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Register a new userExploring the Spectral Shape of Gravitational Waves Induced by Primordial Scalar Perturbations and Connection with the Primordial Black Hole Scenarios
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There is a growing expectation that the gravitational wave detectors will start probing the stochastic gravitational wave backgrounds in the following years. We explore the spectral shapes of gravitational waves induced to second order by scalar perturbations and presumably have been produced in the early universe. We calculate the gravitational wave spectra generated during radiation and kination eras together with the associated primordial black hole counterpart. We employ power spectra for the primordial curvature perturbation generated by $alpha$-attractors and nonminimal derivative coupling inflation models as well as Gaussian and delta-type shapes. We demonstrate the ability of the tensor modes to constrain the spectrum of the primordial curvature perturbations and discriminate among inflationary models. Gravitational wave production during kination and radiation era can also be distinguished by their spectral shapes and amplitudes.
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The possibility that primordial black holes (PBHs) represent all of the dark matter (DM) in the Universe and explain the coalescences of binary black holes detected by LIGO/Virgo has attracted a lot of attention. PBHs are generated by the enhancement of scalar perturbations which inevitably produce the induced gravitational waves (GWs). We calculate the induced GWs up to the third-order correction which not only enhances the amplitude of induced GWs, but also extends the cutoff frequency from $2k_*$ to $3k_*$. Such effects of the third-order correction lead to an around $10%$ increase of the signal-to-noise ratio (SNR) for both LISA and pulsar timing array (PTA) observations, and significantly widen the mass range of PBHs in the stellar mass window accompanying detectable induced GWs for PTA observations including IPTA, FAST and SKA. On the other hand, the null detections of the induced GWs by LISA and PTA experiments will exclude the possibility that all of the DM is comprised of PBHs and the GW events detected by LIGO/Virgo are generated by PBHs.
Primordial black holes (PBHs) from the early Universe have been connected with the nature of dark matter and can significantly affect cosmological history. We show that coincidence dark radiation and density fluctuation gravitational wave signatures associated with evaporation of $lesssim 10^9$ g PBHs can be used to explore and discriminate different formation scenarios of spinning and non-spinning PBHs spanning orders of magnitude in mass-range, which is challenging to do otherwise.
Recent observational constraints indicate that primordial black holes (PBHs) with the mass scale $sim 10^{-12}M_{odot}$ can explain most of dark matter in the Universe. To produce this kind of PBHs, we need an enhance in the primordial scalar curvature perturbations to the order of ${mathcal{O}(10^{-2})}$ at the scale $ k sim 10^{12}~rm Mpc^{-1}$. Here, we investigate the production of PBHs and induced gravitational waves (GWs) in the framework of textbf{$k$-inflation}. We solve numerically the Mukhanov-Sasaki equation to obtain the primordial scalar power spectrum. In addition, we estimate the PBHs abundance $f_{text{PBH}}^{text{peak}}$ as well as the energy density parameter $Omega_{rm GW,0}$ of induced GWs. Interestingly enough is that for a special set of model parameters, we estimate the mass scale and the abundance of PBHs as $sim{cal O}(10^{-13})M_{odot}$ and $f_{text{PBH}}^{text{peak}}=0.96$, respectively. This confirms that the mechanism of PBHs production in our inflationary model can justify most of dark matter. Furthermore, we evaluate the GWs energy density parameter and conclude that it behaves like a power-law function $Omega_{rm GW}sim (f/f_c)^n$ where in the infrared limit $fll f_{c}$, the power index reads $n=3-2/ln(f_c/f)$.
Ultralight primordial black holes (PBHs) with masses $lesssim 10^{15}$g and subatomic Schwarzschild radii, produced in the early Universe, are expected to have evaporated by the current cosmic age due to Hawking radiation. Based on this assumption, a number of constraints on the abundance of ultralight PBHs have been made. However, Hawking radiation has thus far not been verified experimentally. It would, therefore, be of interest if constraints on ultralight PBHs could be placed independent of the assumption of Hawking-radiation. In this paper, we explore the possibility of probing these PBHs, within a narrow mass range, using gravitational-wave (GW) data from the two LIGO detectors. The idea is that large primordial curvature perturbations that result in the formation of PBHs, would also generate GWs through non-linear mode couplings. These induced GWs would produce a stochastic background. Specifically, we focus our attention on PBHs of mass range $sim 10^{13} - 10^{15}$g for which the induced stochastic GW background peak falls in the sensitivity band of LIGO. We find that, for both narrow and broad Gaussian PBH mass distributions, the corresponding GW background would be detectable using presently available LIGO data, provided we neglect the existing constraints on the abundance of PBHs, which are based on Hawking radiation. Furthermore, we find that these stochastic backgrounds would be detectable in LIGOs third observing run, even after considering the existing constraints on PBH abundance. A non-detection should enable us to constrain the amplitude of primordial curvature perturbations as well as the abundance of ultralight PBHs. We estimate that by the end of the third observing run, assuming non-detection, we should be able to place constraints that are orders of magnitude better than currently existing ones.
Primordial Black Holes (PBH) from peaks in the curvature power spectrum could constitute today an important fraction of the Dark Matter in the Universe. At horizon reentry, during the radiation era, order one fluctuations collapse gravitationally to form black holes and, at the same time, generate a stochastic background of gravitational waves coming from second order anisotropic stresses in matter. We study the amplitude and shape of this background for several phenomenological models of the curvature power spectrum that can be embedded in waterfall hybrid inflation, axion, domain wall, and boosts of PBH formation at the QCD transition. For a broad peak or a nearly scale invariant spectrum, this stochastic background is generically enhanced by about one order of magnitude, compared to a sharp feature. As a result, stellar-mass PBH from Gaussian fluctuations with a wide mass distribution are already in strong tension with the limits from Pulsar Timing Arrays, if they constitute a non negligible fraction of the Dark Matter. But this result is mitigated by the uncertainties on the curvature threshold leading to PBH formation. LISA will have the sensitivity to detect or rule out light PBH down to $10^{-14} M_{odot}$. Upcoming runs of LIGO/Virgo and future interferometers such as the Einstein Telescope will increase the frequency lever arm to constrain PBH from the QCD transition. Ultimately, the future SKA Pulsar Timing Arrays could probe the existence of even a single stellar-mass PBH in our Observable Universe.
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