CMB and 21cm bounds on early structure formation boosted by primordial black hole entropy fluctuations

TL;DR

This paper investigates how primordial black hole-induced entropy fluctuations influence early structure formation and how WIMP annihilation in these structures affects CMB and 21cm signals, providing new constraints on dark matter models.

Contribution

It introduces the impact of PBH entropy fluctuations on early structure formation and derives new bounds on PBH and WIMP parameters from CMB and 21cm observations.

Findings

PBH entropy fluctuations can significantly enhance early structure formation.

Latest Planck data constrains PBH fraction to less than 0.1% for sub-solar mass PBHs.

21cm measurements impose even tighter bounds on PBH abundance.

Abstract

The dark matter (DM) can consist of the primordial black holes (PBHs) in addition to the conventional weakly interacting massive particles (WIMPs). The Poisson fluctuations of the PBH number density produce the isocurvature perturbations which can dominate the matter power spectrum at small scales and enhance the early structure formation. We study how the WIMP annihilation from those early formed structures can affect the CMB (in particular the E-mode polarization anisotropies and $y$-type spectral distortions) and global 21cm signals. Our studies would be of particular interest for the light (sub-GeV) WIMP scenarios which have been less explored compared with the mixed DM scenarios consisting of PBHs and heavy ($\gtrsim 1$ GeV) WIMPs. For instance, for the self-annihilating DM mass $m_{\chi}=1$ MeV and the thermally averaged annihilation cross section $\langle \sigma v \rangle \sim 10^{-30} \rm cm^3/s$, the latest Planck CMB data requires the PBH fraction with respect to the whole DM to be at most ${\cal O}(10^{-3})$ for the sub-solar mass PBHs and an even tighter bound (by a factor $\sim 5$) can be obtained from the global 21-cm measurements.