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Register a new userSpins of primordial black holes formed in the matter-dominated phase of the Universe
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Angular momentum plays very important roles in the formation of PBHs in the matter-dominated phase if it lasts sufficiently long. In fact, most collapsing masses are bounced back due to centrifugal force, since angular momentum significantly grows before collapse. As a consequence, most of the formed PBHs are rapidly rotating near the extreme value $a_{*}=1$, where $a_{*}$ is the nondimensional Kerr parameter at their formation. The smaller the density fluctuation $sigma_{H}$ at horizon entry is, the stronger the tendency towards the extreme rotation. Combining the effect of angular momentum with that of anisotropy, we estimate the black hole production rate. We find that the production rate suffers from suppression dominantly due to angular momentum for a smaller value of $sigma_{H}$, while due to anisotrpopy for a larger value of $sigma_{H}$. We argue that matter domination significantly enhances the production of PBHs despite the suppression. If the matter-dominated phase does not last so long, the effect of the finite duration significantly suppresses PBH formation and weakens the tendency towards large spins. (abridged)
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The standard deviation of the initial values of the nondimensional Kerr parameter $a_{*}$ of primordial black holes (PBHs) formed in the radiation-dominated phase of the universe is estimated to the first order of perturbation for the narrow power spectrum. Evaluating the angular momentum at turn around based on linearly extrapolated transfer functions and peak theory, we obtain the expression $sqrt{langle a_{*}^{2} rangle} simeq 4.0times 10^{-3} (M/M_{H})^{-1/3}sqrt{1-gamma^{2}}[1-0.072 log_{10}(beta_{0}(M_{H})/(1.3times 10^{-15}))]^{-1}$, where $M_{H}$, $beta_{0}(M_{H})$, and $gamma$ are the mass within the Hubble horizon at the horizon entry of the overdense region, the fraction of the universe which collapsed to PBHs at the scale of $M_{H}$, and a quantity which characterizes the width of the power spectrum, respectively. This implies that for $Msimeq M_{H}$, the higher the probability of the PBH formation, the larger the standard deviation of the spins, while PBHs of $Mll M_{H}$ formed through near-critical collapse may have larger spins than those of $Msimeq M_{H}$. In comparison to the previous estimate, the new estimate has the explicit dependence on the ratio $M/M_{rm H}$ and no direct dependence on the current dark matter density. On the other hand, it suggests that the first-order effect can be numerically comparable to the second-order one.
With approximately 50 binary black hole events detected by LIGO/Virgo to date and many more expected in the next few years, gravitational-wave astronomy is shifting from individual-event analyses to population studies. We perform a hierarchical Bayesian analysis on the GWTC-2 catalog by combining several astrophysical formation models with a population of primordial black holes. We compute the Bayesian evidence for a primordial population compared to the null hypothesis, and the inferred fraction of primordial black holes in the data. We find that these quantities depend on the set of assumed astrophysical models: the evidence for primordial black holes against an astrophysical-only multichannel model is decisively favored in some scenarios, but it is significantly reduced in the presence of a dominant stable-mass-transfer isolated formation channel. The primordial channel can explain mergers in the upper mass gap such as GW190521, but (depending on the astrophysical channels we consider) a significant fraction of the events could be of primordial origin even if we neglected GW190521. The tantalizing possibility that LIGO/Virgo may have already detected black holes formed after inflation should be verified by reducing uncertainties in astrophysical and primordial formation models, and it may ultimately be confirmed by third-generation interferometers.
We investigate primordial black hole formation in the matter-dominated phase of the Universe, where nonspherical effects in gravitational collapse play a crucial role. This is in contrast to the black hole formation in a radiation-dominated era. We apply the Zeldovich approximation, Thornes hoop conjecture, and Doroshkevichs probability distribution and subsequently derive the production probability $beta_{0}$ of primordial black holes. The numerical result obtained is applicable even if the density fluctuation $sigma$ at horizon entry is of the order of unity. For $sigmall 1$, we find a semi-analytic formula $beta_{0}simeq 0.05556 sigma^{5}$, which is comparable with the Khlopov-Polnarev formula. We find that the production probability in the matter-dominated era is much larger than that in the radiation-dominated era for $sigmalesssim 0.05$, while they are comparable with each other for $sigmagtrsim 0.05$. We also discuss how $sigma$ can be written in terms of primordial curvature perturbations.
One of the crucial windows for distinguishing astrophysical black holes from primordial black holes is through the redshift evolution of their respective merger rates. The low redshift population of black holes of astrophysical origin is expected to follow the star formation rate. The corresponding peak in their merger rate peaks at a redshift smaller than that of the star formation rate peak ($z_p approx 2$), depending on the time delay between the formation and mergers of black holes. Black holes of primordial origin are going to be present before the formation of the stars, and the merger rate of these sources at high redshift is going to be large. We propose a joint estimation of a hybrid merger rate from the stochastic gravitational wave background, which can use the cosmic history of merger rates to distinguish between the two populations of black holes. Using the latest bounds on the amplitude of the stochastic gravitational wave background amplitude from the third observation run of LIGO/Virgo, we obtain weak constraints at $68%$ C.L. on the primordial black hole merger rate index $2.56_{-1.76}^{+1.64}$ and astrophysical black hole time delay $6.7_{-4.74}^{+4.22}$ Gyr. We should be able to distinguish between the different populations of black holes with the forthcoming O5 and A+ detector sensitivities.
Black Holes are possibly the most enigmatic objects in our Universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past 4 years. Nevertheless, they remain active playgrounds for strong gravity and quantum effects, where novel aspects of the elusive theory of quantum gravity may be hard at work. In this review article, we provide an overview of the strong motivations for why Quantum Black Holes may be radically different from their classical counterparts in Einsteins General Relativity. We then discuss the observational signatures of quantum black holes, focusing on gravitational wave echoes as smoking guns for quantum horizons (or exotic compact objects), which have led to significant recent excitement and activity. We review the theoretical underpinning of gravitational wave echoes and critically examine the seemingly contradictory observational claims regarding their (non-)existence. Finally, we discuss the future theoretical and observational landscape for unraveling the Quantum Black Holes in the Sky.
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