We propose a novel method to probe gravity-induced entanglement. We consider the gravitational interaction between a particle trapped in a shallow potential and a harmonic oscillator. The harmonic oscillator is in a quantum superposition of two frequencies and only one of these states can excite the trapped particle via resonance. Once the excited particle is detected, the quantum state of the oscillator is collapsed, which can be observed as the sudden disappearance of the superposition of oscillator frequencies. Thus, the sudden decoherence, which is only triggered by particle detection, can be a smoking gun evidence of gravity-induced entanglement. Since the probability of particle excitation increases linearly with time, the total probability is multiplied by repeating experiments. We will also discuss experimental implementations using optomechanics.

In the semiclassical ($c\to \infty$) limit, 4-point heavy-light-light-heavy correlators in 2D conformal field theory exhibit periodic Euclidean singularities. Periodic singularities in Euclidean time are a general feature of thermal correlators, even at weak coupling. Therefore, the bulk significance of this observation (in particular, the role of the horizon) is somewhat obscure. Explicit numerical computations of finite-$c$ Virasoro blocks furthermore suggest that their departure from semiclassical blocks may begin already at half the period. In this paper, we provide a bulk understanding of these facts and clarify the role of the horizon. We present a bulk geodesic Witten diagram calculation of semiclassical Virasoro blocks in coordinates that are naturally adapted to the Bañados-Teitelboim-Zanelli black hole. This allows a bulk geometric interpretation for boundary time separation. In this language, half of a thermal time period is the boundary timescale at which the light operator geodesic straddles the Euclidean horizon, capturing both the role of the horizon and the associated timescale. This timescale arises in a calculation that does not involve a periodic thermal circle on the bulk or the boundary.

While the two derivative action of gravitation is specified uniquely, higher derivative operators are also allowed with coefficients that are not specified uniquely by effective field theory. We focus on a four derivative operator in which the Riemann tensor couples directly to the electromagnetic field $a$ $R_{\mu \nu \alpha \beta }{F}^{\mu \nu }{F}^{\alpha \beta }$. We compute the corresponding corrections to the Shapiro time delay in the solar system and compare this to data from the Cassini probe. We place an observational upper bound on the coefficient $a$ at 95% confidence $|a|<26(1000\mathrm{km}{)}^2$. By way of motivation, we also compare this to a weak gravity conjecture (WGC) prediction of a bound on the coefficients $a$, $b$ of four derivative operators involving the graviton and the photon; this includes the above term $a$ $R_{\mu \nu \alpha \beta }{F}^{\mu \nu }{F}^{\alpha \beta }$ as well as $b$ $F^4$. We show that by using the observed value of the $b$ coefficient from measurements of light by light scattering, which arises in the Standard Model from integrating out the electron, the WGC predicted bound for $a$ is $a\lesssim 7.8(1000\mathrm{km}{)}^2$. This is consistent with the above observational bound, but is intriguingly close and can be further probed in other observations.

We show that the null geodesic radial action for unbound orbits in the Kerr spacetime, and consequently the scattering angle, can be resummed in terms of hypergeometric functions, extending previous results [Ivanov et al., Resummation of universal tails in gravitational waveforms, arXiv:2504.07862.]. We provide explicit expressions as series expansions in powers of the Kerr rotational parameter up to the fourth order included. We finally use the Mano-Suzuki-Takasugi formalism to prove the relation between the renormalized angular momentum and the radial action highlighted in previous works.

We present a large family of twisting and expanding solutions to the Einstein-Maxwell equations of algebraic type D, for which the two double principal null directions (PNDs) of the Weyl tensor are not aligned with the null eigendirections of the Faraday tensor. In addition to systematically deriving this new class, we present its various metric forms and convenient parametrizations. We show that in Boyer-Lindquist-type coordinates these solutions depend on seven parameters, namely the Kerr and NUT (Newman-Unti-Tamburino) twist parameters $a$ and $l$, mass parameter $m$, acceleration $\alpha$, strength of the Maxwell field $|c|$, and angular parameters $\beta$, $\gamma$ that represent two duality rotations of the Faraday tensor, which include the rotation between the electric and magnetic charges generating the aligned part of the Maxwell field. This coordinate parametrization, analogous to the Griffiths-Podolský form of the Plebański-Demiański solutions, allows us to perform various limits, explicitly identify the subcases, and determine the physical interpretation of the new class. Interestingly, by considering the limit with no acceleration ($\alpha \to 0$), one obtains either the famous Kerr-Newman-NUT black holes (if the parameter $|c|$ remains constant) or the novel Kerr-Bertotti-Robinson black holes, announced recently in our work [Kerr black hole in a uniform Bertotti-Robinson magnetic field: An exact solution, Phys. Rev. Lett. (to be published), 10.1103/rfgv-ybz5.] (if $|c|\to \infty$ while $\alpha |c|=\text{const}.$). We may thus conclude that this new class of spacetimes represents twisting charged accelerating black holes, immersed in an external magnetic (or electric) field. In the nontwisting subcase, we obtain the previously known solution of Van den Bergh-Carminati.

We present the complete set of covariant equations that govern the locally rotationally symmetric torsion spacetimes sourced by Weyssenhoff fluid in Einstein-Cartan-Sciama-Kibble gravity. Using these equations, we can explore in detail the peculiar relationship between conformal structure and torsion. We develop a comprehensive scheme to categorize these torsional spacetimes into distinct classes. We explicitly analyze the properties of each class and obtain novel analytical solutions to the gravitational field equations.

In this study, we investigate how Earth’s gravitational field affects Hong-Ou-Mandel (HOM) interference experiments conducted in a terrestrial laboratory. To second order, we calculate the relativistic time delay from the null geodesic equation (particle perspective), while the phase shift and the associated effective time delay are derived from the Klein-Gordon equation (wave perspective). Since gravity influences both the temporal and spatial parts of the phase shift, these two time delays differ and lead to different coincidence probabilities. The previous HOM experiment conducted on a rotating platform suggests that the wave perspective can explain the experimental results. We further explore the frame dragging and redshift effects in an arbitrarily oriented rectangular interferometer under two distinct scenarios with different photon paths, measuring one effect in each scenario. We find that both effects can be amplified by increasing the number of light loops. Additionally, we emphasize that the next-to-leading order Sagnac effect, arising from gravitational acceleration, is comparable to the Thomas precession, the geodetic effect, and the Lense-Thirring effect. To detect the leading order Sagnac effect and the redshift effect caused by gravitational acceleration, we estimate the number of loops that photons should travel in the interferometer. Furthermore, we propose that the difference between two HOM patterns can be used as a probe to detect gravitational effects on quantum systems.

Aether scalar-tensor theory is a relativistic alternative gravity model that behaves like cold dark matter on cosmological scales while predicting the MOND force-law in astrophysical systems. The theory correctly predicts the cosmic microwave background and linear matter power spectra, and the mass discrepancies observed across the Universe. We derive and solve the equations governing neutron stars in aether scalar tensor theory at first-order in slow rotation, finding that the theory predicts approximate universal relations between the moment of inertia and the compactness ($I–C$ relations) that differ from their general relativity counterparts. These relations may enable tests of aether scalar-tensor theory using x-ray observations of pulsars and gravitational wave observations of binary neutron star mergers.

We examine gravitational waves (GWs) from binary black holes as possible suitable systems for investigating the physical validity of theories predicting the relative locality (RL) effect, an effect arising in the $\kappa$-Minkowski noncommutative spacetime, a central property in theories of quantum gravity (QG) phenomenology. Hence, we are taking a step towards realizing the purpose of the phenomenological effort of having observational evidence to put constraints on QG approaches. In particular, we show that the RL effect induces an uncertainty in the observed rotational frequency $\widehat{\omega }$ during the inspiral phase. This uncertainty becomes stronger with increasing observational distance. It also increases with decreasing orbital radius, and it statistically accumulates as an increasing variance of $\widehat{\omega }$ over successive cycles. In terms of the post-Newtonian deviations, the uncertainty contributes at 1.25th order, an order that has not yet been directly constrained in GW analyses.

It was recently proposed by Rosato et al. and Oshita et al. that the black hole graybody factors, as stable observables at relatively high frequencies, are more relevant quantities than quasinormal modes in modeling ringdown spectral amplitudes. The proposed alternative was primarily motivated by the observed spectral instability of quasinormal modes, which was shown to be highly sensitive to ultraviolet metric perturbations. It was argued that the overall contributions of spectrally unstable quasinormal modes conspire to produce stable observables through collective interference effects. Regge poles, which govern the analytic structure of the $S$-matrix in the complex angular momentum plane, also serve as an effective tool for assessing the scattering processes of black holes. In this regard, the present study investigates the Regge poles and the total absorption cross sections for black hole metrics with discontinuity. Specifically, these quantities are evaluated at given frequencies and scattering angles, which account for the contributions of the graybody factors at different angular momenta. To this end, we generalize the matrix method to evaluate the Regge poles in black hole metrics with discontinuity. To ascertain our approach, the numerical results are compared with those obtained using a modified version of the continued fraction method. The obtained Regge pole spectrum is then used to calculate the scattering amplitude and cross section. We show that the stability of these observables as functions of the frequency can be readily interpreted in terms of the stability of the Regge pole spectrum, particularly the low-lying modes. Nonetheless, destabilization still occurs at larger frequencies, characterized by the emergence of a bifurcation in the spectrum. The latter further evolves, leading to more significant deformation in the Regge poles, triggered by ultraviolet metric perturbations moving farther away from the black hole. However, based on the validity of the WKB approximation, it is argued that such an instability in the spectrum is not expected to cause significant observable implications.

We revisit the modelling of black hole ringdown beyond general relativity (GR), emphasizing the limitations of approaches that rely solely on shifted quasinormal mode (QNM) frequencies. Starting from modified Teukolsky equations in such scenarios, we classify the distinct types of deviations that can arise—those shifting QNM frequencies, and those introducing additional frequencies associated with extra fields. We then construct the most general ansatz for metric perturbations in this context and discuss its implications for QNM modelling and theory-agnostic tests of GR using gravitational wave data.

The peak of the matter power spectrum, known as the turnover (TO) scale, is determined by the horizon size at the time of matter-radiation equality. This scale can serve as a standard ruler, independent of other features in the matter power spectrum, such as baryon acoustic oscillations (BAO). Here, we present the first detection of the turnover in the galaxy autopower spectrum, utilizing the distribution of quasars and luminous red galaxies (LRG) measured by the Dark Energy Spectroscopic Instrument (DESI) during its first year of survey operations in a model-independent manner. To avoid confirmation bias, we first analyze the data using data blinding methods designed for the DESI baryon acoustic oscillation, redshift space distortion and scale-dependent bias signals. We measure the angle-averaged dilation distance $D_{\mathrm{V}}(z=1.651)=(38.1\pm 2.5)r_{\mathrm{H}}$ from the quasars and $D_{\mathrm{V}}(z=0.733)=(21.8\pm 1.0)r_{\mathrm{H}}$ from the LRG sample in units of the horizon $r_{\mathrm{H}}$ at the matter-radiation-equality epoch. Combining these two constraints and assuming a flat $\mathrm{\Lambda }\mathrm{CDM}$ model with three standard neutrino species, we can translate this into a constraint of ${\mathrm{\Omega }}_{\mathrm{m}}{h}^2=0.139_{-0.046}^{+0.036}$. We can break the ${\mathrm{\Omega }}_{\mathrm{m}}\text{−}{H}_0$ degeneracy with low-redshift distance measurements from type-Ia supernova (SN) data from $\mathrm{Pantheon}+$, we obtain a sound-horizon free estimate of the Hubble-Lemaître parameter of $H_0={65.2}_{-6.2}^{+4.9}\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, consistent with sound-horizon dependent DESI measurements. On the other hand, combining the DESI BAO and TO, we find a truly DESI-only measurement of $H_0={74.0}_{-3.5}^{+7.2}\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, in line with DESI-only full-shape results where the sound-horizon scale is marginalized out. This discrepancy in $H_0$ can be reconciled in a $w_0{w}_a\mathrm{CDM}$ cosmology, where the combination of DESI BAO and TO data yields $H_0=66.5\pm 7.2\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$.

In the past two decades, significant advancements have been made in observational techniques to enhance our understanding of the universe and its evolutionary processes. However, our knowledge of the postinflation reheating phase remains limited due to its small-scale dynamics. Traditional observations, such as those of the cosmic microwave background, primarily provide insights into large-scale dynamics, making it challenging to glean information about the reheating era. In this paper, our primary aim is to explore how the generation of gravitational wave (GW) spectra, resulting from electromagnetic fields generated during inflation, can offer valuable insights both into the magnetogenesis model and reheating dynamics. We investigate how the spectral shape of GWs varies across different frequency ranges, depending on the model parameters, the magnetogenesis parameters, and reheating dynamics. For this, we consider a well-known nonhelical magnetogenesis model, where the usual electromagnetic kinetic term is coupled with a background scalar. Notably, for such a scenario, we observe distinct spectral shapes with sufficiently high amplitudes for different reheating histories with the equation of state parametrized by ($w_{\mathrm{re}}$). We identify spectral breaks in the GW spectra for both $w_{\mathrm{re}}<1/3$ and $w_{\mathrm{re}}>1/3$ scenarios. We find that future GW experiments, such as BBO, LISA, SKA, and DECIGO, are well within the reach of observing those distinct spectral shapes and can potentially shed light on the underlying mechanism of the reheating phase. Finally, we attempt to constrain the nonhelical models under consideration utilizing the latest NANOGrav 15-year observation campaign. We find that, in order to explain the observed GW spectrum in the nano-Hz range, the required magnetic field spectrum needs to be very stiff. However, such a stiff magnetic field spectrum in turn yields a strongly blue-tilted GW spectrum, which conflicts with other cosmological observations.

We analyze Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillation, cosmic microwave background, and supernova data to explore the physical origin of the DESI indication for dynamical dark energy. Beyond the standard linear parametrization, $w(a)=w_0+(1-a)w_a$, we explore truncated alternatives and quintessence models. We conclude that there is compelling evidence for dark energy to be decaying in the late Universe, but the evidence for a phantom behavior, i.e., $w<-1$ at some redshift, is less significant. Models without phantom behavior are compatible with the data at the $2\sigma$ CL. Furthermore, we examine a concrete, theoretically well-motivated quintessence scenario with a Higgs-like potential, allowing for a direct comparison with parametrized approaches and testing its consistency with current observations. This framework enables a broader investigation of late-time cosmic evolution and reveals a 93.8% preference for a future transition into an anti–de Sitter space, which may ultimately lead to a cosmological collapse of our Universe.

We analyze strong gravitational lensing by a spherically symmetric mass distribution within the Einstein-Straus–de Sitter framework in a spatially open universe with negative curvature ($k=-1$). Applying the theory to the lensed quasar SDSS $\mathrm{J}1004+4112$, we identify a critical threshold for the current scale factor $a_0$ of approximately $2.6\times {10}^{27}\mathrm{m}$, below which the effects of negative spatial curvature on lensing observables become significant, corresponding to a current curvature density of $|{\mathrm{\Omega }}_{k0}|\gtrsim 0.0025$. In particular, for ${\mathrm{\Omega }}_{k0}=-0.15$, the light bending increases slightly by $\sim 1\%$, while the time delay exhibits a more substantial increase of $\sim 10\%$. Beyond this threshold, however, the lensing observables are found to be insensitive to the current scale factor and converge to those characteristic of a spatially flat universe. Importantly, our results indicate that, even in scenarios where spatial curvature would otherwise enhance lensing observables, the effect of the cosmological constant does remain present and acts to reduce light bending, corroborating the claim of Rindler and Ishak.

In this study, we build upon the findings of Del Popolo et al. (2013) by further analyzing the influence of dynamical friction on the evolution of cosmological perturbations within the framework of the spherical collapse model (SCM) in a Universe dominated by generalized Chaplygin gas (GCG). Specifically, we investigate how dynamical friction alters the growth rate of density perturbations, the effective sound speed, the equation-of-state parameter w, and the evolution of the cosmic expansion rate. Our results demonstrate that dynamical friction significantly delays the collapse process compared to the standard SCM. Accurate computation of these parameters is crucial for obtaining consistent results and reliable physical interpretations when employing the GCG model. Furthermore, our analysis confirms that the suppression of perturbation growth due to dynamical friction is considerably more pronounced than that caused by shear and rotation, as previously indicated by Del Popolo et al. (2013). This enhanced suppression effectively addresses the instability issues, such as oscillations or exponential divergences in the dark-matter power spectrum, highlighted in linear perturbation studies, such as those by Sandvik et al. (2004).

Which of the multiple models of causal and stable relativistic viscous fluids that have been developed is best suited to describe neutron stars? The modeling of out-of-equilibrium effects in these relativistic, astrophysical objects must be done with care, as simple Newtonian intuition fails to remain causal. Radial stability of neutron stars is one of the primary conditions for the viability of such out-of-equilibrium models. In this paper, we study radial perturbations of neutron stars for the Eckart, the Bemfica-Disconzi-Noronha-Kovtun, and the Müller-Israel-Stewart fluid models of relativistic viscous fluids. We find that for small viscosity, the three models have the same stability properties: they are always stable to bulk and shear viscosity, but they can be unstable to heat conductivity if certain thermodynamic conditions are violated. For the latter case, we derive a necessary criterion for stability to heat conductivity that applies to all three fluids. Moreover, we show that the additional degrees of freedom introduced by the Bemfica-Disconzi-Noronha-Kovtun and the Müller-Israel-Stewart models force the perturbations to evolve on fast timescales. Specifically, the Bemfica-Disconzi-Noronha-Kovtun model has additional oscillatory perturbations that propagate with the speed of second sound, while the Müller-Israel-Stewart model MIS only exhibits decaying behavior on the fast timescale. This work therefore establishes the first formal results and criteria for radial stability of these three out-of-equilibrium fluid models on the nontrivial, relativistic background of neutron stars.

We present new constraints on the neutron star equation of state (EOS) and mass distribution using a unified Bayesian inference framework that incorporates the latest NICER measurements, including PSR J0614-3329, alongside gravitational-wave data, radio pulsar masses, and nuclear theory. By systematically comparing four inference scenarios—varying in the inclusion of PSR J0614-3329 and in the pulse profile model used for PSR $\mathrm{J}0030+0451$—we quantify the impact of observational and modeling choices on dense matter inference. We find that systematics from hotspot geometry dominate EOS uncertainties: the choice of hotspot geometry for PSR $\mathrm{J}0030+0451$ leads to significant shifts in the inferred stiffness of the EOS and maximum neutron star mass. In contrast, PSR J0614-3329 mildly softens the EOS at low densities, reducing the radius at $1.4M_{\odot }$ by $\sim 100\mathrm{m}$. A Bayesian model comparison yields a Bayes factor of ${\log }_{10}\mathrm{BF}\approx 1.58$ in favor of the $\mathrm{ST}+\mathrm{PDT}$ hotspot model over PDT-U, providing strong evidence that multimessenger EOS inference can statistically discriminate between competing NICER pulse profile models. These results highlight the critical role of NICER systematics in dense matter inference and the power of joint analyses in breaking modeling degeneracies.

Axions have attracted attention as promising candidates for dark matter (DM). Although axions have been intensively searched for, they have not been observed yet. Recently, novel experiments to search for axion DM have been proposed that use optical cavities to amplify polarization rotation of laser light induced by the axion-photon interaction. One such experiment employs a ring cavity composed of four mirrors. However, its sensitivity to the axion-photon coupling $g_{a\gamma }$ in the low axion mass region is limited due to a reflection phase difference between s- and p-polarizations. In this paper, we propose a new method to improve the sensitivity using zero phase shift mirrors and a wavelength tunable laser. Moreover, the laser makes it easier to scan the high axion mass region by tuning the reflection phase difference between s- and p-polarizations. We experimentally confirmed that the phase difference generated upon reflection on a zero phase shift mirror satisfied the requirement of $8.6\times {10}^{-3}\mathrm{deg}$, which corresponds to the half width at half maximum (HWHM) of the cavity for p-polarization with the mirror fixed on a folded cavity and a wavelength tunable laser.

Newborn pulsars resulting from core-collapse supernovae are promising sources of high-energy (HE) cosmic rays and neutrinos. In this work, we focus on HE neutrinos generated through interactions between protons accelerated in relativistic pulsar winds and the supernova (SN) ejecta. Using a binned likelihood analysis, we evaluate the detection prospects of these neutrinos with IceCube and explore their potential for probing the initial spin period ($P$) and magnetic field strength ($B$) of newborn pulsars. Noticing a degeneracy in neutrino signal with $B/P^2$ across a broad parameter space, we find that HE neutrinos from a Galactic newborn pulsar ($d=10\mathrm{kpc}$) with $(B/{10}^{12}\mathrm{G})(P/\mathrm{ms}{)}^{-2}\gtrsim 0.003$ can be detected at $\gtrsim 3\sigma$ significance. Even in the absence of a nearby pulsar, the diffuse flux from the cosmologic population of pulsars could be probed by next-generation detectors like IceCube-Gen2 and Giant Radio Array for Neutrino Detection 200k. For Galactic pulsars with $(B/{10}^{12}\mathrm{G})(P/\mathrm{ms}{)}^{-2}\gtrsim 0.08$, the combination $B/P^2$ can be measured with an accuracy of better than 20% at the $3\sigma$ confidence level. Additionally, we show that for pulsars with detectable HE neutrino signals, the emission can be clearly distinguished from neutrinos produced by interactions between SN ejecta and the circumstellar medium, due to their distinct temporal and spectral features.