New cosmological constraints on primordial black holes (0912.5297v2)

Abstract: We update the constraints on the fraction of the Universe going into primordial black holes in the mass range 10^9--10^17 g associated with the effects of their evaporations on big bang nucleosynthesis and the extragalactic photon background. We include for the first time all the effects of quark and gluon emission by black holes on these constraints and account for the latest observational developments. We then discuss the other constraints in this mass range and show that these are weaker than the nucleosynthesis and photon background limits, apart from a small range 10^13--10^14 g, where the damping of cosmic microwave background anisotropies dominates. Finally we review the gravitational and astrophysical effects of nonevaporating primordial black holes, updating constraints over the broader mass range 1--10^50 g.

• The paper refines constraints on PBH abundance by analyzing particle emissions from evaporation that impact both big bang nucleosynthesis and the extragalactic photon background.
• It reevaluates BBN limits for PBHs within 10^9–10^13 g, demonstrating how meson and antinucleon injections modify neutron-to-proton ratios and element synthesis.
• The study establishes tighter limits on PBH density via extragalactic gamma-ray spectrum analysis, thereby constraining their role as dark matter candidates.

Overview of New Cosmological Constraints on Primordial Black Holes

The paper "New Cosmological Constraints on Primordial Black Holes" presents an updated analysis of the constraints on the cosmological abundance of primordial black holes (PBHs) across a vast mass range. These constraints are primarily determined by examining the effects of PBH evaporation on big bang nucleosynthesis (BBN) and the extragalactic photon background.

The research explores PBHs with masses ranging from $10^9$ to $10^{17}$ grams, with additional considerations for PBHs that do not evaporate by the present epoch. The authors, B.J. Carr, Kazunori Kohri, Yuuiti Sendouda, and Jun'ichi Yokoyama, further explore the broader implications of PBHs across various cosmological and astrophysical phenomena.

Key Highlights and Numerical Results

1. Evaporation Process and Effects:
   • The paper provides a detailed analysis of the emission of particles, including quarks and gluons, during PBH evaporation. These emissions influence both the BBN and the extragalactic photon background.
   • Using the latest observational data and theoretical methods, such as quark and gluon jet fragmentation, the paper refines our understanding of high-energy particle emissions from PBHs.
2. BBN Constraints:
   • The constraints from BBN are reevaluated, highlighting that PBHs with lifetimes shorter than 1 second are not significantly limited by BBN constraints.
   • For PBHs with mass $M = 10^9\text{{--}}10^{13}$ grams, the paper places stringent limits on the abundance based on their impact on nucleosynthesis, particularly through the injection of mesons and antinucleons increasing neutron-to-proton ratios and dissociating synthesized elements.
3. Extragalactic Photon Background:
   • The paper updates the constraints on the PBH density parameter $\Omega_{\text{PBH}}$, employing comprehensive photon spectrum calculations.
   • A tighter limit on $\beta'(M)$, the fraction of the Universe's mass in PBHs at initial formation, is established based on the extragalactic gamma-ray background, leading to a revised constraint of $\beta'(M_*) < 3 \times 10^{-27}$.
4. Complementary Constraints:
   • The paper also discusses other observational bounds such as those from cosmic neutrinos, the Galactic gamma-ray background, and CMB anisotropies.
   • Particularly, limits from the potential damping of CMB anisotropies by energy injections from evaporating PBHs stand out in the mass range of $10^{13}\text{{--}}10^{14}$ grams, which can dominate over other constraints in this narrow mass window.
5. Implications for Large PBHs:
   • Larger, nonevaporating PBHs impose constraints through dynamical and gravitational effects on large-scale structure, lensing, and accretion-induced modifications to the CMB spectrum.
   • These bounds are essential for understanding the role of PBHs as possible dark matter candidates, provided their mass range is consistent with microlensing and wide binary disruption constraints.

Speculative Theoretical and Practical Implications

The theoretical implications of these refined constraints are significant for models of the early Universe, such as those predicting the formation of PBHs during high-density fluctuations or phase transitions. Additionally, given the constraints on the mass spectrum, PBHs as dark matter candidates face stringent limits, especially for those within the mass range of $10^{15}$ grams.

Practically, this research has implications for interpreting gamma-ray and cosmic-ray data and aids in designing future observational strategies to detect or further constrain PBH populations. The paper also provides a foundation for testing models of new physics beyond the Standard Model, where PBHs could play a role in baryogenesis or as sources of exotic relic particles.

In conclusion, this paper offers a comprehensive and updated framework for understanding the role and limitations of primordial black holes in cosmology, influencing both the theoretical modeling of the early Universe and the interpretation of cosmological observations. Future studies may extend these constraints with more sophisticated models or the inclusion of additional observational data, further elucidating the potential of PBHs in cosmological structure formation and dark matter.