We analyze the near horizon symmetry of the extremal Kerr black hole within the framework of Poincaré gauge theory (PG) for two important limiting cases; Riemannian and teleparallel solution. We show that the algebra of canonical generators is realized by Virasoro algebra, with central charge which depends on the black hole horizon radius. The conformal entropy of the black hole is obtained via the Cardy formula.

The bumblebee gravity model, with a vector field nonminimally coupled to gravity, is a natural extension of the Einstein-Maxwell theory. In this theory, a black hole can carry a vector hair, making the metric deviate from the Schwarzschild metric. To investigate the detectability of the vector hair, we consider an extreme-mass-ratio inspiral (EMRI) system, where a stellar-mass black hole inspiraling into a supermassive black hole. We find that, with a one-year observation of an EMRI by a space-based gravitational-wave detector, we can probe the vector charge as small as $Q\sim {10}^{-3}$ in the bumblebee gravity model, which is about three orders of magnitude tighter comparing to current EHT observations.

In addition to the well-known quasinormal modes, the gravitational modes of the Kerr geometry also include sets of total-transmission modes. Each mode can be considered as an element of a sequence of modes parameterized by the angular momentum of the black hole. One family of gravitational total-transmission modes of Kerr have been known for some time. Modes in this family connect to a Schwarzschild limit where the mode frequency is finite and purely imaginary. Recently, what was thought to be an additional branch of this original family of modes was discovered. However, this new branch is actually a part of one of two entirely new families of total-transmission modes. Modes in these new families, surprisingly, connect to a Schwarzschild limit where the mode frequencies exist at complex infinity. We have numerically constructed full sets of sequences of gravitational total-transmission modes for harmonic indices $\ell =[2,8]$. Using these numerical sequences, we have been able to construct analytic asymptotic expansions for the mode frequencies and their associated separation constants. The asymptotic expansion for the separation constant used in constructing the total-transmission modes seems to be valid for general complex values of the oblateness parameter.

We revisit the classic system of a spherically symmetric black hole in general relativity (i.e., a Schwarzschild black hole) surrounded by a geometrically thin accretion disk. Our purpose is to examine whether one can determine three parameters of this system (i.e., black hole mass $M$, distance between the black hole and an observer $r_o$, inclination angle $i$) solely by observing the accretion disk and the black hole shadow. A point in our analysis is to allow $r_o$ to be finite, which is set to be infinite in most relevant studies. First, it is shown that one can determine the values of $(r_o/M,i)$, where $M/r_o$ is the so-called angular gravitational radius, from the size and shape of shadow. Then, it is shown that if one additionally knows the accretion rate $\dot{M}$ (respectively, mass $M$) by any independent theoretical or observational approach, one can determine the values of $(M,r_o,i)$ [respectively, $(\dot{M},r_o,i)$] without degeneracy, in principle, from the value of flux at any point on the accretion disk.

One of the key ingredients for making binary waveform predictions in a beyond-general-relativity (GR) theory of gravity is understanding the energy and angular momentum carried by gravitational waves and any other radiated fields. Identifying the appropriate energy functional is unclear in Hassan-Rosen bigravity, a ghost-free theory with one massive and one massless graviton. The difficulty arises from the new degrees of freedom and length scales which are not present in GR, rendering an Isaacson-style averaging calculation ambiguous. In this article we compute the energy carried by gravitational waves in bigravity starting from the action, using the canonical current formalism. The canonical current agrees with other common energy calculations in GR, and is unambiguous (modulo boundary terms), making it a convenient choice for quantifying the energy of gravitational waves in bigravity or any diffeomorphism-invariant theories of gravity. This calculation opens the door for future waveform modeling in bigravity to correctly include backreaction due to emission of gravitational waves.

In the analysis of a binary black hole coalescence, it is necessary to include gravitational self-interactions in order to describe the transition of the gravitational wave signal from the merger to the ringdown stage. In this paper we study the phenomenology of the generation and propagation of nonlinearities in the ringdown of a Schwarzschild black hole, using second-order perturbation theory. Following earlier work, we show that the Green’s function and its causal structure determines how both first-order and second-order perturbations are generated, and hence highlight that both of these solutions share some physical properties. In particular, we discuss the sense in which both linear and quadratic quasinormal modes (QNMs) are generated in the vicinity of the peak of the gravitational potential barrier (loosely referred to as the light ring). Among the second-order perturbations, there are solutions with linear QNM frequencies (whose amplitudes are thus renormalized from their linear values), as well as quadratic QNM frequencies with a distinct spectrum. Moreover, we show using a Wentzel-Kramers-Brillouin analysis that, in the eikonal limit, waves generated inside the light ring propagate towards the black hole horizon, and only waves generated outside propagate towards an asymptotic observer. These results might be relevant for recent discussions on the validity of perturbation theory close to the merger. Finally, we argue that even if nonlinearities are small, quadratic QNMs may be detectable and would likely be useful for improving ringdown models of higher angular harmonics and future tests of gravity.

Our present understanding of the reheating phase is incomplete due to a lack of observations. Apart from its cosmological implications, the reheating should play a vital role in particle physics and inflation model building. Conventionally, reheating dynamics are modeled by invoking arbitrary coupling among the inflaton and daughter fields. Such an approach lacks robust cosmological predictions, due to its arbitrary couplings, and is difficult to verify through observation. In this paper, we propose a minimal reheating scenario, where the inflaton is coupled with all the daughter fields only gravitationally. Besides being successful in reheating the Universe, the scenario offers a strong cosmological prediction of the primordial gravitational wave spectrum and discards a large number of possible models of dark matter and inflation that are otherwise consistent with Planck.

We investigate the out-of-equilibrium production of scalar dark matter (DM) from the inflaton condensate during inflation and reheating. We assume that this scalar couples only to the inflaton via a direct quartic coupling and is minimally coupled to gravity. We consider all possible production regimes: purely gravitational, weak direct coupling (perturbative), and strong direct coupling (nonperturbative). For each regime, we use different approaches to determine the dark matter phase space distribution and the corresponding relic abundance. For the purely gravitational regime, scalar dark matter quanta are copiously excited during inflation resulting in an infrared (IR) dominated distribution function and a relic abundance which overcloses the universe for a reheating temperature $T_{\mathrm{reh}}>34\mathrm{GeV}$. A nonvanishing direct coupling induces an effective DM mass and suppresses the large IR modes in favor of ultraviolet (UV) modes and a minimal scalar abundance is generated when the interference between the direct and gravitational couplings is maximal. For large direct couplings, backreaction on the inflaton condensate is accounted for by using the Hartree approximation and lattice simulation techniques. Since scalar DM candidates can behave as noncold dark matter, we estimate the impact of such species on the matter power spectrum and derive the corresponding constraints from the Lyman-$\alpha$ measurements. We find that they correspond to a lower bound on the DM mass of $\gtrsim 3\times {10}^{-4}\mathrm{eV}$ for purely gravitational production, and $\gtrsim 20\mathrm{eV}$ for direct coupling production. We discuss the implications of these results.

We explore a model introduced by Cyr-Racine et al. [Phys. Rev. Lett. 128, 201301 (2022)] that resolves the Hubble tension by invoking a “mirror world” dark sector with energy density a fixed fraction of the “ordinary” sector of $\mathrm{\Lambda }+\mathrm{cold}$ dark matter. Although it reconciles cosmic microwave background and large-scale structure observations with local measurements of the Hubble constant, $H_0$, the model requires a value of the primordial helium mass fraction, $Y_p=0.170\pm 0.025$, that is discrepant with observations and with the predictions of big bang nucleosynthesis. We consider a variant of the model with standard helium mass fraction but with the value of the electromagnetic fine-structure constant, $\alpha$, slightly different during photon decoupling from its present value. If $\alpha$ at that epoch is lower than its current value by $\mathrm{\Delta }\alpha \simeq -2\times {10}^{-5}$, then we can achieve the same Hubble tension resolution as in the work by Cyr-Racine et al. but with consistent helium abundance. As an example of such time evolution of $\alpha$, we consider a toy model of an ultralight scalar field, with mass $m<4\times {10}^{-29}\mathrm{eV}$, coupled to electromagnetism, which evolves after photon decoupling at redshift $z\simeq {10}^3$, and that with appropriate coupling appears to be consistent with late-time constraints on $\alpha$ variation and the weak equivalence principle.

The nonattractor evolution in ultraslow-roll (USR) inflation results in the amplification of superhorizon curvature perturbations and then induces a strong and detectable stochastic gravitational wave background. In this paper, we search for such a stochastic gravitational wave background in data from the third LIGO-Virgo observing run and place constraints on the USR inflationary models. The $e$-folding number of the USR phase are constrained to be $\mathrm{\Delta }N\lesssim 2.9$ at the 95% confidence level and the power spectrum of curvature perturbations amplified during the USR phase is constrained to be ${\log }_{10}{P}_{R\mathrm{p}}<-1.7$ at the scales $2.9\times {10}^5{\mathrm{pc}}^{-1}\lesssim k\lesssim 1.7\times {10}^{11}{\mathrm{pc}}^{-1}$. Besides, we forecast the ability of future experiments to constrain USR inflation, and find $P_{R\mathrm{p}}\lesssim {10}^{-3.6}$ for LISA and Taiji, $P_{R\mathrm{p}}\lesssim {10}^{-3.3}$ for Cosmic Explore and Einstein Telescope, $P_{R\mathrm{p}}\lesssim {10}^{-5.5}$ for DECIGO and Big Bang Observer, and $P_{R\mathrm{p}}\lesssim {10}^{-5.2}$ for Square Kilometre Array.

We study the anisotropies of gravitational waves induced by weak hypermagnetic fields which are randomly distributed and oriented during the electroweak phase transition in the early Universe. The theory setup of this study is the Standard Model plus a real singlet scalar field, which can produce the needed strongly first-order electroweak phase transition. Then we investigate how the hypermagnetic fields can convert to magnetic fields, and we compute the departure of the energy difference between the symmetric phase and the broken phase when the magnetic fields are turned on. It is found that the presence of the hypermagnetic fields can increase the Euclidean action, and thus can decrease nucleation temperature, which can lead to a supercool plasma. We point out that the hypermagnetic field can enhance the gravitational wave production from a first-order electroweak phase transition, and the inhomogeneity of the primordial hypermagnetic field can lead to anisotropies of gravitational waves. By examining three well-motivated distributions of hypermagnetic fields, we calculate the corresponding angular power spectra of stochastic gravitational wave backgrounds and find that they can be significantly larger than the contributions of the Sachs-Wolfe effects and integrated Sachs-Wolfe effects. Our results show that the anisotropies of gravitational waves could provide a novel probe to the primordial hypermagnetic field in the electroweak phase transition epoch.

We investigate the skewness of galaxy number density fluctuations as a possible probe to test gravity theories. We find that the specific linear combination of the skewness parameters corresponds to the coefficients of the second-order kernels of the density contrast, which can be regarded as the consistency relation and used as a test of general relativity and modified gravity theories. We also extend the analysis of the skewness parameters from real space to redshift space and derive the redshift-space skewness consistency relation.

Recent interferometric observations by the Event Horizon Telescope have resolved the horizon-scale emission from sources in the vicinity of nearby supermassive black holes. Future space-based interferometers promise to measure the “photon ring”—a narrow, ring-shaped, lensed feature predicted by general relativity, but not yet observed—and thereby open a new window into strong gravity. Here we present aart: an adaptive analytical ray-tracing code that exploits the integrability of light propagation in the Kerr spacetime to rapidly compute high-resolution simulated black hole images, together with the corresponding radio visibility accessible on very long space-ground baselines. The code samples images on a nonuniform adaptive grid that is specially tailored to the lensing behavior of the Kerr geometry and is therefore particularly well-suited to studying photon rings. This numerical approach guarantees that interferometric signatures are correctly computed on long baselines, and the modularity of the code allows for detailed studies of equatorial sources with complex emission profiles and time variability. To demonstrate its capabilities, we use aart to simulate a black hole movie of a stochastic, nonstationary, nonaxisymmetric equatorial source; by time-averaging the visibility amplitude of each snapshot, we are able to extract the projected diameter of the photon ring and recover the shape predicted by general relativity.

The gravitational waves emitted by massive black hole binaries in the Laser Interferometer Space Antenna (LISA) band can be lensed. Wave-optics effects in the lensed signal are crucial when the Schwarzschild radius of the lens is smaller than the wavelength of the radiation. These frequency-dependent effects can enable us to infer the lens parameters, possibly with a single detection alone. In this work, we assess the observability of wave-optics effects with LISA by performing an information-matrix analysis using analytical solutions for both point-mass and singular isothermal sphere lenses. We use gravitational-waveform models that include the merger, ringdown, higher harmonics, and aligned spins to study how waveform models and source parameters affect the measurement errors in the lens parameters. We find that previous work underestimated the observability of wave-optics effects and that LISA can detect lensed signals with higher impact parameters and lower lens masses.

Indirect detection opens a unique window for probing thermal dark matter (DM): the same annihilation process that determined the relic abundance in the early Universe drives the present day astrophysical signal. While TeV-scale particles weakly coupled to the Standard Model face undoubted challenges from decades of null searches, the scenario remains compelling, and simple realizations such as Higgsino DM remain largely unexplored. The fate of such scenarios could be determined by gamma-ray observations of the center of the Milky Way with imaging atmospheric Cherenkov telescopes (IACTs). We consider the ultimate sensitivity of current IACTs to a broad range of TeV-scale DM candidates—including specific ones such as the wino, Higgsino, and quintuplet. To do so, we use realistic mock H.E.S.S.-like observations of the inner Milky Way halo, and provide a careful assessment of the impact of recent Milky Way mass modeling, instrumental and astrophysical background uncertainties in the Galactic Center region, and the theoretical uncertainty on the predicted signal. We find that the dominant systematic for IACT searches in the inner Galaxy is the unknown distribution of DM in that region, however, beyond this the searches are currently statistically dominated indicating a continued benefit from more observations. For two-body final states at 1 TeV, we find a H.E.S.S.-like observatory is sensitive to $⟨\sigma v⟩\sim 3\times {10}^{-26}-4\times {10}^{-25}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$, except for neutrino final states, although we find results competitive with ANTARES. In addition, the thermal masses for the wino and quintuplet can be probed; the Higgsino continues to be out of reach by at least a factor of a few. Our conclusions are also directly relevant to the next generation Cherenkov Telescope Array, which remains well positioned to be the discovery instrument for thermal DM.

Standard sirens—gravitational wave (GW) sources with an electromagnetic (EM) counterpart—can be used to measure the Hubble constant directly which should help to ease the existing Hubble tension. However, if the source has a relative velocity to the expanding universe on top of its motion due to the Hubble flow, a relativistic redshift affects the redshift of the EM counterpart and the apparent distance of the GW source, and thus it needs to be corrected to obtain accurate measurements. We study the effect of such a relative velocity on GWs for a source in an expanding universe showing that the total redshift of the wave is equal to the product of the relativistic redshift and the cosmological redshift. We further find that a relative velocity of the source changes its apparent distance by a factor $(1+z_{\mathrm{rel}}{)}^2$ in contrast to a linear factor for the cosmological redshift. We discuss that the additional factor for the relativistic redshift is a consequence of a velocity-dependent amplitude for GWs. We consider the effect of the relative velocity on the chirp mass and the apparent distance of the source an observer would infer when ignoring this velocity. We find that for different astrophysical scenarios the error, i.e., the deviation between the value inferred and the actual value, can range between 0.1% and 7% for the chirp mass while the error in the apparent distance can be between 0.25% and 15%. Furthermore, we consider the error introduced in the measurement of the Hubble constant using standard sirens for two cases: (i) when the effect of velocity on the redshift of the EM counterpart is considered but not on the apparent distance obtained from GWs and (ii) when the effect of the relative velocity is ignored completely. We find that in the first case the error can reach 1% for a source moving due to the peculiar velocity of its host galaxy and that in the second case the error can be more than 5% for a source at the distance of GW150914 with the same velocity.

We show that dark-matter candidates with large masses and large nuclear interaction cross sections are detectable with terrestrial radar systems. We develop our results in close comparison to successful radar searches for tiny meteoroids, aggregates of ordinary matter. The path of a meteoroid (or suitable dark-matter particle) through the atmosphere produces ionization deposits that reflect incident radio waves. We calculate the equivalent radar echoing area or “radar cross section” for dark matter. By comparing the expected number of dark-matter-induced echoes with observations, we set new limits in the plane of dark-matter mass and cross section, complementary to preexisting cosmological limits. Our results are valuable because (A) they open a new detection technique for which the reach can be greatly improved and (B) in case of a detection, the radar technique provides differential sensitivity to the mass and cross section, unlike cosmological probes.

Binary neutron star (BNS) postmerger gravitational-wave emission can occur in the aftermath of a BNS merger—provided the system avoids prompt collapse to a black hole—as a quasistable hypermassive remnant experiences quadrupolar oscillations and nonaxisymmetric deformations. The postmerger gravitational-wave spectrum possesses a characteristic peak frequency that has been shown to be dependent on the binary chirp mass and the neutron star equation of state (EOS), rendering postmerger gravitational waves a powerful tool for constraining neutron star composition. Unfortunately, the BNS postmerger signal is emitted at high ($\gtrsim 1.5\mathrm{kHz}$) frequencies, where ground-based gravitational-wave detectors suffer from reduced sensitivity. It is therefore unlikely that postmerger signals will be detected with sufficient signal-to-noise ratio (SNR) until the advent of next-generation detectors. However, by employing empirical relations derived from numerical relativity simulations, we can combine information across an ensemble of BNS mergers, allowing us to obtain EOS constraints with many low-SNR signals. We present a hierarchical Bayesian method for deriving constraints on $R_{1.6}$, the radius of a $1.6M_{\odot }$ neutron star, through an ensemble analysis of binary neutron star mergers. We apply this method to simulations of the next two LIGO-Virgo-KAGRA observing runs, O4 and O5, as well as an extended four-year run at $\mathrm{A}+$ sensitivity, demonstrating the potential of our approach to yield EOS information from the postmerger signal with current-generation detectors. The $\mathrm{A}+$ four-year scenario is predicted to improve the constraint on $R_{1.6}$ from the currently available multimessenger-based 95% credible interval (C.I.) uncertainty of $R_{1.6}=12.07_{-0.77}^{+0.98}$ to $R_{1.6}=11.91_{-0.56}^{+0.80}\mathrm{km}$, a 22% reduction of the 95% C.I. width.

The discovery of Standard-Model like Higgs at 125 GeV may raise more questions than the answers it provides. In particular, the hierarchy problem remains unsolved, and the Standard Model Higgs quartic self-coupling becomes negative below the Planck scale, necessitating new physics beyond the Standard Model. In this work we investigate a popular scenario, extensions of the Standard Model with vectorlike fermion fields, such as the ones present in models with extra dimensions or in Higgs composite models, using a model independent approach. Since fermions decrease the Higgs quartic coupling at high energies, only exacerbating the self-coupling problem, we introduce first an additional scalar, which by itself is enough to overcome the vacuum stability limit, and then explore the effects of vectorlike fermions in singlet, doublet and triplet representations. For each model, we identify the allowed fermion masses and mixing angles with the third family fermions required to satisfy the vacuum stability condition, and compare different representations. Allowed fermion masses emerge at around 1 TeV, raising hope that these will be found at the LHC. We also examine corrections to oblique parameters $\mathbb{S}$ and $\mathbb{T}$ from additional scalar and vectorlike quarks which also impose constraints on mixing and mass splitting of both sectors, but these restrictions are relatively weak compared to the vacuum stability.

In this work we present an attempt to describe the $X_1(2900)$ found by the LHCb collaboration, in the experimental data on the invariant mass spectrum of $D^{-}{K}^{+}$, as a three-meson molecular state of the $K\rho \overline{D}$ system. We discuss that the interactions in all the subsystems are attractive in nature, with the $\rho \overline{D}$ interaction generating ${\overline{D}}_1(2420)$ and the $K\rho$ resonating as $K_1(1270)$. We find that the system can form a three-body state but with a mass higher than that of $X_1(2900)$. We investigate the $K\rho D$ system too, finding that the three-body dynamics generates an isoscalar state, which can be related to $D_{s1}^{*}(2860)$, and an exotic isovector state. This latter state has a mass similar to that of the $X_0(2900)$ and $X_1(2900)$ states found by LHCb, but a very small width ($\sim 7.4\pm 0.9\mathrm{MeV}$) and necessarily requires more than two quarks to describe its properties. We hope that our findings will encourage experimental investigations of the isovector $K\rho D$ state. Finally, in the pursuit of finding a description for $X_1(2900)$, we study the $\pi {\overline{K}}^{*}{D}^{*}$ system where ${\overline{K}}^{*}{D}^{*}$ forms $0^{+}$, $1^{+}$, and $2^{+}$ states. We do not find a state that can be associated with $X_1(2900)$.