We present new families of solutions of $D$-dimensional Einstein-Maxwell theory depending on one variable for all space-time signatures. The solutions found can be thought of as generalized Melvin solutions including flux tubes, domain walls, and cosmological space-times. Explicit examples are given in four and five space-time dimensions.

Observational data of the Sagittarius A* (Sgr A*) shadow released by the Event Horizon Telescope are used to investigate eventual deviations in the black hole shadow radius, aiming to seek physics beyond the Standard Model coming from extra-dimensional theory. We consider the braneworld scenario described by the Randall-Sundrum model and determine the black hole shadow radius correction owing to the higher dimension. From data of the shadow radius in units of black hole mass determined by Keck- and Very Large Telescope Interferometer (VLTI)-based estimates, we imposed restrictions on the deviation obtained, and one sets an upper limit to the curvature radius of anti–de Sitter (${\mathrm{AdS}}_5$) spacetime ${\ell }_{(\text{Keck})}\lesssim 3.8\times {10}^{-2}\mathrm{AU}$ and ${\ell }_{(\mathrm{VLTI})}\lesssim 1.6\times {10}^{-2}\mathrm{AU}$ (at 68% confidence level), respectively.

We study the axial gravitational perturbations of slowly rotating compact objects which are assumed to be supported by anisotropic fluids. We find that the gravitational perturbations decouple from the matter perturbations for axial sectors. We obtain a master wave equation whose potential is fully determined by the metric functions. This equation makes the calculations of gravitational quasi-normal modes for rotating compact objects extremely easy in specific background configurations.

In this work, we investigate the quasinormal modes of the Poincaré thick brane with a finite extra dimension. Unlike the case with an infinite extra dimension, the gravitational effective potential exhibits three distinct shapes within different ranges of the parameter $n$ in the warp factor: harmonic oscillator potential, Pöschl-Teller potential, and volcanolike potential. We then study various types of perturbations in this system. Utilizing a combination of analytical, semianalytical, and numerical methods, we obtain the quasinormal modes of the perturbed fields. Our findings reveal a set of discrete quasinormal modes for the thick brane, similar to those of black holes. Interestingly, when $n=1$, the quasinormal modes exhibit purely imaginary behavior. This study may provide a new way to detect the existence of extra dimensions.

Measuring accurate long-duration gravitational waves from extreme mass ratio inspirals (EMRIs) could provide scientifically fruitful knowledge of massive black hole populations and robust tests for general relatively during the LISA mission. However, the immense computational requirements surrounding EMRI data processing and analysis makes their detection and analysis challenging. We further develop and explore a sparse dictionary learning (SDL) algorithm to expeditiously reconstruct EMRI gravitational waveforms lasting as long as 1 year. A suite of year-long EMRI systems are studied to understand the detection and accurate waveform retrieval prospects of the method. We show that full-year EMRIs can be reconstructed within 2 minutes, some with a false alarm rate less than $0.001{\mathrm{yr}}^{-1}$ and with 1.16 day time windows with mismatch as low as 0.06. This provides an encouraging prospect to use the SDL method for long-duration gravitational wave searches like that for EMRIs in this study.

We study the polarization modes of gravitational waves in general Einstein-vector theory with an arbitrary constant background vector field under a Minkowski background. We compare these polarization modes with those of other vector-tensor theories and constrain the parameter spaces based on the gravitational-wave event GW170817 with its electromagnetic counterpart GRB170817A and observations of pulsar timing arrays. The presence of the background vector field leads to the anisotropy of space and a rich variety of gravitational wave contents. Our results reveal that the polarization modes of gravitational waves depend on the parameter spaces. There are at least two and at most five independent polarization modes in one parameter space. In different parameter spaces, some mixture modes are allowed, including tensor-vector, tensor-scalar, tensor-vector-scalar, vector-scalar, and scalar-scalar mixture modes, as well as five independent modes (excluding $P_l$): $P_{+}$, $P_{\times }$, $P_x$, $P_y$, and $P_b$. In all regions of the parameter spaces, there are always two tensor modes, which can be either independent of or mixed with other modes. The independent $P_b$ mode consistently exhibits the same speed as light. If the speed of tensorial gravitational waves strictly equals that of light, only the $P_{+}$, $P_{\times }$, and $P_b$ modes are permitted. Furthermore, through comparisons of some vector-tensor theories and based on the observations of gravitational waves, the $P_b$ mode is expected to be allowed.

Accretion is a common phenomenon associated with any astrophysical compact object, which is best described by plasma, a state of matter composed of electrons and heavy ions. In this paper, we analyze the linear dynamics of electromagnetic (EM) fields propagating through the accreting plasma around static and spherically symmetric horizonless, exotic compact objects (ECOs). The general equations governing the propagation of EM waves in such a background exhibits quasibound states, whose characteristic frequencies differ from the black hole (BH) values, for both the axial and the polar modes, as well as for homogeneous and inhomogeneous plasma distributions. Moreover, the real and imaginary parts of these quasibound frequencies depict an oscillatory behavior with the plasma frequency, characteristic of the ECOs considered. The amplitude of these oscillations depend on the nonzero reflectivity of the surface of the compact object, while the oscillation length depends on its compactness. This results in slower decay of the quasibound states with time for certain parameter space of the plasma frequency, compared to BHs, making these ECOs more prone to instabilities.

By working with the covariant phase space formalism, we have shown that not only can the Hamiltonian conjugate to a Killing vector field $\xi$ be expressed as the sum of the associated Noether charge and $\xi$ contracted with the Hilbert action boundary term for $F(R_{abcd})$ gravity, but it can also be written as its contraction with another $\xi$ independent tensor field. With this, we have proven the equivalence of Noether charge and Hilbert action boundary term formulas for the stationary black hole entropy in $F(R_{abcd})$ gravity, which is further substantiated by our explicit computation using both formulas.

Phase-locking configuration was proposed as a baseline optical implementation for the space-borne gravitational wave detectors. By assigning one laser as the reference, termed the master, the remaining five lasers, dubbed slaves, are no longer free-running but phase-locked to form a master-slave arrangement. As a baseline optical implementation for the space-borne gravitational wave detectors, such an approach addresses the relative central frequency drift occurring to individual lasers over time. It is crucial for the effectiveness of the interference measurements, as the resultant beat note frequencies are guaranteed to remain in the phasemeter range. Moreover, it aims to reduce hardware redundancy and effectively decrease the total number of independent laser noise and data streams. The present study focuses on the implication of the time-delay interferometry (TDI) algorithm when implementing the master-slave configuration governed by phase-locking. We examine the modifications to the data streams’ structure and the reduction in the number of independent optical links compared to the standard TDI formalism. From a geometric TDI perspective, we analyze the valid TDI combinations by exhausting the solution space up to sixteen links. We show that a geometric TDI combination can always be reiterated regarding the modified data streams associated with the phase-locking scheme, resulting in a size equal to or shorter than the original TDI solution. Notably, for a specific phase-locking scheme, it is demonstrated that an originally sixteen-link TDI combination typically shrinks down to its counterpart with a smaller number of links, effectively simplifying the solutions’ forms. The analysis can be extended to all six distinct phase-locking schemes, and the aim is to identify TDI combinations that offer enhanced performance.

This work focuses on gravitational perturbations of black holes in the self-accelerating branch of the minimal theory of massive gravity (MTMG). This theory is a healthy extension of general relativity (GR) which displays the feature of massive tensor modes, without additional polarizations, strong-coupling issues nor requiring screening mechanisms. We proceed by implementing a newly developed technique that, instead of considering a second-order Schrödinger-like reformulation of perturbation equations, relies on a first-order formulation and solves it asymptotically, before numerically deriving the quasinormal modes. We find that the black holes of MTMG are stable, and that their quasinormal modes smoothly differ from the GR ones, for nonvanishing values of the graviton mass. This work hence confirms the fact that GR is a smooth limit of MTMG, and opens the exciting possibility of a clean test, performed for instance by the LISA detector.

The expansion and upgrade of the global network of ground-based gravitational-wave detectors promises to improve our capacity to infer the sky localization of transient sources, enabling more effective multimessenger follow-ups. At the same time, the increase in the signal-to-noise ratio of detected events allows for more precise estimates of the source parameters. This study aims to assess the performance of advanced-era networks of ground-based detectors, focusing on the Hanford, Livingston, Virgo, and KAGRA instruments. We use full Bayesian parameter estimation procedures to predict the scientific potential of a network. Assuming a fixed configuration of the LIGO Hanford and Livingston detectors, we find that the addition of the Virgo detector is beneficial to the sky localization starting from a binary neutron star range of 20 Mpc and improves significantly from 40 Mpc onward for both a single and double LIGO detector network, reducing the inferred mean sky area by up to 95%. Similarly, the KAGRA detector tightens the constraints, starting from a binary neutron star range of 10 Mpc. Looking at highly spinning binary black holes, we find significant improvements with increasing sensitivity in constraining the intrinsic source parameters when adding Virgo to the two LIGO detectors. Finally, we also examine the impact of the low-frequency cutoff data on the signal-to-noise ratio. We find that existing 20 Hz thresholds are sufficient and propose a metric to monitor this to study detector performance. Our findings quantify how future enhancements in detector sensitivity and network configurations will improve the localization of gravitational-wave sources and allow for more precise identification of their intrinsic properties.

We study gravitational perturbations on the near-horizon region of extremal and near-extremal rotating black holes in a general higher-derivative extension of Einstein gravity. We find a decoupled modified Teukolsky equation that rules the gravitational perturbations and that separates into an angular and a radial equation. The angular equation leads to a deformation of the spin-weighted spheroidal harmonics, while the radial equation takes the same form as in Kerr except for a modification of the angular separation constants. We provide a detailed analysis of the corrections to these angular separation constants and find analytic results for axisymmetric modes as well as in the eikonal limit. As an application, we reproduce recent results that show that extremal Kerr black holes in higher-derivative gravity become singular under certain deformations and extend them by including parity-breaking corrections, which we show lead to the same effect. Finally, we obtain constraints on the form of the full modified Teukolsky radial equation by demanding that it has the right near-horizon limit. These results serve as a stepping stone towards the study of quasinormal modes of near-extremal black holes in higher-derivative extensions of general relativity.

Lacking terrestrial experimental data, our best constraints on the behavior of matter at high densities up to and above nuclear density arise from observations of neutron stars. Current constraints include those based on measurements of stellar masses, radii, and tidal deformabilities. Here we explore how orbits of primordial black holes—should they exist—inside neutron stars could provide complementary constraints on the nuclear equation of state (EOS). Specifically, we consider a sample of candidate EOSs, construct neutron star models for these EOSs, and compute orbits of primordial black holes inside these stars. We discuss how the pericenter advance of eccentric orbits, i.e. orbital precession, results in beat phenomena in the emitted gravitational wave signal. Observing this beat frequency could constrain the nuclear EOS and break possible degeneracies arising from other contraints, as well as provide information about the host star.

The energy extraction from an electrostatic black hole by the decay or splitting of electrically charged particles is analyzed. We determine the energetic conditions that make the extraction process viable and present a general expression for the efficiency in terms of the parameters of the electrostatic black hole and the decaying particles. We also examine the conditions that optimize the efficiency of the extraction process. We analyze two particular cases: the first one is the extraction process from a Reissner-Nordström black hole, for charged test particles with nonvanishing angular momentum; the second one and more interesting corresponds to the energy extraction from a Reissner-Nordström de Sitter black hole. For the latter there are two regions where the energy extraction is possible, the generalized ergosphere and a cosmological ergosphere induced by the cosmological horizon. Under certain conditions the two ergospheres get connected and cover the whole region between the event horizon and the cosmological horizon, and therefore, the energy extraction is possible at any point in the vicinity of the black hole. Moreover, the efficiency of the energy extraction can be the same for different breakup points and also there is the possibility of a different efficiency for the same breakup point. The conditions that maximize the efficiency are determined as well.

In this paper we analyze the asymptotic symmetries of the three-dimensional Chern-Simons supergravity for a supersymmetric extension of the semisimple enlargement of the Poincaré algebra, also known as AdS-Lorentz superalgebra, which is characterized by two fermionic generators. We propose a consistent set of asymptotic boundary conditions for the aforementioned supergravity theory, and we show that the corresponding charge algebra defines a supersymmetric extension of the semisimple enlargement of the ${\mathfrak{bms}}_3$ algebra, with three independent central charges. This asymptotic symmetry algebra can alternatively be written as the direct sum of three copies of the Virasoro algebra, two of which are augmented by supersymmetry. Interestingly, we show that the flat limit of the obtained asymptotic algebra corresponds to a deformed super-${\mathfrak{bms}}_3$ algebra, being the charge algebra of the minimal Maxwell supergravity theory in three dimensions.

It is shown that an exactly static and spherically symmetric black hole event horizon cannot be embedded in a time-dependent geometry. Forcing it to do so results in a naked null singularity at the would-be horizon. Therefore, since the Universe is expanding, black holes must couple to the cosmological expansion, which was suggested as the growth mechanism for supermassive black holes in galaxies, with implications for the dark energy puzzle.

The Starobinsky model of cosmological inflation in four spacetime dimensions is reviewed with the emphasis on the impact of quantum gravity corrections. As a specific example of the quantum corrections, the Grisaru-Zanon quartic curvature terms in the gravitational effective action of closed superstrings are chosen. Those quartic curvature terms are compared to the Bel-Robinson tensor squared in a flat Friedman universe, and the upper bound on the effective string coupling constant is found by demanding unitarity (causality) and the absence of ghosts. It is found that the quantum corrections to the observables (tilts) of the cosmic microwave background radiation in the Starobinsky inflation may be of the same order of magnitude as the next-to-next-to-next classical contributions in the Starobinsky model with respect to the inverse powers of the e-folding number at the horizon crossing.

Cosmic magnification on the observed galaxy overdensity is a promising weak gravitational lensing tracer. Current cosmic magnification reconstruction algorithms, the analytical method of blind separation (ABS) and constrained internal linear combination (cILC), intend to disentangle the weak lensing signal using the magnification response in various flux bins. In this work, we reveal an unrecognized systematic bias arising from the difference between galaxy bias and the galaxy-lensing cross-correlation bias, due to the mismatch between the weak lensing kernel and the redshift distribution of photometric objects. It results into a galaxy-lensing degeneracy, which invalidates ABS as an exact solution. Based on the simulated cosmoDC2 galaxies, we verify that the recovered weak lensing amplitude by ABS is biased low by $\sim 10\%$. cILC, including a modified version proposed here, also suffers from systematic bias of comparable amplitude. Combining flux and color information leads to significant reduction in statistical errors, but fails to eliminate the aforementioned bias. With the presence of this newly found systematic, it remains a severe challenge in blindly and robustly separating the cosmic magnification from the galaxy intrinsic clustering.

We study Monge-Ampère gravity (MAG) as an effective theory of cosmological structure formation through optimal transport theory. MAG is based on the Monge-Ampère equation, a nonlinear version of the Poisson equation, that relates the Hessian determinant of the potential to the density field. We explain how MAG emerges from a conditioned system of independent and indistinguishable Brownian particles, through the large deviation principle, in the continuum limit. To numerically explore this highly nonlinear theory, we develop a novel $N$-body simulation method based on semidiscrete optimal transport. Our results obtained from the very first $N$-body simulation of Monge-Ampère gravity with over 100 millions particles show that on large scales, Monge-Ampère gravity is similar to the Newtonian gravity but favors the formation of anisotropic structures such as filaments. At small scales, MAG has a weaker clustering and is screened in high-density regions. Although here we study the Monge-Ampère gravity as an effective rather than a fundamental theory, our novel highly performant optimal transport algorithm can be used to run high-resolution simulations of a large class of modified theories of gravity, such as Galileons, in which the equations of motion are second-order and of Monge-Ampère type.

Cosmic strings are a common prediction in many grand unified theories and a promising source of stochastic gravitational waves (GWs) from the early Universe. In this paper, we point out that the GW signal from cosmic strings produced at a comparatively low energy scale, $v\lesssim {10}^9\mathrm{GeV}$, exhibits several novel features that are not present in the case of high-scale cosmic strings. Our findings notably include (i) a sharp cutoff frequency $f_{\mathrm{cut}}$ in the GW spectrum from the fundamental oscillation mode on closed string loops and (ii) an oscillating pattern in the total GW spectrum from all oscillation modes whose local minima are located at integer multiples of $f_{\mathrm{cut}}$. These features reflect the fact that string loops produced in the early Universe fail to shrink to zero size because of GW emission within the age of the Universe, if their tension is low enough. In addition, they offer an exciting opportunity to directly probe the discrete spectrum of oscillation modes on closed string loops in GW observations. For strings produced at a scale $v\sim {10}^9\mathrm{GeV}$, the novel features in the GW spectrum are within the sensitivity reach of future experiments such as BBO and DECIGO.