We present a nongeometric derivation of $\mathcal{N}=1$ Super Yang-Mills by focusing on the consistency of interactions that extend the free vector supermultiplet rather than assuming gauge invariance under extended symmetries. By utilizing a superspace first-order description, the theory is given in closed form as a third-order polynomial, which includes a single cubic interaction term instead of an infinite series, thus eliminating the need for a special gauge. The geometrical interpretation of the theory emerges, as opposed to being presupposed.

Zero-point fluctuations in the background of a cosmic string provide an opportunity to study the effects of topology in quantum field theory. We use a scattering theory approach to compute quantum corrections to the energy density of a cosmic string, using the “ballpoint pen” and “flowerpot” models to allow for a nonzero string radius. For computational efficiency, we consider a massless field in $2+1$ dimensions. We show how to implement precise and unambiguous renormalization conditions in the presence of a deficit angle, and make use of Kontorovich-Lebedev techniques to rewrite the sum over angular momentum channels as an integral on the imaginary axis.

Quadratic quasinormal mode, generated at second order in black hole perturbation theory, are a promising target for testing gravity in the nonlinear regime with next-generation gravitational wave detectors. While their frequencies have long been known, their amplitudes remain poorly studied. We introduce regular variables and compute amplitudes for Schwarzschild black holes with the Leaver algorithm. We find a nonlinear ratio $\mathcal{R}\simeq 0.154e^{-0.068i}$ for the most excited $\ell =4$ mode, matching results from numerical relativity. We also predict new low-frequency $\ell =2$ quadratic modes.

We investigate static boson star solutions in the so-called Einstein-Friedberg-Lee-Sirlin (E-FLS) theory and perform a complete analysis of the solution space in this model. We study the phenomenological aspects of E-FLS stars with an emphasis on the analysis of circular timelike orbits and circular photon orbits. In order to study the astrophysical signatures of such stars, their images were obtained under the assumption that they are surrounded by a geometrically thin accretion disk. Here we consider two different accretion disk models, namely the optically thin and optically thick disk models. We present a selection of our findings for the astrophysical images of E-FLS stars and discuss their relevance as possible black hole mimickers.

We construct Einstein-Maxwell-scalar theories that admit regular electric black holes. Such a Maxwell-scalar theory is equivalent to some nonlinear electrodynamics (NLED) at the level of equations of motion, but it has the advantage of circumventing the no-go theorem of regular electric black holes under a given Lagrangian of NLED. We study the thermodynamics and show that the mass of the regular black hole can be determined solely by the Maxwell field, without having to know the metric profile function. Our formalism allows us to study the applications of the electrically charged regular black holes in areas that were previously available only to the nonregular ones.

It has recently been revealed [R. Gervalle and M. S. Volkov, Phys. Rev. Lett. 133, 171402 (2024)] that magnetically charged black holes of the composed Einstein-Weinberg-Salam field theory can support bound-state hairy configurations of electroweak fields. In the present paper we study, using analytical techniques, the physical and mathematical properties of the supported linearized electroweak fields (spatially regular electroweak “clouds”) in the dimensionless large-mass $\mu \equiv m_{\mathrm{w}}{r}_{+}\gg 1$ regime of the composed black-hole-field system (here $m_{\mathrm{w}}$ is the mass of the supported $W$-boson field and $r_{+}$ is the outer horizon radius of the central supporting black hole). In particular, we derive a remarkably compact formula for the discrete resonance spectrum $\{{\mu }_k(n){\}}_{k=0}^{k=\infty }$ that characterizes the composed black-hole-linearized-field configurations, where the integer $n\equiv 2Pe\in \mathbb{Z}$ characterizes the discrete charge parameter $P$ of the central magnetic Reissner-Nordström black hole and $e$ is the electron charge. The physical significance of the analytically derived resonant spectrum stems from the fact that, in the dimensionless large-charge $n\gg 1$ regime, the fundamental (largest) eigenvalue ${\mu }_0(n)$ determines the critical existence line of the composed Einstein-Weinberg-Salam theory, a boundary line that separates hairy magnetic-black-hole-electroweak-field bound-state configurations from bald magnetic Reissner-Nordström black holes.

We develop a relativistic scenario of fast magnetic reconnection process, for general magnetohydrodynamical plasmas around Kerr black holes. Generalizing the Petschek model, we study various properties of the reconnection layer in distinct configurations. When current sheet forms in the zero-angular-momentum (ZAMO) frame which corotates with the black hole, the reconnection rate for both radial and azimuthal configurations is decreased by spacetime curvature. However, when the current sheet forms in a non-ZAMO frame, which rotates either faster or slower than the black hole, detail analysis establishes that for any given slow rotations (subrelativistic at most) and mildly relativistic inflow, the ZAMO observer will find asymmetric reconnection rates for radial configuration. It is decreased on one side of the current sheet and is increased on the other side in comparison to the unrotation limit. This is valid to both the Sweet–Parker and the Petschek scenario. The results clarify the effects of rotation on the reconnection layer in the laboratory frame in the flat spacetime limit.

The inclusion of higher derivative terms in the gravitational action brings about corrections to the original forms of black hole solutions and thermodynamics. A simple and valuable approach emerged over two decades ago, which states that the first order corrections to black hole thermodynamics, caused by higher derivative terms, can be achieved without explicitly solving the perturbed metric. However, a direct application of this approach may potentially yield incorrect answers as it comes to anti-de Sitter (AdS) black holes, which poses a challenge to the valuable approach. In this paper, we identify that the subtlety associated with AdS black holes is that certain higher derivative terms may alter the asymptotic structure of the spacetime. By taking this subtlety into account, we successfully enhance the original approach and resolve the mentioned problem. Furthermore, we make an interesting observation that the extremalization analysis that underlies this approach also plays a fundamental role in the perturbation theories of quantum mechanics and other disciplines.

Black-hole spacetimes that possess stationary equatorial matter rings are known to exist in general relativity. We here reveal the existence of black-hole spacetimes that support nonequatorial matter rings. In particular, it is proved that rapidly rotating Kerr black holes in the dimensionless large-spin regime $\overline{a}>{\overline{a}}_{\mathrm{crit}}=\sqrt{\{7+\sqrt{7}\cos [\frac{1}{3}\arctan (3\sqrt{3})]-\sqrt{21}\sin [\frac{1}{3}\arctan (3\sqrt{3})]\}/12}\simeq 0.78$ can support a pair of nonequatorial massive scalar rings that are negatively coupled to the Gauss-Bonnet curvature invariant of the spinning spacetime (here $\overline{a}\equiv J/M^2$ is the dimensionless angular momentum of the central supporting black hole). We explicitly prove that these nonequatorial scalar rings are characterized by the dimensionless functional relation $-\frac{57+28\sqrt{21}\cos [\frac{1}{3}\arctan (\frac{1}{3\sqrt{3}})]}{8(1+\sqrt{1-{\overline{a}}^2}{)}^6}·\frac{\overline{\eta }}{{\overline{\mu }}^2}\to 1^{+}$ in the large-mass $\overline{\mu }\equiv M\mu \gg 1$ regime (here $\{\overline{\eta }<0,\mu \}$ are, respectively, the nontrivial coupling parameter of the composed Einstein-Gauss-Bonnet-massive-scalar field theory and the proper mass of the supported nonminimally coupled scalar field).

Detecting subsolar objects in black hole binary mergers is considered a smoking gun signature of primordial black holes. Their supposedly vanishing tidal Love number is generically thought to help distinguish them from other subsolar and more deformable compact objects, such as neutron stars. We show that a large and detectable Love number of primordial black holes can be generated in the presence of even small disturbances of the system, thus potentially jeopardizing their discovery. However, such small perturbations are not tightly bound and are therefore disrupted before the mergers. We show that they leave a characteristic signature in the gravitational waveform that could be observed with current and future gravitational wave detectors. Thus, they may still hint towards the primordial nature of the black holes in the merger. Finally, we demonstrate that disregarding possible environmental effects in the matched-filter search for subsolar gravitational wave events can lead to a decreased sensitivity in the detectors.

Gibbons and Schiller have raised the physically interesting conjecture that forces in general relativity are bounded from above by the mathematically compact relation $\mathcal{F}\le c^4/4G$. In the present compact paper we explicitly prove, using the nonlinearly coupled Einstein-matter field equations, that the force function $\mathcal{F}\equiv 4\pi r^{2}p(r)$ in stable self-gravitating horizonless matter configurations is characterized by the upper bound $\mathcal{F}\le c^4/G$ [here $p(r)$ is the radial pressure inside the self-gravitating matter configuration].

In the present work, we investigate the gravitational lensing effects of two quantum-modified black hole models recently proposed in effective quantum gravity. The light deflection angles are calculated for both the weak-field and strong-field limits. Furthermore, using the data for the supermassive black holes SgrA* and M87*, we calculate the lensing observables in the strong-field limit. We find that the quantum parameter plays a role analogous to the electric charge in weak gravitational lensing. In the strong-field limit, in contrast, the effects of the quantum parameter on the deflection angle, the angular separation, and the relative magnification are opposite to those of the electric charge, the scalar charge, and the quantum parameters in some gravity theories. The results indicate the crucial difference between the classical black holes and the two quantum-modified black hole models that depend on the quantum correction, making them a valuable tool for distinguishing these black hole models.

Superradiant instability can form clouds around rotating black holes (BHs) composed of ultralight bosonic fields, such as axions. A BH with such a cloud in a binary system exhibits rich phenomena, and gravitational waves from the BH merger provide a means to probe axions. For the first time, we study the evolution of axion clouds in a binary system during the inspiral phase, including axion self-interaction effects. When the self-interaction is significant, unlike in the negligible case, two types of clouds coexist through mode coupling. We examine the evolution of the system considering the effects of dissipation caused by both self-interaction and tidal interaction. For tidal interaction, in addition to the processes of emission to infinity and absorption by the BH, indirect emission via transitions (both resonant and off-resonant) is also considered as a second-order perturbation. Our results demonstrate that the signatures of axion self-interaction are imprinted in the modification of the gravitational wave phase. Furthermore, we find the possibility of a dynamical instability called bosenova during the binary inspiral phase.

Estimating the parameters of compact binaries which coalesce and produce gravitational waves is a challenging Bayesian inverse problem. Gravitational-wave parameter estimation lies within the class of multifidelity problems, where a variety of models with differing assumptions, levels of fidelity, and computational cost are available for use in inference. In an effort to accelerate the solution of a Bayesian inverse problem, cheaper surrogates for the best models may be used to reduce the cost of likelihood evaluations when sampling the posterior. Importance sampling can then be used to reweight these samples to represent the true target posterior, incurring a reduction in the effective sample size. In cases when the problem is high dimensional, or when the surrogate model produces a poor approximation of the true posterior, this reduction in effective samples can be dramatic and render multifidelity importance sampling ineffective. We propose a novel method of tempered multifidelity importance sampling in order to remedy this issue. With this method the biasing distribution produced by the low-fidelity model is tempered, changing the chi-squared divergence between the two distributions and thereby affecting the efficiency of importance sampling. There is an optimal temperature that maximizes the efficiency in this setting, and we propose a low-cost strategy for approximating this optimal temperature using samples from the untempered distribution. In this paper, we motivate this method by applying it to Gaussian target and biasing distributions. Finally, we apply it to a series of problems in gravitational wave parameter estimation and demonstrate improved efficiencies when applying the method to real gravitational wave detections.

The study of perturbations around black hole backgrounds in general relativity and Einstein-Maxwell theory has a long history, going back to the work of Regge and Wheeler in the 1950s. As part of a broader investigation of perturbations around black holes in supergravity, we describe here our results for the perturbations around the Gibbons-Maeda static charged black holes in a class of Einstein-Maxwell-Dilaton theories. Our analysis follows the general strategy developed by Chandrasekhar and Xanthopoulos for the perturbations of the Reissner-Nordström black hole. Here, the analysis is considerably more involved, because of the presence of the dilaton field, which couples to the other polar modes. We nonetheless find that the problem is completely solvable, in the sense that one can separate variables and eventually describe all the perturbations in terms of diagonalised second-order radial equations. We are able to prove the mode stability of all the Gibbons-Maeda black hole solutions.

After studying null geodesics in the equatorial plane in NUT spacetime, we show that there are unstable photon rings in this plane. Next, we transform the metric of this plane to the isotropic coordinates and introduce its equivalent two-parameter index of refraction. Utilizing the analog gravity concepts, we assign this index of refraction to a metamaterial analog of this plane, and by ray-tracing simulation find its photon rings. Furthermore, we extend our analysis to the charged NUT spacetime and employ wave optics to numerically solve Maxwell’s equations for electromagnetic waves within an inhomogeneous medium assigned with this spacetime’s three-parameter index of refraction. We investigate the optical properties of such metamaterial analogs for designing enhanced and fine-tuned optical devices.

Boyle and Turok’s $CPT$-symmetric universe model posits that the Universe was symmetric at the big bang, addressing numerous problems in both cosmology and the Standard Model of particle physics. We extend this model by incorporating the symmetric conditions at the end of the Universe, which impose constraints on the allowed perturbation modes. These constrained modes conflict with the integer wave vectors required by the global spatial geometry in a closed universe. To resolve this conflict, only specific values of curvature are permissible, and in particular the curvature density is constrained to be ${\mathrm{\Omega }}_K\in \{-0.016,-0.011,-0.004,\dots \}$, consistent with Planck observations.

Chain inflation is an alternative to slow-roll inflation in which the inflaton tunnels along a large number of consecutive minima in its potential. In this work we perform the first comprehensive calculation of the gravitational wave (GW) spectrum of chain inflation. In contrast to slow-roll inflation the latter does not stem from quantum fluctuations of the gravitational field during inflation, but rather from the bubble collisions during the first-order phase transitions associated with vacuum tunneling. Our calculation is performed within an effective theory of chain inflation which builds on an expansion of the tunneling rate capturing most of the available model space. The effective theory can be seen as chain inflation’s analog of the slow-roll expansion in rolling models of inflation. The near scale-invariance of the scalar power spectrum translates to a quasiperiodic shape of the inflaton potential in chain inflation, with the tunneling rate changing very slowly during the e-folds leading to cosmic microwave background observables. We show that chain inflation produces a very characteristic double-peak GW spectrum: a faint high-frequency peak associated with the gravitational radiation emitted during inflation, and a strong low-frequency peak associated with the graceful exit from chain inflation (marking the transition to the radiation-dominated epoch). There exist very exciting prospects to test the gravitational wave signal from chain inflation at the aLIGO-aVIRGO-KAGRA network, at LISA and /or at pulsar timing array experiments. A particularly intriguing possibility we point out is that chain inflation could be the source of the stochastic gravitational wave background recently detected by NANOGrav, PPTA, EPTA, and CPTA. We also show that the gravitational wave signal of chain inflation is often accompanied by running/ higher running of the scalar spectral index to be tested at future cosmic microwave background experiments.

This paper studies gravitational lensing degeneracies in the wave-optics regime, focusing on lensed gravitational waves (GWs). Considering lensing degeneracies as rescaling (or transformations) of arrival time delay surface, we can divide them into local and global types. Local degeneracies only affect the time delay surface in localized regions, whereas global degeneracies rescale the whole time delay surface by a constant while keeping the various observed image properties unchanged. We show that local degeneracies can be broken in the wave-optics regime since lensing effects become sensitive to the overall arrival time delay surface and not only to the time delay values at the image positions. On the other hand, global degeneracies (such as similarity transformation, prismatic transformation, and mass-sheet degeneracy) multiply the amplification factor by a constant factor (let us say, $\lambda$). However, in GW lensing, as the GW signal amplitude depends on the source distance, it turns out that $\lambda$ is completely degenerate with the Hubble constant, similar to what we see in geometric optics. Hence, with the lensing of GWs, global degeneracies are as hard to break in wave optics as they are in geometric optics.

New physics beyond the Standard Model (BSM) with an extra neutral boson can be constrained from the observation of SN1987A, since productions of this neutral boson in a supernova (SN) could accelerate the SN cooling and potentially lead to a period of the neutrino burst incompatible with the observation. The constraint to the model is formulated by the condition $L_{\mathrm{NB}}\le 3\times {10}^{52}\mathrm{erg}/\mathrm{s}$ according to Raffelt with $L_{\mathrm{NB}}$ the luminosity of BSM neutral boson. Computing the above luminosity in the large coupling case, the so-called trapping regime, is nontrivial since the luminosity is a competition between the large production rate and the efficient absorption or decay rate of the neutral boson. We illustrate such a subtlety using $U(1{)}_{L_{\mu }-L_{\tau }}$ model as an example where the $Z^{\prime }$ luminosity, $L_{Z^{\prime }}$, from the neutrinosphere is calculated. We calculate $Z^{\prime }$ production, absorption, and decay rates through pair-coalescence, semi-Compton, loop-bremsstrahlung from proton-neutron scattering, and their inverse processes in a benchmark SN simulation with muons. We point out that, as the coupling constant $g_{Z^{\prime }}$ increases, $L_{Z^{\prime }}$ shall be approaching to a constant plateau value for a given $m_{Z^{\prime }}$ instead of monotonically decreasing down to zero as obtained in the previous literature. We demonstrate that this plateau phenomenon can be understood by physical arguments and justified by numerical calculations. With a different result on $L_{Z^{\prime }}$ from the previous one, we discuss impacts on the constraints to $U(1{)}_{L_{\mu }-L_{\tau }}$ parameter space by SN1987A. The implication of our result to the similar constraint on a generic BSM model with an extra neutral boson is also discussed.