LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies

TL;DR

This paper explores how primordial black holes, suggested by LIGO's gravitational wave detections, could explain early universe halo formation and the observed near-IR cosmic infrared background fluctuations, linking dark matter to observable cosmic signals.

Contribution

It demonstrates that primordial black holes can significantly increase early halo abundance, explaining IR background fluctuations and X-ray correlations, a novel link between dark matter and cosmic background observations.

Findings

Primordial black holes can dominate small-scale density fluctuations.

Enhanced early halo formation explains IR background anisotropies.

Gas accretion onto PBHs accounts for X-ray and IR background coherence.

Abstract

LIGO's discovery of a gravitational wave from two merging black holes (BHs) of similar masses rekindled suggestions that primordial BHs (PBHs) make up the dark matter (DM). If so, PBHs would add a Poissonian isocurvature density fluctuation component to the inflation-produced adiabatic density fluctuations. For LIGO's BH parameters, this extra component would dominate the small-scale power responsible for collapse of early DM halos at z>10, where first luminous sources formed. We quantify the resultant increase in high-z abundances of collapsed halos that are suitable for producing the first generation of stars and luminous sources. The significantly increased abundance of the early halos would naturally explain the observed source-subtracted near-IR cosmic infrared background (CIB) fluctuations, which cannot be accounted for by known galaxy populations. For LIGO's BH parameters this increase is such that the observed CIB fluctuation levels at 2 to 5 micron can be produced if only a tiny fraction of baryons in the collapsed DM halos forms luminous sources. Gas accretion onto these PBHs in collapsed halos, where first stars should also form, would straightforwardly account for the observed high coherence between the CIB and unresolved cosmic X-ray background in soft X-rays. We discuss modifications possibly required in the processes of first star formation if LIGO-type BHs indeed make up the bulk or all of DM. The arguments are valid only if the PBHs make up all, or at least most, of DM, but at the same time the mechanism appears inevitable if DM is made of PBHs.