1,721,029 research outputs found
Footprints of the QCD crossover on cosmological gravitational waves at pulsar timing arrays
Full text linkPulsar timing arrays (PTAs) have reported evidence for a stochastic gravitational wave (GW) background at nanohertz frequencies, possibly originating in the early Universe. We show that the spectral shape of the low-frequency (causality) tail of GW signals sourced at temperatures around T≳1 GeV is distinctively affected by confinement of strong interactions (QCD), due to the corresponding sharp decrease in the number of relativistic species, and significantly deviates from ∼f3 commonly adopted in the literature. Bayesian analyses in the NANOGrav 15 years and the previous international PTA datasets reveal a significant improvement in the fit with respect to cubic power-law spectra, previously employed for the causality tail. While no conclusion on the nature of the signal can be drawn at the moment, our results show that the inclusion of standard model effects on cosmological GWs can have a decisive impact on model selection
NANOGrav Data Hints at Primordial Black Holes as Dark Matter
No full textThe NANOGrav Collaboration has recently published strong evidence for a stochastic common-spectrum process that may be interpreted as a stochastic gravitational wave background. We show that such a signal can be explained by second-order gravitational waves produced during the formation of primordial black holes from the collapse of sizeable scalar perturbations generated during inflation. This possibility has two predictions: (i) the primordial black holes may comprise the totality of the dark matter with the dominant contribution to their mass function falling in the range (10-15÷10-11)M⊙ and (ii) the gravitational wave stochastic background will be seen as well by the Laser Interferometer Space Antenna experiment
On the primordial black hole mass function for broad spectra
Full text linkWe elaborate on the mass function of primordial black holes in the case in which the power spectrum of the curvature perturbation is broad. For the case of a broad and flat spectrum, we argue that such a mass function is peaked at the smallest primordial black mass which can be formed and possesses a tail decaying like M−3/2, where M is the mass of the primordial black hole
Primordial black holes from broad spectra: Abundance and clustering
Full text linkA common mechanism to form primordial black holes in the early universe is by enhancing at small-scales the scalar perturbations generated during inflation. If these fluctuations have a large enough amplitude, they may collapse into primordial black holes upon horizon re-entry. Such primordial black holes may comprise the totality of the dark matter. We offer some considerations about the formation and clustering of primordial black holes when the scalar perturbations are characterised by a broad spectrum. Using the excursion set method, as well as the supreme statistics, we show that the cloud-in-cloud phenomenon, for which small mass primordial black holes may be absorbed by bigger mass ones, is basically absent. This is due to the fact that the formation of a primordial black hole is an extremely rare event. We also show that, from the point of view of mass distribution, broad and narrow spectra give similar results in the sense that the mass distribution is tilted towards a single mass. Furthermore, we argue that primordial black holes from Gaussian broad spectra are not clustered at formation, their distribution is dominantly Poissonian
Imprints of spinning particles on primordial cosmological perturbations
No full textIf there exist higher-spin particles during inflation which are light compared to the Hubble rate, they may leave distinct statistical anisotropic imprints on the correlators involving scalar and graviton fluctuations. We characterise such signatures using the dS/CFT3 correspondence and the operator product expansion techniques. In particular, we obtain generic results for the case of partially massless higher-spin states
Constraining the initial primordial black hole clustering with CMB distortion
Full text linkThe merger rate of primordial black holes depends on their initial clustering. In the absence of primordial non-Gaussianity correlating short and large scales, primordial black holes are distributed à la Poisson at the time of their formation. However, primordial non-Gaussianity of the local type may correlate primordial black holes on large scales. We show that future experiments looking for cosmic microwave background μ distortion would test the hypothesis of initial primordial black hole clustering induced by local non-Gaussianity, while existing limits already show that significant non-Gaussianity is necessary to induce primordial black hole clustering
Bubble correlation in first-order phase transitions
Full text linkMaking use of both the stochastic approach to the tunneling phenomenon and the threshold statistics, we offer a simple argument to show that critical bubbles may be correlated in first-order phase transitions and biased compared to the underlying scalar field spatial distribution. This happens though only if the typical energy scale of the phase transition is sufficiently high. We briefly discuss possible implications of this result, e.g., the formation of primordial black holes through bubble collisions
Stable wormholes in scalar-tensor theories
Full text linkWe reconsider the issue of whether scalar-tensor theories can admit stable wormhole configurations supported by a non-trivial radial profile for the scalar field. Using a recently proposed effective theory for perturbations around static, spherically symmetric backgrounds, we show that scalar-tensor theories of “beyond Horndeski” type can have wormhole solutions that are free of ghost and gradient instabilities. Such solutions are instead forbidden within the more restrictive “Horndeski” class of theories
Bayesian evidence for both astrophysical and primordial black holes: mapping the GWTC-2 catalog to third-generation detectors
No full textWe perform a hierarchical Bayesian analysis of the GWTC-2 catalog to investigate the mixed scenario in which the merger events are explained by black holes of both astrophysical and primordial origin. For the astrophysical scenario we adopt the phenomenological model used by the LIGO/Virgo collaboration and we include the correlation between different parameters inferred from data, the role of the spins in both the primordial and astrophysical scenarios, and the impact of accretion in the primordial scenario. Our best-fit mixed model has a strong statistical evidence relative to the single-population astrophysical model, thus supporting the coexistence of populations of black-hole mergers of two different origins. In particular, our results indicate that the astrophysical mergers account for roughly four times the number of primordial black hole events and predict that third-generation detectors, such as the Einstein Telescope and Cosmic Explorer, should detect up to hundreds of mergers from primordial black hole binaries at redshift zgsim30
Threshold for primordial black holes. II. A simple analytic prescription
Full text linkPrimordial black holes could have been formed in the early universe from nonlinear cosmological perturbations reentering the cosmological horizon when the Universe was still radiation dominated. Starting from the shape of the power spectrum on superhorizon scales, we provide a simple prescription, based on the results of numerical simulations, to compute the threshold δc for primordial black hole formation. Our procedure takes into account both the nonlinearities between the Gaussian curvature perturbation and the density contrast and, for the first time in the literature, the nonlinear effects arising at horizon crossing, which increase the value of the threshold by about a factor two with respect to the one computed on superhorizon scales
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