In many astrophysical scenarios the charge of the black hole is often neglected due to unrealistically large values of the charge required for the Reissner-Nordström spacetime metric. However, black holes may possess small electric charge due to various selective accretion mechanisms. In this paper we investigate the effect of a small hypothetical electric charge of a Schwarzschild black hole on the ionization of a freely falling neutral particle and subsequent escape of the ionized particle from the black hole. We show that the energy of ionized particle can grow ultrahigh and discuss distinguishing signatures of particle acceleration by weakly charged black holes. We also discuss a possible application of the proposed mechanism as an alternative cosmic ray acceleration scenario. In particular we show that the Galactic center supermassive black hole is capable to act as a PeVatron of protons. The presented mechanism can serve as a simple toy model of a nonrotating compact object acting as a particle accelerator with a potential astrophysical implementations related to the cosmic ray physics and beyond.

In this present work, we suggest studying the neutron star with a conformal coupling by using the analytical expressions of realistic equations of state. The equations of perturbed and nonperturbed metrics, scalar field, and the pressure of the matter are derived in order to observe the effects of the conformal coupling on the mass, radius, and tidal Love number of polar and axial types. The numerical and the analytical analyses show a different comportment of our model from general relativity by varying the value of the parameters in the model. We showed also for a particular case that the scalar field becomes vanished outside the star which will give us the opportunity to investigate the behavior tidal Love number of our model. Finally, a comparison between the results of our model and the outcome from the signal of gravitational waves, coming from the fusion of binary neutron star, was performed using the universal relations $C-\mathrm{\Lambda }$.

Mass-radius relations of homogeneous cold spheres are obtained for six solid materials commonly found in terrestrial planets. An additional degeneracy in the (exo)planet profiles is discussed together with their properties concluded from our findings in the framework of Palatini $f(\mathcal{R})$ gravity. Moreover, a new test of gravity has been proposed: The results presented here will allow us to test and constrain models of gravity by the use of seismic data acquired from earthquakes and marsquakes.

Several formulations of the first order approach to unimodular gravity are presented. There is always a particular one such that it is classically equivalent to the second order formulation; this we call educated. It is often at variance with the naive approach, in which the Lagrangian is taken as given exactly by the same expression as in the second order formulation; only the number and character of the independent variables changes. Namely, typically some of the momenta are now considered as coordinates. The ensuing Hamiltonians are thereby discussed, and their physical differences pointed out.

We study the effect of scattering gravitational radiation off the static background curvature, up to second order in Newton constant, known in the literature as tail and tail-of-tail processes, for generic electric and magnetic multipoles. Starting from the multipole expansion of composite compact objects, and as expected due to the known electric quadrupole case, both long- and short-distance (UV) divergences are encountered. The former disappear from properly defined observables, the latter are renormalized, and their associated logarithms give rise to a classical renormalization group flow. UV divergences alert for incompleteness of the multipolar description of the composite source and are expected not to be present in a UV-complete theory, as explicitly derived in the literature for the case of conservative dynamics. Logarithmic terms from tail-of-tail processes associated to generic magnetic multipoles are computed in this work for the first time.

We construct conformally scalar NUT-like dyon black holes in nonlinear electrodynamics (NLE) which possess conformal and duality-rotation symmetries and are characterized by a free dimensionless parameter. The thermodynamic first law of the black hole is formulated. Then, we explore the strong gravitational effects of the black hole, mainly focusing on the innermost stable circular orbits (ISCOs) of massive particles and shadows formed by the photons. One of our interesting results is that if the black hole is endowed with a positive real conformal scalar charge, rendering it Reissner-Nordström-like, the radii of the ISCOs and the shadow both increase with the increasing NLE parameter, signifying the increasing nonlinearity of the electromagnetic field.

We investigate a particular regularization of big bang singularity, which remains within the domain of 4-dimensional general relativity but allowing for degenerate metrics. We study the geodesics and geodesic congruences in the modified Friedmann-Lemaitre-Robertson-Walker universe. In particular, we calculate the expansion of timelike and null geodesic congruences. Based on these results, we also briefly discuss the cosmological singularity theorems.

The four laws of black hole mechanics have been put forward for a long time. However, the zeroth law, which states that the surface gravity of a stationary black hole is a constant on the event horizon, still lacks universal proof in various modified gravitational theories. In this paper, we study the zeroth law in a special Horndeski gravity, which is an interesting gravitational theory with a nonminimally coupled scalar field. After assuming that the nonminimally coupled scalar field has the same symmetries with the spacetime, the minimally coupled matter fields satisfy the dominant energy condition and the Horndeski gravity has a smooth limit to Einstein gravity when the coupling constant approaches zero, we prove the zeroth law based on the gravitational equation in Horndeski gravity without any assumption to the spacetime symmetries.

The primordial black hole (PBH) is an effective candidate for dark matter. In this work, the PBH abundance $f$ is calculated in peak theory, with one or two perturbations in the inflaton potential. We construct an antisymmetric perturbation that can create a perfect plateau in the inflaton potential, leading inflation to the ultraslow-roll stage. During this stage, the power spectrum of primordial curvature perturbation is remarkably enhanced on small scales, generating abundant PBHs. The PBH abundance $f\sim 0.1$ can be achieved in one or two typical mass windows at ${10}^{-17}{M}_{\odot }$, ${10}^{-13}{M}_{\odot }$, and $30M_{\odot }$, without spoiling the nearly scale-invariant power spectrum on large scales. For comparison, $f$ is calculated in two approximate methods of peak theory (with different spectral moments) and also in the Press-Schechter theory. It is found that the Press-Schechter theory systematically underestimates $f$ by 2 or 3 orders of magnitude compared with peak theory.

We compute numerically the reheating temperature due to the gravitational production of conformally-coupled superheavy particles during the phase transition from the end of inflation to the beginning of kination in two different quintessential inflation (QI) scenarios, namely Lorentzian quintessential inflation and $\alpha$-attractors in the context of quintessential inflation ($\alpha$-QI). Once these superheavy particles have been created, they must decay into lighter ones to form a relativistic plasma, whose energy density will eventually dominate the one of the inflaton field in order to reheat our Universe after its inflation period with a very high temperature—in both cases greater than ${10}^7\mathrm{GeV}$—contrary to the usual belief that heavy masses suppress the particle production and, thus, lead to an inefficient reheating temperature. Finally, we will show that the overproduction of gravitational waves during this phase transition, when one deals with our models, does not disturb the big bang nucleosynthesis success.

Cubic Galileon massive gravity is a development of de Rham–Gabadadze-Tolley massive gravity theory in which the space of the Stueckelberg field is broken. We consider the cubic Galileon term as a scalar field coupled to the graviton field. We present a detailed study of the cosmological aspects of this theory of gravity. We analyze self-accelerating solutions of the background equations of motion to explain the accelerated expansion of the Universe. Exploiting the latest Union2 type Ia supernovae (SNIa) dataset, which consists of 557 SNIa, we show that cubic Galileon massive gravity theory is consistent with the observations. We also examine the tensor perturbations within the framework of this model and find an expression for the dispersion relation of gravitational waves and show that it is consistent with the observational results.

We compute the spectrum of tensor perturbations in warm inflation. We find that the spectrum, besides the standard component $\propto H^2/M_P^2$ associated to the amplification of the tensor vacuum fluctuations, acquires a component $\propto {\ell }_{\mathrm{mfp}}{T}^5/M_P^4$, where ${\ell }_{\mathrm{mfp}}$ and $T$ are, respectively, the mean free path and the temperature of the thermal degrees of freedom. The new contribution is due to the direct production of gravitational waves by the thermal bath and can exceed the standard one in a viable region of parameter space. This contribution is dominated by thermal fluctuations at scales longer than ${\ell }_{\mathrm{mfp}}$.

Previously, we have shown in Li et al. [Phys. Rev. D 97, 122001 (2018)] that very weak $\gamma$-ray excesses ($\sim 2\sigma$) appear in some Milky Way dwarf spheroidal galaxies (dSphs, including candidates) and the combination analysis of 12 nearby dSphs yields a local significance of $>4\sigma$. In this work, we adopt a longer dataset (i.e., the 12 years of Fermi-LAT data) and the latest Fermi-LAT software as well as background models to update the searches of $\gamma$-ray emission from these sources. Very weak $\gamma$-ray excesses ($>2\sigma$) are found in the directions of three dSphs, including Reticulum II, Bootes II and Willman 1. In the direction of Reticulum II, the peak test statistic (TS) value of the excess reaches $\sim 11$. However, different from the previous analysis with the 9 years of Fermi-LAT data, now the location of the gamma-ray emission is significantly away from the center of Reticulum II because of the enhancement of “offset” $\gamma$ rays above 10 GeV since 2017. The detected weak excess is likely due to the contamination of an astrophysical $\gamma$-ray source with a TS value of $\sim 22$, irrelevant to the dark matter inside Reticulum II. The possible excesses in the directions of Bootes II and Willman 1 are weaker with lower peak TS values ($\sim 7$). If interpreted as annihilation of dark matter particles into ${\tau }^{+}{\tau }^{-}$, the dark mass of $m_{\chi }\sim 14\mathrm{GeV}$ and $\sim 80\mathrm{GeV}$ is found for Bootes II and Willman 1, respectively. Much more data are needed to clarify whether these two potential signals are real and then reveal their origins.

We map the likelihood of GW190521, the heaviest detected binary black hole (BBH) merger, by sampling under different mass and spin priors designed to be uninformative. We find that a source-frame total mass of $\sim 150M_{\odot }$ is consistently supported, but posteriors in mass ratio and spin depend critically on the choice of priors. We confirm that the likelihood has a multimodal structure with peaks in regions of mass ratio representing very different astrophysical scenarios. The unequal-mass region ($m_2/m_1<0.3$) has an average likelihood $\sim e^6$ times larger than the equal-mass region ($m_2/m_1>0.3$) and a maximum likelihood $\sim e^2$ larger. Using ensembles of samples across priors, we examine the implications of qualitatively different BBH sources that fit the data. We find that the equal-mass solution has poorly constrained spins and at least one black hole mass that is difficult to form via stellar collapse due to pair instability. The unequal-mass solution can avoid this mass gap entirely but requires a negative effective spin and a precessing primary. Either of these scenarios is more easily produced by dynamical formation channels than field binary coevolution. Drawing representative samples from each region of the likelihood map, we find a sensitive comoving volume time $\mathcal{O}(10)$ times larger in the mass gap region than the gap-avoiding region. Considering $D_{\mathrm{com}}^3\mathcal{L}$ to account for the distance effect, the likelihood of these representative samples still reverses the advantage to favor the gap-avoiding scenario by a factor of $\mathcal{O}(100)$ before including mass and spin priors. Posteriors are easily driven away from this high-likelihood region by common prior choices meant to be uninformative, making GW190521 parameter inference sensitive to the assumed mass and spin distributions of mergers in the source’s astrophysical channel. This may be a generic issue for similarly heavy events given current detector sensitivity and waveform degeneracies.

In environments with high dense neutrino gases, such as in core-collapse supernovae, the neutrinos can experience collective neutrino oscillation due to their self-interactions. In particular, fast flavor conversion driven by the crossings in the neutrino angular distribution can affect explosion mechanism, nucleosynthesis, and neutrino observation. We perform the numerical computation of nonlinear flavor evolution on the neutrino angular distribution with tiny crossings expected to be generated in the preshock region. We demonstrate that the fast instability is triggered and a cascade develops under a realistic three-flavor model considering muon production and weak magnetism in the supernova dynamics. The tiny crossing excites specific spatial modes, and then the flavor instability propagates into other modes which otherwise remain stable due to the nonlinear effects. Our results indicate that fast flavor conversion can rise in the preshock region and have a sufficient impact on the flavor contents.

The detection of gravitational radiation, emitted in the aftermath of the excitation of neutron star quasinormal modes, has the potential to provide unprecedented access to the properties of matter in the star interior, and shed new light on the dynamics of nuclear interactions at microscopic level. Of great importance, in this context, will be the sensitivity to the modeling of three-nucleon interactions, which are known to play a critical role in the high-density regime. We report the results of a calculation of the frequencies and damping times of the fundamental mode, carried out using the equation of state of Akmal, Pandharipande and Ravenhall as a baseline, and varying the strength of the isoscalar repulsive term of the Urbana IX potential within a range consistent with multimessenger astrophysical observations. The results of our analysis indicate that repulsive three-nucleon interactions strongly affect the stiffness of the equation of state, which in turn determines the pattern of the gravitational radiation frequencies, largely independent of the mass of the source. The observational implications are also discussed.

In recent years, the usefulness of astrophysical objects as dark matter (DM) probes has become more and more evident, especially in view of null results from direct-detection and particle-production experiments. The potentially observable signatures of DM gravitationally trapped inside a star, or another compact astrophysical object, have been used to forecast stringent constraints on the nucleon–dark matter interaction cross section. Currently, the probes of interest are at high redshifts, Population III (Pop III) stars that form in isolation or in small numbers, in very dense DM minihalos at $z\sim 15–40$, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. None of these objects are truly single component and, as such, capture rates calculated with the common assumption made in the literature of single-component capture, i.e., capture of DM by multiple scatterings with one single type of nucleus inside the object, are not accurate. In this paper, we present an extension of this formalism to multicomponent objects and apply it to Pop III stars, thereby investigating the role of He in the capture rates of Pop III stars. As expected, we find that the inclusion of the heavier He nuclei leads to an enhancement of the overall capture rates, further improving the potential of Pop III stars as dark matter probes.

We show that interacting quark stars (IQSs) composed of interacting quark matter (IQM), including the strong interaction effects such as perturbative QCD corrections and color superconductivity, can be compact enough to feature a photon sphere that is essential to the signature of gravitational wave echoes. We utilize an IQM equation of state unifying all interacting phases by a simple reparametrization and rescaling, through which we manage to maximally reduce the number of degrees of freedom into one dimensionless parameter $\overline{\lambda }$ that characterizes the relative size of strong interaction effects. It turns out that gravitational wave echoes are possible for IQSs with $\overline{\lambda }\gtrsim 10$ at large center pressure. Rescaling the dimension back, we illustrate its implication on the dimensional parameter space of the effective bag constant $B_{\mathrm{eff}}$ and the superconducting gap $\mathrm{\Delta }$ with variations of the perturbative QCD parameter $a_4$ and the strange quark mass $m_s$ in their empirical range. We calculate the rescaled gravitational wave echo frequencies ${\overline{f}}_{\mathrm{echo}}$ associated with IQSs, from which we obtain a simple scaling relation for the minimal echo frequency $f_{\mathrm{echo}}^{\min }\approx 5.76\sqrt{B_{\mathrm{eff}}/{(100\mathrm{MeV})}^4}\mathrm{kHz}$ at the large $\overline{\lambda }$ limit.

The precise calibration of the strain readout of the LIGO gravitational wave observatories is paramount to the accurate interpretation of gravitational wave events. This calibration is traditionally done by imparting a known force on the test masses of the observatory via radiation pressure. Here, we describe the implementation of an alternative calibration scheme: the Newtonian calibrator. This system uses a rotor consisting of both quadrupole and hexapole mass distributions to apply a time-varying gravitational force on one of the observatory’s test masses. The force produced by this rotor can be predicted to $<1\%$ relative uncertainty and is well resolved in the readout of the observatory. This system currently acts as a cross-check of the existing absolute calibration system.

We investigate the relations between transverse momentum dependent parton distributions (TMDs) and generalized parton distributions (GPDs) in a light-front quark-diquark model motivated by soft-wall AdS/QCD. Many relations are found to have similar structure in different models. It is found that a relation between the Sivers function and the GPD $E_q$ can be obtained in this model in terms of a lensing function. The quark orbital angular momentum is calculated and the results are compared with the results in other similar models. Implications of the results are discussed. Relations among different TMDs in the model are also presented.