Testing scenarios of primordial black holes being the seeds of supermassive black holes by ultracompact minihalos and CMBμdistortions

Abstract

Supermassive black holes and intermediate mass black holes are believed to exist in the Universe. There is no established astrophysical explanation for their origin and considerations have been made in the literature that those massive black holes (MBHs) may be primordial black holes (PBHs), black holes which are formed in the early universe (well before the matter-radiation equality) due to the direct collapse of primordial overdensities. This paper aims at discussing the possibility of excluding the PBH scenario as the origin of the MBHs. We first revisit the constraints on PBHs obtained from the CMB distortion that the seed density perturbation causes. By adopting a recent computation of the CMB distortion sourced by the seed density perturbation and the stronger constraint on the CMB distortion set by the COBE/FIRAS experiment used in the literature, we find that PBHs in the mass range 6 × 10 4 M ⊙ ∼ 5 × 10 13 M ⊙ are excluded. Since PBHs lighter than 6 × 10 4 M ⊙ are not excluded from the non-observation of the CMB distortion, we propose a new method which can potentially exclude smaller PBHs as well. Based on the observation that large density perturbations required to create PBHs also result in the copious production of ultracompact minihalos (UCMHs), compact dark matter halos formed at around the recombination, we show that weakly interacting massive particles (WIMPs) as dark matter annihilate efficiently inside UCMHs to yield cosmic rays far exceeding the observed flux. Our bound gives severe restriction on the compatibility between the particle physics models for WIMPs and the PBH scenario as the explanation of MBHs.2

The synergy between CMB spectral distortions and anisotropies

“…In particular, their abundance is believed to be intrinsically related to a possible non-Gaussianity of the density perturbations [150,151]. Moreover, it has been argued that a potential detection of a PBH might rule out several WIMP models [152][153][154][155][156][157].…”

Formation of supermassive primordial black holes by Affleck-Dine mechanism

“…PBH formation requires large density fluctuations on small scales, which leads to the CMB spectral distortion due to the Silk damping [29,30]. In fact the µ-distortion of CMB gives a stringent constraint on the amplitude of the power spectrum of the curvature perturbations, P ζ 10 −4 for k ∼ 1 − 10 4 Mpc, from which PBHs with masses between 4 × 10 13 M and 4 × 10 2 M are excluded [31].…”

Constraints on small-scale primordial power by annihilation signals from extragalactic dark matter minihalos

“…See also Refs. [26,27]. In addition, since the density of dark matter is relatively high inside minihalos, annihilation there could be substantial if dark matter particles annihilate.…”