Inflationary Primordial Black Holes as All Dark Matter (1701.02544v3)

Abstract: Following a new microlensing constraint on primordial black holes (PBHs) with $\sim10^{20}$--$10^{28}\,\mathrm{g}$~[1], we revisit the idea of PBH as all Dark Matter (DM). We have shown that the updated observational constraints suggest the viable mass function for PBHs as all DM to have a peak at $\simeq 10^{20}\,\mathrm{g}$ with a small width $\sigma \lesssim 0.1$, by imposing observational constraints on an extended mass function in a proper way. We have also provided an inflation model that successfully generates PBHs as all DM fulfilling this requirement.

• The paper establishes that a narrowly peaked PBH mass function around 10^20 g can account for all dark matter.
• It employs a double inflation model with tailored potential parameters to generate the large density perturbations needed for PBH formation.
• Numerical results conform to stringent microlensing and astrophysical constraints, prompting future observational campaigns.

Inflationary Primordial Black Holes as All Dark Matter: A Summary

The paper "Inflationary Primordial Black Holes as All Dark Matter" by Inomata et al. revisits the hypothesis that primordial black holes (PBHs) can account for the entirety of dark matter (DM). The paper is motivated by updated microlensing constraints on PBHs, particularly those derived from the Subaru Hyper Suprime-Cam (HSC) data. The authors elucidate the conditions under which PBHs of a certain mass range could constitute all of the DM observed today, focusing on PBHs originating from fluctuations during cosmic inflation.

Key Findings and Methodology

1. Mass Function and Observational Constraints:
   • The paper discusses the implications of recent astrophysical constraints, especially those affecting PBHs with masses in the range of $\sim10^{20}$ to $10^{28} \, \mathrm{g}$.
   • A crucial result presented indicates that the viable mass range for PBHs to constitute all dark matter is narrowly peaked around $\simeq 10^{20} \, \mathrm{g}$ with a small width $\sigma \lesssim 0.1$. This follows a careful application of observational constraints to an extended mass function of PBHs.
2. Inflationary Model:
   • The authors propose a specific inflationary scenario that could generate PBHs fulfilling the mass criteria. The model involves a double inflation mechanism where large superhorizon fluctuations are generated during cosmic inflation.
   • The paper describes how the balance between the potential features and initial conditions contributes to the formation of PBHs of the desired mass. Techniques involve a particular choice of parameters in potential formulations to achieve stability, initial conditions, and consistent post-inflation behavior.
3. Numerical Results and Constraints:
   • Detailed calculations illustrate the scalar power spectrum required for PBH formation. The paper explores how large density perturbations with $\mathcal{P}_\zeta \sim 10^{-2}$ are needed for effective PBH formation, given observational constraints.
   • The model shows consistency with the constraints from extra-galactic gamma-ray background, gravitational lensing, and constraints related to accretion and the cosmic microwave background.

Implications and Speculative Outlook

The findings imply a distinct possibility for PBHs to constitute all dark matter under narrowly defined conditions. The implications are two-fold:

• Theoretical: This reinforces the feasibility of a non-particle dark matter candidate, which doesn't require the physics beyond the Standard Model, given that PBHs arise naturally from known mechanisms during inflation.
• Practical: It invites further observational campaigns, potentially employing upcoming space-based gravitational wave detectors and microlensing surveys, to probe or refute the presence of such PBHs rigorously.

The paper also alludes to the potential alignment with hypotheses involving high-energy scales ($v \gtrsim 10^{15}$ GeV) that address electroweak vacuum stability during inflation.

Future Directions

• Extended Observations: Continued microlensing observations, possibly using extended data sets from Subaru HSC or other observatories, could further constrain or close the $\simeq 10^{20} \, \mathrm{g}$ mass window.
• Model Refinement: Refinement of inflationary models that account for enhanced non-Gaussian statistics at play during the inflationary epoch could better fit observational data.
• Cross-disciplinary Analysis: Integration of new data on white dwarf abundance and behavior with theoretical modeling could bring additional clarity to the role and parameter constraints of PBHs as DM.

In essence, Inomata et al.'s work is a meticulous examination of the PBH hypothesis for dark matter, presenting rigorous analysis against, and consistent navigation through, updated observational constraints. It stands as a significant contribution to understanding the interplay between inflationary physics and dark matter phenomenology.