Charged-particle decay and suppression of small-scale power

Abstract

We study the suppression of the small-scale power spectrum due to the decay of charged matter to dark matter prior to recombination. Prior to decay, the charged particles couple to the photon-baryon fluid and participate in its acoustic oscillations. However, after decaying to neutral dark matter the photon-baryon fluid is coupled only gravitationally to the newly-created dark matter. This generically leads to suppression of power on length scales that enter the horizon prior to decay. For decay times of approximately 3.5 years this leads to suppression of power on subgalactic scales, bringing the observed number of Galactic substructures in line with observation. Decay times of a few years are possible if the dark matter is purely gravitationally interacting, such as the gravitino in supersymmetric models or a massive Kaluza-Klein graviton in models with universal extra dimensions.

Figures (3)

• Figure 1: The evolution of the comoving wavenumber $k=3.0~{\rm Mpc}^{-1}$ density perturbations in the early Universe for dark matter (dashed line) and baryons (dotted line). The dark-matter perturbation always grows under the influence of gravity while the baryonic perturbation oscillates due to a competition between gravity and the photon pressure.
• Figure 2: The evolution of the comoving wavenumber $k=30.0~{\rm Mpc}^{-1}$ (left panel), $k=3.0~{\rm Mpc}^{-1}$ (center panel), and $k=0.3~{\rm Mpc}^{-1}$ density perturbations in the early Universe for dark matter in the $\Lambda$CDM model (dashed line) and in the model with $\tau=3.5~{\rm yr}$ (solid line). The '$\beta$' perturbation is represented by the dotted line. Due to being sourced by the low amplitude '$\beta$' perturbations at early times the dark matter perturbation in the model with a decaying charged component is suppressed relative to the standard $\Lambda$CDM case for $k = 3.0~{\rm Mpc}^{-1}$. For $k \gg 3.0~{\rm Mpc}^{-1}$ (very small scales) $\delta_{d}$ tracks the oscillations in $\delta_{\beta}$ before decay, while for $k \ll 3.0~{\rm Mpc}^{-1}$ (large scales) $\delta_{d}$ follows the standard growing evolution.
• Figure 3: The linear-theory power spectrum of matter density fluctuations in the standard $\Lambda$CDM model (dashed line), and in the charged decay to dark matter model (solid line) with $\tau = 3.5~{\rm yr}$. The charged decay model matches the standard $\Lambda$CDM model on length scales larger than $0.3~{\rm Mpc}$, but power drops of sharply below $0.3~{\rm Mpc}$.