Cannibalism's lingering imprint on the matter power spectrum

Abstract

The early universe may have contained internally thermalized dark sectors that were decoupled from the Standard Model. In such scenarios, the relic dark thermal bath, composed of the lightest particle in the dark sector, can give rise to an epoch of early matter domination prior to Big Bang Nucleosynthesis, which has a potentially observable impact on the smallest dark matter structures. This lightest dark particle can easily and generically have number-changing self-interactions that give rise to “cannibal” behavior. We consider cosmologies where an initially sub-dominant cannibal species comes to temporarily drive the expansion of the universe, and we provide a simple map between the particle properties of the cannibal species and the key features of the enhanced dark matter perturbation growth in such cosmologies. We further demonstrate that cannibal self-interactions can determine the small-scale cutoff in the matter power spectrum even when the cannibal self-interactions freeze out prior to cannibal domination.

Dark matter self-interactions in the matter power spectrum

“…This requires a modification of the publicly available code to include self-interactions, allowing DM to interpolate between perfect fluid and free streaming behavior. Related work can be found in [14][15][16]. While our analysis differs in several aspects our findings are consistent with the results presented in [14].…”

Snowmass2021 Cosmic Frontier White Paper: Dark Matter Physics from Halo Measurements

“…After the inflaton decays into radiation, other oscillating scalar fields may come to dominate the energy density of the Universe; this phenomenon is a generic consequence of stabilized moduli in string theories [109]. It is also possible that unstable massive particles could temporarily dominate the Universe prior to BBN, and hidden sector theories of dark matter frequently include EMDEs [110][111][112][113][114][115][116]. Another possible deviation from radiation domination is a period of kination, during which the kinetic energy of a scalar field dominates the energy density of the Universe [117][118][119].…”

“…If T RH is the radiation temperature when the Universe became radiation dominated, then an EMDE or a period of kination enhances the abundance of microhalos with masses less than 30M ⊕ [(10 MeV)/T RH ] 3 [5]. The formation time and size of the smallest microhalos depends on the temperature of the dark matter particles [133][134][135][136], the properties of the particles that dominated the energy density during the EMDE [115,116,137], and the duration of the EMDE [137,138]. If dark matter is sufficiently cold and density perturbations remain in the linear regime during the EMDE, most of the dark matter is contained in microhalos at redshifts as high as 200, long before halos are expected to form in standard cosmologies [135,137,139].…”

Astrophysical and Cosmological Probes of Dark Matter

“…An early matter-dominated era also enhances dark matter density perturbations on scales that enter the horizon prior to the onset of the final radiation-dominated epoch [409]. Significant progress has been made in understanding the impact this has on the abundance of sub-Earth-mass microhalos [409,410] and how the minimum halo mass depends on the properties of dark matter [411][412][413][414], as well as the properties of the particle responsible for the early matter-dominated era [415][416][417][418]. These microhalos provide a new observational probe of the early Universe; their impact on the dark matter annihilation rate can be constrained using the isotropic gamma-ray background [413,415,419], and they can be detected gravitationally using pulsar timing arrays [420][421][422][423] and observations of stellar microlensing events in galaxy clusters [424,425].…”