We study the thermalization of smeared particle detectors that couple locally to any operator in a quantum field theory in curved spacetimes. We show that if the field state satisfies the Kubo-Martin-Schwinger condition with inverse temperature $\beta$ with respect to the detector’s local notion of time evolution, reasonable assumptions ensure that the probe thermalizes to the temperature $1/\beta$ in the limit of long interaction times. Our method also imposes bounds on the size of the system with respect to its proper acceleration and spacetime curvature in order to accurately probe the Kubo-Martin-Schwinger temperature of the field. We then apply this formalism to a uniformly accelerated detector probing the Minkowski vacuum of any $CPT$ symmetric quantum field theory, and show that the detector thermalizes to the Unruh temperature, independently of the operator it couples to. This exemplifies yet again the robustness of the Unruh effect, even when arbitrary smeared detectors are used to probe general operators in a quantum field theory.

We investigate the relation between the subleading soft graviton theorem and asymptotic symmetries in gravity in even dimensions $d=2+2m$ higher than four. After rewriting the subleading soft graviton theorem as a Ward identity, we argue that the charges of such identity generate $\mathrm{Diff}(S^{2m})$. In order to show that, we propose suitable commutation relation among certain components of the metric fields. As a result, all $\mathrm{Diff}(S^{2m})$ transformations are symmetries of gravitational scattering.

We discuss the energy loss due to gravitational radiation of binaries composed of exotic objects whose horizon boundary conditions are replaced with reflective ones. Our focus is on the extreme mass-ratio inspirals in which the central heavier black hole is replaced with an exotic compact object. We show, in this case, a modulation of the energy loss rate depending on the evolving orbital frequency occurs and leads to two different types of modifications to the gravitational wave phase evolution; the oscillating part directly corresponding to the modulation in the energy flux, and the nonoscillating part coming from the quadratic order in the modulation. This modification can be sufficiently large to detect with future space-born detectors.

Constituted with a massive black hole and a stellar mass compact object, extreme mass ratio inspiral (EMRI) events hold unique opportunity for the study of massive black holes, such as by measuring and checking the relations among the mass, spin and quadrupole moment of a massive black hole, putting the no-hair theorem to test. TianQin is a planned space-based gravitational wave observatory and EMRI is one of its main types of sources. It is important to estimate the capacity of TianQin on testing the no-hair theorem with EMRIs. In this work, we use the analytic kludge waveform with quadrupole moment corrections and study how the quadrupole moment can be constrained with TianQin. We find that TianQin can measure the dimensionless quadrupole moment parameter with accuracy to the level of ${10}^{-5}$ under suitable scenarios. The choice of the waveform cutoff is found to have significant effect on the result: if the Schwarzschild cutoff is used, the accuracy depends strongly on the mass of the massive black hole, while the spin has negligible impact; if the Kerr cutoff is used, however, the dependence on the spin is more significant. We have also analyzed the cases when TianQin is observing simultaneously with other detectors such as LISA.

Although the entropy of black holes in any diffeomorphism invariant theory of gravity can be expressed as the Wald entropy, the issue of whether the entropy always obeys the second law of black hole thermodynamics remains open. Since the nonminimal coupling interaction between gravity and the electromagnetic field in the general quadric corrected Einstein-Maxwell gravity can sufficiently influence the expression of the Wald entropy, we check whether the Wald entropy of black holes in the quadric corrected gravity still satisfies the second law. A quasistationary accreting process of black holes is first considered, which describes that black holes are perturbed by matter fields and eventually settle down to a stationary state. Two assumptions that the matter fields should obey the null energy condition and that a regular bifurcation surface exists on the background spacetime are further proposed. According to the two assumptions and the Raychaudhuri equation, we demonstrate that the Wald entropy monotonically increases along the future event horizon under the linear order approximation of the perturbation. This result indicates that the Wald entropy of black holes in the quadric corrected gravity strictly obeys the linearized second law of thermodynamics.

In lower dimensions, charged anti–de Sitter (AdS) black holes in an extended phase space, where the cosmological constant is interpreted as the thermodynamic pressure, are typically absent of liquid/gas phase transitions. We investigate the criticality of lower-dimensional, charged, dilatonic, asymptotically AdS (${\mathrm{CDAdS}}_d$) black holes generated from consistent truncations of ${\mathrm{RNAdS}}_{d+2}$ black objects. We demonstrate that ${\mathrm{CDAdS}}_d$ black holes in $d<4$ can exhibit rich van der Waals behavior and confirm that the associated critical exponents match those expected from mean field theory.

Assuming that the vacuum energy-momentum tensor is not exceptionally large, we consider 4D evaporating black holes with spherical symmetry and evaluate the proper distance $\mathrm{\Delta }L$ between the timelike apparent horizon and the surface of the collapsing matter after it has entered the apparent horizon. We show that $\mathrm{\Delta }L$ can never be larger than $\mathcal{O}(n^{3/2}{\ell }_p)$ when the black hole has evaporated to $1/n$ of its initial mass, as long as $n\ll a^{2/3}/{\ell }_p^{2/3}$ (where $a$ is the Schwarzschild radius and ${\ell }_p$ is the Planck length). For example, the distance between the matter and the apparent horizon must be Planckian at the Page time.

Recently, it has been found that, with the Rényi statistics, the asymptotically flat Schwarzschild black hole can be in thermal equilibrium with infinite heat reservoir at a fixed temperature when its event horizon radius is larger than the characteristic length scale $L_{\lambda }=1/\sqrt{\pi \lambda }$, where $\lambda$ is the nonextensivity parameter. In the Rényi extended phase space with the $PdV$ work term, an off shell free energy in the canonical ensemble with the thermodynamic volume as an order parameter is considered to identify a first-order Hawking-Page (HP) phase transition as a solid-liquid phase transition. It has the latent heat of fusion from solid (corresponding to thermal radiation) to liquid (corresponding to black hole) in the form of $\sim 1/\sqrt{\lambda }$; this is evident in the absence of the HP phase transition in the case of an asymptotically flat Schwarzschild black hole from the Gibbs-Boltzmann statistics ($\lambda =0$). Moreover, we investigate the generalized second law of black hole thermodynamics (GSL) in Rényi statistics by considering the black hole as a working substance in a heat engine. Interestingly, an efficiency $\eta$ of the black hole in a Carnot cycle takes the form ${\eta }_c=1-T_{\mathrm{C}}/T_{\mathrm{H}}$. This confirms the validity of the GSL in the Rényi extended phase space.

It has been known that the Schwarzschild–de Sitter (Sch-dS) black hole may not be in thermal equilibrium and also be found to be thermodynamically unstable in the standard black hole thermodynamics. In the present work, we investigate the possibility to realize the thermodynamical stability of the Sch-dS black hole as an effective system by using the Rényi statistics, which includes the nonextensive nature of black holes. Our results indicate that the nonextensivity allows the black hole to be thermodynamically stable, which gives rise to the lower bound on the nonextensive parameter. By comparing the results to ones in the separated system approach, we find that the effective temperature is always smaller than the black hole horizon temperature and the thermodynamically stable black hole in the effective approach is always larger than the one in the separated approach at a certain temperature. There exists only the zeroth-order phase transition from the hot gas phase to the black hole phase for the effective system, while it is possible to have transitions of both zeroth order and first order for the separated system.

In this work, we explore the existence of traversable wormhole solutions supported by double gravitational layer thin shells and satisfying the null energy condition (NEC) throughout the whole spacetime, in a quadratic-linear form of the generalized hybrid metric-Palatini gravity. We start by showing that for a particular quadratic-linear form of the action, the junction conditions on the continuity of the Ricci scalar $R$ and the Palatini Ricci scalar $\mathcal{R}$ of the theory can be discarded without the appearance of undefined distribution terms in the field equations. As a consequence, a double gravitational layer thin shell arises at the separation hypersurface. We then outline a general method to find traversable wormhole solutions satisfying the NEC at the throat and provide an example. Finally, we use the previously derived junction conditions to match the interior wormhole solution to an exterior vacuum and asymptotic flat solution, thus obtaining a full traversable wormhole solution supported by a double gravitational layer thin shell and satisfying the NEC. Unlike the wormhole solutions previously obtained in the scalar-tensor representation of this theory, which were scarce and required fine-tuning, the solutions obtained through this method are numerous and exist for a wide variety of metrics and actions.

It has long been known that gravitational waves from compact binary coalescing sources are responsible for a first-order displacement memory effect experienced by a pair of freely falling test masses. This constant displacement is sourced from the nonvanishing final gravitational-wave strain present in the wave’s after zone, often referred to as the nonlinear memory effect, and is of the same order of magnitude as the strain from the outgoing quadrupole radiation. Hence, this prediction of general relativity is verifiable experimentally by measurement of the final relative separation between test masses that comprise gravitational-wave detectors. In a separate context, independent calculations have demonstrated that exact, sandwich, plane-wave spacetimes exhibit a velocity memory effect; a nonzero relative velocity, gained by a pair of test masses in free fall, after the passage of a gravitational wave. In this paper, we find that in addition to the known constant displacement memory effect test masses experience, a velocity memory effect at leading order arises due to the nonlinear nature of gravitational waves from compact binary sources. We discuss the magnitude of the first-order velocity memory effect in the context of observing gravitational-wave radiation from super massive binary black hole mergers in LISA.

The observed discrepancy of the Hubble parameter measurements in the local universe with the cosmic microwave background (CMB) data may indicate a new physics. It is vital to test the alternative models that reconcile the Hubble tension with other cosmological observations in this direction. The CMB lensing is a crucial observation that relates the early universe perturbations to the matter’s late-time distribution. In this work, we study the prediction of the $ü\mathrm{\Lambda }\mathrm{CDM}$ as a solution for $H_0$ tension for CMB lensing and the low- and high-$\ell$’s temperature (TT) power spectrum internal inconsistency. We show that this model relaxes the low- and high-$\ell$’s TT mild inconsistency and the CMB lensing tensions simultaneously. Accordingly, $ü\mathrm{\Lambda }\mathrm{CDM}$ having the same number of free parameters as $\mathrm{\Lambda }\mathrm{CDM}$ with lensing amplitude $A_L$ added, has a better fit with $\mathrm{\Delta }{\chi }^2=-3.3$.

We release relike (reionization effective likelihood), a fast and accurate effective likelihood code based on the latest Planck 2018 data that allows one to constrain any model for reionization between $6<z<30$ using five constraints from the CMB reionization principal components (PC). We tested the code on two example models which showed excellent agreement with sampling the exact Planck likelihoods using either a simple Gaussian PC likelihood or its full kernel density estimate. This code enables a fast and consistent means for combining Planck constraints with other reionization data sets, such as kinetic Sunyaev-Zeldovich effects, line-intensity mapping, luminosity function, star formation history, quasar spectra, etc., where the redshift dependence of the ionization history is important. Since the PC technique tests any reionization history in the given range, we also derive model-independent constraints for the total Thomson optical depth ${\tau }_{\mathrm{PC}}={0.0619}_{-0.0068}^{+0.0056}$ and its $15\le z\le 30$ high redshift component ${\tau }_{\mathrm{PC}}(15,30)<0.020$ (95% C.L.). The upper limits on the high-redshift optical depth is a factor of $\sim 3$ larger than those reported in the Planck 2018 cosmological parameter paper using the FlexKnot method and we validate our results with a direct analysis of a two-step model which permits this small high-$z$ component.

Fitting half-integer generalized Laguerre functions to the evolved, real-space dark matter and halo correlation functions provides a simple way to reconstruct their initial shapes. We show that this methodology also works well in a wide variety of realistic, assembly biased, velocity biased and redshift-space distorted mock galaxy catalogs. We use the linear point feature in the monopole of the redshift-space distorted correlation function to quantify the accuracy of our approach. We find that the linear point estimated from the mock galaxy catalogs is insensitive to the details of the biasing scheme at the subpercent level. However, the linear point scale in the nonlinear, biased, and redshift-space distorted field is systematically offset from its scale in the unbiased linear density fluctuation field by more than 1%. In the Laguerre reconstructed correlation function, this is reduced to sub-percent values, so it provides comparable accuracy and precision to methods that reconstruct the full density field before estimating the distance scale. The linear point in the reconstructed density fields provided by these other methods is likewise precise, accurate, and insensitive to galaxy bias. All reconstructions depend on some input parameters, and marginalizing over uncertainties in the input parameters required for reconstruction can degrade both accuracy and precision. The linear point simplifies the marginalization process, enabling more realistic estimates of the precision of the distance scale estimate for negligible additional computational cost. We show this explicitly for Laguerre reconstruction.

An observer in relative motion to the cosmic microwave background (CMB) rest frame is sensitive to both aberration and Doppler effects. Both effects introduce similar but nonidentical off-diagonal couplings in the spherical harmonic coefficients. The CMB temperature dipole may have additional contributions from an intrinsic component, which in turn produces different aberration and Doppler couplings. Moreover, the standard conversion from intensity measurements into temperature also introduces spurious Doppler-like couplings. In order to learn about the intrinsic dipole it is therefore important to measure both aberration and Doppler couplings in an independent manner while also removing the spurious contributions from unit conversion, which are degenerate with the dipole. Here we present a pipeline to measure independently the dipole, Doppler and aberration effects on the CMB considering realistic beaming, noise and mask effects. Our pipeline results in independent and unbiased estimators which have uncertainties only $\simeq 20\%$ larger than the simple theoretical expectations. We discuss the achievable precision in each measurement for Planck 2018 and also forecast them for future ground-based experiments with the Simons Observatory and CMB stage IV experiment (CMB-S4). An alternative pipeline is present in order to cross-check results and improve robustness.

The low density nuclear matter equation of state is strongly constrained by nuclear properties; however, for constraining the high density equation of state it is necessary to resort to indirect information obtained from the observation of neutron stars, compact objects that may have a central density several times nuclear matter saturation density, $n_0$. Taking a metamodeling approach to generate a huge set of equations of state that satisfy nuclear matter properties close to $n_0$ and that do not contain a first-order phase transition, the possibility of constraining the high density equation of state (EOS) was investigated. The entire information obtained from the GW170817 event for the probability distribution of $\tilde{\mathrm{\Lambda }}$ was used to make a probabilistic inference of the EOS, which goes beyond the constraints imposed by nuclear matter properties. Nuclear matter properties close to saturation, below $2n_0$, do not allow us to distinguish between equations of state that predict different neutron star (NS) maximum masses. This is, however, not true if the equation of state is constrained at low densities by the tidal deformability of the NS merger associated to GW170817. Above $3n_0$, differences may be large, for both approaches, and, in particular, the pressure and speed of sound of the sets studied do not overlap, showing that the knowledge of the NS maximum mass may give important information on the high density EOS. Narrowing the maximum mass uncertainty interval will have a sizable effect on constraining the high density EOS.

The flux of ultrahigh energy cosmic rays reaching the Earth is affected by the interactions with the cosmic radiation backgrounds as well as with the magnetic fields that are present along their trajectories. We combine the simprop cosmic ray propagation code with a routine that allows us to account for the average effects of a turbulent magnetic field on the direction of propagation of the particles. We compute in this way the modification of the spectrum which is due to the magnetic horizon effect, both for primary nuclei as well as for the secondary nuclei resulting from the photodisintegration of the primary ones. We also provide analytic parametrizations of the attenuation effects, as a function of the magnetic field parameters and of the density of cosmic ray sources, which make it possible to obtain the expected spectra in the presence of the magnetic fields from the spectra that would be obtained in the absence of magnetic fields. The discrete nature of the distribution of sources with finite density also affects the spectrum of cosmic rays at the highest energies where the flux is suppressed due to the interactions with the radiation backgrounds, and parametrizations of these effects are obtained.

We perform force-free simulations for a neutron star orbiting a black hole, aiming at clarifying the main magnetosphere properties of such binaries toward their innermost stable circular orbits. Several configurations are explored, varying the orbital separation, the individual spins and misalignment angle among the magnetic and orbital axes. We find significant electromagnetic luminosities, $L\sim {10}^{42–46}[B_{\mathrm{pole}}/{10}^{12}\mathrm{G}{]}^2\mathrm{erg}/\mathrm{s}$ (depending on the specific setting), primarily powered by the orbital kinetic energy, being about one order of magnitude higher than those expected from unipolar induction. The systems typically develop current sheets that extend to long distances following a spiral arm structure. The intense curvature of the black hole produces extreme bending on a particular set of magnetic field lines as it moves along the orbit, leading to magnetic reconnections in the vicinity of the horizon. For the most symmetric scenario (aligned cases), these reconnection events can release large-scale plasmoids that carry the majority of the Poynting fluxes. On the other hand, for misaligned cases, a larger fraction of the luminosity is instead carried outwards by large-amplitude Alfvén waves disturbances. We estimate possible precursor electromagnetic emissions based on our numerical solutions, finding radio signals as the most promising candidates to be detectable within distances of $\lesssim 200\mathrm{Mpc}$ by forthcoming facilities like the Square Kilometer Array.

X-ray pulse profile modeling of PSR $\mathrm{J}0740+6620$, the most massive known pulsar, with data from the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement’s implications for the neutron star equation of state (EoS), employing a nonparametric EoS model based on Gaussian processes and combining information from other x-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors EoSs that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of 6.9. For such EoSs, the transition mass from the hadronic to the hybrid branch is constrained to lie outside $(1,2){\mathrm{M}}_{\odot }$. We also find that the conformal sound-speed bound is violated inside neutron star cores, which implies that the core matter is strongly interacting. The squared sound speed reaches a maximum of $0.75_{-0.24}^{+0.25}{c}^2$ at ${3.60}_{-1.89}^{+2.25}$ times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR $\mathrm{J}0740+6620$’s central density is only ${3.57}_{-1.3}^{+1.3}$ times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in $\beta$-equilibrium.

Subhalos at subgalactic scales ($M\lesssim {10}^7{M}_{\odot }$ or $k\gtrsim {10}^3{\mathrm{Mpc}}^{-1}$) are pristine test beds of dark matter (DM). However, they are too small, diffuse and dark to be visible, in any existing observations. In this paper, we develop a complete formalism for weak and strong diffractive lensing, which can be used to probe such subhalos with chirping gravitational waves (GWs). Also, we show that Navarro-Frenk-–White (NFW) subhalos in this mass range can indeed be detected individually, albeit at a rate of $\mathcal{O}(10)$ or less per year at Big Bang Observer and others—limited by small merger rates and large required $\mathrm{SNR}\gtrsim 1/\gamma (r_0)\sim {10}^3$. It becomes possible as NFW scale radii $r_0$ are of the right size comparable to the GW Fresnel length $r_F$, and unlike all existing probes, their lensing is more sensitive to lighter subhalos. Remarkably, our formalism further reveals that the frequency dependence of weak lensing (which is actually the detectable effect) is due to shear $\gamma$ at $r_F$. Not only is it consistent with an approximate scaling invariance, but it also offers a new way to measure the mass profile at a successively smaller scale of chirping $r_F\propto f^{-1/2}$. Meanwhile, strong diffraction that produces a blurred Einstein ring has a universal frequency dependence, allowing only detections. These are further demonstrated through semianalytic discussions of power-law profiles. Our developments for a single lens can be generalized and will promote diffractive lensing to a more concrete and promising physics in probing DM and small-scale structures.