We compute the scaling dimensions of a family of fixed-charge operators at the infrared fixed point of the $O(N)$ model featuring cubic interactions in $d=6-\epsilon$ for arbitrary $N$ to leading and subleading order in the charge but to all orders in the couplings. The results are used to analyze the conjectured equivalence with the $O(N)$ model displaying quartic interactions at its ultraviolet fixed point. This is performed by comparing the cubic model scaling dimensions against the known large $N$ results for the quartic model and demonstrating that they match. Our results reinforce the conjectured equivalence and further provide novel information on the finite $N$ physics stemming from the computations in the cubic model just below six dimensions.

It was recently found that, when linearized in the absence of matter, 58 cases of the general gravitational theory with quadratic curvature and torsion are (i) free from ghosts and tachyons and (ii) power-counting renormalizable. We inspect the nonlinear Hamiltonian structure of the eight cases whose primary constraints do not depend on the curvature tensor. We confirm the particle spectra and unitarity of all these theories in the linear regime. We uncover qualitative dynamical changes in the nonlinear regimes of all eight cases, suggesting at least a broken gauge symmetry, and possibly the activation of negative kinetic energy spin-parity sectors and acausal behavior. Two of the cases propagate a pair of massless modes at the linear level, and were interesting as candidate theories of gravity. However, we identify these modes with vector excitations, rather than the tensor polarizations of the graviton. Moreover, we show that these theories do not support a viable cosmological background.

The linear-order effects of radiation-reaction on the classical scattering of two point masses, in general relativity, are derived by a variation-of-constants method. Explicit expressions for the radiation-reaction contributions to the changes of 4-momentum during scattering are given to linear order in the radiative losses of energy, linear-momentum, and angular momentum. The polynomial dependence on the masses of the 4-momentum changes is shown to lead to nontrivial identities relating the various radiative losses. At order $G^3$ our results lead to a streamlined classical derivation of results recently derived within a quantum approach. At order $G^4$ we compute the needed radiative losses to next-to-next-to-leading-order in the post-Newtonian expansion, thereby reaching the absolute fourth and a half post-Newtonian level of accuracy in the 4-momentum changes. We also provide explicit expressions, at the absolute sixth post-Newtonian accuracy, for the radiation-graviton contribution to conservative $O(G^4)$ scattering. At orders $G^5$ and $G^6$ we derive explicit theoretical expressions for the last two hitherto undetermined parameters describing the fifth-post-Newtonian dynamics. Our results at the fifth-post-Newtonian level confirm results of [Nucl. Phys. B965, 115352 (2021)] but exhibit some disagreements with results of [Phys. Rev. D 101, 064033 (2020)].

In the context of whether a massive compact object recently observed in the GW190814 event is a neutron star (NS) or not, we study the role of the parameters $\kappa$ and ${\mathrm{\Lambda }}_c$ of the Eddington-inspired Born-Infeld (EiBI) gravity theory in the NS mass-radius relation, moment of inertia, and tidal deformability. The results are compared to recent observational constraints extracted from the analysis of NS observation data. The NS core equation of state (EoS) is calculated using the relativistic mean-field model with the G3 parameter set. In the hyperon sector, the SU(3) and hyperon potential depths are used to determine the hyperon coupling constants. For the inner and outer crusts, we use the crust EoS from Miyatsu et al. [Astrophys. J. 777, 4 (2013)]. We also maintain the sound speed to not exceed $c/\sqrt{3}$ at high densities. We find that, in general, the NS mass significantly depends on the value of $\kappa$, and the radius $R$ is sensitive to the value of ${\mathrm{\Lambda }}_c$. Moreover, as ${\mathrm{\Lambda }}_c$ is equal to zero or less than the accepted bound of the cosmological constant, the NS within the EiBI theory is compatible with observational constraints, including $2.0M_{\odot }$ mass, canonical radius $R_{1.4M_{\odot }}$, moment of inertia, and tidal deformation. Our investigation also reveals that the $2.6M_{\odot }$ mass compact object and current observational constraint of canonical radius $R_{1.4M_{\odot }}$ can simultaneously be satisfied only when the ${\mathrm{\Lambda }}_c$ value is unphysically too large and negative. Therefore, within the specific EoS employed in this work, we conclude that the secondary object with $2.6M_{\odot }$ observed in the GW190814 event [1] is not likely a static (or a slow-rotating) NS within the EiBI gravity theory.

The deflection and gravitational lensing of light and massive particles in arbitrary static, spherically symmetric and asymptotically (anti–)de Sitter spacetimes are considered in this work. We first proved that for spacetimes whose metric satisfies certain conditions, the deflection of null rays with fixed closest distance will not depend on the cosmological constant $\mathrm{\Lambda }$, while that of timelike signals and the apparent angle in gravitational lensing still depend on $\mathrm{\Lambda }$. A two-step perturbative method is then developed to compute the change of the angular coordinate and total travel time in the weak field limit. The results are quasipower series of two small quantities, with the finite distance effect of the source/detector naturally taken into account. These results are verified by applying to some known asymptotically de Sitter spacetimes. Using an exact gravitational lensing equation, we solved the apparent angles of the images and time delays between them and studied the effect of $\mathrm{\Lambda }$ on them. It is found that generally, a small positive $\mathrm{\Lambda }$ will decrease the apparent angle of images from both sides of the lens and increase the time delay between them. The time delay between signals from the same side of the lens but with different energy however, will be decreased by $\mathrm{\Lambda }$.

In this paper, we provide a tractable example of a polyhomogeneous solution space for electromagnetism at null infinity in four dimensions. The memory effect for electromagnetism is then derived from the polyhomogeneous solution space. We also comment on the connection between the electromagnetic memories and asymptotic symmetries.

Theories of gravity that obey the weak equivalence principle have the same parametrized post-Newtonian parameter $\gamma$ for all particles at all energies. The large Shapiro time delays of extragalactic sources allow us to put tight constraints on differences in $\gamma$ between photons of different frequencies from spectral lag data, since a nonzero $\mathrm{\Delta }\gamma$ would result in a frequency-dependent arrival time. The majority of previous constraints have assumed that the Shapiro time delay is dominated by a few local massive objects, although this is a poor approximation for distant sources. In this work we consider the cosmological context of these sources by developing a source-by-source, Monte Carlo–based forward model for the Shapiro time delays by combining constrained realizations of the local density field using the Bayesian origin reconstruction from galaxies algorithm with unconstrained large-scale modes. Propagating uncertainties in the density field reconstruction and marginalizing over an empirical model describing other contributions to the time delay, we use spectral lag data of gamma ray bursts from the BATSE satellite to constrain $\mathrm{\Delta }\gamma <2.1\times {10}^{-15}$ at $1\sigma$ confidence between photon energies of 25 keV and 325 keV.

The motion of spinning test particles around a traversable wormhole is investigated using the Mathisson-Papapetrous-Dixon equations, which couple the Riemann tensor with the antisymmetric tensor $S^{\alpha \beta }$, related to the spin of the particle. Hence, we study the effective potential, circular orbits, and innermost stable circular orbit (ISCO) of spinning particles. We found that the spin affects significantly the location of the ISCO, in contrast with the motion of nonspinning particles, where the ISCO is the same in both the upper and lower universes. On the other hand, since the dynamical four-momentum and kinematical four-velocity of the spinning particle are not always parallel, we also consider a superluminal bound on the particles motion. In the case of circular orbits at the ISCO, we found that the motion of particles with an adimensional spin parameter lower (greater) than $s=-1.5$ (1.5) is forbidden. The spin interaction becomes important for Kerr black hole orbiting super massive wormholes (SMWH). The motion of spinning test particles around a rotating wormhole is in process, and we will present it in a new manuscript soon.

We present and discuss a new family of topological hairy dyonic black hole solutions in asymptotically anti–de Sitter space. The coupled Einstein-Maxwell-scalar gravity system, that carries both the electric and magnetic charges is solved, and exact hairy dyonic black hole solutions are obtained analytically. The scalar field profiles that give rise to such black hole solutions are regular everywhere. The hairy solutions are obtained for planar, spherical, and hyperbolic horizon topologies. In addition, analytic expressions of regularized action, stress tensor, conserved charges, and free energies are obtained. We further comment on different prescriptions for computing the black hole mass with hairy backgrounds. We analyze the thermodynamics of these hairy dyonic black holes in canonical and grand canonical ensembles, and we find that both electric and magnetic charges have a constructive effect on the stability of the hairy solution. For the case of planar and hyperbolic horizons, we find thermodynamically stable hairy black holes that are favored at low temperatures compared to the nonhairy counterparts. We further find that, for a spherical hairy dyonic black hole, the thermodynamic phase diagram resembles to that of a Van der Waals fluid not only in canonical but also in the grand canonical ensemble.

We study the Bogomol’nyi-Prasad-Sommerfield solutions of the asymptotically flat, stationary microstate geometries with biaxisymmetry and reflection symmetry in the five-dimensional ungauged minimal supergravity. We show that the angular momenta of the microstate geometry with a small number of centers (at least, five centers) have lower bounds, which are slightly smaller than those of the maximally spinning Breckenridge-Myers-Peet-Vafa (BMPV) black hole. Therefore, there exists a certain narrow parameter region such that the microstate geometry with a small number of the centers admits the same angular momenta as the BMPV black hole. Moreover, we investigate the dependence of the topological structure of the evanescent ergosurfaces on the magnetic fluxes through the 2-circles between two centers.

We have studied the spacetime of a Kerr black hole immersed in Melvin magnetic field, and found Melvin-Kerr black hole shadow becomes more elongated as the magnetic field parameter $B$ increases. We found not only that an unstable light ring could exist but also that a stable light ring could exist. The unstable light ring radius increases with the increase of $B$, but it is the opposite for a stable light ring. For a certain rotation parameter $a$, both the prograde and retrograde unstable (stable) light rings exist when $B$ is less than a critical value $B_c$ for retrograde light ring. In this case, the shadows of Melvin-Kerr black hole have two regions corresponding to the prograde and retrograde stable photon orbits on both sides of the middle main shadow. As $B$ continues to increase, there is only the prograde unstable (stable) light ring. In this case, the region corresponding to the prograde stable photon orbits emerges in the life of the main shadow. The absence of the retrograde unstable (stable) light ring makes the Melvin-Kerr black hole shadow a half-panoramic (equatorial) shadow. When $B$ is bigger than $B_c$ for a prograde light ring, neither the prograde nor retrograde unstable (stable) light ring exists. In this case, the shadow of Melvin-Kerr black hole becomes a panoramic (equatorial) shadow. In addition, there also exist some self-similar fractal structures in the shadow of Melvin-Kerr black hole arising from the chaotic motion of photon.

We report new topological invariants in four dimensions that are generalizations of the Nieh-Yan topological invariant. The new topological invariants are obtained through a systematic method along the lines of the one used to get the Nieh-Yan form, but involving an $SO(4,1)$ [$SO(5)$] connection constructed out from an $SO(3,1)$ [$SO(4)$] connection and three $SO(3,1)$ [$SO(4)$] tensor 1-forms. We give explicit expressions of the new 4-forms that give rise to the new topological invariants for particular choices of these 1-forms and show that the Nieh-Yan form arises as a particular case.

We show that axionlike particles that only couple to invisible dark photons can generate visible $B$ mode signals around the reionization epoch. The axion field starts rolling shortly before reionization, resulting in a tachyonic instability for the dark photons. This generates an exponential growth of the dark photon quanta sourcing both scalar metric modes and gravitational waves that leave an imprint on the reionized baryons. The tensor modes modify the cosmic microwave background (CMB) polarization at reionization, generating visible $B$ mode signatures for the next generation of CMB experiments for parameter ranges that satisfy the current experimental constraints.

We propose an experiment that the entanglement between two macroscopic mirrors suspended at the end of an equal-arm interferometer is destroyed by the noise of gravitons through bremsstrahlung. By calculating the correlation function of the noise, we obtain the decoherence time from the decoherence functional. We estimate that the decoherence time induced by the noise of gravitons in squeezed states stemming from inflation is approximately 20 s for 40 km long arms and 40 kg mirrors. Our analysis shows that observation of the decoherence time of quantum entanglement has the potential to detect gravitons indirectly. This indirect detection of gravitons would give strong evidence of quantum gravity.

Primordial black holes (PBHs) have been proposed to explain at least a portion of dark matter. Observations have put strong constraints on PBHs in terms of the fraction of dark matter which they can represent, $f_{\mathrm{PBH}}$, across a wide mass range—apart from the stellar-mass range of $20M_{\odot }\lesssim M_{\mathrm{PBH}}\lesssim 100M_{\odot }$. In this paper, we explore the possibility that such PBHs could serve as point-mass lenses capable of altering the gravitational-wave (GW) signals observed from binary black hole (BBH) mergers along their line-of-sight. We find that careful GW data analysis could verify the existence of such PBHs based on the fitting factor and odds ratio analyses. When such a lensed GW signal is detected, we expect to be able to measure the redshifted mass of the lens with a relative error $\mathrm{\Delta }{M}_{\mathrm{PBH}}/M_{\mathrm{PBH}}\lesssim 0.3$. If no such lensed GW events were detected despite the operation of sensitive GW detectors accumulating large numbers of BBH mergers, it would translate into a stringent constraint of $f_{\mathrm{PBH}}\lesssim {10}^{-2}-{10}^{-5}$ for PBHs with a mass larger than $\sim 10M_{\odot }$ by the Einstein Telescope after one year of running, and $f_{\mathrm{PBH}}\lesssim 0.2$ for PBHs with mass greater than $\sim 50M_{\odot }$ for advanced LIGO after ten years of running.

We study the dynamics of a homogeneous, isotropic, and positively curved universe in the presence of a SU(2) gauge field or a triplet of mutually orthogonal vector fields. In the SU(2) case we use the previously known ansatz for the gauge-field configuration, but the case without non-Abelian symmetries is more nontrivial and we develop a new ansatz. We in particular consider axion-SU(2) inflation and inflation with vector fields having $\mathrm{U}(1)\times \mathrm{U}(1)\times \mathrm{U}(1)$ symmetry, and analyze their dynamics in detail numerically. Novel effects of the spatial curvature come into play through vector fields, which causes unconventional preinflationary dynamics. It is found that the closed universe with vector fields is slightly more stable against collapse than that filled solely with an inflaton field.

We study the evolution of nonlinear superhorizon perturbations in a universe dominated by a complex scalar field. The analysis is performed adopting the gradient expansion approach, in the constant mean curvature slicing. We derive general solutions valid to second order in the ratio $H^{-1}/L$ for scalar field inhomogeneities of size $L$ subject to an arbitrary canonical potential. We work out explicit solutions for the quadratic and the quartic potentials and discuss their relevance in setting initial conditions required for the simulations of primordial black hole formation.

We study in some detail an interacting cosmological model based on two canonical scalar fields starting from a Lagrangian description. Contrary to other more phenomenological approaches where nonrelativistic matter and dark energy are cosmological perfect fluids, and where a source term is added by hand at the level of the continuity equations, here within Einstein’s theory we model the dark sector, which dominates the evolution of the universe, as two minimally coupled scalar fields, out of which the first play the role of dark matter and the second plays the role of dark energy. We compute both the deceleration parameter and the distance modulus versus redshift, and we demonstrate that the model is capable of explaining the current cosmic acceleration. We find that a negative coupling constant implies two distinctive features, which comprise two robust predictions of the model studied in the present work, and which are the following: (a) a transient acceleration phase, and (b) a collapsing Universe, or in other words an initial expansion which is followed by a contraction leading eventually to a big crunch.

Lambda cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) is increasingly facing challenges in late-time cosmology, notably in the Hubble parameter $H(z)$ at redshift zero known as $H_0$-tension. Extensions to $\mathrm{\Lambda }\mathrm{CDM}$ have been approached mostly by models with an extra parameter. In this work, we provide a framework for evaluating the viability of models of late-time cosmology by investigating unparticle cosmology as a case study. Unparticle cosmology proposes a scale invariant contribution by a dimensionless parameter $\delta$. We focus on $H_0$-tension in comparison with scale invariant and $\mathrm{\Lambda }\mathrm{CDM}$ models. The dynamical behavior of unparticle cosmology with and without a cosmological constant shows that for most values of $\delta \in [-6,1]$ (corresponding to $d_u\in [-2,3/2]$), the late-time Universe will be $\mathrm{\Lambda }$ and matter dominated, respectively, with negligible contribution of unparticles. It predicts $H(z)$ in the future to be zero or constant, the latter pointing to a stable de Sitter phase for our Universe. Our data analysis shows $\delta =-2.06\pm 0.46$ as the best fit to the data. Consequently, the conventional value of $d_u=3/2\equiv \delta =1$ is ruled out by $6.6\sigma$. However, there is a pronounced gap between the models and what is demanded by observational data in the $qQ$-diagram for all $\delta$. We conclude that unparticle cosmology fails to come to the rescue of challenges to late-time $\mathrm{\Lambda }\mathrm{CDM}$, but it provides a pointer to holographic dark energy.

In an early work, we applied a QCD-based equation of state (EOS) to the study of the stellar structure of self-bound strange stars, obtaining sequences with maximum masses larger than two solar masses and radii ranging from 8 to 12 km. In this work, we update the previous calculations and compare them with the most recent data, including the very recent determination of the mass and radius of the massive pulsar PSR $\mathrm{J}0740+6620$ performed by the NICER and XMM-Newton Collaborations. Our equation of state is similar to the MIT bag model one, but it includes repulsive interactions, which turn out to be essential to reproduce the accumulated experimental information. We find that our EOS is still compatible with all astrophysical observations, but the parameter window is now narrower.