Primordial black hole ringdown: The irreducible stochastic gravitational wave background

Abstract

Independently from the formation mechanism of primordial black holes in the early Universe, their generation is accompanied by a ringdown phase during which they relax to a stationary configuration and gravitational waves under the form of quasinormal modes are emitted. Such gravitational waves generate an irreducible and unavoidable stochastic background which is testable by current and future experiments. In particular, for primordial black holes with masses exceeding $10^{14}\,M_{\odot}$, the associated stochastic background lies within the frequency range accessible to current and upcoming cosmic microwave background experiments, thereby providing a direct observational way to probe the existence of such extremely massive objects.

Figures (1)

• Figure 1: Left panel: Current constraints on the PBH abundance for a monochromatic population of non-spinning PBHs (see the main text for a description). The maximum value of the PBH abundance, namely $f^{\text{\tiny max}}_\text{\tiny PBH}(M)$, is shown by a black line. Right panel: Prediction for the present GW abundance associated to the SGWB induced by PBH ringdown. The black lines denote the predictions assuming a QNM amplitude $|A|=( 10^{-5},10^{-3},10^{-2},10^{-1})$ and $f_\text{\tiny PBH}= f^{\text{\tiny max}}_\text{\tiny PBH}(M)$, while solid (and dotted) gray lines are obtained fixing $|A|=10^{-2}$ and assuming $f_\text{\tiny PBH}=1$$(10^{-5})$. The plot also shows the constraints derived from ongoing experiments (solid colored lines) and sensitivity from future missions (dashed colored lines) (see the main text for a description). We stress that the black and gray lines should not be interpreted as a continuous signal but as a set of points; given a monochromatic population of PBHs with mass $M$, one can determine first the peak frequency at which the signal is produced, $f_\text{\tiny GW}$, and then the corresponding amplitude at the peak of the signal, $h_0^2\,\Omega_{\text{\tiny GW},0}(f_\text{\tiny GW})$.