It is conventionally assumed that the physics underlying the Peccei-Quinn (PQ) mechanism for addressing the strong $CP$ problem is at very high energies, orders of magnitude above the weak scale. However, this may not be the case in general and the associated PQ boson $\varphi$, besides the signature state, i.e., the ultralight axion $a$, may emerge well below the weak scale. We consider this possibility and examine some of the conditions for its viability. The example model proposed here may also provide the requisite Standard Model Higgs mass parameter, without invoking new scalars above the giga-electron-volt (GeV) scale. The corresponding parameter space can maintain finite naturalness against quantum corrections. This scenario, depending on the choice of parameters, can potentially be constrained by flavor data. We point out that the current mild excess in $B^{+}\to K^{+}\nu \overline{\nu }$, reported by the Belle II experiment, could in principle be explained in this setup as $B^{+}\to K^{+}\varphi$ and $B^{+}\to K^{+}a$, with both $\varphi$ and $a$ escaping the detector as missing energy. For a sufficiently heavy PQ boson, in the GeV regime (as the preferred explanation of the Belle II excess), one can separate these two contributions, due to the difference in $K^{+}$ momenta. In this case, the axion may also affect lighter meson, e.g., kaon, decays while $\varphi$ would not be a kinematically allowed final state.

We develop a framework to compute the tidal response of a Kerr-like compact object in terms of its reflectivity, compactness, and spin, both in the static and the frequency-dependent case. Here we focus on the low-frequency regime, which can be solved fully analytically. We highlight some remarkable novel features, in particular, the following: (i) Even in the zero-frequency limit, the tidal Love numbers (TLNs) depend on the linear-in-frequency dependence of the object’s reflectivity in a nontrivial way. (ii) Intriguingly, although the static limit of the (phenomenologically more interesting) frequency-dependent TLNs is continuous, it differs from the strictly static TLNs for compact objects other than black holes. This shows that earlier findings regarding the static TLNs of ultracompact objects correspond to a measure-zero region in the parameter space, though the logarithmic behavior of the TLNs in the black hole limit is retained. (iii) In the nonrotating case, the TLNs generically vanish in the zero-frequency limit (just like for a black hole), except when the reflectivity is $\mathcal{R}=1+\mathcal{O}(M\omega )$, in which case they vanish with a model-dependent scaling, which is generically logarithmic, in the black-hole limit. The TLNs initially grow with frequency, for any nonzero reflectivity, and then display oscillations and resonances tied up with the quasinormal modes of the object. (iv) For rotating compact objects, the TLNs decrease when the reflectivity decreases or the rotation parameter increases. Our results lay the theoretical groundwork to develop model-independent tests of the nature of compact objects using tidal effects in gravitational-wave signals.

Asymmetric emission of gravitational waves during a compact binary coalescence results in the loss of linear momentum and a corresponding “kick” or recoil on the binary’s center of mass. This leads to a direction-dependent Doppler shift of the ringdown gravitational waveform. We quantify the measurability of the kick imparted to the remnant black hole in a binary black hole merger. Future ground- and space-based gravitational-wave detectors will measure this effect to within $\sim 2\%$ to $\sim 30\%$ for a subset of their expected observed sources. Certain binary configurations in the LISA band may allow a sub-percent-level measurement of this effect. This direct measurement of black hole kicks can also facilitate a novel test of general relativity based on linear momentum balance. We formulate this kick consistency test via measurement of a null variable that quantifies the difference between the inferred kick (using numerical relativity) and that observed via the Doppler-shifted ringdown signal. This null variable can be constrained (at 90% confidence) to $\sim 10\%$ to 30% with Cosmic Explorer and to $\sim 3\%$ to 12% with LISA.

With the strong evidence for a gravitational wave (GW) background in the nanohertz frequency band from pulsar timing arrays, the detection of continuous GWs from individual supermassive black hole binaries is already at the dawn. Utilizing continuous GWs to test theories of gravity, especially to test the polarizations of GWs is becoming more and more realistic. In this theoretical study, assuming a detection of signals from individual supermassive binary black holes, we use the null stream to estimate the capability of identifying the nontensorial polarizations of GWs. We consider cases for the nontensorial polarizations where the dipole radiation and quadrupole radiation dominate separately. With a frequentist method, we estimate the threshold of the nontensor-to-tensor relative amplitude above which extra polarizations can be detected. We also conduct Bayesian analysis to estimate parameters with the null stream data. Our treatment provides a data-analysis methodology using the null stream to probe the nontensorial GW polarizations with pulsar timing arrays.

We show that quantum dynamics of any systems with $SU(1,1)$ symmetry are governed by the same underlying geometry of an anti-de Sitter spacetime in $2+1$ dimensions (${\mathrm{AdS}}_{2+1}$). Using complexity geometry, quantum evolutions are mapped to trajectories in ${\mathrm{AdS}}_{2+1}$. Whereas the time measured in laboratories becomes either the proper time or the proper distance, quench dynamics follow the geodesics of ${\mathrm{AdS}}_{2+1}$. Such a geometric approach provides us with a unified interpretation of a wide range of prototypical phenomena that appear disconnected. For instance, the light cone of ${\mathrm{AdS}}_{2+1}$ underlies expansions of unitary fermions released from harmonic traps, the onsite of parametric amplifications, and the exceptional points that represent the $PT$ symmetry breaking in non-Hermitian systems. Our work provides a transparent means to optimize quantum controls by exploiting the shortest paths in the complexity geometry. It also allows experimentalists to engineer the corresponding geometries by manipulating the quantum dynamics.

We derive the cutoff length scale of the quadratic gravity in $d\ge 5$ dimensional spacetime by demanding that the quantum focusing conjecture for the smeared quantum expansion holds at the classical level. The cutoff scale has a different dependence on the spacetime dimension depending on the sign of the coupling constant of the quadratic gravity. We also investigate a concrete example of the five-dimensional Schwarzschild spacetime and directly confirm that the quantum focusing conjecture holds when the quantum expansion is smeared over the scale larger than our cutoff scale.

We show that higher form fields, specifically 2- and 3-forms in four spacetime dimensions, suffer from loss of hyperbolicity when they have self interaction. The equations of motion lose their wavelike characteristic in certain parts of the configuration space, and such problematic regions can be reached dynamically from healthy initial data. These findings are analogous to recent results for vector fields, which are equivalently 1-forms, however, the details of the pathology can be different for higher forms. We also show that the possibility of more than one self-interaction term in higher form fields leads to a variety in time evolution characteristics, where hyperbolicity is lost in different parts of the configuration space as the interaction changes.

Searches for gravitational wave events use models, or templates, for the signals of interest. The templates used in current searches in the LIGO-Virgo-KAGRA data model the dominant quadrupole mode $(\ell ,|m|)=(2,2)$ of the signals and omit subdominant higher-order modes (HMs) such as $(\ell ,|m|)=(3,3)$, (4, 4), which are predicted by general relativity. This omission reduces search sensitivity to black hole mergers in interesting parts of parameter space, such as systems with high masses and asymmetric-mass ratios. We develop a new strategy to include HMs in template banks: instead of making templates containing a combination of different modes, we separately store normalized templates corresponding to (2, 2), (3, 3), and (4, 4) modes. To model aligned-spin (3, 3), (4, 4) waveforms corresponding to a given (2, 2) waveform, we use a combination of post-Newtonian formulas and machine learning tools. In the matched-filtering stage, one can filter each mode separately with the data and collect the time series of signal-to-noise ratios (SNRs). This leads to a HM template bank whose matched-filtering cost is just $\approx 3\times$ that of a quadrupole-only search (as opposed to $\approx 100\times$ in previously proposed HM search methods). Our method is effectual and generally applicable for template banks constructed with either stochastic or geometric placement techniques. New gravitational wave candidate events that we detect using our HM banks and details for combining the different SNR mode time series are presented in accompanying papers [ et al., arXiv:2312.06631; et al., Phys. Rev. D 110, 044063 (2024)]. Additionally, we discuss nonlinear compression of (2, 2)-only geometric placement template banks using machine learning algorithms.

The ringdown gravitational wave signal arising, e.g., in the final stage of a black hole binary merger, contains important information about the properties of the remnant and can potentially be used to perform clean tests of general relativity. However, interpreting the ringdown signal, in particular, when it is the loudest, requires understanding the role of nonlinearities and their potential impact on modeling this phase using quasinormal modes. Here, we focus on a particular nonlinear effect arising from the change in the black hole’s mass and spin due to the partial absorption of a quasinormal perturbation. We isolate and systematically study this third-order, secular effect by evolving the equations governing linear metric perturbations on the background of a spinning black hole, but allowing the properties of the background to evolve in a prescribed way. We find that this leads to the excitation of quasinormal modes with higher polar angular number, retrograde modes (counterrotating with respect to the black hole), and overtones, as well as giving rise to a component of the signal at early times that cannot be fully described using quasinormal modes. Quantifying these effects, we find that they may be relevant in analyzing the ringdown in black hole mergers.

We present a coordinate-free background space construction of Euclidean Jackiw-Teitelboim gravity. It is written as a gauge theory obtained from the Killing vectors and conformal Killing vectors of a hyperboloid embedded in a three-dimensional background. A novel feature of the gauge theory is that vanishing field strength does not necessarily imply that the gauge potentials (written in the embedding space) are pure gauges, not even locally. On the other hand, the projection of the potentials along the hyperboloid are pure gauges, as is the case in the standard approach. As is usual, metric tensors are dynamically generated from the classical solutions of the theory, which here do not rely on coordinate charts on the two-dimensional surface. We find a special class of solutions whereby the derived metric tensor on the surface is the induced metric from the background space. The gauge theory construction given here has a natural generalization to a noncommutative space, which does not require the use of coordinates, symbols, or a star product.

In the post-Newtonian (PN) expansion, we extend the determination of quasicircular orbital parameters to be used by subsequent full numerical simulations to the 3.5PN order, and find that this leads to lower eccentricities, $e$, than with our previous method that used up to 3PN order. We also supplement the computation of the radial infall due to radiation reaction and the location of the center of mass to 3.5PN order, providing explicit formulas. In addition, we consider the small mass ratio limit by explicitly including the Schwarzschild and Kerr limits, the later in quasi-isotropic as well as in our standard use of Arnowitt-Deser-Misner transverse-traceless (ADMTT) coordinates. We evolve binaries with a $q=1/16$ mass ratio by using 3PN, 3.5PN, $3.5\mathrm{PN}+\mathrm{Schwarzschild}$, $3.5\mathrm{PN}+\mathrm{KerrQISO}$ and $3.5\mathrm{PN}+\mathrm{KerrADMTT}$ quasicircular data for three different configurations where the larger hole intrinsic spins are ${\chi }^z=-0.8$, $-0.4$ and $+0.8$. Using different measures of eccentricity from the black hole trajectories and from the waveform amplitudes and phases, we determine a systematic reduction of eccentricities with respect to the 3PN initial values by factors of up to an order of magnitude, and reaching the desired $e\sim {10}^{-3}$ threshold.

In this work, static and spherically symmetric solutions of the general relativity coupled to linear/nonlinear electrodynamics and a dust fluid (GR-ED-DF) are studied. We demonstrate that these solutions can be categorized into two sets, both having an electromagnetically charged metric, but with the following conditions: (i) a variable redshift function, without a dust fluid as source, and (ii) a constant redshift function, with a dust fluid as source. Using (i), we provide a simple proof of the nonexistence of static and spherically symmetric traversable Morris-Thorne wormhole solutions with variable redshift functions in GR-ED-DF. Whereas using (ii), we construct several static and spherically symmetric traversable Morris-Thorne wormholes with constant redshift function in GR-ED-DF where the source of gravity consists of a dust fluid having negative energy density and an electromagnetic field described by a physically reasonable model of linear/nonlinear electrodynamics with Lagrangian density $\mathcal{L}(\mathcal{F})$ depending only on the electromagnetic invariant $\mathcal{F}=F_{\alpha \beta }{F}^{\alpha \beta }/4$ where $F_{\alpha \beta }$ are the components of the electromagnetic field tensor. We show that in the limit of the weak electromagnetic field, each of our solutions become a traversable wormhole of the electromagnetically charged Ellis-Bronnikov wormhole type. Additionally, we present a theorem that establishes when an electrically and magnetically charged Ellis-Bronnikov wormhole, with null $\mathcal{F}$, can be supported by a GR-ED-DF theory.

A generic black hole merger occurs through a restructuring of creases (sharp edges) on the event horizon. This process is studied for a black hole merger in the limit of infinite mass ratio, for which constructing the event horizon reduces to finding a null hypersurface that asymptotes to a Rindler horizon in the Kerr spacetime. Geometrical properties of the creases on this horizon are determined and the results are compared with the predictions of an exact local description of the event horizon in a generic merger. The crease set is shown to have finite area. A recently proposed expression for the gravitational entropy of a crease is shown to diverge at the instant of merger. Caustics on (and off) the event horizon are determined by exploiting the correspondence with the problem of gravitational lensing by a Kerr black hole. Caustics form an “astroid tube” in spacetime, two edges of which lie on the event horizon of the merger. Perturbative expressions for the location of this tube are presented, extending previous work to much higher perturbative order.

We investigate compact stars in the noninteger power (NIP) model of $f(R)$ gravity theory, which includes the higher-curvature correction to the Einstein-Hilbert action. The mass-radius relation of the compact stars in the NIP model predicts large deviations from those in the general relativity in the low-mass region, potentially allowing us to test the NIP model by future astrophysical observations. We also study the nonvanishing scalar hair surrounding the compact star and demonstrate that the chameleon mechanism works efficiently. They result in distinct scalar profiles inside and outside the star, which implies screening the fifth force mediated by the scalar field.

In the study of rotating neutron stars the $I$-Love-$Q$ relations refer to the existence of various approximate, equation of state-independent relations involving the moment of inertia, the Love number and the quadrupole moment. These relations are relevant for observational astrophysics, since they allow (in theory) the inference of any two quantities within the $I$-Love-$Q$ triad out of the third one alone. However, the quantities involved in the relations are, in fact, normalized by a parameter $M_0$ that arises in the usual perturbative analytical approach as the mass of the background configuration. Since $M_0$ is not the mass of the rotating star $M_S$, it is not an observational quantity, which may affect the application of the relations to actual observations. This situation is usually ignored in most studies by taking $M_0$ to be the mass of the star, an approximation that can, in some cases, be inconsistent. In this paper we extract the value of $M_0$ using an extended version of the universal relations that involves a fourth parameter, $\delta M$, proportional to the difference $M_S-M_0$. We analyze to which degree this extended set of relations yields a more precise inference of compact star properties and equation of state parameters.

Nonsingular horizonless ultracompact objects provide a simple resolution to the black holes singularity problem. It has been shown that, if these objects are compact enough to exhibit the presence of the light ring required to mimic the phenomenology of general relativity black holes, they must have at least one additional light ring. The stability of the inner light ring has been proven under the assumptions of Einstein equations and the validity of the null energy condition. Since this can have important repercussions on the instability of a horizonless ultracompact object and the existence of the latter requires some modified gravitational dynamics and/or exotic matter, it is desirable to obtain a model-independent proof of the stability of the additional light ring. In this paper, we prove the stability of the inner light ring without any assumption on the dynamics of the theory, while assuming that the outer light ring has the same properties as the Kerr light ring. Given the stringent observational constraints on the geometry at the outer light ring scale, our result now rests solely on geometric considerations and applies to any theory of gravity with any matter content that cannot be ruled out by observations.

This work investigates the impact of different modified gravity (MG) models on the large-scale structures (LSS) properties in relation to the cosmic web (CW), using $N$-body simulations of $f(R)$ and normal branch of Dvali-Gabadadze-Porrati (nDGP) models. We analyze the impact of the MG effect on the density field through density distribution and clustering statistics, and assess its influence on halo properties by examining the halo mass function and spin. We find that the probability density function of dark matter density fields shift toward lower densities for stronger variants of $f(R)$ and nDGP. Additionally, when segregated into CW environments, the stronger variants show a higher mean density in knots, and a lower mean density in voids compared to $\mathrm{\Lambda }\mathrm{CDM}$. For higher-order clustering statistics relative to $\mathrm{\Lambda }\mathrm{CDM}$, the scale-dependent $f(R)$ variants exhibit a greater nonmonotonic deviation as a function of scale when segregated into environments, compared to nDGP. Additionally, the halo mass function separated into CW environments shows a similar behavior, introducing complex trends as a function of mass for $f(R)$ and nDGP models. We also report up to a $\sim 15\%$ enhancement in the angular momentum of halos in $f(R)$ gravity models compared to $\mathrm{\Lambda }\mathrm{CDM}$, with similar differences when considering environmental segregation. We demonstrate that this difference in the spin arises largely due to different tidal torquing across the various MG models. Therefore, studying higher-order statistics of the cosmological fields and halo properties separated into CW components probes the additional physics contained within the MG models. We conclude that considering the effect of CW in MG studies increases the constraining power of these LSS statistics, and can further aid the distinction between the cosmologies that have an identical expansion history to the standard $\mathrm{\Lambda }\mathrm{CDM}$ but differing underlying physics, such as the MG models presented in this work.

We probed the capability of third-generation gravitational wave (GW) interferometers, such as the Einstein Telescope and Cosmic Explorer, to constrain a cosmological model with an interacting dark sector. We focused on GW events with a detected electromagnetic counterpart being the $\gamma$ or x emission of gamma-ray burst, and a kilonova emission. We assumed that the first one is detected by the THESEUS satellite while the second one is detected by the Vera Rubin Observatory. We probed three different interaction kernels and found that the posterior estimation of the cosmological parameters is biased due to the existing degeneracies between the dark and matter sectors. We also found that introducing an external prior on the matter density parameter breaks the degeneracy, removes the bias results, and improves the accuracy on the dark sector parameters.

In this paper, we present a novel approach to accelerate the Bayesian inference process, focusing specifically on the nested sampling algorithms. Bayesian inference plays a crucial role in cosmological parameter estimation, providing a robust framework for extracting theoretical insights from observational data. However, its computational demands can be substantial, primarily due to the need for numerous likelihood function evaluations. Our method utilizes the power of deep learning, employing feed forward neural networks to approximate the likelihood function dynamically during the Bayesian inference process. Unlike traditional approaches, our method trains neural networks on-the-fly using the current set of live points as training data, without the need for pretraining. This flexibility enables adaptation to various theoretical models and datasets. We perform the hyperparameter optimization using genetic algorithms to suggest initial neural network architectures for learning each likelihood function. Once sufficient accuracy is achieved, the neural network replaces the original likelihood function. The implementation integrates with nested sampling algorithms and has been thoroughly evaluated using both simple cosmological dark energy models and diverse observational datasets. Additionally, we explore the potential of genetic algorithms for generating initial live points within nested sampling inference, opening up new avenues for enhancing the efficiency and effectiveness of Bayesian inference methods.

In this work, we develop a simulation-based model to predict the density split (DSS) and second-order shear and clustering statistics. A simulation-based model has the potential to model highly non-linear scales where current DSS models fail. To build this model, we use the abacussummit N-body simulation suite from which we measure all necessary statistics and train an emulator based on cosmopower. In that context, we discuss possible improvements for future emulators to make the measurement less noisy and biased, resulting in more accurate and precise model predictions. Regarding the emulator’s accuracy, we find that the most important aspect is the average of the summary statistics over multiple-shot noise realizations of the foreground galaxies. However, these results probably depend on the chosen number density of the foreground galaxies. Regarding the parameter forecast based on preliminary LOWZxUNIONS data, we find that DSS has more constraining power to derive cosmological parameters than second-order statistics and that the joint analysis with second-order statistics is particularly useful for extracting parameters of the galaxy-halo connection.