We study a point scalar charge in circular orbit around a topological star, a regular, horizonless soliton emerging from dimensional compactification of Einstein-Maxwell theory in five dimensions, which could describe qualitative properties of microstate geometries for astrophysical black holes. This is the first step towards studying extreme mass-ratio inspirals around these objects. We show that when the particle probes the spacetime close to the object, the scalar-wave flux deviates significantly from the corresponding black hole case. Furthermore, as the topological star approaches the black-hole limit, the inspiral can resonantly excite its long-lived modes, resulting in sharp features in the emitted flux. Although such resonances are too narrow to produce detectable dephasing, we estimate that a year-long inspiral down to the innermost stable circular orbit could accumulate a significant dephasing for most configurations relative to the black hole case. While a full parameter-estimation analysis is needed, the generically large deviations are likely to be within the sensitivity reach of future space-based gravitational-wave detectors.

In this work, we study a non-geometrical perturbation to the stealth field, which means the background remains invariant. The stealh is homogeneous in a universe whose source is dust and demand that perturbation unchanged density. As a regular procedure, we introduce a parameter $\lambda$ to perturb the scalar field equation and get an intriguing expression of the equation, similar to a series expansion in $\lambda$. From this procedure, we distinguish and approach to discriminate solutions, and the numerical solutions show that the most significant contribution to the solution comes from the linear term of $\lambda$.

In general relativity, the tangential speed of objects in stable circular orbits is not uniquely described by the orbital radius and the mass present inside the orbital radius. This work presents a static, spherically symmetric spacetime metric which produces stable circular orbits whose speed approaches a constant value at high radii. The orbital speed is independent of the mass contained within the orbital radius, however, there is pressure throughout the spacetime. The stress energy tensor of this metric is evaluated numerically using the mass of the Milky Way's central black hole, the orbital speed of its distant satellites, and three different values of a unitless 'mass tuning' parameter B. These B 'tune' the amount of mass present, without violating the Weak Energy Condition (WEC) at any evaluated spacetime point. The metric can be merged with a Friedmann-Robertson-Walker metric, in which case it achieves isotropy and obeys the Friedmann equations at cosmological distances.

We explore the quantum state transition of photon orbital angular momentum (OAM) in the present of gravitational waves (GWs) and demonstrate the potential of a new photonic single-arm GW detection technique. The interaction is calculated based on the framework of the wave propagation in linearized gravity theory and canonical quantization of the electromagnetic field in curved spacetime. It is demonstrated that when a photon possessing OAM of 1 interacts with GWs, it may relinquish its OAM and produce a central signal that may be detected. The detector provides a high and steady rate of detected photons in the low-frequency range ($<1$ Hz), opens a potential window to identify GWs in the mid-frequency range ($1\sim10$ Hz), which is absent in other contemporary GW detectors, and establishes a selection rule for GW frequencies in the high-frequency range ($>10$ Hz), allowing for the adjustment of detector parameters to focus on specific GW frequencies. Furthermore, the detector is insensitive to seismic noise, and the detectable photon count rate is proportional to the square of the GW amplitude, making it more advantageous for determining the distance of the source compared to current interferometer detectors. This technique not only facilitates the extraction of GW information but also creates a new approach for identifying and selecting GW signals.

A relativistic self-gravitating equilibrium system with steady flow as well as spherical symmetry is discovered. The energy-momentum tensor contains the contribution of a current related to the flow and the metric tensor does an off-diagonal component to balance with the flow momentum. The presence of the off-diagonal component of the metric implies the radial motion of the reference frame, which gives rise to a problem how the relativistic effect is included in thermodynamic observables for such a general relativistic system. This problem is solved by taking an instantaneously rest frame in which geometric thermodynamic observables read as previously and giving them the special relativistic effect emerged from the inverse transformation to the original frame pointwise. The solution of the thermodynamic observables in accord with the laws of thermodynamics and the theory of relativity is presented. Finally the relativistic structure equations for the equilibrium are derived, from which the general relativistic Poisson equation as well as the heat conduction one are developed exactly.

In this work we investigate the spontaneous symmetry breaking (SSB) induced by a classical background spacetime's curvature, via the 2 particle irreducible (2PI) non-perturbative effective action formalism. We use the standard Schwinger-DeWitt local expansion of the Feynman propagator, appropriate to probe the effect of spacetime curvature on the local or short scale physics. Recently it was shown using perturbative computations that such SSB is possible with a scalar with a quartic self interaction, positive rest mass squared and positive non-minimal coupling. Here we confirm in the two loop Hartree approximation that curvature can indeed induce SSB for such a theory. SSB for such a model is not possible in a flat spacetime. The 2PI technique does not only resum the self energy resulting in mass generation, but also resums, as we have discussed, curvature terms through such mass generation. We have explicitly discussed our results in the context of the de Sitter spacetime, although our calculations are valid for any non-singular curved spacetime. We show that, in contrast to the perturbative results, SSB is possible with a vanishing non-minimal coupling. These results are further extended to the case of an $O(N)$ symmetric scalar field theory. Restoration of the broken symmetry in the thermal case is also briefly discussed.

Explicit example, where the Hawking temperature of a black hole horizon is compatible with the black hole's R\'enyi entropy thermodynamic description, is constructed. It is shown that for every static, spherically symmetric, vacuum black hole space-time, a corresponding black hole solution can be derived, where the Hawking temperature is identical with the R\'enyi temperature, i.e. the one obtained from the R\'enyi entropy of the black hole via the 1st law of thermodynamics. In order to have this Hawking-R\'enyi type thermodynamic property, the black holes must be surrounded by an anisotropic fluid in the form of a Kiselev metric, where the properties of the fluid are uniquely determined by the mass of the black hole, $M$, and the R\'enyi parameter, {\lambda}. In the simplest Schwarzschild scenario, the system is found to be thermodynamically unstable, and the 3rd law of thermodynamics seems to play the role of a cosmic censor via placing an upper bound on the black hole's mass, by which preventing the black hole from loosing its horizon(s).

We investigate the capability of the Taiji space-based gravitational wave observatory to detect stochastic gravitational wave backgrounds produced by first-order phase transitions in the early universe. Using a comprehensive simulation framework that incorporates realistic instrumental noise, galactic double white dwarf confusion noise, and extragalactic compact binary backgrounds, we systematically analyze Taiji's sensitivity across a range of signal parameters. Our Bayesian analysis demonstrates that Taiji can robustly detect and characterize phase transition signals with energy densities exceeding $\Omega_{\text{PT}} \gtrsim 1.4 \times 10^{-11}$ across most of its frequency band, with particularly strong sensitivity around $10^{-3}$ to $10^{-2}$ Hz. For signals with amplitudes above $\Omega_{\text{PT}} \gtrsim 1.1 \times 10^{-10}$, Taiji can determine the peak frequency with relative precision better than $10\%$. These detection capabilities would enable Taiji to probe electroweak-scale phase transitions in various beyond-Standard-Model scenarios, potentially revealing new physics connected to baryogenesis and dark matter production. We quantify detection confidence using both Bayes factors and the Deviance Information Criterion, finding consistent results that validate our statistical methodology.

A horizonless ultracompact object can have a stable antiphoton sphere, which causes the strong deflection of photons inside the unstable photon sphere, leading to the formation of distinctive inner photon rings. In this work, we present analytical descriptions for the shape, thickness and interference pattern of higher-order inner photon rings. By taking the static spherically symmetric Schwarzschild star with a photon sphere as an example, we find that its inner photon rings can be more non-circular and thicker than the outer ones, and show that the inclusion of the inner photon rings can give rise to new features in the interferometric pattern. Our formulae can also be applied to other ultracompact objects, providing a convenient way to study the observational properties of their higher-order photon rings.

We explore the gravitational baryogenesis paradigm in the homogeneous and isotropic cosmology of generalized coupling gravity and, in particular, of the so-called Minimal Exponential Measure Model (MEMe). We show that, also in this theory, the time derivative of the Ricci scalar couples with matter currents and can preserve an unbalance in the baryon-antibaryon number beyond thermal equilibrium. Using the current bounds on the ratio of baryon number to entropy density, we can considerably improve the known constraints on the parameter q that characterizes the MEMe model. This estimate also allows us to draw stringent constraints on the spatial curvature of the cosmological model.

In this note, we investigate the stability of the dark energy model from time crystals proposed in [1]. We emphasize two ingredients, the coupling of the scalar field to gravity, and the fact that these time crystals are on an expanding FRW background, which play a crucial role in the field's dynamics. The Hubble parameter, which contributes a drag term to the equations of motion, grows with time until the scale factor diverges. When taken into account, these factors also alleviate the stability concern of [2].

From a purely geometric (kinematic) perspective, black holes in four dimensional spacetimes can have event horizons with arbitrary topologies. It is only when energy conditions are imposed that the horizon's shape is constrained to be spherical. Despite this, exploring exotic horizon topologies remains theoretically intriguing since it allows to unveil structural aspects of General Relativity and gain intuition on energy condition violations. In the axisymmetric case, besides the well-known spherical topology, only a toroidal topology is consistent with the symmetry. Complete solutions, describing the entire exterior region of such toroidal black holes without singularities, have not been reported yet. To the best of our knowledge, the construction we present here is the first explicit example of a toroidal black hole solution in four spacetime dimensions that is free of singularities in the external region.

In this study, we investigated the effects of incorporating barotropic fluids on cosmological solutions within the general relativity (GR) framework. We proposed a modified version of the barotropic fluid with the EoS, $p=\zeta _0 \rho +\zeta _1 \rho \left(t-t_0\right){}^{-2 n}$, where $\zeta_0$, $\zeta_1$, $t_0$ and $n$ are some constants. Our goal is to explore if this type of EoS might help explain the universe's development, concentrating on the scenario where the universe bounces instead of singularities. Interestingly the generic solutions derived from our model are sufficiently adaptable to illustrate the bounce scenario, cosmic inflation and late-time dark-energy behaviour. The parameters $\zeta_0$, $\zeta_1$, $t_0$, and $n$ define the universe's phase in this non-singular solution. We investigated several elements of cosmic development, including as the energy density, deceleration parameter, and energy conditions, in order to validate our model. Stability analysis showed that the perturbations approach to zero as the time evolves, indicating the model is stable under scalar perturbation. Additionally, we looked at the statefinder diagnostics and Hubble flow dynamics to get more understanding of the model's dark energy and inflationary behaviour, respectively. Additionally, we conducted a study of the models' relevance to the observational datasets from BAO, DESI and Pantheon+SH0ES.

We investigate the static solutions with rotational symmetry in the nonprojectable Ho\v{r}ava theory in 2+1 dimensions. We consider all inequivalent terms of the effective theory, including the cosmological constant. We find two distinct types of solutions: the first one corresponds to a Lifshitz solution, while the second one is obtained through a coordinate transformation of the equations of motion. This exact solution does not exhibit Lifshitz behavior and features a naked singularity.

In this article, we revisit the initial data rigidity theorem of Eichmair, Galloway and Mendes (arxiv:2009.09527). The goal is to strengthen their result by showing that the initial data sets concerned carry a vector field that is lightlike and parallel in an ambient sense. This will be used in a second step to show that among the spacetimes satisfying the dominant energy condition there exists locally essentially one spacetime extending these initial data sets. This local uniqueness theorem also applies in the context of other initial data rigidity theorems. Notably, the one in the spin case due the author (arxiv:2304.02331) and a recent study of the mass zero case in the positive energy theorem due to Hirsch and Zhang (arxiv:2403.15984).

We construct postcarrollian gravity models in two, three, and four spacetime dimensions by applying algebraic expansion methods. As a byproduct, we present the most general postcarrollian 2d dilaton gravity model, construct its solutions and discuss some boundary aspects, including Schwarzian-type boundary actions. In 3d, we propose Brown-Henneaux-like boundary conditions, generalizing a corresponding Carrollian analysis, and derive the postcarrollian asymptotic symmetry algebra with its central extensions.

We investigate the sine model, a one-dimensional tight-binding Hamiltonian featuring hoppings with a sinusoidal dependence on position, and demonstrate the formation of synthetic horizons where electronic wave packets exhibit exponential slowdown. Interestingly, employing the exact transformation between this model and the Harper equation, which describes the eigenstates of a square lattice tight-binding model subjected to a perpendicular magnetic field, we find that analogous semi-classical horizons can emerge in a quantum Hall setup at half-filling for specific values of the magnetic flux. Furthermore, by applying sudden quenches to the sine model's hopping profile, we observe the emergence of thermal states characterized by an Unruh temperature. Our numerical calculations of this temperature reveal a non-universal behavior, suggesting the involvement of physical mechanisms beyond a simple low-energy description.

The Su-Schrieffer-Heeger (SSH) model, a prime example of a one-dimensional topologically nontrivial insulator, has been extensively studied in flat space-time. In recent times, many studies have been conducted to understand the properties of the low-dimensional quantum matter in curved spacetime, which can mimic the gravitational event horizon and black hole physics. However, the impact of curved spacetime on the topological properties of such systems remains unexplored. Here, we investigate the curved spacetime (CST) version of the SSH model by introducing a position-dependent hopping parameter. We show, using different topological markers, that the CST-SSH model can undergo a topological phase transition. We find that the topologically non-trivial phase can host zero-energy edge modes, but those edge modes are asymmetric, unlike the usual SSH model. Moreover, we find that at the topological transition point, a critical slowdown takes place for zero-energy wave packets near the boundary, indicating the presence of a horizon, and interestingly, if one moves even a slight distance away from the transition point, wave packets start bouncing back and reverse the direction before reaching the horizon. A semiclassical description of the wave packet trajectories also supports these results.

We build slowly rotating anisotropic neutron stars using the Hartle-Thorne formalism, employing three distinct anisotropy models--Horvat, Bowers-Liang, and a covariant model--to characterize the relationship between radial and tangential pressure. We analyze how anisotropy influences stellar properties such as the mass-radius relation, angular momentum, moment of inertia, and binding energy. Our findings reveal that the maximum stable mass of non-rotating stars depends strongly on the anisotropy model, with some configurations supporting up to 60% more mass than their isotropic counterparts with the same central density. This mass increase is most pronounced in the models where the anisotropy grows toward the star's surface, as seen in the covariant model. Furthermore, slowly rotating anisotropic stars adhere to universal relations for the moment of inertia and binding energy, regardless of the chosen anisotropy model or equation of state.

This paper develops a framework for the Hamiltonian quantization of complex Chern-Simons theory with gauge group $\mathrm{SL}(2,\mathbb{C})$ at an even level $k\in\mathbb{Z}_+$. Our approach follows the procedure of combinatorial quantization to construct the operator algebras of quantum holonomies on 2-surfaces and develop the representation theory. The $*$-representation of the operator algebra is carried by the infinite dimensional Hilbert space $\mathcal{H}_{\vec{\lambda}}$ and closely connects to the infinite-dimensional $*$-representation of the quantum deformed Lorentz group $\mathscr{U}_{\mathbf{q}}(sl_2)\otimes \mathscr{U}_{\widetilde{\mathbf{q}}}(sl_2)$, where $\mathbf{q}=\exp[\frac{2\pi i}{k}(1+b^2)]$ and $\widetilde{\mathbf{q}}=\exp[\frac{2\pi i}{k}(1+b^{-2})]$ with $|b|=1$. The quantum group $\mathscr{U}_{\mathbf{q}}(sl_2)\otimes \mathscr{U}_{\widetilde{\mathbf{q}}}(sl_2)$ also emerges from the quantum gauge transformations of the complex Chern-Simons theory. Focusing on a $m$-holed sphere $\Sigma_{0,m}$, the physical Hilbert space $\mathcal{H}_{phys}$ is identified by imposing the gauge invariance and the flatness constraint. The states in $\mathcal{H}_{phys}$ are the $\mathscr{U}_{\mathbf{q}}(sl_2)\otimes \mathscr{U}_{\widetilde{\mathbf{q}}}(sl_2)$-invariant linear functionals on a dense domain in $\mathcal{H}_{\vec{\lambda}}$. Finally, we demonstrate that the physical Hilbert space carries a Fenchel-Nielsen representation, where a set of Wilson loop operators associated with a pants decomposition of $\Sigma_{0,m}$ are diagonalized.

By generalizing the construction of genuine multi-entropy ${\rm GM}[\mathtt{q}]$ for genuine multi-partite entanglement proposed in the previous paper arXiv:2504.01625, we give a prescription on how to construct ${\rm GM}[\mathtt{q}]$ systematically for any $\mathtt{q}$. The crucial point is that our construction naturally fits to the partition number $p(\mathtt{a})$ of integer $\mathtt{a}$. For general $\mathtt{q}$, ${\rm GM}[\mathtt{q}]$ contains $N (\mathtt{q}) = p(\mathtt{q})-p(\mathtt{q}-1)-1$ number of free parameters. Furthermore, these give $N (\mathtt{q})+1$ number of new diagnostics for genuine $\mathtt{q}$-partite entanglement. Especially for $\mathtt{q}=4$ case, this reproduces not only the known diagnostics pointed out by arXiv:1406.2663, but also a new diagnostics for quadripartite entanglement. We also study these ${\rm GM}[\mathtt{q}]$ for $\mathtt{q} = 4, 5$ in holography and show that these are of the order of ${\cal{O}}\left(1/G_N \right)$ both analytically and numerically. Our results give evidence that genuine multipartite entanglement is ubiquitous in holography. We discuss the connection to quantum error correction and the role of genuine multipartite entanglement in bulk reconstruction.

We apply the caustic technique to samples of galaxy clusters stacked in redshift space to estimate the gravitational potential in the cluster's outer region and test modifications to the standard theory of gravity. We separate 122 galaxy clusters from the HeCS-SZ, HeCS-redMapper, and HeCS samples into four samples with increasing mass; we estimate four robust, highly constraining caustic profiles for these samples. The caustic masses of the four stacked clusters agree within $ 10\%$ with the corresponding median values of each cluster sample. By adopting the NFW density profile to model the gravitational potential, we recover the caustic profile $\mathcal{A}(r)$ up to radius $r_{\rm p} \sim 4.0\, {\rm Mpc}$. This comparison is a first-order validation of the mass-concentration relation for galaxy clusters expected in the $\Lambda$CDM model. We thus impose this correlation as a prior in our analysis. Based on our stacked clusters, we estimate the value of the filling factor, which enters the caustic technique, $\mathcal{F}_{\beta} = 0.59\pm 0.05$; we derive this value using real data alone and find it consistent with the value usually adopted in the literature. We then use the caustic profiles $\mathcal{A}(r)$ of the stacked clusters to constrain the chameleon gravity model. We find that the caustic profiles provide a stringent upper limit of $|f_{\rm R0}| \lesssim 4 \times 10^{-6}$ at $95\%$ C.L. limits in the $f(\mathcal{R})$ scenario. The formalism developed here shall be further refined to test modifications to gravity in the extended outer weak gravitational regions of galaxy clusters.

We construct a family of supersymmetric solutions in Type IIB supergravity of the form ${\rm WAdS}_3\times {\rm WS}^3\times T^4$, where ${\rm WAdS}_3$ and ${\rm WS^3}$ denote a warped anti-de Sitter spacetime and a warped 3-sphere, respectively, while $T^4$ denotes an internal 4-torus. These backgrounds are constructed by uplifting corresponding solutions in the $D=6$, $\mathcal{N}=(1,1)$ ungauged supergravity resulting from the compactification of Type IIB supergravity on a $T^4/\mathbb{Z}_2$-orientifold. More specifically, the supersymmetric solutions are ${\rm WAdS}_3\times {\rm WS}^3\times T^4$ with lightlike warped AdS$_3$ and ${\rm WAdS}_3\times {\rm S}^3\times T^4$ in which the warping of AdS$_3$ is generic. Moreover, we also construct solutions in the form of a warped product $\mathrm{LM}^3_{\zeta,\omega}\times_{{\rm w}} \mathrm{S}^3\times T^4$ of a 2-parameter deformation $\mathrm{LM}^3_{\zeta,\omega}$ of ${\rm AdS}_3$ and a three-sphere. We discuss the relation of these backgrounds to known solutions.

The combination of independent cosmological datasets is a route towards precision and accurate inference of the cosmological parameters if these observations are not contaminated by systematic effects. However, the presence of unknown systematics present in differrent datasets can lead to a biased inference of the cosmological parameters. In this work, we test the consistency of the two independent tracers of the low-redshift cosmic expansion, namely the supernovae dataset from Pantheon$+$ and the BAO dataset from DESI DR2 using the distance duality relation which is a cornerstone relation in cosmology under the framework of General Relativity. We find that these datasets violate the distance duality relation and show a signature of redshift evolution, hinting toward unaccounted physical effects or observational artifacts. Coincidentally this effect mimics a redshift evolving dark energy scenario when supernovae dataset and DESI datasets are combined without accounting for this inconsistency. Accounting for this effect in the likelihood refutes the previous claim of evidence of non-cosmological constant as dark energy model from DESI DR2, and shows a result consistent with cosmological constant with $w_0= -0.92\pm 0.08$ and $w_a= -0.49^{+0.33}_{-0.36}$. This indicates that the current conclusion from DESI DR2 in combination with Pantheon$+$ is likely due to the combination of two inconsistent datasets resulting in precise but inaccurate inference of cosmological parameters. In the future, tests of this kind for the consistency between different cosmological datasets will be essential for robust inference of cosmological parameters and for deciphering unaccounted physical effects or observational artifacts from supernovae and BAO datasets.

During the late stages of a binary neutron star inspiral, dynamical tides induced in each star by its companion become significant and should be included in complete gravitational-wave (GW) modeling. We investigate the coupling between the tidal field and quasi-normal modes in hybrid stars and show that the discontinuity mode ($g$-mode)--intrinsically associated with first-order phase transitions and buoyancy--can rival the contribution of the fundamental $f$-mode. We find that the $g$-mode overlap integral can reach up to $\sim 10\%$ of the $f$-mode value for hybrid star masses in the range 1.4-2.0$M_{\odot}$, with the largest values generally associated with larger density jumps. This leads to a GW phase shift due to the $g$-mode of $\Delta \phi_g \lesssim 0.1$-$1$ rad (i.e., up to $\sim$5\%-10\% of $\Delta \phi_f$), with the largest shifts occurring for masses near the phase transition. At higher masses, the shifts remain smaller and nearly constant, with $\Delta \phi_g \lesssim 0.1$ rad (roughly $\sim 1\%$ of $\Delta \phi_f$). These GW shifts may be relevant even at the design sensitivity of current second-generation GW detectors in the most optimistic cases. Moreover, if a $g$-mode is present and lies near the $f$-mode frequency, neglecting it in the GW modeling can lead to systematic biases in neutron star parameter estimation, resulting in radius errors of up to $1\%-2\%$. These results show the importance of dynamical tides to probe neutron stars' equation of state, and to test the existence of dense-matter phase transitions.