Formation of black hole and emission of gravitational waves

Numerical simulations were performed for the formation process of rotating black holes. It is suggested that Kerr black holes are formed for wide ranges of initial parameters. The nature of gravitational waves from a test particle falling into a Kerr black hole as well as the development of 3D numerical relativity for the coalescing binary neutron stars are discussed.     

Primordial black holes and third order scalar induced gravitational waves

The process of primordial black hole (PBH) formation is inevitably accompanied by scalar induced gravitational waves (SIGWs). The strong correlation between PBH and SIGW signals may offer a promising approach to detecting PBHs in upcoming gravitational wave experiments, such as the Laser Interferometer Space Antenna (LISA). We investigate third order SIGWs during a radiation-dominated era in the case of the monochromatic primordial power spectrum \begin{document}$ \mathcal{P}_{\zeta}=A_{\zeta}k_*\delta\left(k-k_*\right) $\end{document}. For LISA observations, the relationships between the signal-to-noise ratio (SNR) and monochromatic primordial power spectrum are studied systematically, revealing that the effects of third order SIGWs extend the cutoff frequency from \begin{document}$ 2f_* $\end{document} to \begin{document}$ 3f_* $\end{document} and lead to an approximately 200% increase in the SNR for the frequency band from \begin{document}$ 10^{-5} $\end{document}Hz to \begin{document}$ 1.6\times 10^{-3} $\end{document}Hz, corresponding to PBHs with masses in the range \begin{document}$4\times 10^{-12}M_{\odot} \sim 10^{-7}M_{\odot}$\end{document}. We find that there is a critical value, \begin{document}$ A_*=1.76\times 10^{-2} $\end{document}, for the amplitude of the monochromatic primordial power spectra, such that when \begin{document}$ A_{\zeta}>A_* $\end{document}, the energy density of third order SIGWs is larger than that of second order SIGWs.     

Search for electron-antineutrinos associated with gravitational-wave events GW150914, GW151012, GW151226, GW170104, GW170608, GW170814, and GW170817 at Daya Bay

The establishment of a possible connection between neutrino emission and gravitational-wave (GW) bursts is important to our understanding of the physical processes that occur when black holes or neutron stars merge. In the Daya Bay experiment, using the data collected from December 2011 to August 2017, a search was performed for electron-antineutrino signals that coincided with detected GW events, including GW150914, GW151012, GW151226, GW170104, GW170608, GW170814, and GW170817. We used three time windows of ±10, ±500, and ±1000 s relative to the occurrence of the GW events and a neutrino energy range of 1.8 to 100 MeV to search for correlated neutrino candidates. The detected electron-antineutrino candidates were consistent with the expected background rates for all the three time windows. Assuming monochromatic spectra, we found upper limits (90% confidence level) of the electron-antineutrino fluence of (1.13 ? 2.44)×10¹¹ cm^?2 at 5 MeV to 8.0×10⁷ cm^?2 at 100 MeV for the three time windows. Under the assumption of a Fermi-Dirac spectrum, the upper limits were found to be (5.4 ? 7.0)×10⁹ cm^?2 for the three time windows.     

The LIGO-Virgo O3 Run and the Multi-Messenger Investigations of Compact Binary Mergers

The third observing run (O3) of Advanced LIGO and Advanced Virgo started in April 2019 and ended in March 2020 due to the pandemic. From the O3 run three catalogs of compact binary mergers, GWTC-2, GWTC-2.1, and GWTC-3, that include also some exceptional events, are produced by the LIGO/Virgo Collaboration. The paper will review the science results about compact binary mergers during the O3 run and the follow-up of gravitational wave candidate events involving the whole electromagnetic spectrum and neutrinos. No confirmed counterpart is found during the O3 run for any candidate. The impact of detected events on astrophysics and cosmology will also be discussed. The paper will also briefly summarize additional multi-messenger investigations involving candidates not initially associated to gravitational events.     

The tune of love and the nature(ness) of spacetime

The black hole information paradox is among the most outstanding puzzles in physics. I argue here there is yet another black hole quandary which, in light of the recent direct detection of gravitational waves by Advanced LIGO, reveals a new window to probe the nature of spacetime in the forthcoming era of ‘precision gravity.'     

On the Reconstruction of the Scalar Mode of GW170817 in Scalar-Tensor Gravity Theory

In general metric theory of gravity, a gravitational wave is allowed to have up to six polarizations: two scalar and two vector modes in addition to tensor modes. In case the number of laser-interferometric gravitational wave telescopes is larger than the number of polarizations of a gravitational wave, all the polarizations can be individually reconstructed. Since it depends on theories of gravity which polarizations the gravitational waves have, the investigation of polarizations is important for the test of theories of gravity. In order to test the scalar–tensor gravity theory, one of important alternative theories of gravity, the scalar mode of GW170817 observed by LIGO Livingstone, Hanford and Virgo is reconstructed without prior information about any tensor–scalar gravity theories. The upper limit of the scalar mode in term of the band-limited root-sum-square of the amplitude is $$ with the time window of 2 [s] and frequency window of ≈60–120 [Hz]. It is also studied how much the tensor modes are leaked into the reconstructed scalar mode, and it is found that the reconstructed scalar mode contains roughly 30% of energy leaked from the tensor modes.     

Strong gravitational lensing for photon coupled to Weyl tensor in Kiselev black hole

The objective of the present work is to highlight the phenomena of strong gravitational lensing and deflection angle for the photon coupling with the Weyl tensor in a Kiselev black hole. Here, we have extended the prior work of Chen and Jing (S. Chen and J. Jing, JCAP, 10: 002 (2015)) for a Schwarzschild black hole to a Kiselev black hole. For this purpose, the equation of motion for the photons coupled to the Weyl tensor, null geodesic, and equation of photon sphere in a Kiselev black hole spacetime have been formulated. It is found that the equation of motion of the photons depends not only on the coupling between the photons and the Weyl tensor, but also on the polarization direction of the photons. There is a critical value of the coupling parameter, \begin{document}$ \alpha$\end{document} , for the existence of the marginally circular photon orbit outside the event horizon, which depends on the parameters of the black hole and the polarization direction of the photons. Further, the polarization directions of the coupled photons and the coupling parameter, \begin{document}$ \alpha$\end{document}; both modify the features of the photon sphere, angle of deflection, and functions \begin{document}$ (\bar{a}$\end{document} and \begin{document}$ \bar{b})$\end{document} owing to the strong gravitational lensing in the Kiselev black hole spacetime. In addition to this, the observable gravitational lensing quantities and the shadows of the Kiselev black hole spacetime are presented in detail.     

Gravitational perturbations of the Einstein-Euler-Heisenberg black hole

The prospect of using gravitational wave detections via the quasinormal modes (QNMs) to test modified gravity theories is exciting area of current research. Gravitational waves (GWs) emitted by a perturbed black hole (BH) will decay as a superposition of their QNMs of oscillations at the ringdown phase. In this work, we investigate the QNMs of the Einstein-Euler-Heisenberg (EEH) BH for both axial and polar gravitational perturbations. We obtain master equations with the tetrad formalism, and the quasinormal frequencies of the EEH BH are calculated in the 6th order Wentzel-Kramers-Brill?uin approximation. It is interesting to note that the QNMs of the EEH BH would differ from those of the Reissner-Nordstr?m BH under the EH parameter, which indicates the EH parameter would affect the gravitational perturbations for the EEH BH.     

Central black hole masses of galaxies

In this paper, the stellar velocity dispersions in the host galaxies are used to estimate the central black hole masses for a sample of elliptical galaxies. We find that the central black hole masses are in the range of 10^{(5.5-9.5)}M_⊙. Based on the estimated masses in this paper and those by Woo & Urry (2002) and the measured host galaxy absolute magnitude, a relation, log (M_{BH}/M_⊙) = -(0.25±4.3×10^{-3})M_R + (2.98±0.208) is found for central black hole mass and the host galaxy magnitude. Some discussions are presented.     

Quasistationary collapse to the extreme Kerr black hole

It is shown that the extreme Kerr black hole, i.e. the one with the maximal angular momentum for a given mass, is the only candidate for a black hole limit of rotating fluid bodies in equilibrium.     

Constrained instanton and black hole creation

It is generally accepted that the Einstein theory of general relativity governs the evolution of the universe. However, the singularity theorem of Hawking and Penrose[1] shows that under the most reasonable physical conditions, a spacetime singularity, where physics laws and even causality are broken down, is unavoidable. One has to impose the boundary or initial conditions at the edge of spacetime for the evolution of the universe. This is the so-called first cause problem. The no-boundary …     

Thermodynamics of regular black hole

We investigate thermodynamics for a magnetically charged regular black hole (MCRBH), which comes from the action of general relativity and nonlinear electromagnetics, comparing with the Reissner–Norström (RN) black hole in both four and two dimensions after dimensional reduction. We find that there is no thermodynamic difference between the regular and RN black holes for a fixed charge Q in both dimensions. This means that the condition for either singularity or regularity at the origin of coordinate does not affect the thermodynamics of black hole. Furthermore, we describe the near-horizon AdS₂ thermodynamics of the MCRBH with the connection of the Jackiw–Teitelboim theory. We also identify the near-horizon entropy as the statistical entropy by using the AdS₂/CFT₁ correspondence.     

Entropy and topology of the Kerr—de Sitter black hole

By using the path integral method of Gibbons and Hawking, the entropy of the Kerr-de Sitter black hole is investigated under the microcanonical ensemble. We find that the entropy is one eighth the sum of the products of the Euler number of its cosmological horizon and event horizon with their respective areas. It is shown that the origin of the entropy of the black hole is related to the topology of its instanton.     

Primordial black hole as a source of the boost factor

Primordial black holes (PBHs) accumulate weakly interacting massive particles (WIMPs) around them and form ultracompact minihalos (UCMHs), if the WIMP is a dominant component of the dark matter (DM). In this Letter, we discuss that the UCMHs seeded by the PBHs with sub-earth mass enhance the WIMP annihilation in the present Universe and can successfully explain the positron and/or electron excess in cosmic ray observed by PAMELA/Fermi experiments. The signal is very similar to that from a decaying dark matter, which can explain the PAMELA and/or Fermi anomaly without conflict with any constraints as long as the decay mode is proper. In this scenario, the boost factor can be as large as 10⁵. In addition, we discuss testability of our scenario by gamma-ray point source and gravitational-wave experiments.     

Violation of the second law of black hole physics by tachyons

It is shown that the interaction of a class of positive energy tachyons decreases the area of the horizon of theT-S, ∂=3 metric even in the case of a reversible transformation (M _ir=constant). This is a violation of the second law of black hole physics.     

Pre-low-mass X-ray binaries containing a black hole: investigating a detection mechanism

This work investigates the feasibility of detecting close, detached, black hole-red dwarf binaries, which are expected to be evolutionary precursors of low-mass X-ray binaries (LMXBs). Although this pre-low-mass X-ray binary (pre-LMXB) phase of evolution is predicted theoretically, as yet no such systems have been identified observationally. The calculations presented here suggest that the X-ray luminosity of black hole wind accretion in a pre-LMXB system could exceed the intrinsic X-ray luminosity of the red dwarf secondary star, thereby providing a detection mechanism. However, there is significant uncertainty regarding the efficiency of the conversion of gravitational potential energy to X-ray luminosity resulting from accretion onto a black hole, for example energy may be lost via advection across the event horizon. Still, sources with X-ray luminosities greater than that expected for a red dwarf star, but whose positions coincide with that of a red dwarf would represent candidate pre-LMXB systems. These candidates should be surveyed for the radial velocity shifts that would occur as a result of the orbital motion of a red dwarf star within a close binary system containing a black hole.      

Cosmological black holes: A black hole in the Einstein-de Sitter universe

As an example of a dynamical cosmological black hole, a spacetime that describes an expanding black hole in the asymptotic background of the Einstein-de Sitter universe is constructed. The black hole is primordial in the sense that it forms ab initio with the big bang singularity and its expanding event horizon is represented by a conformal Killing horizon. The metric representing the black hole spacetime is obtained by applying a time dependent conformal transformation on the Schwarzschild metric, such that the result is an exact solution with a matter content described by a two-fluid source. Physical quantities such as the surface gravity and other effects like perihelion precession, light bending and circular orbits are studied in this spacetime and compared to their counterparts in the gravitational field of the isolated Schwarzschild black hole. No changes in the structure of null geodesics are recorded, but significant differences are obtained for timelike geodesics, particularly an increase in the perihelion precession and the non-existence of circular timelike orbits. The solution is expressed in the Newman-Penrose formalism.     

Expanding the Area of Gravitational Entropy

I describe how gravitational entropy is intimately connected with the concept of gravitational heat, expressed as the difference between the total and free energies of a given gravitational system. From this perspective one can compute these thermodyanmic quantities in settings that go considerably beyond Bekenstein's original insight that the area of a black hole event horizon can be identified with thermodynamic entropy. The settings include the outsides of cosmological horizons and spacetimes with NUT charge. However the interpretation of gravitational entropy in these broader contexts remains to be understood.     

Dirac quasinormal modes of Born-Infeld black hole spacetimes

Quasinormal modes (QNMs) for massless and massive Dirac perturbations of Born-Infeld black holes (BHs) in higher dimensions are investigated. Solving the corresponding master equation in accordance with hypergeometric functions and the QNMs are evaluated. We discuss the relationships between QNM frequencies and spacetime dimensions. Meanwhile, we also discuss the stability of the Born-Infeld BH by calculating the temporal evolution of the perturbation field. Both the perturbation frequencies and the decay rate increase with increasing dimension of spacetime n. This shows that the Born-Infeld BHs become more and more unstable at higher dimensions. Furthermore, the traditional finite difference method is improved, so that it can be used to calculate the massive Dirac field. We also elucidate the dynamic evolution of Born-Infeld BHs in a massive Dirac field. Because the number of extra dimensions is related to the string scale, there is a relationship between the spacetime dimension n and the properties of Born-Infeld BHs that might be advantageous for the development of extra-dimensional brane worlds and string theory.