[Paper Review] Scrutinizing the Primordial Black Holes Interpretation of PTA Gravitational Waves and JWST Early Galaxies

Yann Gouttenoire, Sokratis Trifinopoulos+2 more|arXiv (Cornell University)|Jul 4, 2023

Cosmology and Gravitation TheoriesPhysics and Astronomy18 citations

TL;DR

The paper tests whether supermassive primordial black holes (PBHs) can jointly explain the PTA gravitational-wave background and JWST’s early massive galaxies, finding most PBH-based explanations incompatible with UV luminosity function and large-scale structure constraints; a narrow seed-based JWST-compatible region may survive only under extreme primordial non-Gaussianities.

ABSTRACT

Recent observations have granted to us two unique insights into the early universe: the presence of a low-frequency stochastic gravitational wave background detected by the NANOGrav and Pulsar Timing Array (PTA) experiments and the emergence of unusually massive galaxy candidates at high redshifts reported by the James Webb Space Telescope (JWST). In this letter, we consider the possibility that both observations have a common origin, namely primordial black holes (PBHs) in the mass range between $10^{6}~M_{\odot}$ and $10^{13}~M_{\odot}$. While superheavy PBHs act as seeds for accelerated galaxy formation capable of explaining the JWST extreme galaxies, they can also form binary mergers that source gravitational waves which can be potentially identified as the PTA signal. The analysis is performed taking into account the constraints on the relevant region of the PBH parameter space including the novel bound imposed by the Ultraviolet Luminosity Function of galaxies observed by the Hubble Space Telescope. We conclude that PTA's and JWST's interpretations in terms of PBH binary mergers and Poissonian gas of PBHs, respectively, are strongly excluded.

Motivation & Objective

• Assess whether PBHs in the mass range 10^6–10^13 solar masses can explain the PTA GW background via PBH binaries.
• Assess whether PBHs can seed the rapid formation of JWST-detected massive galaxies at high redshift.
• Incorporate constraints from UV galaxy luminosity functions (HST UV LF), large-scale structure, and other bounds.
• Compare PBH-based explanations with environmental and non-Gaussianity considerations to identify viable parameter space.

Proposed method

• Model the GW energy density from PBH binaries with a broken-law fit to Omega_GW(h^2) and a peak at f_peak ~ 5000 M_sun/M_PBH for PBH binaries.
• Compute the PBH-induced isocurvature contribution to the matter power spectrum using a Poisson term P_iso(k) with a cut-off k_cut, and predict the halo mass function via Sheth-Tormen with a top-hat window.
• Perform a Bayesian analysis of PTA data (NANOGrav 12.5-year and IPTA DR2) to derive posteriors for PBH mass M_PBH and abundance f_PBH.
• Incorporate JWST-era galaxy formation via Poisson and seed effects to evaluate the ability of PBHs to produce the observed massive galaxies at z~8.
• Impose UV LF constraints from HST data to bound PBH-induced isocurvature perturbations and compare with LSS/dynamical friction limits.

Experimental results

Research questions

• RQ1 Can supermassive PBHs in the 10^6–10^13 M_sun range explain the NANOGrav/PTA stochastic GW background through binary mergers?
• RQ2 Can PBHs efficiently seed or enhance early galaxy formation to account for JWST's massive high-z galaxies?
• RQ3 What constraints do HST UV luminosity functions, large-scale structure, and μ-distortion bounds impose on the PBH parameter space (M_PBH, f_PBH)?
• RQ4 Is there a region in PBH parameter space that simultaneously fits PTA and JWST observations without violating constraints, and under what non-Gaussianity conditions?

Key findings

• PTA interpretation via PBH binary mergers is strongly excluded by UV LF constraints and subject to LSS bounds.
• JWST interpretation via PBH Poisson fluctuations to boost early galaxies is also excluded by UV LF and structure formation limits.
• A narrow region with 10^8 ≤ M_PBH/M_sun ≤ 10^9 and f_PBH ~ 10^-2 can compatibly explain both PTA and JWST signals at the 90% CL, but this region is tightly constrained by UV LF and environmental considerations.
• The UV LF bound derived from HST data is more stringent than the prior LSS bounds for PBHs in this mass range.
• PBH seeding of JWST-like galaxies could remain viable for f_PBH ≲ 10^-3 only if extremely large primordial non-Gaussianities are assumed (p ≲ 0.5), otherwise ruled out.
• The note discusses that clustering scenarios may evade some constraints, but are still challenging under more aggressive k_cut choices.