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We revisit cosmic microwave background (CMB) constraints on primordial black hole dark matter. Spectral distortion limits from COBE/FIRAS do not impose a relevant constraint. Planck CMB anisotropy power spectra imply that primordial black holes with $m_{BH}gtrsim 5~M_{odot}$ are disfavored. However, this is susceptible to sizeable uncertainties due to the treatment of the black hole accretion process. These constraints are weaker than those quoted in earlier literature for the same observables.
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The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has yielded unprecedented measurements of the soft gamma-ray spectrum of our Galaxy. Here we use those measurements to set constraints on dark matter (DM) that decays or annihilates into photons with energies $Eapprox 0.02-2$ MeV. First, we revisit the constraints on particle DM that decays or annihilates to photon pairs. In particular, for decaying DM, we find that previous limits were overstated by roughly an order of magnitude. Our new, conservative analysis finds that the DM lifetime must satisfy $taugtrsim 5times 10^{26},{rm s}times (m_{chi}/rm MeV)^{-1}$ for DM masses $m_{chi}=0.054-3.6$ MeV. For MeV-scale DM that annihilates into photons INTEGRAL sets the strongest constraints to date. Second, we target ultralight primordial black holes (PBHs) through their Hawking radiation. This makes them appear as decaying DM with a photon spectrum peaking at $Eapprox 5.77/(8pi G M_{rm PBH})$, for a PBH of mass $M_{rm PBH}$. We use the INTEGRAL data to demonstrate that, at 95% C.L., PBHs with masses less than $1.2times 10^{17}$ g cannot comprise all of the DM, setting the tightest bound to date on ultralight PBHs.
Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{odot}$ are interesting candidates for dark matter, as they sit in a narrow window between microlensing and cosmic microwave background constraints. There are however tight constraints from the binary merger rate observed by the LIGO and Virgo experiments. In deriving these constraints, PBHs were treated as point Schwarzschild masses, while the more careful analysis in an expanding universe we present here, leads to a time-dependent mass. This implies a stricter set of conditions for a black hole binary to form and means that black holes coalesce much more quickly than was previously calculated, namely well before the LIGO/Virgos observed mergers. The observed binaries are those coalescing within galactic halos, with a merger rate consistent with data. This reopens the possibility for dark matter in the form of LIGO-mass PBHs.
Seven observations point towards the existence of primordial black holes (PBH), constituting the whole or an important fraction of the dark matter in the Universe: the mass and spin of black holes detected by Advanced LIGO/VIRGO, the detection of micro-lensing events of distant quasars and stars in M31, the non-detection of ultra-faint dwarf satellite galaxies with radius below 15 parsecs, evidences for core galactic dark matter profiles, the correlation between X-ray and infrared cosmic backgrounds, and the existence of super-massive black holes very early in the Universes history. Some of these hints are newly identified and they are all intriguingly compatible with the re-constructed broad PBH mass distribution from LIGO events, peaking on PBH mass $m_{rm PBH} approx 3 M_odot$ and passing all other constraints on PBH abundances. PBH dark matter also provides a new mechanism to explain the mass-to-light ratios of dwarf galaxies, including the recent detection of a diffuse galaxy not dominated by dark matter. Finally we conjecture that between 0.1% and 1% of the events detected by LIGO will involve a PBH with a mass below the Chandrasekhar mass, which would unambiguously prove the existence of PBH.
The renewed interest in the possibility that primordial black holes (PBHs) may constitute a significant part of the dark matter has motivated revisiting old observational constraints, as well as developing new ones. We present new limits on the PBH abundance, from a comprehensive analysis of high-resolution, high-redshift Lyman-$alpha$ forest data. Poisson fluctuations in the PBH number density induce a small-scale power enhancement which departs from the standard cold dark matter prediction. Using a grid of hydrodynamic simulations exploring different values of astrophysical parameters, {we obtain a marginalized upper limit on the PBH mass of $f_{rm PBH}M_{rm PBH} sim 60~M_{odot}$ at $2sigma$, when a Gaussian prior on the reionization redshift is imposed, preventing its posterior distribution to peak on very high values, which are disfavoured by the most recent estimates obtained both through Cosmic Microwave Background and Inter-Galactic Medium observations. Such bound weakens to $f_{rm PBH}M_{rm PBH} sim 170~M_{odot}$, when a conservative flat prior is instead assumed. Both limits significantly improves previous constraints from the same physical observable.} We also extend our predictions to non-monochromatic PBH mass distributions, ruling out large regions of the parameter space for some of the most viable PBH extended mass functions.
Dark matter coupled solely gravitationally can be produced through the decay of primordial black holes in the early universe. If the dark matter is lighter than the initial black hole temperature, it could be warm enough to be subject to structure formation constraints. In this paper we perform a more precise determination of these constraints. We first evaluate the dark matter phase-space distribution, without relying on the instantaneous decay approximation. We then interface this phase-space distribution with the Boltzmann code CLASS to extract the corresponding matter power spectrum, which we find to match closely those of warm dark matter models, albeit with a different dark matter mass. This mapping allows us to extract constraints from Lyman-$alpha$ data without the need to perform hydrodynamical simulations. We robustly rule out the possibility, consistent with previous analytic estimates, of primordial black holes having come to dominate the energy density of the universe and simultaneously given rise to all the DM through their decay. Consequences and implications for dark radiation and leptogenesis are also briefly discussed.
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