The finite local conformally noninvariant $R^2$ term emerges in the one-loop effective action of the model of quantum gravity based on the Weyl-squared classical action. This term is related to the $\square R$ contribution to the conformal anomaly, which in a wide class of regularization schemes is determined by the second Schwinger-DeWitt (or Gilkey-Seeley) coefficient of the heat kernel expansion for inverse propagators of the theory. The calculation of this term requires evaluating the contributions of the fourth-order derivative minimal and of the second-order nonminimal operators in the tensor and vector sectors of the theory, corresponding to metric, ghost, and gauge-fixing operators. To ensure the correctness of existing formulas, we derived (and confirmed) the result using a special technique of calculations, based on the heat-kernel representation of the Euclidean Green’s function and the method of universal functional traces.

We analyze the general structure of the three-point functions involving conserved higher-spin currents $J_s≔J_{\alpha (i)\dot{\alpha }(j)}$ belonging to any Lorentz representation in four-dimensional conformal field theory. Using the constraints of conformal symmetry and conservation equations, we computationally analyze the general structure of three-point functions $⟨J_{s_1}{J}_{s_2}^{\prime }{J}_{s_3}^{\prime \prime }⟩$ for arbitrary spins and propose a classification of the results. For bosonic vectorlike currents with $i=j$, it is known that the number of independent conserved structures is $2\min (s_i)+1$. For the three-point functions of conserved currents with arbitrarily many dotted and undotted indices, we show that in many cases the number of structures deviates from $2\min (s_i)+1$.

We study the universal logarithmic coefficient of the entanglement entropy in a sphere for free fermionic field theories in a $d=4$ Minkowski spacetime. As a warm-up, we revisit the free massless spin-$1/2$ field case by employing a dimensional reduction to the $d=2$ half-line and a subsequent numerical real-time computation on a lattice. Surprisingly, the area coefficient diverges for a radial discretization but is finite for a geometric regularization induced by the mutual information. The resultant universal logarithmic coefficient $-11/90$ is consistent with the literature. For the free massless spin-$3/2$ field, the Rarita-Schwinger field, we also perform a dimensional reduction to the half-line. The reduced Hamiltonian coincides with the spin-$1/2$ one, except for the omission of the lowest total angular momentum modes. This gives a universal logarithmic coefficient of $-71/90$. We discuss the physical interpretation of the universal logarithmic coefficient for free higher-spin field theories without a stress-energy tensor.

We consider quantum-gravitational corrections to the Oppenheimer-Snyder metric describing time-dependent dust ball collapse. The interior metric also describes Friedmann-Lemaître-Robertson-Walker cosmology and our results are interpreted in that context. The exterior corrections are an example of quantum hair and are shown to persist throughout the collapse. Our results show that the quantum hair survives throughout the horizon formation and the internal state of the resulting black hole is accessible to outside observers.

One of the critical components of loop quantum gravity and cosmology (LQC)—the Thiemann regularization procedure—is nonunique. Different choices of particular prescriptions lead to models that differ in both mathematical structures and physical predictions. Here we briefly recall a set of such prescriptions proposed in the literature in the context of isotropic LQC on the example of a flat universe with massless scalar matter content. For the one least investigated so far, further called the Yang-Ding-Ma prescription, a detailed analysis of its mathematical structure and resulting quantum dynamics is performed, confirming and extending the results obtained so far by phenomenological methods. To probe the dynamics, a relatively robust method (working in the approximation of the macroscopic universe) of evaluating quantum trajectories is devised. This method is a variant of a semiclassical treatment that allows one to express the trajectories analytically as a function of the internal clock and a set of certain central moments—constants of motion encoding quantum corrections up to an arbitrary order in a systematic manner. As a test of the method’s robustness, an analogous evaluation of the quantum trajectory in volume is performed for those of other prescriptions, for which it is applicable. The limitations of the treatment are further briefly discussed.

A new method to work out the Hermitian correspondence of a $\mathcal{PT}$-symmetric quantum mechanical Hamiltonian is proposed. In contrast to the conventional method, the new method ends with a local Hamiltonian of the form $\frac{1}{2}{p}^2+\frac{1}{2}{m}^2{x}^2+v(x)$ without any higher-derivative terms. This method is available for Hamiltonians with general antilinear symmetries in the perturbative regime. Possible extensions to multivariable quantum mechanics and quantum field theories are discussed.

Quantum entanglement harvesting in the relativistic setup attracted a lot of attention in recent times. Acquiring more entanglement within two qubits may be very desirable to establish fruitful communication between them. On the other hand use of reflecting boundaries in a spacetime has close resemblance to the cavity quantum-optomechanical systems. Here, in presence of two reflecting boundaries, we study the generation of entanglement between two uniformly accelerated Unruh-DeWitt detectors which are interacting with the background scalar fields. Like no-boundary and single-boundary situations, entanglement harvesting is possible for their motions in opposite Rindler wedges. We observe that the reflecting boundaries can play double roles. In some parameter space it causes suppression, while in other parameter space we can have enhancement of entanglement compared to no-boundary and single-boundary cases. Thus increase of boundaries has significant impact in this phenomena and a suitable choices of parameters provides desirable increment of it.

Within nonlinear electrodynamics (NED), photons follow null geodesics of an effective geometry, which is different from the geometry of the spacetime itself. Over the last years, several works were dedicated to investigate the motion of photons in the effective geometry of NED-based magnetically charged regular black hole (RBH) solutions. We study the light rings, shadows, and gravitational lensing of the electrically charged RBH solution proposed by Irina Dymnikova (ID), which is a static and spherically symmetric spacetime with a NED source. We show that the shadow associated with the effective geometry can be almost 10% bigger that the one associated with the standard geometry. We also find that the ID solution may mimic the shadow properties of the Reissner-Nordström BH, for low-to-extreme values of the electric charge. Besides that, by using the backwards ray-tracing technique, we obtain that ID and Reissner-Nordström BH solutions can have a very similar gravitational lensing, for some values of the correspondent electric charges. We also show that the motion of photons in the effective geometry can be interpreted as a nongeodesic curve submitted to a four-force term, from the perspective of an observer in the standard geometry.

In this work, we present novel analytical solutions for static and spherically symmetric wormhole geometries threaded by an anisotropic distribution of matter conformally coupled to a scalar ghost field. We explore the main features of the theory, such as the dynamics of the scalar field and matter throughout the wormhole, as well as the role played by the nonminimal coupling. Furthermore, coupled ghosts in the presence of a scalar potential are considered, and traversability conditions are analyzed within such geometrical scheme. More specifically, we find analytical solutions in which, although the energy density of the ghost is strictly negative, the energy density of matter may attain positive values.

We present a methodology based on the implementation of a fully connected neural network algorithm to estimate the temporal evolution of the high-frequency gravitational wave emission for a core collapse supernova (CCSN). For this study, we selected a fully connected deep neural network (DNN) regression model because it can learn both linear and nonlinear relationships between the input and output data, it is more appropriate for handling large-dimensional input data, and it offers high performance at a low computational cost. To train the Machine Learning (ML) algorithm, we construct a training dataset using synthetic waveforms, and several CCSN waveforms are used to test the algorithm. We performed a first-order estimation of the high-frequency gravitational wave emission on real interferometric LIGO data from the second half of the third observing run (O3b) with a two detector network (L1 and H1). The relative error associated with the estimate of the slope of the resonant frequency versus time for the GW from CCSN signals is within 13% for the tested candidates included in this study up to different Galactic distances (1.0, 2.3, 3.1, 4.3, 5.4, 7.3, and 10 kpc). This method is, to date, the best estimate of the temporal evolution of the high-frequency emission in real interferometric data. Our methodology of estimation can be used in future studies focused on physical properties of the progenitor. The distances where comparable performances could be achieved for Einstein Telescope and Cosmic Explorer roughly rescale with the noise floor improvements.

Both circular and epicyclic motion of test particles along equatorial circular orbits in the revisited Kerr–de Sitter spacetime are analyzed. We present relations for specific energy, specific angular momentum, and Keplerian angular velocity of particles on equatorial circular orbits, and discuss criteria for the existence and stability of such orbits giving limits on spacetime parameters. Finally, we discuss the epicyclic motion along equatorial circular orbits obtaining relations for radial and vertical epicyclic frequencies. The results are compared with those for standard Kerr–de Sitter geometry.

In this work, we propose a model of the gravitational collapse of dark matter in the presence of quintessence or phantomlike scalar fields. Our treatment is based on the principles of general relativity up to virialization. We have chosen a spherical patch that starts to collapse gravitationally as it happens in top-hat collapse. It is seen that although the dark matter sector collapses the dark energy sector does keep a profile that is almost similar to the dark energy profile for the background expanding Friedmann-Lemaitre-Robertson-Walker (FLRW) universe for suitable model parameters. It is observed that in order to formulate the problem in the general relativistic setting one has to abandon the idea of a closed FLRW isolated collapsing patch. General relativity requires an external generalized Vaidya spacetime to be matched with the internal spherical patch whose dynamics is guided by the FLRW metric. It is shown that almost all collapses are accompanied by some flux of matter and radiation in the generalized Vaidya spacetime. Some of the spherical regions of the Universe are seen not to collapse but expand eternally, producing voidlike structures. Whether a spherical region will collapse or expand depends upon the initial values of the system and other model parameters. As this work shows that collapsing structures must emit some form of radiation, this may be taken as an observational signature of our proposal.

Using the complete classification of the bases in the rotating black hole background, we separate superradiance from the Hawking effect. We first find that there is spontaneous particle creation for fermions by the potential outside the black hole horizon for the frequencies inside the superradiant regime, i.e. $\omega <k{\mathrm{\Omega }}_H$. However, these particles do not enhance the total flux from the black hole. For the superradiance particle to became real, its negative energy counterpart has to be canceled by the positive energy Hawking radiation mode at the horizon. Since due to the Pauli’s principle this cancellation must be one-to-one, the superradiance effect cannot add anything to the total black hole flux. For an extremal black hole, the Hawking temperature is zero, the horizon is not populated with thermal modes, and fermions can be emitted through the superradiance mechanism. On the other hand, a macroscopic flux of fermions infalling to the black hole is the opposite process of Hawking radiation. A positive energy-infalling particle must cancel out a negative energy thermal mode at the horizon, which leaves a net positive energy mode that crosses the horizon. Since there is finite thermal particle density at the horizon, this implies that there is a maximal fermion infalling rate which is also controlled by the Hawking temperature.

This work discusses the motion of extended test bodies as described by the Mathisson-Papapetrou-Dixon (MPD) equations in the pole-dipole-quadrupole approximation. We focus on the case that the quadrupole is solely induced by the spin of the body which is assumed to move on a circular equatorial orbit in a Kerr background. To fix the center of mass of the MPD body we use two different spin supplementary conditions (SSCs): the Tulczyjew-Dixon SSC and the Mathisson-Pirani SSC. We provide the frequencies of the circular equatorial orbits for the pole-dipole-(spin induced) quadrupole approximation of the body for both SSCs. In the process we develop an explicit four-velocity four-momentum relation for a pole-dipole-quadrupole body under the Mathisson-Pirani SSC.

We study extreme mass-ratio binary systems in which a stellar mass compact object spirals into a supermassive black hole surrounded by a scalar cloud. Scalar clouds can form through superradiant instabilities of massive scalar fields around spinning black holes and can also serve as a proxy for dark matter halos. Our framework is fully relativistic and assumes that the impact of the cloud on the geometry can be treated perturbatively. As a proof of concept, here we consider a point particle in circular, equatorial motion around a nonspinning black hole surrounded either by a spherically symmetric or a dipolar nonaxisymmetric scalar cloud, but the framework can in principle be generalized to generic black hole spins and scalar cloud geometries. We compute the leading-order power lost by the point particle due to scalar radiation and show that, in some regimes, it can dominate over gravitational-wave emission. We confirm the presence of striking signatures due to the presence of a scalar cloud that had been predicted using Newtonian approximations, such as resonances that can give rise to sinking and floating orbits, as well as “sharp features” in the power lost by the particle at given orbital radii. Finally, for a spherically symmetric scalar cloud, we also compute the leading-order corrections to the black-hole geometry and to the gravitational-wave energy flux, focusing only on axial metric perturbations for the latter. We find that, for noncompact clouds, the corrections to the (axial) gravitational-wave fluxes at high frequencies can be understood in terms of a gravitational-redshift effect, in agreement with previous works.

We develop a modified CCZ4 formulation of the Einstein equations in $d+1$ spacetime dimensions for general relativity plus a Gauss-Bonnet term, as well as for the most general parity-invariant scalar-tensor theory of gravity up to four derivatives. We demonstrate well-posedness for both theories and provide full expressions for their implementation in numerical relativity codes. As a proof of concept, we study the so-called “stealth scalarization” induced by the spin of the remnant black hole after the merger. As in previous studies using alternative gauges, we find that the scalarization occurs too late after the merger to impact the tensor waveform, unless the parameters are finely tuned. Naively increasing the coupling to accelerate the growth of the scalar field risks a breakdown of the effective field theory, and therefore well-posedness, as the evolution is pushed into the strongly coupled regime. Observation of such an effect would therefore rely on the detection of the scalar radiation that is produced during scalarization. This work provides a basis on which further studies can be undertaken using codes that employ a moving-punctures approach to managing singularities in the numerical domain. It is therefore an important step forward in our ability to analyze modifications of general relativity in gravitational wave observations.

Quadratic theories of gravity with second order equations of motion provide an interesting model for testing deviations from general relativity in the strong gravity regime. However, they can suffer from a loss of hyperbolicity, even for initial data that is in the weak coupling regime and free from any obvious pathology. This effect has been studied in a variety of cases including isolated black holes and binaries. Here we explore the loss of hyperbolicity in spin-induced scalarization of isolated Kerr black holes in a scalar-Gauss-Bonnet theory of gravity, employing the modified CCZ4 formulation that has recently been developed. We find that, as in previous studies, hyperbolicity is lost when the scalar field and its gradients become large, and identify the breakdown in our evolutions with the physical modes of the purely gravitational sector. We vary the gauge parameters and find the results to be independent of their value. This, along with our use of a different gauge formulation to previous works, supports the premise that the loss of hyperbolicity is dominated by the physical modes. Since scalar-Gauss-Bonnet theories can be viewed as effective field theories (EFTs), we also examine the strength of the coupling during the evolution. We find that at the moment when hyperbolicity is lost the system is already well within the regime where the EFT is no longer valid. This reinforces the idea that the theories should only be applied within their regime of validity, and not treated as complete theories in their own right.

This paper considers the problem of searching for quiet, long-duration, and broadband gravitational wave signals, such as stellar-mass binary black hole binaries, in mock LISA data. We propose a method that combines a semicoherent likelihood with the use of a particle-swarm optimizer capable of efficiently exploring a large parameter space. The semicoherent analysis is used to widen the peak of the likelihood distribution over parameter space, congealing secondary peaks and thereby assisting in localizing the posterior bulk. An iterative strategy is proposed, using particle-swarm methods to initially explore a wide, loosely coherent likelihood and then progressively constraining the signal to smaller regions in parameter space by increasing the level of coherence. The properties of the semicoherent likelihood are first demonstrated using the well-studied binary neutron star signal GW170817. As a proof of concept, the method is then successfully applied to a simplified search for a stellar-mass binary black hole in zero-noise LISA data. Finally, we conclude by discussing what remains to be done to develop this into a fully capable search and how the method might also be adapted to tackle the EMRI search problem in LISA.

Atom gradiometers have emerged as compelling broadband probes of scalar ultralight dark matter (ULDM) candidates that oscillate with frequencies between approximately ${10}^{-2}\mathrm{Hz}$ and ${10}^3\mathrm{Hz}$. ULDM signals with frequencies greater than $\sim 1\mathrm{Hz}$ exceed the expected Nyquist frequency of atom gradiometers, and so are affected by aliasing and related phenomena, including signal folding and spectral distortion. To facilitate the discovery of super-Nyquist ULDM signals, in this work we investigate the impact of these effects on parameter reconstruction using a robust likelihood-based framework. We demonstrate that accurate reconstruction of ULDM parameters can be achieved as long as the experimental frequency resolution is larger than the ULDM signal linewidth. Notably, as ULDM candidates whose frequencies differ by integer multiples of the sampling frequency are identified at the same aliased frequency, our discovery analysis recovers discrete islands in parameter space. Our study represents the first comprehensive exploration of aliasing in the context of dark matter direct detection and paves the way for enhanced ULDM detection strategies with atom gradiometers.

Supernova neutrino boosted dark matter ($\mathrm{SN}\nu$ BDM) and its afterglow effect have been shown to be a promising signature for beyond Standard Model (bSM) physics. The time-evolution feature of $\mathrm{SN}\nu$ BDM allows for possibly direct inference of DM mass $m_{\chi }$, and results in significant background suppression with improving sensitivity. This paper extends the earlier study [Y.-H. Lin et al., Phys. Rev. Lett. 130, 111002 (2023)] and provides a general framework for computing the $\mathrm{SN}\nu$ BDM fluxes for a supernova that occurs at any location in our galaxy. A bSM $U(1{)}_{L_{\mu }-L_{\tau }}$ model with its gauge boson coupling to both DM and the second and third generation of leptons is considered, which allows for both DM-$\nu$ and DM-$e$ interactions. Detailed analysis of the temporal profile, angular distribution, and energy spectrum of the $\mathrm{SN}\nu$ BDM are performed. Unique signatures in $\mathrm{SN}\nu$ BDM allowing extraction of $m_{\chi }$ and detail features that contain information of the underlying interaction type are discussed. Expected sensitivities on the above new physics model from Super-Kamiokande, Hyper-Kamiokande, and DUNE detections of BDM events induced by the next galactic SN are derived and compared with the existing bounds.