Constraints on Primordial Black Holes (2002.12778v2)

Abstract: We update the constraints on the fraction of the Universe that may have gone into primordial black holes (PBHs) over the mass range $10^{-5}\text{--}10^{50}$ g. Those smaller than $\sim 10^{15}$ g would have evaporated by now due to Hawking radiation, so their abundance at formation is constrained by the effects of evaporated particles on big bang nucleosynthesis, the cosmic microwave background (CMB), the Galactic and extragalactic $\gamma$-ray and cosmic ray backgrounds and the possible generation of stable Planck mass relics. PBHs larger than $\sim 10^{15}$ g are subject to a variety of constraints associated with gravitational lensing, dynamical effects, influence on large-scale structure, accretion and gravitational waves. We discuss the constraints on both the initial collapse fraction and the current fraction of the CDM in PBHs at each mass scale but stress that many of the constraints are associated with observational or theoretical uncertainties. We also consider indirect constraints associated with the amplitude of the primordial density fluctuations, such as second-order tensor perturbations and $\mu$-distortions arising from the effect of acoustic reheating on the CMB, if PBHs are created from the high-$\sigma$ peaks of nearly Gaussian fluctuations. Finally we discuss how the constraints are modified if the PBHs have an extended mass function, this being relevant if PBHs provide some combination of the dark matter, the LIGO/Virgo coalescences and the seeds for cosmic structure. Even if PBHs make a small contribution to the dark matter, they could play an important cosmological role and provide a unique probe of the early Universe.

• The paper provides comprehensive constraints on PBHs by analyzing their evaporation effects on BBN, CMB, and gamma-ray backgrounds.
• It employs microlensing and dynamical analyses to limit PBH contributions to dark matter across distinct mass ranges.
• The study integrates accretion and gravitational wave data to evaluate extended PBH mass functions and their cosmological implications.

Constraints on Primordial Black Holes

The paper "Constraints on Primordial Black Holes" by Carr, Kohri, Sendouda, and Yokoyama provides a comprehensive overview of the various constraints on the primordial black holes (PBHs) across a wide mass range. Since PBHs have been proposed to account for dark matter (DM), gravitational wave events, and even the seeds of supermassive black holes, understanding these constraints is crucial for evaluating their cosmological relevance.

Evaporation Constraints

PBHs with masses below $10^{15}\, \mathrm{g}$ would have evaporated by now due to Hawking radiation. The primary constraints for such small PBHs come from the effects of evaporated particles on big bang nucleosynthesis (BBN), the cosmic microwave background (CMB) spectrum, and the extragalactic gamma-ray background. For instance, modifications to light element abundances due to additional entropy injection by evaporating PBHs during BBN provide stringent limits, constraining $\beta(M)$ (the fraction of PBH mass relative to the universe's mass at formation) to be extremely small across a considerable range of masses.

Lensing and Dynamical Constraints

Lensing constitutes a significant class of constraints, as PBHs can act as lenses for background light sources like stars, quasars, or the CMB itself. Microlensing searches in the Magellanic Clouds and more refined analyses involving quasar microlensing have excluded PBHs as the sole DM candidates in various mass windows, primarily in the range $$10^{-7} \, \mathrm{M_{\odot}} < M < 100 \, \mathrm{M_{\odot}}$$.

For larger PBHs, dynamical effects such as the disruption of wide binaries, star clusters, and even larger cosmic structures by PBHs provide additional indirect constraints. These constraints are based on the gravitational interactions that PBHs would inevitably have if they were abundant enough to constitute a significant fraction of DM.

Accretion and CMB Constraints

PBHs can accrete surrounding matter, especially during epochs when the Universe is matter-dominated. Such accretion impacts the CMB anisotropy and spectral distortion due to energy injection into the intergalactic medium. The paper discusses these effects, placing constraints on PBH masses that would significantly heat or ionize baryonic matter, thus affecting CMB observations. While previous analyses suggested strong suppression of PBHs due to these constraints, recent revisions have relaxed some of these limits but remain critical at higher masses.

Gravitational Waves and Non-Gaussianity

The detections of gravitational waves from binary black holes by LIGO/Virgo have opened another venue of constraints. PBHs as sources of these mergers provide compelling observational opportunities but also demand consistency with merger rates and the stochastic gravitational wave background. Constraints on second-order gravitational waves, which would have been induced by the scalar perturbations that also formed PBHs, further restrict the parameter space of PBH formation.

Extended Mass Functions

The paper acknowledges that PBHs may not have a monochromatic mass function; rather, it could be extended due to the complexity of formation scenarios. Therefore, combined constraints must consider integration over extended mass functions, which is discussed using methods like the lognormal distribution. Exploring these extended distributions helps to more accurately determine how PBHs can fit within the cosmological framework and their possible contribution to DM.

Implications and Future Directions

The paper posits that while PBHs cannot account for all of DM across most permitted ranges due to the severe constraints outlined, they could still serve as interesting probes of the early Universe's conditions, as well as local structures like globular clusters. Furthermore, the authors highlight the significance of multi-frequency and multimodal observation strategies, including gravitational waves, to further probe PBH contributions across scales that are currently ambiguously understood or constrained.

In conclusion, while the constraints on PBHs are comprehensive and span various physical phenomena, the paper of PBHs remains a promising domain to understand high-energy physics and early Universe conditions in greater depth. With upcoming observatories and refined theoretical models, we anticipate richer insights into these potential cosmological remnants.