Can we identify primordial black holes? The role of subsolar gravitational wave events

TL;DR

This work addresses whether subsolar-mass binaries detected via gravitational waves can be confidently identified as primordial black holes (PBHs) rather than very light neutron stars (NSs). It employs the TaylorF2 inspiral waveform with tidal effects, using Bayesian inference to bound the tidal parameter $\tilde{\Lambda}$ for an SSM200308-like event across O4 and 3G detector networks (ET+2CE). The key finding is that PBHs yield zero tidal phase ($\delta{\psi_{\rm tidal}}=0$), enabling clear discrimination from NSs once tidal information is available, with 3G detectors achieving $\Delta\tilde{\Lambda} \sim 10^{2}$ precision; O4 already delivers precise mass measurements. The results have significant implications: if PBHs are responsible, one can infer the PBH abundance $f_{\rm PBH}$ and constrain PBH dark-matter scenarios; if the objects are NSs, tidal measurements at subsolar masses would tightly constrain the NS equation of state, including the possibility of exotic forms like quark stars.

Abstract

The detection of a subsolar object in a compact binary merger is regarded as one of the most compelling signatures of a population of primordial black holes (PBHs). We critically examine whether such systems can be distinguished from stellar binaries, such as those composed of neutron stars (NSs), which could also populate the subsolar mass range. Unlike PBHs, the gravitational-wave signal from stellar binaries is affected by tidal effects, which increase by several orders of magnitude as the mass decreases. We forecast the capability of current and future gravitational-wave (GW) detectors to constrain tidal effects in putative subsolar binaries. We also discuss the broader implications that the detection of a subsolar merger would have for both cosmology and nuclear physics.

Figures (2)

• Figure 1: Current and future sensitivity curves for the Livingston and Hanford LIGO detectors during the O3, O4, and O5 observing runs, alongside those of the next-generation Einstein Telescope (ET) and Cosmic Explorer (CE). The black dashed line indicates the GW amplitude ($2 |\tilde{h} (f)| \sqrt{f}$), where $\tilde{h} (f)$ is defined in Eq. \ref{['hf']}, for the inspiral phase of an SSM200308-like merger. Source ref.:Crescimbeni:2024cwh
• Figure 2: Posterior distributions of the source-frame masses $(m_1,m_2)$ and the $\tilde{\Lambda}$ parameter for O4 (left) and ET+2CE (right). The 2D contours correspond to 68$\%$ and 95$\%$ credible intervals. The dashed lines in the 1D histograms indicate 1$\sigma$ intervals relative to the median. Red lines mark the injected values. Source ref.:Crescimbeni:2024cwh