General Relativity and Quantum Cosmology (gr-qc)

Abstract: Modified Tolman VII (MTVII), in the presence of an additional parameter, can increase the compactness of compact object. If the compactness is in the ultracompact regime, the quasinormal modes~(QNMs) of the trapped mode as well as the gravitational echoes can be more viable. Starting with the MTVII model, we introduce nonlocality into the matter sector and analyze the effective potential, the QNMs, and the gravitational echoes of the compact and ultracompact object in the nonlocal model. The nonlocal gravity version of MTVII~(NGMTVII) is parametrized by the nonlocal parameters~($\tilde{\beta} $), free parameter ($ \alpha $), and the compactness ($ \mathcal{C}$). We have found that NGMTVII can reach $ \mathcal{C}_{max}=0.414 $ with $ \tilde{\beta}_{max} = 3 $ and $ \alpha=0, $ which is significantly more compact than the MTVII model. We have also found that for relatively small value of $\tilde{\beta}$ and the compactness $ \mathcal{C} \lesssim 0.277$~(with $M=2.15$ solar masses, $R=11.5$ km at $\mathcal{C}=0.277$), the causality condition and the dominant energy condition~(DEC) are satisfied. For the perturbation, the quasinormal modes are calculated using Bohr-Sommerfeld (BS) fitting and it is found that the nonlocality produces more trapped modes than the original (MTVII) counterpart. At high compactness, gravitational echoes are simulated numerically.

Abstract: The advent of gravitational wave astronomy has seen a huge influx of new predictions for potential discoveries of beyond the Standard Model fields. The coupling of all fundamental fields to gravity, together with its dominance on large scales, makes gravitational physics a rich laboratory to study fundamental physics. This holds especially true for the search for the elusive dark photon, a promising dark matter candidate. The dark photon is predicted to generate instabilities in a rotating black hole spacetime, birthing a macroscopic Bose-Einstein condensate. These condensates can especially form around super massive black holes, modifying the dynamical inspiralling process. This then opens another window to leverage future space-borne gravitational wave antennas to join the hunt for the elusive dark matter particle. This study builds a preliminary model for the gravitational waveform emitted by such a dressed extreme mass-ratio inspiral. Comparing these waveforms to the vacuum scenario allows projections to the potential constrainability on the dark photon mass by space-borne gravitational wave antennas. The superradiant instability of a massive vector field on a Kerr background is calculated and the modification to the dynamics of an inspiralling solar mass-scale compact object is determined with approximations on the backreaction effect of the cloud on the compact object. The end result is the projection that the LISA mission should be able to constrain the dark photon mass using extreme mass ratio inspirals in the range $[1.8 \times 10^{-17}, 4.47 \times 10^{-16}]$ eV.

Abstract: This article provides a detailed investigation into the motion of the surrounding particles around a polymer black hole in loop quantum gravity (LQG). Using effective potential, the critical bound orbits and innermost stable circular orbits (ISCO) are analyzed. The study finds that the radii and angular momentum of the critical bound orbits decrease with an increase in the parameter $A_\lambda$ which labels the LQG effects, while the energy and angular momentum of the ISCO also decreases with an increase in $A_\lambda$. Based on these findings, we then explore the periodic orbits of the polymer black hole in LQG using rational numbers composed of three integers. Our results show that the rational numbers increase with the energy of particles and decrease with the increase of angular momentum based on a classification scheme. Moreover, compared to a Schwarzschild black hole, the periodic orbits in a polymer black hole in LQG consistently have lower energy, providing a potential method for distinguishing a polymer black hole in LQG from a Schwarzschild black hole. Finally, we also examine the gravitational wave radiations of the periodic orbits of a test object which orbits a supermassive polymer black hole in LQG, which generates intricate GW waveforms that can aid in exhibiting the gravitational structure of the system.

Abstract: In the light of a general scheme of non-Hermitian $\mathcal{PT}$-symmetric Hamiltonian we apply the tetrad-based method to probe the idea of Weyl semimetal black hole analogy. We evaluate the tunneling probability by making use of the conventional null-geodesic approach wherein the associated Hawking radiation is described as a quantum tunneling process across a classically forbidden potential barrier which the event horizon imposes. Our estimate for the tunneling probability is independent of the non-Hermitian parameter that appears in the guiding Hamiltonian.

Abstract: This paper discusses the leading-order correction induced by cosmological perturbations on the average expansion rate of an expanding spacetime, containing one or many perfect fluids. The calculation is carried out up to the second order in the perturbations, and is kept as general as possible. In particular, no approximation such as a long-wavelength or a short-wavelength limit is invoked, and all three types of perturbations (scalar, vector, and tensor) are considered. First, the average value of the expansion rate is computed over a three-dimensional space-like surface where the total density of the fluids is constant. Then, a formula is derived relating that average value to the one over any other surface, on which a different scalar property of the fluids is constant. Moreover, the general formulas giving the correction to the average expansion rate are applied, in particular, to the case of a spacetime containing a single fluid with a constant equation of state. The sign and the effective equation of state of the corresponding back-reaction effect in the first Friedmann equation are examined.

Abstract: From a classical analysis, we show that gravitational waves in a cosmological medium with equation of state $\omega=-1/3$ can follow a London-like equation, implying that some gravitational wave solutions present a decay for certain wavelengths. This scenario, corresponding to a cosmic string cosmology, induces an attenuation temporal scale on the gravitational wave propagation. We discuss on how these solutions impose a limit on the wavelength of the waves that can propagate, which depends on the type of spatial curvature and the energy density content of this type of cosmology.

Abstract: The Effective Field Theory (EFT) of perturbations on an arbitrary background geometry with a timelike scalar profile was recently constructed in the context of scalar-tensor theories. In this paper, we use this EFT to study quasinormal frequencies of odd-parity perturbations on a static and spherically symmetric black hole background. Keeping a set of operators that can accommodate shift-symmetric quadratic higher-order scalar-tensor theories, we demonstrate the computation for two examples of hairy black holes, of which one is the stealth Schwarzschild solution and the other is the Hayward metric accompanied by a non-trivial scalar field. We emphasize that this is the first phenomenological application of the EFT, opening a new possibility to test general relativity and modified gravity theories in the strong gravity regime.

Abstract: We give a simplified approach to Kunzinger & S\"amann's theory of Lorentzian length spaces in the globally hyperbolic, regularly localizable case; these provide a nonsmooth framework for general relativity. We close a gap in this setting, by showing consistency of two potentially different notions of timelike geodesic segments used in the literature. We propose a nonsmooth reformulation of Penrose' null energy condition in terms of the timelike curvature-dimension conditions of Cavalletti & Mondino (and Braun), and discuss its consistency and stability properties. This yields new insights even in the smooth setting.