Primordial black hole constraints for extended mass functions (1705.05567v3)

Abstract: We revisit the cosmological and astrophysical constraints on the fraction of the dark matter in primordial black holes (PBHs) with an extended mass function. We consider a variety of mass functions, all of which are described by three parameters: a characteristic mass and width and a dark matter fraction. Various observations then impose constraints on the dark matter fraction as a function of the first two parameters. We show how these constraints relate to those for a monochromatic mass function, demonstrating that they usually become more stringent in the extended case than the monochromatic one. Considering only the well-established bounds, and neglecting the ones that depend on additional astrophysical assumptions, we find that there are three mass windows, around $4\times 10^{-17}M_\odot,$ $2\times 10^{-14}M_\odot$ and $25-100M_\odot$, where PBHs can constitute all dark matter. However, if one includes all the bounds, PBHs can only constitute of order $10\%$ of the dark matter.

• The paper establishes that extended PBH mass functions impose significantly tighter dark matter constraints compared to monochromatic models.
• The paper employs a comprehensive methodology integrating microlensing, gravitational wave, and CMB observations to map the PBH parameter space.
• The paper concludes that even under relaxed assumptions, PBHs could account for at most about 10% of dark matter.

Primordial Black Hole Constraints for Extended Mass Functions: An Overview

The paper presented in "Primordial black hole constraints for extended mass functions" addresses an important topic in cosmology: understanding the potential contribution of primordial black holes (PBHs) to dark matter. Authored by Bernard Carr, Martti Raidal, Tommi Tenkanen, Ville Vaskonen, and Hardi Veermäe, the paper revisits the cosmological and astrophysical constraints on PBHs, particularly when the PBHs have extended mass functions—those that are not confined to a single mass scale.

Overview of the Research

The research investigates various mass functions for PBHs, characterized by a characteristic mass and width, along with the fraction of PBHs constituting dark matter. The paper analyzes how observational constraints on these parameters differ between extended mass functions and the more commonly discussed monochromatic ones, where all PBHs have approximately the same mass. The authors assert that constraints are typically more stringent for extended mass functions compared to monochromatic.

Notably, the paper identifies three mass windows—around $4 \times 10^{-17} M_{\odot}$, $2 \times 10^{-14} M_{\odot}$, and within the range $25-100 M_{\odot}$—where PBHs could theoretically account for all dark matter, assuming certain astrophysical assumptions are set aside. However, if one considers a comprehensive set of constraints, PBHs would only represent about 10% of dark matter at best.

Key Contributions and Findings

1. Extended Mass Functions: The researchers provide a thorough methodology for applying constraints to extended PBH mass functions. This is significant because if the mass distribution of PBHs is broad, evaluating their dark matter contribution using monochromatic constraints becomes invalid.
2. Numerical Results: The paper finds that wider mass functions make it harder for PBHs to evade current constraints, challenging assertions in the literature that extended mass functions could comfortably circumvent these limitations. The analysis rules out the possibility that wide mass functions with a width $\sigma \gtrsim 1$ could constitute all dark matter.
3. Dynamical and Astrophysical Constraints: The work highlights a diverse set of observational constraints, such as those from microlensing, gravitational waves, and cosmic microwave background anisotropies. It discusses the evolving nature of these constraints and the need for ongoing updates in light of emerging astrophysical observations.
4. Illustrative Figures: The paper provides graphical representations of the constraints based on various assumptions, allowing researchers to see how the parameter space varies for different PBH mass functions.

Implications and Future Directions

The implications of this research are both practical and theoretical. Practically, the findings guide astrophysical searches for PBHs, suggesting that more nuanced models need to be considered in future observational campaigns. Theoretically, the research deepens the understanding of how PBHs could be distributed across different mass ranges, informing models of the early universe and PBH formation scenarios.

Given contemporary interest in the role of PBHs, especially following gravitational wave detections by LIGO, this research offers a sobering reflection on the limitations of PBH contributions to dark matter. It suggests that if PBHs are to be a significant dark matter component, future studies must actively reconcile observations with model predictions, taking into account both existing constraints and any novel phenomena that might affect the formation and evolution of PBHs.

Overall, this paper reflects the complex interplay between theory, observation, and cosmological models as researchers continue to address the dark matter puzzle. As the landscape of cosmological observations evolves, the methodologies and results discussed in this paper will serve as a foundation for further exploration into the diverse hypotheses regarding the composition of dark matter, including, but not limited to, primordial black holes.