Visualizing Invisible Dark Matter Annihilation with the CMB and Matter Power Spectrum

TL;DR

This paper explores how invisible dark matter annihilation affects cosmic microwave background and matter distribution, revealing potential observable signatures that could help detect sub-MeV dark matter.

Contribution

It introduces a novel framework linking invisible dark matter annihilation to observable cosmological signatures in the CMB and matter power spectrum.

Findings

Residual annihilation causes scale-dependent ΔN_eff in CMB

Phase shifts in acoustic peaks due to dark radiation

Modifications to matter power spectrum similar to warm dark matter

Abstract

We study the cosmological signatures of Invisibly Annihilating Dark Matter (IAnDM), where DM annihilates into dark radiation particles that are decoupled from the Standard Model (SM). In the simple benchmark model we consider here, such invisible annihilation determines the relic abundance of DM via dark thermal freeze-out. We demonstrate that IAnDM may reveal itself through observable, novel signatures that are correlated: scale-dependent $\Delta N_\text{eff}$ (number of extra effective neutrinos) in the Cosmic Microwave Background (CMB) spectrum due to DM residual annihilation, while the phase of acoustic peaks shifts towards the opposite direction relative to that due to SM neutrinos, resembling the effect due to scattering (fluidlike) thermal dark radiation; in addition, IAnDM induces modifications to the matter power spectrum that resemble those due to warm dark matter. Current data are sensitive to IAnDM with masses up to $\sim200$ keV, while future observations will improve this reach, especially if the late-time DM annihilation cross section is enhanced relative to the standard thermal value, which can be realized in a variety of scenarios. This work also demonstrates a new possibility of realizing thermal sub-MeV DM with observable signals.