Inflationary primordial black holes for the LIGO gravitational wave events and pulsar timing array experiments (1611.06130v2)

Abstract: Primordial black holes (PBHs) are one of the candidates to explain the gravitational wave (GW) signals observed by the LIGO detectors. Among several phenomena in the early Universe, cosmic inflation is a major example to generate PBHs from large primordial density perturbations. In this paper, we discuss the possibility to interpret the observed GW events as mergers of PBHs which are produced by cosmic inflation. The primordial curvature perturbation should be large enough to produce a sizable amount of PBHs and thus we have several other probes to test this scenario. We point out that the current pulsar timing array (PTA) experiments already put severe constraints on GWs generated via the second-order effects, and that the observation of the cosmic microwave background (CMB) puts severe restriction on its $\mu$ distortion. In particular, it is found that the scalar power spectrum should have a very sharp peak at $k \sim 10^{6}$ Mpc$^{-1}$ to fulfill the required abundance of PBHs while evading constraints from the PTA experiments together with the $\mu$ distortion. We propose a mechanism which can realize such a sharp peak. In the future, simple inflation models that generate PBHs via almost Gaussian fluctuations could be probed/excluded.

• The paper finds that a sharp peak in the scalar power spectrum can generate PBHs whose mergers explain LIGO gravitational wave events.
• It constrains the required amplification of primordial density perturbations to about 𝒫ζ ~ 0.01 to satisfy PTA and CMB μ distortion limits.
• The study explores a double inflation model that may avoid current observations, suggesting future experiments like SKA or PIXIE could test these scenarios.

Inflationary Primordial Black Holes: Implications for LIGO and Pulsar Timing Array

In the paper titled "Inflationary primordial black holes for the LIGO gravitational wave events and pulsar timing array experiments," the authors explore the potential of primordial black holes (PBHs) as a source of the gravitational wave (GW) signals observed by the LIGO detectors. Cosmic inflation, a major phenomenon in the early Universe, is proposed as a mechanism for generating these PBHs from large primordial density perturbations. The paper thoroughly examines how these perturbations might produce PBHs capable of explaining the LIGO events through merger processes.

The central thesis of the paper hinges on whether the observed GW events can be interpreted as mergers of PBHs formed during cosmic inflation. To satisfy the required abundance of PBHs and evade constraints from pulsar timing array (PTA) experiments and cosmic microwave background (CMB) $\mu$ distortion measurements, the authors argue for a sharp peak in the scalar power spectrum at $k \sim 10^{6}$\,Mpc$^{-1}$. They propose a mechanism capable of generating such a peak, suggesting that future simplistic inflation models that produce PBHs through Gaussian fluctuations may be subject to being ruled out.

Several technical aspects of PBH formation are discussed, highlighting the relation between PBH mass and the primordial perturbation scale. A significant portion of the paper is devoted to deriving constraints on the curvature perturbation spectrum necessary for generating the PBHs associated with LIGO events. The authors show that PTA experiments impose rigorous limits on GWs induced by these perturbations. Additionally, the $\mu$ distortion of the CMB spectrum further constrains the magnitude of scalar perturbations at the required scales.

One of the notable numerical results in the paper is the evaluation of the induced GW density parameter $\Omega_\text{GW} h^2$, which the authors find to be considerable compared to current PTA constraints. They find that the necessary PBH formation would demand a curvature spectrum amplification near $\mathcal P_\zeta \sim 0.01$, which is visibly substantial against the existing constraints. The results indicate a worrying scenario for instances where PBHs are formed via high $\gamma$ values.

The researchers explore a double inflation model as a concrete example where sufficient PBHs might be generated without conflicting with PTA or $\mu$ distortion constraints. Despite some scenarios being marginally held against current observations, the model proposes that specific parameter sets could avoid constraints with more precise, high-sensitivity future observations like SKA or PIXIE.

Implications of this paper are significant both in theoretical and practical domains. The interaction of inflationary physics with observable cosmic phenomena like GWs raises questions about the nature of the early Universe and the role of PBHs in current cosmological models. The work provides a foundation inviting further exploration of inflationary scenarios that could be tested or excluded with upcoming experimental advancements.

Moreover, this research challenges simple inflationary models, suggesting that only highly specific mechanisms may produce PBHs viable for explaining the LIGO events. As observational technologies evolve, the constraints outlined in this paper provide a pivotal reference for guiding searches in the field of astrophysical and cosmological black hole phenomena. Future developments could substantially shape our understanding of inflation, PBHs, and their contributions to the observable Universe.