercises. The boundary between WIMPs and light DM models represents a difference in detection method and lies around the transition between nuclear recoils and electron recoils in liquid xenon experiments LZ:2025igz.