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General Relativity and Quantum Cosmology (gr-qc)

Tue, 23 May 2023

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1.Thermal analysis and Joule-Thomson expansion of black hole exhibiting metric-affine gravity

Authors:Muhammad Yasir, Xia Tiecheng, Faisal Javed, G. Mustafa

Abstract: This study examines a recently hypothesized black hole, which is a perfect solution of metric-affine gravity with a positive cosmological constant, and its thermodynamic features as well as the Joule-Thomson expansion. We develop some thermodynamical quantities, such as volume, Gibbs free energy, and heat capacity, using the entropy and Hawking temperature. We also examine the first law of thermodynamics and thermal fluctuations, which might eliminate certain black hole instabilities. In this regard, a phase transition from unstable to stable is conceivable when the first law order corrections are present. Besides that, we study the efficiency of this system as a heat engine and the effect of metric-affine gravity for physical parameters $q_e$, $q_m$, $\kappa_{\mathrm{s}}$, $\kappa_{\mathrm{d}}$ and $\kappa_{\mathrm{sh}}$. Further, we study the Joule-Thomson coefficient, and the inversion temperature and also observed the isenthalpic curves in the $T_i -P_i$ plane. In metric-affine gravity, a comparison is made between the Van der Waals fluid and the black hole to study their similarities and differences.

2.Can electromagnetic charge inhabit in Rastall gravity?

Authors:Bobir Toshmatov, Zdeněk Stuchlík, Bobomurat Ahmedov

Abstract: One of the eminent generalizations of theory of general relativity is the Rastall gravity which was {constructed} based on the assumption of the non-conserved energy-momentum tensor of the matter field. Despite in the literature several solutions of black holes in the Rastall gravity coupled to the electromagnetic field have been presented, in the current paper we argue that the Rastall gravity with non-conserved energy-momentum tensor (with $\lambda\neq0$ and $R\neq0$) cannot couple to the electrodynamics, i.e., the electromagnetically charged black hole solution cannot be obtained in this case. This statement is adequate for both linear and nonlinear electrodynamics with the electric, magnetic, or dyonic charges coupled to the Rastall gravity.

3.Modified Friedmann Equations from Maxwell-Weyl Gauge Theory

Authors:Salih Kibaroğlu

Abstract: This study investigates the possibility of a homogeneous and isotropic cosmological solution within the context of the Maxwell-Weyl gauge theory of gravity. To achieve this, we utilize the Einstein-Yang-Mills theory as an analogy and represent the Maxwell gauge field in terms of two time-dependent scalar fields. Then, we present the modified Friedmann equations, which incorporate the contributions of the Maxwell gauge field, as well as the effective cosmological constant, which is a function of the Dirac scalar field.

4.Spins of primordial black holes formed with a soft equation of state

Authors:Daiki Saito, Tomohiro Harada, Yasutaka Koga, Chul-Moon Yoo

Abstract: We investigate the probability distribution of the spins of primordial black holes (PBHs) formed in the universe dominated by a perfect fluid with the linear equation of state $p=w\rho$, where $p$ and $\rho$ are the pressure and energy density of the fluid, respectively. We particularly focus on the parameter region $0<w\leq 1/3$ since the larger value of the spin is expected for the softer equation of state than that of the radiation fluid ($w=1/3$). The angular momentum inside the collapsing region is estimated based on the linear perturbation equation at the turn-around time which we define as the time when the linear velocity perturbation in the conformal Newtonian gauge takes the minimum value. The probability distribution is derived based on the peak theory with the Gaussian curvature perturbation. We find that the root mean square of the non-dimensional Kerr parameter $\sqrt{\langle a_{*}^2\rangle}$ is approximately proportional to $(M/M_{H})^{-1/3}(6w)^{-(1+2w)/(1+3w)}$, where $M$ and $M_{H}$ are the mass of the PBH and the horizon mass at the horizon entry, respectively. Therefore the typical value of the spin parameter decreases with the value of $w$. We also evaluate the mass and spin distribution $P(a_{*}, M)$, taking account of the critical phenomena. We find that, while the spin is mostly distributed in the range of $10^{-3.9}\leq a_{*}\leq 10^{-1.8}$ for the radiation-dominated universe, the peak of the spin distribution is shifted to the larger range $10^{-3.0}\leq a_{*}\leq 10^{-0.7}$ for $w=10^{-3}$.

5.Probing high frequency gravitational waves with pulsars

Authors:Asuka Ito, Kazunori Kohri, Kazunori Nakayama

Abstract: We study graviton-photon conversion in magnetosphere of a pulsar and explore the possibility of detecting high frequency gravitational waves with pulsar observations. It is shown that conversion of one polarization mode of photons can be enhanced significantly due to strong magnetic fields around a pulsar. We also constrain stochastic gravitational waves in frequency range of $10^{8}-10^{9}\,$Hz and $10^{13}-10^{27}\,$Hz by using data of observations of the Crab pulsar and the Geminga pulsar. Our method widely fills the gap among existing high frequency gravitational wave experiments and boosts the frequency frontier in gravitational wave observations.

6.Kinematic topologies of black holes

Authors:Jiayu Yin, Jie Jiang, Ming Zhang

Abstract: We investigate the kinematic topologies of light rings (LRs) and massive particle rings (PRs) encircling spherical and axisymmetric black holes. Our results demonstrate that the global topology number of LRs is consistently -1, independent of the spacetime background's asymptotic property. Additionally, we show that the global topology of PRs varies, with a value of 0 in asymptotically flat and Anti-de Sitter spacetime but -1 in asymptotic de Sitter spacetime.

7.Bayesian Time Delay Interferometry for Orbiting LISA: Accounting for the Time Dependence of Spacecraft Separations

Authors:Jessica Page, Tyson Littenberg

Abstract: Previous work demonstrated effective laser frequency noise (LFN) suppression for Laser Interferometer Space Antenna (LISA) data from raw phasemeter measurements using a Markov Chain Monte Carlo (MCMC) algorithm with fractional delay interpolation (FDI) techniques to estimate the spacecraft separation parameters required for time-delay interferometry (TDI) under the assumption of a rigidly rotating LISA configuration. Including TDI parameters in the LISA data model as part of a global fit analysis pipeline enables gravitational wave inferences to be marginalized over uncertainty in the spacecraft separations. Here we extend the algorithm's capability to perform data-driven TDI on LISA in Keplerian orbits, which introduce a time-dependence in the arm-length parameters and at least $\mathcal{O}$(M) times greater computational cost since the filter must be applied for every sample in the time series of sample size M. We find feasibility of arm-length estimation on $\sim$day-long time scales by using a novel Taylor-expanded version of the fractional delay interpolation filter that allows half of the filter computation to be calculated and stored before MCMC iterations and requires shorter filter lengths than previously reported. We demonstrate LFN suppression for orbiting LISA using accurate arm-length estimates parameterized by Keplerian orbital parameters under the assumption of unperturbed analytical Keplerian orbits, and explore the potential extension of these methods to arbitrary numerical orbits.