General Relativity and Quantum Cosmology (gr-qc)

Abstract: We analyse several aspects of detectors on uniformly accelerated, rotating trajectories in de Sitter ($\Lambda>0$) and anti-de Sitter ($\Lambda<0$) spacetimes, focusing particularly on the periodicity, in (Euclidean) proper time $\tau_{\rm traj}$, of geodesic interval $\tau_{\rm geod}$ between two events on the trajectory. We show that for $\Lambda<0$, $\tau_{\rm geod}$ is periodic in ${\rm i} \tau_{\rm traj}$ for specific values of torsion and acceleration. The periodicity disappears in the limit $\Lambda \to 0$, yielding the well known Minkowski result. The results for stationary rotational trajectories in arbitrary curved spacetime are expressed as a perturbative expansion in curvature. Relevance of the work for Unruh-de Witt detectors in curved spacetimes is highlighted.

Abstract: We examine the anisotropy originated from a first-order vacuum phase transitions through three-dimensional numerical simulations. We apply Bianchi Type-I metric to our model that has one scalar field minimally coupled to the gravity. We calculate the time evolution of the energy density for the shear scalar and the directional Hubble parameters as well as the power spectra for the scalar field and the gravitational radiation although there are a number of caveats for the tensor perturbations in Bianchi Type-I universe. We run simulations with different mass scales of the scalar field, therefore, in addition to investigation of anisotropy via the shear scalar, we also determine at which mass scale the phase transition completes successfully, hence, neglecting the expansion of the Universe does not significantly affect the results. Finally, we showed that such an event may contribute to the total anisotropy depending on the mass scale of the scalar field and the initial population of nucleated bubbles.

Abstract: The geometries with SL$(2,\mathbb{R})$ and some axial U$(1)$ isometries are called ``near-horizon extremal geometries" and are found usually, but not necessarily, in the near-horizon limit of the extremal black holes. We present a new member of this family of solutions in five-dimensional Einstein-Hilbert gravity that has only one non-zero angular momentum. In contrast with the single-rotating Myers-Perry extremal black hole and its near-horizon geometry in five dimensions, this solution has a non-vanishing and finite entropy. Although there is a uniqueness theorem that prohibits the existence of such single-rotating near-horizon geometries in five-dimensional general relativity, this solution has a curvature singularity at one of the poles, which breaks the smoothness conditions in the theorem.

Abstract: Reduced Order Quadrature (ROQ) methods can greatly reduce the computational cost of Gravitational Wave (GW) likelihood evaluations, and therefore greatly speed up parameter estimation analyses, which is a vital part to maximize the science output of advanced GW detectors. In this paper, we do an in-depth study of ROQ techniques applied to GW data analysis and present novel algorithms to enhance different aspects of the ROQ bases construction. We improve upon previous ROQ construction algorithms allowing for more efficient bases in regions of parameter space that were previously challenging. In particular, we use singular value decomposition (SVD) methods to characterize the waveform space and choose a reduced order basis close to optimal and also propose improved methods for empirical interpolation node selection, greatly reducing the error added by the empirical interpolation model. To demonstrate the effectiveness of our algorithms, we construct multiple ROQ bases ranging in duration from 4s to 256s for compact binary coalescence (CBC) waveforms including precession and higher order modes. We validate these bases by performing likelihood error tests and P-P tests and explore the speed up they induce both theoretically and empirically with positive results. Furthermore, we conduct end-to-end parameter estimation analyses on several confirmed GW events, showing the validity of our approach in real GW data.

Abstract: We often encounter a situation that black hole solutions can be regarded as continuous deformations of simpler ones, or modify general relativity by continuous parameters. We develop a general framework to compute high-order perturbative corrections to quasinormal mode frequencies in such deformed problems. Our method has many applications, and allows to compute numerical values of the high-order corrections very accurately. For several examples, we perform this computation explicitly, and discuss analytic properties of the quasinormal mode frequencies for deformation parameters.

Abstract: The magnification effect of wormholes and black holes has been extensively researched. It is crucial to provide a finite distance analysis to understand this magnification phenomenon better. In this article, the rotational Simpson-Visser metric (RSV) is chosen as the focus of research. By calculating the deflection of light in RSV metric, we determine the resulting magnification effect, then applied the RSV metric to specific examples such as the Ellis-Bronnikov wormhole, Schwarzschild black hole, and Kerr black hole (or wormhole) to analyze the magnification. We find that Ellis-Bronnikov wormhole only has single magnification peaks, while Kerr black hole has one to three magnification peaks. In addition, the article's findings suggest that the lensing effect of the Central Black Hole of the Milky Way Galaxy exhibits magnification of multiple peaks. However, it should be noted that these effects are not observable from Earth.

Abstract: TianQin and LISA are space-based laser interferometer gravitational wave (GW) detectors planned to be launched in the mid-2030s. Both detectors will detect low-frequency GWs around $10^{-2}\,{\rm Hz}$, however, TianQin is more sensitive to frequencies above this common sweet-spot while LISA is more sensitive to frequencies below $10^{-2}\,{\rm Hz}$. Therefore, TianQin and LISA will be able to detect the same sources but with different accuracy for different sources and their parameters. We study the detection distance and the detection accuracy with which TianQin and LISA will be able to detect some of the most important astrophysical sources -- massive black hole binaries, stellar-mass black holes binaries, double white dwarfs, extreme mass ratio inspirals, light and heavy intermediate mass ratio inspirals, as well as the stochastic gravitational background produced by binaries. We, further, study the detection distance and detection accuracy from joint detection. We compare the results obtained by the three detection scenarios highlighting the gains from joint detection as well as the contribution of TianQin and LISA to a combined study of astrophysical sources. In particular, we consider the different orientations, lifetimes, and duty cycles of the two detectors to explore how they can give a more complete picture when working together.

Abstract: We evaluate several neural-network architectures, both convolutional and recurrent, for gravitational-wave time-series feature extraction by performing point parameter estimation on noisy waveforms from binary-black-hole mergers. We build datasets of 100,000 elements for each of four different waveform models (or approximants) in order to test how approximant choice affects feature extraction. Our choices include \texttt{SEOBNRv4P} and \texttt{IMRPhenomPv3}, which contain only the dominant quadrupole emission mode, alongside \texttt{IMRPhenomPv3HM} and \texttt{NRHybSur3dq8}, which also account for high-order modes. Each dataset element is injected into detector noise corresponding to the third observing run of the LIGO-Virgo-KAGRA (LVK) collaboration. We identify the Temporal Convolutional Network (TCN) architecture as the overall best performer in terms of training and validation losses and absence of overfitting to data. Comparison of results between datasets shows that the choice of waveform approximant for the creation of a dataset conditions the feature extraction ability of a trained network. Hence, care should be taken when building a dataset for the training of neural networks, as certain approximants may result in better network convergence of evaluation metrics. However, this performance does not necessarily translate to data which is more faithful to numerical relativity simulations. We also apply this network on actual signals from LVK runs, finding that its feature-extracting performance can be effective on real data.

Abstract: For a black hole, the appearance of a shadow is due to the light rays entering the event horizon, and the unstable photon sphere determines the boundary of shadow. Our research indicates that even in the presence of a complete photon sphere without an event horizon, a shadow will not be formed. We present the images of Konoplya-Zhidenko compact object with or without complete photon sphere, and investigate the influence of unstable photon circular orbits (UPCOs) and stable photon circular orbits (SPCOs) on the images of Konoplya-Zhidenko compact object. When the event horizon is absent, the unstable prograde and retrograde light rings can also exist, so dose the complete photon sphere. But the dark shadow doesn't emerge, and the image of the complete photon sphere appears as an infinite number of relativistic Einstein rings. For this case, the light rays pass through the photon sphere, but eventually escape to infinity. For some parameter values, only the unstable retrograde light ring can exist, which leads to an incomplete photon sphere. In this case, the dark shadow also doesn't emerge, and the image of the incomplete photon sphere appears as an infinite number of relativistic Einstein arcs. Furthermore, in Konoplya-Zhidenko naked singularity spacetime, the stable LRs and SPCOs can also exist, but they have no effect on the naked singularity image. This study may contribute to future astronomical observations, and aid in verifying the cosmic censorship conjecture and various gravitational theories.

Abstract: Recent images from the Event Horizon Telescope of accreting supermassive black holes (BHs), along with upcoming observations with better sensitivity and angular resolution, offer exciting opportunities to deepen our understanding of spacetime in strong gravitational fields. A significant focus for future BH imaging observations is the direct detection of the "photon ring," a narrow band on the observer's sky that collects extremely lensed photons. The photon ring consists of self-similarly nested subrings which, in spherically-symmetric spacetimes, are neatly indexed by the maximum number of half-loops executed around the BH by the photons that arrive in them. Each subring represents an entire "higher-order" image of the horizon-scale accretion flow. Furthermore, this self-similarity is controlled by a single critical lensing exponent linked to the radial (in)stability of photon orbits near the critical (circular) photon orbit, solely determined by the spacetime geometry. However, extracting such information about the spacetime geometry can be challenging because the observed photon ring is also influenced by the structure of the emitting region. To address this, we conducted a comprehensive study by varying (a) a wide range of emission-zone morphology models and (b) families of spacetime metrics. We find that the lensing exponent can be reliably determined from future observations. This exponent can provide access to the $rr-$component of the spacetime metric, as well as significantly narrow down currently accessible BH parameter spaces. Additionally, the width of the first-order photon subring serves as yet another important discriminator of the spacetime geometry. Finally, observations of flaring events across different wavelengths might reveal time-delayed secondary images, with the delay time providing a promising new way to independently estimate the BH shadow size.