Primordial Black Holes as Dark Matter: Recent Developments

TL;DR

PBHs present a non-particle dark matter candidate with formation tied to horizon-scale physics and myriad formation pathways. The paper surveys formation mechanisms, the impact of non-Gaussianity and non-sphericity, and how extended mass functions can escape monochromatic constraints. It also discusses claimed observational signatures, a unified thermal-history scenario that produces multiple PBH mass peaks, and the possibility of PBHs coexisting with particle DM or leaving Planck-mass relics. Future gravitational-wave and cosmological data are highlighted as critical tests for PBH scenarios, including their roles as SMBH seeds and contributors to structure formation.

Abstract

Although the dark matter is usually assumed to be some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to $10^{16}$ - $10^{17}\,$g, $10^{20}$ - $10^{24}\,$g and $10$ - $10^{3}\,M_{\odot}$. The last possibility is contentious but of special interest in view of the recent detection of black-hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they have only a small fraction of the dark-matter density. In particular, those larger than $10^{3}\,M_{\odot}$ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark-matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than $10^{12}\,M_{\odot}$ could provide an interesting dark component.

Figures (7)

• Figure 1: Constraints on $f( M )$ for a monochromatic mass function, from evaporations (red), lensing (blue), gravitational waves (GW) (gray), dynamical effects (green), accretion (light blue), CMB distortions (orange) and large-scale structure (purple). Evaporation limits come from the extragalactic $\gamma$-ray background (EGB), the Voyager positron flux (V) and annihilation-line radiation from the Galactic centre (GC). Lensing limits come from microlensing of supernovae (SN) and of stars in M31 by Subaru (HSC), the Magellanic Clouds by EROS and MACHO (EM) and the Galactic bulge by OGLE (O). Dynamical limits come from wide binaries (WB), star clusters in Eridanus II (E), halo dynamical friction (DF), galaxy tidal distortions (G), heating of stars in the Galactic disk (DH) and the CMB dipole (CMB). Large-scale structure constraints derive from the requirement that various cosmological structures do not form earlier than observed (LSS). Accretion limits come from X-ray binaries (XB) and Planck measurements of CMB distortions (PA). The incredulity limits (IL) correspond to one PBH per relevant environment (galaxy, cluster, Universe). There are four mass windows (A, B, C, D) in which PBHs could have an appreciable density. Possible constraints in window D are discussed in Section \ref{['sec:Primordial-Black-Hole-versus-Particle-Dark-Matter']} but not in the past literature.
• Figure 2: Sketch of the limits shown in Figure \ref{['fig:contraints-large']} for different redshifts. Here, we break down the large-scale structure limit into its individual components from clusters (Cl), Milky Way galaxies (Gal) and dwarf galaxies (dG), as these originate from different redshifts (cf. Reference Carr:2018rid). Further abbreviations are defined in the caption of Figure \ref{['fig:contraints-large']}.
• Figure 3: Equation-of-state parameter $w$ as a function of temperature $T$, from Reference Carr:2019kxo. The grey vertical lines correspond to the masses of the electron, pion, proton/neutron, $W / Z$ bosons and top quark, respectively. The grey dashed horizontal lines correspond to $g_{*} = 100$ and $w = 1 / 3$.
• Figure 4: The mass spectrum of PBHs with spectral index $n_{\mathrm{s}} = 0.965$ (red, dashed), $0.97$ (blue, solid), $0.975$ (green, dotted), from Reference Carr:2019kxo. The grey vertical lines corresponds to the electroweak and QCD phase transitions and $e^{+}e^{-}$ annihilation. Also shown are the constraints associated with microlensing (M), wide-binaries (W), accretion (A), Eridanus (E) and X-ray observations (X). The vertical lines correspond to the gravitational-wave events GW190425 Abbott:2020uma, GW190814 Abbott:2020khf and GW190521 Abbott:2020mjqAbbott:2020tfl.
• Figure 5: Expected probability distribution of PBH mergers with masses $m_{1}$ and $m_{2}$ (in solar units), assuming a PBH mass function with $n_{\mathrm{s}} = 0.97$ and the LIGO sensitivity for the O2 run. The solid and dashed white lines correspond to mass ratios $q \equiv m_{2} / m_{1}$ of $0.1$ and $0.5$, respectively. (1) corresponds to the peak for neutron-star mergers without electromagnetic counterparts. The mergers of stellar black holes are not expected within the red-bouned regions, which are: (2) events with one black hole above $100\,M_\odot$; (3) mergers of subsolar objects, which might be taken to be neutron stars, with objects at peak of black-hole distribution; (4) mergers of objects in mass gap; (5) a subdominant population of mergers with low mass ratios. The colour bar indicates the probability of detection. The green lines indicate the gravitational-wave events GW190425, GW190814 and GW190521, these lying in regions (2), (4) and (5), respectively. Figure adapted from Reference Carr:2019kxo.