Gravitational waves from primordial black hole isocurvature: the effect of non-Gaussianities

Kavli Affiliate: Misao Sasaki

| First 5 Authors: Xin-Chen He, Yi-Fu Cai, Xiao-Han Ma, Theodoros Papanikolaou, Emmanuel N. Saridakis

| Summary:

Ultra-light primordial black holes (PBHs) with masses $M_{rm PBH}<5times
10^8mathrm{g}$ can dominate transiently the energy budget of the Universe and
reheat the Universe through their evaporation taking place before Big Bang
Nucleosynthesis. The isocurvature energy density fluctuations associated to the
inhomogeneous distribution of a population of such PBHs can induce an abundant
production of GWs due to second-order gravitational effects. In this work, we
discuss the effect of primordial non-Gaussianity on the clustering properties
of PBHs and study the effect of a clustered PBH population on the spectral
shape of the aforementioned induced GW signal. In particular, focusing on
local-type non-Gaussianity we find a double-peaked GW signal with the amplitude
of the low-frequency peak being proportional to the square of the non-Gaussian
parameter $tau_mathrm{NL}$. Remarkably, depending on the PBH mass $M_{rm
PBH}$ and the initial abundance of PBHs at formation time, i.e.
$Omega_mathrm{PBH,f}$, this double-peaked GW signal can lie well within the
frequency bands of forthcoming GW detectors, namely LISA, ET, SKA and BBO,
hence rendering this signal falsifiable by GW experiments and promoting it as a
novel portal probing the primordial non-Gaussianity.

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