We investigate the existence and nature of the classical maximum force bound between two black holes with touching horizons. Besides general relativity, the maximum force bound is independent of black hole masses only in Moffat’s theory, Brans-Dicke theory (which is the same as Einstein’s for vacuum), and the higher-dimensional generalization of Einstein’s theory, pure Lovelock gravity, which is characterized by having a single $n$th-order term in the Lovelock polynomial without a sum over lower orders in the action. Further, if the bound is to exist in higher dimensions and is entirely in terms of the velocity of light and the gravitational constant, it uniquely has to be pure Lovelock gravity. In pure Lovelock gravity, the maximum force bound exists in all $3n$-space dimensions and has the value $c^4/4G_n$ where $G_n$ is the corresponding gravitational constant. The absence of mass dependence in the maximum force bound may have relevance for the formation of naked singularities.

We examine analytical, intrinsically non-Abelian, black holes with SU(2) Yang-Mills matter content. Working in the extended thermodynamics scenario of Lovelock black holes, we study the phase structure of a four- and a five-dimensional spherically symmetric configuration. We work in Gauss-Bonnet-Einstein-Yang-Mills gravity with a negative cosmological constant and use Euclidean methods to explore the thermodynamics of the systems. We observe that the solutions belong to the expected universality class of van der Waals and find a reentrant phase transition.

We study the generalized version of energy-momentum squared gravity (EMSG) in the Palatini formalism. This theory allows the existence of a scalar constructed with energy-momentum tensor as $T_{\alpha \beta }{T}^{\alpha \beta }$ in the generic action of the theory. We study the most general form of this theory in the Palatini framework and present the underlying field equations. The equations of motion of a massive test particle have been derived. The weak field limit of the theory is explored and the generalized version of the Poisson equation is obtained. Moreover, we explore the cosmological behavior of the theory with emphasis on bouncing solutions. Some new bouncing solutions for the specific Palatini EMSG model given by the Lagrangian density $\mathcal{L}=R+\beta R^2+\eta T_{\mu \nu }{T}^{\mu \nu }$ are introduced. We show that only the case $\eta >0$ can lead to viable cosmic bounce.

We analyze the spin stability of a binary black hole coalescence when the binary system is described by the Post-Newtonian (PN) equations in the adiabatic regime. The main idea in this work is to make a massive exploration of the solution space in search of chaos. For that, we evolve the PN equations using a CUDA implementation of the RKF78 scheme and study the dynamical behavior of the system. Each initial spin configuration run in the GPU is composed by more than 80000 simulations. The chaos indicator used to characterize the degree of separation of two infinitesimally close trajectories is the Lyapunov exponent. We find zones in the solution space where the separation between nearby trajectories reaches several orders of magnitude bigger than the initial separation. Also, we note that the chaotic behavior can be observed in forward as well as in backward evolution.

We perform the stability analysis for the scalarized charged black holes obtained from Einstein-Maxwell-conformally coupled scalar (EMCS) theory by employing the radial perturbations. The targeting black holes include a single branch of a scalarized charged black hole with the coupling parameter $\alpha >0$ inspired by the constant scalar hairy black hole as well as infinite branches of $n=0(\alpha \ge 8.019),1(\alpha \ge 40.84),2(\alpha \ge 99.89),\dots$ scalarized charged black holes found through the spontaneous scalarization on the Reissner-Nordström black hole. It turns out that the black hole in the single branch and the $n=0$ black hole are stable against radial perturbations, while the $n=1$, 2 excited black holes are unstable in the EMCS theory with both quadratic and exponential couplings.

We propose two models for constant density relativistic perfect-fluid spheres supported by thin-shell configurations. These models are obtained from the Schwarzschild constant density star solution: the first via the collapse of the external layers of the fluid into a thin shell by performing a matching with the exterior Schwarzschild solution at a matching radius smaller than the star radius; and the second via the creation of a vacuum bubble inside the star by matching it with an interior Minkowski spacetime. Both models are shown to satisfy both the weak and the strong energy conditions (WEC and SEC) and can have a compactness arbitrarily close to that of a black-hole without developing singularities at the center, thus being exceptions to the Buchdahl limit. We compute the stability regimes of the models proposed and we show that there are combinations of the star radius $R$ and the matching radius $R_{\mathrm{\Sigma }}$ for which the solutions are stable, the dominant energy condition (DEC) is satisfied, and the radius of the object is smaller than $3M$, implying that these models could be used as models for dark matter or exotic compact objects.

In this work, we revisit a toy model proposed by Camanho et al. [J. High Energy Phys. 10 (2015) 179] and extensively study the possible existence of gravitational phase transition from AdS to dS geometries by adding the Maxwell field as an impurity substitution. We show that the phase transitions proceed via the bubble nucleation of spherical thin shells described by different branches of the solution which host a black hole in the interior. In order to demonstrate the existence of the phase transition, we examine how the free energy and temperature depend on the higher-order gravity coupling ($\lambda$) indicating the possibility of thermalon-mediated phase transition. We observe that the phase transitions of the charged case are possible in which the required (maximum) temperature is lower than that of the neutral case. Interestingly, we also discover that the critical temperature and the coupling $\lambda$ of the phase transitions are modified when having the charge. Notably, our results agree with the claim that the generalized gravitational phase transition is a generic behavior of the higher-order gravity theories even when the matter field is added.

Using the geodesic method and the perturbative approach, we study the deflection of light by time-periodic spherically symmetric gravitational fields. Assuming the weakness of the gravitational field, we derive general formulas that determine the deflection angle in the leading order approximation. The formulas are valid for both time-periodic and static metrics. Using these results, we calculate the deflection angle of a light ray passing through a spherically symmetric oscillating distribution of a self-gravitating scalar field with a logarithmic potential. It turned out that in this case the deflection angle does not depend on time in the leading order.

By allowing torsion into the gravitational dynamics one can promote the cosmological constant $\mathrm{\Lambda }$ to a dynamical variable in a class of quasitopological theories. In this paper we perform a minisuperspace quantization of these theories in the connection representation. If $\mathrm{\Lambda }$ is kept fixed, the solution is a delta-normalizable version of the Chern-Simons (CS) state, which is the dual of the Hartle and Hawking and Vilenkin wave functions. We find that the CS state solves the Wheeler–De Witt equation also if $\mathrm{\Lambda }$ is rendered dynamical by an Euler quasitopological invariant, in the parity-even branch of the theory. In the absence of an infrared (IR) cutoff, the CS state suggests the marginal probability $P(\mathrm{\Lambda })=\delta (\mathrm{\Lambda })$. Should there be an IR cutoff (for whatever reason), the probability is sharply peaked at the cut off. In the parity-odd branch, however, we can still find the CS state as a particular (but not most general) solution, but further work is needed to sharpen the predictions. For the theory based on the Pontryagin invariant (which only has a parity-odd branch) the CS wave function no longer is a solution to the constraints. We find the most general solution in this case, which again leaves room for a range of predictions for $\mathrm{\Lambda }$.

We compute gravitational waves from inspiraling stellar-mass compact objects on the equatorial plane of a massive spinning black hole (BH). Our inspiral orbits are computed by taking into account the adiabatic change of orbital parameters due to gravitational radiation in the lowest order in mass ratio. We employ an interpolation method to compute the adiabatic change at arbitrary points inside the region of orbital parameter space computed in advance. Using the obtained inspiral orbits and associated gravitational waves, we compute power spectra of gravitational waves and the signal-to-noise ratio (SNR) for several values of the BH spin, the masses of the binary, and the initial orbital eccentricity during a hypothetical three-year Laser Interferometer Space Antenna observation before final plunge. We find that (i) the SNR increases as the BH spin and the mass of the compact object increase for the BH mass $M\gtrsim {10}^6{M}_{\odot }$, (ii) the SNR has a maximum for $M\approx {10}^6{M}_{\odot }$, and (iii) the SNR increases as the initial eccentricity increases for $M={10}^6{M}_{\odot }$. We also show that incorporating the contribution from the higher multipole modes of gravitational waves is crucial for enhancing the detection rate.

Cosmic history can be traced considering further curvature contributions inside the gravitational action. Assuming that standard general relativity can be extended by other curvature invariants, we discuss the possibility that an action containing higher-order curvature terms can fit, in principle, the whole universe evolution. In particular, a theory like $F(R,\mathcal{G})$, with $R$ the Ricci scalar and $\mathcal{G}$ the Gauss-Bonnet topological term, contains all the curvature invariants that, depending on the energy regime, can address inflation, and matter dominated and dark energy regimes. In this paper, we investigate this possibility considering how $F(R,\mathcal{G})$ models can lead gravity from ultraviolet to infrared scales. Specifically, we will take into account a cosmographic approach for this purpose.

In this paper, we investigate the potential-driven inflation models with a disformal coupling to Einstein gravity, to find out the effects of such a coupling on these models. We consider a simple coupling form which introduces only one parameter, and three inflation models, namely the chaotic inflation, the Higgs inflation, and the monodromy inflation. We find that the disformal coupling can have some modifications to the observational variables of these models such as the power spectrum, the spectral index as well as the tensor/scalar ratio, although not too large due to the constraints on the disformal coupling parameter. With these modifications, one has the opportunity of improving models that lie on the edge of the favorable regions of Planck observational data, such as monodromy inflation. Moreover, the nontrivial sound speed of tensor perturbations (gravitational waves) may come out, due to the coupling of gravity and kinetic terms of the field.

The standard cosmological structure formation scenario is successful on large scales. Several apparent problems affect it however at galactic scales, such as the small scale problems at low redshift and more recent issues involving early massive galaxy and black hole formation. As these arise where complex baryonic physics becomes important, this is often assumed to be behind the problems. But the same scales are also those where the primordial spectrum is relatively unconstrained, and there are several ways in which it can be modified. We focus on that arising from effects possibly associated with the crossing of high energy cutoff scale by fluctuation modes during inflation. Elementary arguments show that adiabatic evolution cannot modify the near scale invariance, we thus discuss a simple model for the contrary extreme of sudden transition. Numerical calculations and simple arguments suggest that its predictions, for parameters considered here, are more generic than may be expected, with significant modifications requiring a rapid transition. We examine the implications of such a scenario, in this simplest form of sudden jump as well as gradual variants, on the matter power spectrum and halo mass function in light of the limitations imposed by particle production. We show the resulting enhancement and oscillation in the power spectrum on currently nonlinear scales can potentially simultaneously alleviate both the apparent problem of early structure formation and, somewhat counterintuitively, problems at low redshift concerning the abundance of dwarf galaxies, including those too big to fail. We discuss consequences that can observationally constrain the scenario and its parameters, including an inflationary Hubble scale $\lesssim {10}^{-8}{M}_{\mathrm{Pl}}$, while touching on the possibility of simultaneous modification of power on the largest scales.

The sample of neutron stars with a measured mass is growing quickly. With the latest sample, we adopt both a flexible Gaussian mixture model and a Gaussian plus Cauchy-Lorentz component model to infer the mass distribution of neutron stars and use the Bayesian model selection to explore evidence for multimodality and a sharp cutoff in the mass distribution. The two models yield rather similar results. Consistent with previous studies, we find evidence for a bimodal distribution together with a cutoff at a mass of $M_{\max }=2.26_{-0.05}^{+0.12}{M}_{\odot }$ (68% credible interval, for the Gaussian mixture model). If such a cutoff is interpreted as the maximum gravitational mass of nonrotating cold neutron stars, the prospect of forming supramassive remnants is found to be quite promising for the double neutron star mergers with a total gravitational mass less than or equal to $2.7M_{\odot }$ unless the thermal pions could substantially soften the equation of state for the very hot neutron star matter. These supramassive remnants have a typical kinetic rotational energy of approximately $1-2\times {10}^{53}\mathrm{ergs}$. Together with a high neutron star merger rate approximately ${10}^3{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-3}$, the neutron star mergers are expected to be significant sources of EeV ($10^{18}$ eV) cosmic-ray protons.

The total binding energy of compact stars is the sum of the gravitational binding energy $(BE{)}_g$ and the nuclear binding energy $(BE{)}_n$, the last being related to the microphysics of the interactions. While the first is positive (binding) both for hadronic stars and for strange quark stars, the second is large and negative for hadronic stars (antibinding) and either small and negative (antibinding) or positive (binding) for strange quark stars. A hadronic star can convert into a strange quark star with a larger radius because the consequent reduction of $(BE{)}_g$ is overcompensated by the large increase in $(BE{)}_n$. Thus, the total binding energy increases due to the conversion and the process is exothermic. Depending on the equations of state of hadronic matter and quark matter and on the baryonic mass of the star, the contrary is obviously also possible, namely, the conversion of hadronic stars into strange quark stars having smaller radii, a situation more often discussed in the literature. We provide a condition that is sufficient and in most of the phenomenologically relevant cases also necessary in order to form strange quark stars with larger radii while satisfying the exothermicity request. Finally, we compare the two schemes in which quark stars are produced (one having large quark stars and the other having small quark stars) among themselves and with the third-family scenario and we discuss how present and future data can discriminate among them.

We consider the decoherence effects in the propagation of active neutrinos due to the nonforward neutrino scattering processes in a matter background composed of electrons and nucleons. We calculate the contribution to the imaginary part of the neutrino self-energy arising from such processes. Since the initial neutrino state is depleted but does not actually disappear (the initial neutrino transitions into a neutrino of a different flavor but does not decay) those processes should be associated with decoherence effects that cannot be described in terms of the coherent evolution of the state vector. Based on the formalism developed in our previous work for treating the nonforward scattering processes using the notion of the stochastic evolution of the state, we identify the jump operators, as used in the context of the master or Lindblad equation, in terms of the results of the calculation of the nonforward neutrino scattering contribution to the imaginary part of the neutrino self-energy. As a guide to estimating the decoherence effects in situations of practical interest we give explicit formulas for the decoherence terms for different background conditions, and point out some of the salient features in particular the neutrino energy dependence. To establish contact with previous works in which the decoherence terms are treated as phenomenological parameters, we consider the solution to the evolution equation in the two-generation case. We give formulas that are useful for estimating the effects of the decoherence terms under various conditions and environments, including the typical conditions applicable to long baseline experiments, where matter effects are important. In those contexts the effects appear to be small, and indicative that if significant decoherence effects were to be found they would be due to nonstandard contributions to the decoherence terms.

We construct the grand partition function of the system of chiral fermions in a uniform magnetic field from Landau levels, through which all thermodynamic quantities can be obtained. Making use of the Abel-Plana formula, these thermodynamic quantities can be expanded as series with respect to a dimensionless variable $b=2eB/T^2$. We find that the series expansions of the energy density, pressure, magnetization intensity, and magnetic susceptibility contain a singular term with $\ln b^2$, while the particle number density, entropy density, and heat capacity are power series of $b^2$. The asymptotic behaviors of these thermodynamic quantities in extreme conditions are also discussed.

We systematically study the holographic phase transition of the radion field in a five-dimensional warped model which includes a scalar potential with a powerlike behavior. We consider Kaluza-Klein (KK) resonances with masses $m_{\mathrm{KK}}$ at the TeV scale or beyond. The backreaction of the radion field on the gravitational metric is taken into account by using the superpotential formalism. The confinement/deconfinement first order phase transition leads to a gravitational wave stochastic background which mainly depends on the scale $m_{\mathrm{KK}}$ and the number of colors, $N$, in the dual theory. Its power spectrum peaks at a frequency that depends on the amount of tuning required in the electroweak sector. It turns out that the present and forthcoming gravitational wave observatories can probe scenarios where the KK resonances are very heavy. Current aLIGO data already rule out vector boson KK resonances with masses in the interval $m_{\mathrm{KK}}\sim (1–10)\times {10}^5\mathrm{TeV}$. Future gravitational experiments will be sensitive to resonances with masses $m_{\mathrm{KK}}\lesssim {10}^5\mathrm{TeV}$ (LISA), ${10}^8\mathrm{TeV}$ (aLIGO Design) and ${10}^9\mathrm{TeV}$ (ET). Finally, we also find that the big bang nucleosynthesis bound in the frequency spectrum turns into a lower bound for the nucleation temperature as $T_n\gtrsim {10}^{-4}\sqrt{N}{m}_{\mathrm{KK}}$.

We introduce a light dark photon $A_{\mu }^{\prime }$ to the minimal Higgs portal model, by coupling the Higgs boson to “dark QED” containing fermionic dark matter, which gives rise to rich and interesting collider phenomenology. There are two prominent features in such a simple extension—the Higgs boson could have decays into the long-lived dark photon through the “mono-$A^{\prime }$ channel,” or into multiple collimated leptons via “darkonium,” depending on the mixing parameter of the $A_{\mu }^{\prime }$ with the visible photon. We initiate a study on the possibility of probing the parameter space of the model in both the energy and the lifetime frontiers at the Large Hadron Collider.

We propose a renormalizable theory with minimal particle content and symmetries, that successfully explains the number of Standard Model (SM) fermion families, the SM fermion mass hierarchy, the tiny values for the light active neutrino masses, the lepton and baryon asymmetry of the Universe, the dark matter relic density as well as the muon and electron anomalous magnetic moments. In the proposed model, the top quark and the exotic fermions do acquire tree-level masses whereas the SM charged fermions lighter than the top quark gain one-loop level masses. Besides that, the tiny masses for the light active neutrino are generated from an inverse seesaw mechanism at one-loop level.