In higher dimensions, we study degenerate-higher-order-scalar-tensor theories and we derive solutions that resemble the Schwarzschild anti–de Sitter black holes. We compute their thermodynamic quantities following the Wald formalism, satisfying the first law of thermodynamics and a higher dimensional Smarr relation. Constructing a Noether charge with a suitable choice of a spacelike Killing vector, we obtain the shear viscosity of the nongravitational dual field theory, where for a suitable choice of the couplings functions, the Kovtun-Son-Starinets bound is violated. These results are corroborated by the calculation of the Green’s functions following the Kubo formalism.

Boundary conformal field theory (BCFT) and interface conformal field theory (ICFT) attract attention in the context of the information paradox problem. On this background, we develop the idea of the reflected entropy in BCFT/ICFT. We first introduce the left-right reflected entropy in BCFT and show that its holographic dual is given by the area of the entanglement wedge cross section through AdS/BCFT. We also present how to evaluate the reflected entropy in ICFT. By using this technique, we can show the universal behavior of the reflected entropy in some special classes.

In this paper, for a variety of types of regular black hole solutions, we investigate the entropy products of inner and outer horizons. Similar to singular black holes, for the regular ones we find that universality (mass independence) of the entropy product is true for some solutions, and it fails for some others. In the case of regular black holes that respect the universality, we read central charges of the dual CFTs from the entropy product, according to the thermodynamics method introduced in chen et al. [ J. High Energy Phys. 03 (2013) 102]. For these solutions, we also calculate central charges, using the asymptotic symmetry group formalism. The results of these two approaches are the same, which means that universality of the entropy product provides a simple method to find central charges of the dual CFTs.

We deduce, in a general background gauge, the counterterm Lagrangian for pure quantum gravity to one-loop order. As an application, we evaluate the leading quantum correction to the classical gravitational potential, generated by the vacuum polarization. We find that, in specific background gauges, this yields the complete result for the one-loop quantum corrections to the Newtonian potential. This approach is also applied to calculate the $\ln (T)$ contributions in quantum gravity at high-temperature.

We extend General Relativity by promoting Planck scale and the cosmological constant into integration constants, interpreted as fluxes of 4-forms hiding in the theory. When we include the charges of the 4-forms, these “constants” can vary discretely from region to region. We explain how the cosmological constant problem can be solved in this new framework. When the cosmological constant picks up contributions from two different 4-forms, with an irrational ratio of charges, the spectrum of its values is a very fine discretuum. When the charges are mutually irrational, $\frac{2{\kappa }_{\mathtt{eff}}^2{\kappa }^2|{\mathcal{Q}}_i|}{3{\mathcal{T}}_i^2}<1$, the discharge processes populating our discretuum will dynamically relax $\mathrm{\Lambda }$, ceasing as $\mathrm{\Lambda }$ approaches zero. Thus, the theory exponentially favors a huge hierarchy $\mathrm{\Lambda }/M_{\mathrm{Pl}}^4\ll 1$ instead of $\mathrm{\Lambda }/M_{\mathrm{Pl}}^4\simeq 1$.

A finite volume allows tunneling between degenerate vacua in quantum field theory, and leads to remarkable energetic features, arising from the competition of different saddle points in the partition function. We describe this competition for finite temperature at equilibrium, taking into account both static and (Euclidean) time-dependent saddle points. The effective theory for the homogeneous order parameter yields a nonextensive vacuum energy at low temperatures, implying a dynamical violation of the null energy condition.

We investigate quasinormal modes, greybody factor, and Hawking evaporation of a five-dimensional Kerr-anti–de Sitter (${\mathrm{AdS}}_5$) black hole by solving a wave equation for a test massive scalar field in terms of the local Heun function. We clarify the distribution of the quasinormal modes satisfying the ingoing and decaying boundary conditions at the event horizon and conformal infinity, respectively, and their splitting behaviors in the complex frequency plane with respect to a variation of spin parameters. We also point out the existence of purely imaginary modes. Further, we develop a method to extract in/outgoing waves near the AdS boundary, which enables us to define the greybody factor for a wide range of parameter space complementary to previous works. Using the greybody factor, we compute evaporation rates for mass and spins for the Kerr-${\mathrm{AdS}}_5$ black hole via the Hawking radiation of the scalar field.

Gravitational theories invariant under transverse diffeomorphisms and Weyl transformations have the same classical solutions as the corresponding fully diffeomorphism invariant theories. However, they solve some of the problems related to the cosmological constant and in principle allow local energy nonconservation. In the present work, we obtain the Noether charge formalism for these theories. We first derive expressions for the Noether currents and charges corresponding to transverse diffeomorphisms and Weyl transformations, showing that the latter vanish identically. We then use these results to obtain an expression for a perturbation of a Hamiltonian corresponding to evolution along a transverse diffeomorphism generator. From this expression, we derive the first law of black hole mechanics, identifying the total energy, the total angular momentum, the Wald entropy, and the contributions of the cosmological constant perturbations and energy nonconservation. Lastly, we extend our formalism to derive the first law of causal diamonds.

The motion of charged particles in spacetimes containing a submanifold of constant positive or negative curvature is considered, with the electromagnetic tensor proportional to the volume two-form form of the submanifold. In the positive curvature case, this describes spherically symmetric spacetimes with a magnetic monopole, while in the negative curvature case, it is a hyperbolic spacetime with magnetic field uniform along hyperbolic surfaces. Constants of motion are found by considering Poisson brackets defined on a phase space with gauge-covariant momenta. In the spherically-symmetric case, we find a correspondence between the trajectories on the Poincaré cone with equatorial geodesics in a conical defect spacetime. In the hyperbolic case, the analogue of the Poincaré cone is defined as a surface in an auxiliary Minkowski spacetime. Explicit examples are solved for the Minkowski, ${\mathrm{AdS}}_4\times S^2$, and the hyperbolic AdS-Reissner–Nordström spacetimes.

We study gravitational wave memory effect in the Friedmann-Robertson-Walker cosmological model with matter and a cosmological constant. Since the background is curved, gravitational radiation develops a tail part arriving after the main signal that travels along the past light cone of the observer. First we discuss first order gravitational wave sourced by a binary system, and we find that the tail only gives a negligible memory, in accord with previous results. Then we study the nonlinear memory effect coming from induced gravitational radiation sourced by first order gravitational radiation propagating over cosmological distances. In the light cone part of the induced gravitational wave we find a novel term missed in previous studies of the cosmological memory effect. Furthermore, we show that the induced gravitational wave has a tail part that slowly accumulates after the light cone part has passed and grows to a sizeable magnitude over a cosmological timescale. This tail part of the memory effect will be a new component in the stochastic gravitational wave background.

We derive exact, modified geodesic equations for a system of nonspinning, self-gravitating interacting bodies in a class of alternative theories of gravity to general relativity. We use a prescription proposed by Eardley for incorporating the effects of self-gravity within gravitationally bound bodies, in which their masses may depend on invariant quantities constructed from the auxiliary scalar, vector or tensor fields introduced by such theories, evaluated in the vicinity of each body. The forms of the equations are independent of the field equations of the chosen theory. In the case where the masses are strictly constant, the equations reduce to the conventional geodesic equations of general relativity. These equations may be useful tools for deriving equations of motion for compact bodies to high post-Newtonian orders in alternative theories of gravity.

One of the long-standing problems is a quest for regular black hole solutions, in which a resolution of the spacetime singularity has been achieved by some physically reasonable, classical field, before one resorts to the quantum gravity. The prospect of using nonlinear electromagnetic fields for this goal has been limited by the Bronnikov’s no-go theorems, focused on Lagrangians depending on the electromagnetic invariant $F_{ab}{F}^{ab}$ only. We extend Bronnikov’s results by taking into account Lagrangians that depend on both electromagnetic invariants, $F_{ab}{F}^{ab}$ and $F_{ab}\star F^{ab}$, and prove that the tension between the Lagrangian’s Maxwellian weak field limit and boundedness of the curvature invariants persists in more general class of theories.

For a flat $\mathrm{\Lambda }\mathrm{CDM}$ universe, the dipole of the luminosity distance can be utilized to measure the Hubble parameter. It is here shown that this is not the case in more general settings where curvature and cosmic backreaction is permitted. This implies that a discordance between $H(z)$ measurements obtained using such dipole luminosity distance data and $“\mathrm{true}”/\mathrm{actual}$ $H(z)$ data obtained from e.g., cosmic chronometers is a signal of curvature and/or cosmic backreaction. By considering mock future gravitational wave measurements of the Hubble parameter obtained through the dipole luminosity distance, it is shown that already a 1% curvature could in principle just barely show up in the determination. However, for realistic mock data generation using models with as much as 5% curvature, parameter estimates do not yield reliable measures of inconsistency between the false $H(z)$ measurements and true measurements of $H(z)$. At the same time, cosmic backreaction is hard to detect even if it makes up 10% of the “energy budget” in the current Universe, even when considering a highly idealized situation with low errors. The results concerning backreaction are based on specific “scaling solutions” to the backreaction problem and the study shows that the possibility of detecting a signal of backreaction through the dipole of the luminosity distance depends strongly on the particular backreaction model.

The next generation of cosmological observations will be sensitive to small deviations from a pure power law in the primordial power spectrum of the curvature perturbations. In the context of slow-roll inflation, these deviations are expected, they correspond to the so-called running of the spectral index. Their measurement would bring as much information as the discovery of deviations from scale invariance. However, robust parameter inference requires to marginalize over any possible higher-order uncertainties which have been, up to now, not fully determined. We tackle this issue by deriving the inflationary scalar and tensor slow-roll power spectra at next-to-next-to-next to leading order, fully expanded around an observable pivot wave number, for all single-field inflationary models having minimal and nonminimal kinetic terms. Our result therefore encompasses string-inspired inflationary models having a varying speed of sound.

Gravitational waves from a phase transition associated with the generation of the masses of elementary particles are within the reach of future space-based detectors such as LISA. A key determinant of the resulting power spectrum, not previously studied, is the lifetime of the acoustic turbulence which follows. We study decaying acoustic turbulence using numerical simulations of a relativistic fluid in two dimensions. Working in the limit of nonrelativistic bulk velocities, with an ultrarelativistic equation of state, we find that the energy spectrum evolves toward a self-similar broken power law, with a high wave number behavior of $k^{-2.08\pm 0.08}$, cut off at very high $k$ by the inverse width of the shock waves. Our model for the decay of acoustic turbulence can be extended to three dimensions using the universality of the high-$k$ power law and the evolution laws for the kinetic energy and the integral length scale. It is used to build an estimate for the gravitational wave power spectrum resulting from a collection of shock waves, as might be found in the aftermath of a strong first order phase transition in the early universe. The power spectrum has a peak wave number set by the initial length scale of the acoustic waves, and a new secondary scale at a lower wave number set by the integral scale after a Hubble time. Between these scales a distinctive new power law appears. Our results allow more accurate predictions of the gravitational wave power spectrum for a wide range of early universe phase transition scenarios.

Gravitational-wave astronomy is a vibrant field that leverages both classic and modern data processing techniques for the understanding of the Universe. Various approaches have been proposed for improving the efficiency of the detection scheme, with hierarchical matched filtering being an important strategy. Meanwhile, deep learning methods have recently demonstrated both consistency with matched filtering methods and remarkable statistical performance. In this work, we propose a hierarchical detection network (HDN), a novel approach to efficient detection that combines ideas from hierarchical matching and deep learning. The network is trained using a novel loss function, which encodes simultaneously the goals of statistical accuracy and efficiency. We discuss the source of complexity reduction of the proposed model and describe a general recipe for initialization with each layer specializing in different regions. We demonstrate the performance of the HDN with experiments using open LIGO data and synthetic injections, and observe with two-layer models a 79% efficiency gain compared with matched filtering at an equal error rate of 0.2%. Furthermore, we show how training a three-layer HDN initialized using a two-layer model can further boost both accuracy and efficiency, highlighting the power of multiple simple layers in efficient detection.

We study the gravitational-wave background produced by f-mode oscillations of neutron stars triggered by magnetar giant flares. For the gravitational-wave energy, we use analytic formulas obtained via general relativistic magnetohydrodynamic simulations of strongly magnetized neutron stars. Assuming the magnetar giant flare rate is proportional to the star-formation rate, we show the gravitational-wave signal is likely undetectable by third-generation detectors such as the Einstein Telescope and Cosmic Explorer. We calculate the minimum value of the magnetic field and the magnetar giant flare rate necessary for such a signal to be detectable, and discuss these in the context of our current understanding of magnetar flares throughout the Universe.

We investigate the solar modulation effect with the long-time cosmic ray (CR) proton and helium spectra measured by AMS-02 on the timescale of a Bartels rotation (27 days) between May 2011 and May 2017. The time covers the negative heliospheric magnetic field polarity cycle, the polarity reversal period, and the positive polarity cycle. The precise AMS-02 data provide an excellent opportunity to improve the understanding of the time-dependent solar modulation effect. In this work, a two-dimensional solar modulation model is used to compute the propagation of cosmic rays in the heliosphere. The CR propagation equation is numerically solved by the public code solarprop. We find that the drift effect is suppressed during the high solar activity period for a long time but nearly recovered in the first half of 2017. The time-dependent rigidity dependence of the mean free path is critical to reproduce the observations during the polarity reversal period. We also confirm that protons and helium have the same diffusive mean free path. The future monthly AMS-02 and PAMELA data will further confirm the vital assumption about the universal mean free path for all species. The monthly antiproton data will be crucial to break the degeneracy between the diffusion and drift effect and determine the level of drift effect during different epochs.

We investigate the impact of the neutron-star matter equation of state on the $f$- and $p_1$-mode oscillations of neutron stars obtained within the Cowling approximation and linearized general relativity. The $f$- and $p_1$-mode oscillation frequencies, and their damping times are calculated using representative sets of Skyrme Hartree-Fock and relativistic mean-field models, all of which reproduce nuclear systematics and support $2M_{\odot }$ neutron stars. Our study shows strong correlations between the frequencies of $f$- and $p_1$-modes and their damping times with the pressure of $\beta$-equilibrated matter at densities equal to or slightly higher than the nuclear saturation density ${\rho }_0$. Such correlations are found to be almost independent of the composition of the stars. The frequency of the $p_1$-mode of $1.4M_{\odot }$ star is strongly correlated with the slope of the symmetry energy $L_0$ and $\beta$-equilibrated pressure at density ${\rho }_0$. Compared to GR calculations, the error in the Cowling approximation for the $f$-mode is about 30% for neutron stars of low mass, whereas it decreases with increasing mass. The accuracy of the $p_1$-mode is better than 15% for neutron stars of maximum mass, and improves for lower masses and higher number of radial nodes.