NANOGrav Hints to Primordial Black Holes as Dark Matter (2009.08268v2)

Abstract: The NANOGrav Collaboration has recently published a strong evidence for a stochastic common-spectrum process that may be interpreted as a stochastic gravitational wave background. We show that such a signal can be explained by second-order gravitational waves produced during the formation of primordial black holes from the collapse of sizeable scalar perturbations generated during inflation. This possibility has two predictions: $i$) the primordial black holes may comprise the totality of the dark matter with the dominant contribution to their mass function falling in the range $(10^{-15}\div 10^{-11}) M_\odot$ and $ii$) the gravitational wave stochastic background will be seen as well by the LISA experiment.

• The paper presents a model linking NANOGrav’s stochastic gravitational wave background to the formation of primordial black holes during inflation.
• It predicts that primordial black holes with masses between 10⁻¹⁵ and 10⁻¹¹ solar masses could account for all dark matter.
• The study highlights that future missions like LISA will critically test the gravitational wave signatures predicted by the model.

NANOGrav Hints to Primordial Black Holes as Dark Matter: An Expert Overview

The paper in question discusses the implications of the recent findings by the NANOGrav Collaboration, which reported a potential observation of a stochastic gravitational wave (GW) background. The authors propose an explanatory model linking this signal to primordial black holes (PBHs), suggesting that these PBHs could constitute the entirety of dark matter in the Universe.

Primordial Black Holes and Gravitational Waves

Primordial Black Holes are hypothesized entities that could form in the early Universe due to the gravitational collapse of high-density regions. The authors explore the scenario where such collapses generate significant scalar perturbations during inflation, leading to the formation of PBHs. They argue that these perturbations could also induce second-order GWs, which might form the background signal observed by NANOGrav. This link is explored through a detailed examination of the second-order perturbative effects on the tensor metric perturbations.

Numerical Results and Theoretical Predictions

The paper presents two key predictions. Firstly, the PBHs are hypothesized to possess mass ranges between $10^{-15}$ to $10^{-11} M_\odot$, which lack stringent observational constraints and allow them to account for all dark matter. Secondly, the model suggests that the stochastic GW background should be detectable by forthcoming experiments, such as LISA, due to the predicted spectral continuity at lower frequencies observed by NANOGrav.

Implications and Future Directions

The significance of this research lies in its potential to link observed GW signals to a cosmological phenomena that could account for dark matter. If validated, this could reshape understanding of both cosmology and particle physics. Future GW observatories, particularly LISA, are anticipated to provide further data to test the proposed model. Speculatively, this could lead to reevaluating PBH theories or necessitating new constraints on their formation mechanisms.

Conclusion

This paper provides a compelling interpretation of the NANOGrav data through the lens of PBH-related gravitational waves, underscoring a potential avenue for resolving the dark matter conundrum. While further experimental validation is necessary, the robustness of the model's predictions encourages extended exploration into PBHs and their observable cosmic imprints.