In this paper we present a method to study the frequency shift of signals sent from near a Schwarzschild black hole that grows or shrinks through accretion. We construct the numerical solution of Einstein’s equations sourced by a spherical shell of scalar field, with positive energy density to simulate the growth and with negative energy density to simulate the shrink of the black hole horizon. We launch a distribution of null rays at various time slices during the accretion and estimate their energy along their own trajectories. Spatially the bundles of photons are distributed according to the distribution of dust, whose dynamics obeys Euler equations in the test field limit during the evolution of the black hole. With these elements, we construct the frequency shift of photons during the accretion process of growth or contraction of the hole, which shows a variability that depends on the thickness of the scalar field shell or equivalently the timescale of the accretion.

In this paper we construct higher-dimensional minimal theory of mass-varying massive gravity (MTMVMG) where the masslike scalar potential is coupled to a vielbein potential, unlike in the previous literature where it is coupled to metric, such that the number of graviton degrees of freedom in the theory is the same as in general relativity. We then study the cosmological aspects of this theory and show that it has eight critical points: five in the massless sector and three in the massive sector. In contrast to the standard theory of mass-varying massive gravity where the graviton mass asymptotically approaches zero at late times, hence making the contribution of massive gravity to the late-time cosmic expansion minimum, the MTMVMG can provide good descriptions both in the massless and massive sectors. Especially, there are at least two interesting possible scenarios for the late-time cosmology in the theory: the dark energy is either due to the constant graviton mass which comes from the scalar field that becomes frozen after the reheating era, or due to the quintessence paradigm where the scalar field is dynamic. Therefore, if the accelerating expansion of the universe in the massless sector can be explained by standard quintessence paradigm, in the massive sector it has to be explained by the nontrivial interplay between quintessence and massive gravity.

We present the first application of a hierarchical Markov Chain Monte Carlo (MCMC) follow-up on continuous gravitational-wave candidates from real-data searches. The follow-up uses an MCMC sampler to draw parameter-space points from the posterior distribution, constructed using the matched-filter as a log-likelihood. As outliers are narrowed down, coherence time increases, imposing more restrictive phase-evolution templates. We introduce a novel Bayes factor to compare results from different stages: The signal hypothesis is derived from first principles, while the noise hypothesis uses extreme value theory to derive a background model. The effectiveness of our proposal is evaluated on fake Gaussian data and applied to a set of 30 outliers produced by different continuous wave searches on O2 Advanced LIGO data. The results of our analysis suggest all but five outliers are inconsistent with an astrophysical origin under the standard continuous wave signal model. We successfully ascribe four of the surviving outliers to instrumental artifacts and a strong hardware injection present in the data. The behavior of the fifth outlier suggests an instrumental origin as well, but we could not relate it to any known instrumental cause.

Extreme-mass-ratio inspirals (EMRIs), compact binaries with small mass-ratios $\epsilon \ll 1$, will be important sources for low-frequency gravitational wave detectors. Almost all EMRIs will evolve through important transient orbital $r\theta$ resonances, which will enhance or diminish their gravitational wave flux, thereby affecting the phase evolution of the waveforms at $O({\epsilon }^{1/2})$ relative to leading order. While modeling the local gravitational self-force (GSF) during resonances is essential for generating accurate EMRI waveforms, so far the full GSF has not been calculated for an $r\theta$-resonant orbit owing to computational demands of the problem. As a first step we employ a simpler model, calculating the scalar self-force (SSF) along $r\theta$-resonant geodesics in Kerr spacetime. We demonstrate two ways of calculating the $r\theta$-resonant SSF (and likely GSF), with one method leaving the radial and polar motions initially independent as if the geodesic is nonresonant. We illustrate results by calculating the SSF along geodesics defined by three $r\theta$-resonant ratios (1∶3, 1:2, 2∶3). We show how the SSF and averaged evolution of the orbital constants vary with the initial phase at which an EMRI enters resonance. We then use our SSF data to test a previously proposed integrability conjecture, which argues that conservative effects vanish at adiabatic order during resonances. We find prominent contributions from the conservative SSF to the secular evolution of the Carter constant $⟨\dot{\mathcal{Q}}⟩$, but these nonvanishing contributions are on the order of, or less than, the estimated uncertainties of our self-force results. The uncertainties come from residual incomplete removal of the singular field in the regularization process. Higher order regularization parameters, once available, will allow definitive tests of the integrability conjecture.

The first detections of black hole–neutron star mergers (GW200105 and GW200115) by the LIGO-Virgo-Kagra Collaboration mark a significant scientific breakthrough. The physical interpretation of pre- and postmerger signals requires careful cross-examination between observational and theoretical modelling results. Here we present the first set of black hole–neutron star simulations that were obtained with the numerical-relativity code $\mathsf{BAM}$. Our initial data are constructed using the public LORENE spectral library, which employs an excision of the black hole interior. $\mathsf{BAM}$, in contrast, uses the moving-puncture gauge for the evolution. Therefore, we need to “stuff” the black hole interior with smooth initial data to evolve the binary system in time. This procedure introduces constraint violations such that the constraint damping properties of the evolution system are essential to increase the accuracy of the simulation and in particular to reduce spurious center-of-mass drifts. Within $\mathsf{BAM}$ we evolve the $Z4c$ equations and we compare our gravitational-wave results with those of the SXS collaboration and results obtained with the sacra code. While we find generally good agreement with the reference solutions and phase differences $\lesssim 0.5\mathrm{rad}$ at the moment of merger, the absence of a clean convergence order in our simulations does not allow for a proper error quantification. We finally present a set of different initial conditions to explore how the merger of black hole neutron star systems depends on the involved masses, spins, and equations of state.

We consider the circular motion of test particles in the gravitational field of a static and axially symmetric compact object described by the $q$ metric. To this end, we calculate orbital parameters of test particles on accretion disks such as angular velocity ($\mathrm{\Omega }$), total energy ($E$), angular momentum ($L$), and radius of the innermost stable circular orbit ($r_{\mathrm{ISCO}}$) as functions of the mass ($m$) and quadrupole ($q$) parameters of the source. The radiative flux, differential, and spectral luminosity of the accretion disk, which are quantities that can be experimentally measured, are then explored in detail. The obtained results are compared with the corresponding ones for the Schwarzschild and Kerr black holes in order to establish whether black holes may be distinguished from the $q$ metric via observations of the accretion disk’s spectrum.

Atom-interferometer gravitational-wave (GW) observatories, as a new design of ground-based GW detectors due in the near future, are sensitive at a relatively low frequency for GW observations. Taking the proposed atom interferometer Zhaoshan long-baseline Atom Interferometer Gravitation Antenna (ZAIGA), and its illustrative upgrade ($\mathrm{Z}+$) as examples, we investigate how the atom interferometer will complement ground-based laser interferometers in testing the gravitational dipole radiation from binary neutron star (BNS) mergers. A test of this kind is important for a better understanding of the strong equivalence principle lying at the heart of Einstein’s general relativity. To obtain a statistically sound result, we sample BNS systems according to their merger rate and population, from which we study the expected bounds on the parametrized dipole-radiation parameter $B$. Extracting BNS parameters and the dipole radiation from the combination of ground-based laser interferometers and the atom interferometer $\mathrm{ZAIGA}/\mathrm{Z}+$, we are entitled to obtain tighter bounds on $B$ by a few times to a few orders of magnitude, compared to ground-based laser interferometers alone, ultimately reaching the levels of $|B|\lesssim {10}^{-9}$ (with ZAIGA) and $|B|\lesssim {10}^{-10}$ (with $\mathrm{Z}+$).

The gravitational Faraday and its dual spin-Hall effects of light arise in space-times of nonzero angular momentum. These effects were studied in stationary, asymptotically flat space-times. Here we study these effects in arbitrary, nonstationary, asymptotically flat space-times. These effects arise from the interaction between light polarization and space-time angular momentum. As a result of such interaction, the phase velocity of left- and right-handed circularly polarized light becomes different, that results in the gravitational Faraday effect. This difference implies different dynamics of these components, that begin to propagate along different paths—the gravitational spin-Hall effect of light. Due to this effect, the gravitational field splits a multicomponent beam of unpolarized light and produces polarized gravitational rainbow. The component separation is an accumulative effect observed in long range asymptotics. To study this effect, we construct uniform eikonal expansion and derive dynamical equation describing this effect. To analyze the dynamical equation, we present it in the local space and time decomposition form. The spatial part of the equation presented in the related optical metric is analogous to the dynamical equation of a charged particle moving in magnetic field under the influence of the Coriolis force.

We study how by careful monitoring of the presence or absence of tidal deformability (TD) and tidal-heating (TH) in the inspiral signal of compact object binaries in ground-based gravitational wave (GW) detectors, one can test if its components are black holes or not. The former property (TD) is finite for neutron stars but vanishes for black holes (in general relativity), whereas the latter is finite for black holes and negligible for neutron stars, and affects the GW phase evolution of binaries in a distinctly different way. We introduce waveform parameters that characterize the strength of tidal-heating and are zero when there is no horizon. We develop Bayesian methods that use TD and TH for distinguishing the presence or absence of horizons in a binary. This is timely owing to several claims that these stellar-mass objects, especially, with masses heavier than those of neutron stars, may not have a horizon but may be black hole mimickers or exotic compact objects (ECOs). It is also astrophysically important to have the tools to test the presence or absence of horizons in mass-gap binaries and, thereby, help detect the heaviest neutron star or the lightest black hole. A proper accounting of tidal-heating in binary waveform models will also be critical for an unbiased measurement of characteristics of the equation of state of neutron stars in GW observations of binaries containing them—or even to probe the existence of ECOs. We show that purely based on GW waveforms it will not be possible to discern binary horizons in the mass gap in Advanced LIGO, Virgo and KAGRA detectors unless the binary is within a few tens of Mpc. However, third generation ground-based detectors will be able to do so for binaries a few hundred Mpc away.

We obtain one family of dynamic solutions in the Einstein-Maxwell-scalar (EMS) theory. Our solutions could describe the evolving charged black(white) hole or wormhole and its transition, including the case of black bounce/wormhole transition. We compare different wormhole throat definitions and suggest that the usage of trapping horizons is the most suitable choice for tracking the evolution of the dynamic black(white) hole and wormhole, and their conversion in a unified framework. Then we research several evolving processes in the appropriate parameters region, including the charge, the initial condition for the scalar hair, and parameters in our EMS Lagrangian. The results show that the appearance of a degenerate marginal trapped surface is the crucial event for the conversion or transition, particularly in these cases: (i) when the evolving wormhole converts to a black hole, the surface emerges and splits into two trapping horizons; (ii) if the metric would become a black hole but finally fails, two trapping horizons combine as the surface and then vanish; (iii) if the black bounce/wormhole transition happens, one single trapping horizon changes its type.

We investigate vacuum static black hole solutions of Einstein-Gauss-Bonnet gravity with a negative cosmological constant in five dimensions. These are solutions with horizons of nontrivial topologies. The first one possesses a horizon with the topology $S^1\times H^2$, and a varying Gauss-Bonnet coupling constant $\alpha$. By looking into its thermodynamic properties, we find that its specific heat capacity with fixed volume is negative; therefore, it is thermodynamically unstable. The second one is equipped with a so-called “Sol manifold” as its horizon, and interestingly, the product of the Gauss-Bonnet coupling constant $\alpha$ and the cosmological constant $\mathrm{\Lambda }$ is fixed. For the second solution, the total energy is a constant and the entropy vanishes. These results expand our knowledge of both topological black holes in higher dimensions and the property of higher curvature corrections of gravitational theories.

We use numerical relativity to study the merger and ringdown stages of “superkick” binary black hole systems (those with equal mass and antiparallel spins). We find a universal way to describe the mass and current quadrupole gravitational waves emitted by these systems during the merger and ringdown stage: (i) The time evolutions of these waves are insensitive to the progenitor’s parameters (spins) after being normalized by their own peak values. (ii) The peak values, which encode all the spin information of the progenitor, can be consistently fitted to formulas inspired by post-Newtonian theory. We find that the universal evolution of the mass quadrupole wave can be accurately modeled by the so-called Backwards One-Body (BOB) model. However, the BOB model, in its present form, leads to a lower waveform match and a significant parameter-estimation bias for the current quadrupole wave. We also decompose the ringdown signal into seven overtones, and study the dependence of mode amplitudes on the progenitor’s parameters. Such dependence is found to be insensitive to the overtone index (up to a scaling factor). Finally, we use the Fisher matrix technique to investigate how the ringdown waveform can be at least as important for parameter estimation as the inspiral stage. Assuming the Cosmic Explorer, we find the contribution of ringdown portion dominates as the total mass exceeds $\sim 250M_{\odot }$. For massive binary black hole (BBH) systems, the accuracy of parameter measurement is improved by incorporating the information of ringdown—the ringdown sector gives rise to a different parameter correlation from inspiral stage; hence, the overall parameter correlation is reduced in full signal.

Solutions to the two-body problem in general relativity allow us to predict the mass, spin, and recoil velocity of a black-hole merger remnant given the masses and spins of its binary progenitors. We address the inverse problem: given a binary black-hole merger, can we use the parameters measured by gravitational-wave interferometers to determine whether the binary components are of hierarchical origin, i.e., whether they are themselves remnants of previous mergers? If so, can we determine at least some of the properties of their parents? This inverse problem is in general overdetermined. We show that hierarchical mergers occupy a characteristic region in the plane composed of the effective-spin parameters ${\chi }_{\mathrm{eff}}$ and ${\chi }_{\mathrm{p}}$, and therefore a measurement of these parameters can add weight to the hierarchical-merger interpretation of some gravitational-wave events, including GW190521. If one of the binary components has hierarchical origin and its spin magnitude is well measured, we derive exclusion regions on the properties of its parents: for example, we infer that the parents of GW190412 (if hierarchical) must have had unequal masses and low spins. Our formalism is quite general, and it can be used to infer constraints on the astrophysical environment producing hierarchical mergers.

Atomic clocks can measure the gravitational redshift predicted by general relativity with great accuracy and for height differences as little as 1 cm. All existing experiments, however, involve the comparison of two independent clocks at different locations rather than a single clock in a delocalized quantum superposition. Here we present an interferometry scheme employing group-II-type atoms, such as Sr or Yb, capable of measuring the gravitational time dilation in a coherent superposition of atomic wave packets at two different heights. In contrast to other recent proposals, there is no need for pulses that can efficiently diffract both internal states. Instead, the scheme relies on very simple atom optics for which high diffraction efficiencies can be achieved with rather mild requirements on laser power. Furthermore, the effects of vibration noise are subtracted by employing a simultaneous Rb interferometer that acts as an inertial reference. Remarkably, the recently commissioned VLBAI facility in Hannover, a 10-m atomic fountain that can simultaneously operate Yb and Rb atoms and enables up to 2.8 s of free evolution time, meets all the requirements for a successful experimental implementation.

By revising the application of the open quantum system approach to the early universe and extending it to the conditions beyond the Markovian approximation, we obtain a new non-Markovian quantum Boltzmann equation. Throughout the paper, we also develop an extension of the quantum Boltzmann equation to describe the processes that are irreversible at the macroscopic level. This new kinetic equation is, in principle, applicable to a wide variety of processes in the early Universe. For instance, using this equation, one can accurately study the microscopic influence of a cosmic environment on a system of cosmic background photons or stochastic gravitational waves. In this paper, we apply the non-Markovian quantum Boltzmann equation to study the damping of gravitational waves propagating in a medium consisting of decoupled ultrarelativistic neutrinos. For such a system, we study the time evolution of the intensity and the polarization of the gravitational waves. It is shown that, in contrast to intensity and linear polarization that are damped, the circular polarization ($V$ mode) of the gravitational wave (if present) is amplified by propagating through such a medium.

Electroweak baryogenesis (EWBG) is sourced by nonstandard $CP$-violating interactions of the Higgs boson with fermions, usually taken to be the top quark, enhanced by its large Yukawa coupling. Numerous papers have studied EWBG sourced by lighter fermions, including the tau lepton and off-diagonal quark mass terms. We critically reassess the viability of EWBG in these scenarios, comparing the predictions based on the semiclassical (WKB) formalism for the source term to those from the vacuum expectation value insertion approximation (VIA), using updated values for the collision terms, and clarifying discrepancies in the definition of the weak sphaleron rate. The VIA systematically predicts a baryon asymmetry that is orders of magnitude larger than the WKB formalism. We trace this to the differing shapes of the $CP$-violating source terms in the two formalisms, showing that the additional spatial derivative in the WKB source term causes large cancellations when it is integrated over the bubble wall profile. An important exception is a source term from $c‐t$ quark mixing, where the WKB prediction also allows for a realistically large baryon asymmetry. In contrast, the analogous $b‐s$ mixing source is found to be orders of magnitude too small.

Forthcoming surveys will extend the understanding of cosmological large scale structures up to unprecedented redshift. According to this perspective, we present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the determination of the Hubble constant. To this aim, we work within linear perturbation theory and relate the fluctuations of the luminosity distance-redshift relation, in the cosmic concordance model, to the intrinsic uncertainty associated with the measurement of $H_0$ from high-redshift surveys ($0.15\le z\le 3.85$). We first present the detailed derivation of the luminosity distance-redshift relation two-point correlation function and then provide analytical results for all the involved relativistic effects, such as peculiar velocity, lensing, time delay, and (integrated) Sachs-Wolfe, and their angular spectra. Hence, we apply our analytical results to the study of the high-redshift Hubble diagram, according to what has been recently claimed in the literature. Following the specifics of Euclid Deep Survey and LSST, we conclude that the cosmic variance associated with the measurement of the Hubble constant is at most of 0.1%. Our work extends the analysis already done in literature for closer sources, where only peculiar velocity has been taken into account. We then conclude that deep surveys will provide an estimation of the $H_0$ which will be more precise than the one obtained from local sources, at least in regard of the intrinsic uncertainty related to a stochastic distribution of inhomogeneities.

We analyze field fluctuations during an ultra slow-roll phase in the stochastic picture of inflation and the resulting non-Gaussian curvature perturbation, fully including the gravitational backreaction of the field’s velocity. By working to leading order in a gradient expansion, we first demonstrate that consistency with the momentum constraint of general relativity prevents the field velocity from having a stochastic source, reflecting the existence of a single scalar dynamical degree of freedom on long wavelengths. We then focus on a completely level potential surface, $V=V_0$, extending from a specified exit point ${\varphi }_{\mathrm{e}}$, where slow roll resumes or inflation ends, to $\varphi \to +\infty$. We compute the probability distribution in the number of $e$-folds $\mathcal{N}$ required to reach ${\varphi }_{\mathrm{e}}$, which allows for the computation of the curvature perturbation. We find that, if the field’s initial velocity is high enough, all points eventually exit through ${\varphi }_{\mathrm{e}}$ and a finite curvature perturbation is generated. On the contrary, if the initial velocity is low, some points enter an eternally inflating regime despite the existence of ${\varphi }_{\mathrm{e}}$. In that case, the probability distribution for $\mathcal{N}$, although normalizable, does not possess finite moments, leading to a divergent curvature perturbation.

We present here up-to-date neutrino mass limits exploiting the most recent cosmological data sets. By making use of the cosmic microwave background temperature fluctuation and polarization measurements, supernovae Ia luminosity distances, baryon acoustic oscillation observations and determinations of the growth rate parameter, we are able to set the most constraining bound to date, $\sum m_{\nu }<0.09\mathrm{eV}$ at 95% C.L. This very tight limit is obtained without the assumption of any prior on the value of the Hubble constant and highly compromises the viability of the inverted mass ordering as the underlying neutrino mass pattern in nature. The results obtained here further strengthen the case for very large multitracer spectroscopic surveys as unique laboratories for cosmological relics, such as neutrinos: that would be the case of the Dark Energy Spectroscopic Instrument survey and of the Euclid mission.

We investigate the parametric amplification of the zero-point fluctuations in the spin modes of a two-component Bose-Einstein condensate, triggered by the dynamical evolution of the condensate density. We first make use of a Thomas-Fermi approximation to develop a tractable theoretical model of the quantum dynamics of the Bogoliubov excitations in a harmonically trapped condensate with a time-dependent trapping frequency. The predictions of this model are then compared to an ab initio numerical study of the correlation functions of density and spin fluctuations for general spatially inhomogeneous configurations. Results are shown for the two cases of expanding and oscillating condensates: while the quantum excitation of spin modes remains weak and relatively featureless in the case of an expanding condensate, clear and experimentally promising signatures of particle creation are anticipated for the oscillating case under suitable resonance conditions between the density and the spin modes.