In the present work, we construct black-bounce configurations in the context of $k$-essence theory. The solutions have a regular metric function at the origin. The area metric function is linked to the black-bounce area initially considered by Simpson-Visser, ${\mathrm{\Sigma }}^2=x^2+a^2$. Subsequently, the expressions for the scalar field and scalar potential corresponding to the found solutions are determined, exhibiting phantom behavior everywhere due to violation of the null energy condition $({\mathrm{NEC}}^{\varphi })$. The Kretschmann scalar is regular in spacetime, and the geodesics are complete. The energy conditions are analyzed, verifying that the null $({\mathrm{NEC}}_1^{\varphi })$ and dominant energy conditions $({\mathrm{DEC}}_1^{\varphi })$ are violated inside and outside the event horizon. Finally, the extrinsic curvature method is applied to determine the sign of the mass on the junction surface.

The intricacies of black hole ringdown analysis are amplified by the absence of a complete set of orthogonal basis functions for quasinormal modes. Although damped sinusoids effectively fit the ringdown signals from binary black hole mergers, the risk of overfitting remains, due to initial transients and nonlinear effects. In light of this challenge, we introduce two methods for extracting quasinormal modes in numerical simulations and qualitatively study how the transient might affect quasinormal mode fitting. In one method, we accurately fit quasinormal modes by using their spatial functional form at constant time hypersurfaces, while in the other method, we exploit both spatial and temporal aspects of the quasinormal modes. Both fitting methods leverage the spatial behavior of quasinormal eigenfunctions to enhance accuracy, outperforming conventional time-only fitting techniques at null infinity. We also show that we can construct an inner product for which the quasinormal eigenfunctions form an orthonormal (but not complete) set. We then conduct numerical experiments involving linearly perturbed Kerr black holes in horizon penetrating, hyperboloidally compactified coordinates, as this setup enables a more precise isolation and examination of the ringdown phenomenon. From solutions to the Teukolsky equation, describing scattering of an ingoing gravitational wave pulse, we find that the contributions from early-time transients can lead to large uncertainties in the fit to the amplitudes of higher overtones ($n\ge 3$). While the methods we discuss here cannot be applied directly to data from merger observations, our findings underscore the persistence of ambiguities in interpreting ringdown signals, even with access to both temporal and spatial information.

We revisit a question asked by Dyson: “Is a graviton detectable?” We demonstrate that in both Dyson’s original sense and in a more modern measurement-theoretic sense, it is possible to construct a detector sensitive to single gravitons, and in fact a variety of existing and near-term gravitational wave detectors can achieve this. However, while such a signal would be consistent with the quantization of the gravitational field, we draw on results from quantum optics to show how the same signal could just as well be explained via classical gravitational waves. We outline the kind of measurements that would be needed to demonstrate quantization of gravitational radiation and explain why these are substantially more difficult than simply counting graviton clicks or observing gravitational noise in an interferometer, and likely impossible to perform in practice.

Einstein-Maxwell-dilaton-axion (EMDA) gravity provides a simple framework to investigate the signatures of string theory. The axion and the dilaton fields arising in EMDA gravity have important implications in inflationary cosmology and in addressing the late time acceleration of the Universe. It is therefore instructive to explore the implications of such a model in explaining the astrophysical observations. The Kerr-Sen metric represents the exact, stationary, and axisymmetric black hole solution of EMDA gravity. Such a black hole is characterized by the angular momentum $a$ acquired from the axionic field and the dilatonic charge $r_2$ arising from string compactifications. We study the role of spin and the dilaton parameter in modifying the shape and size of the black hole critical curve, which is associated with the projection of the spherical null geodesics on the sky. We compare the theoretically derived critical curve with the Event Horizion Telescope results related to the images of M87* and Sgr A* to obtain constraints on the dilaton parameter $r_2$. We take into account the errors in mass and distance of M87* and Sgr A* while deriving their theoretical critical curve. Our analysis reveals that the image of M87* exhibits a preference toward the Kerr scenario when the critical curve angular diameter is calculated with the central value of mass and distance. When errors in mass and distance are taken into account the allowed range of $r_2$ turns out to be $0\lesssim r_2\lesssim 1$. For Sgr A*, the preferred range of $r_2$ is $0.1\lesssim r_2\lesssim 0.4$ when central values of mass and distance are used to calculate the theoretical critical curve. When error bars in mass and distance are used to calculate the theoretical critical curve of Sgr A*, the preferred range of $r_2$ turns out to be $0\lesssim r_2\lesssim 0.5$. Thus the image of M87* favors the Kerr scenario and allows the Kerr-Sen scenario only when errors in the mass and distance are taken into consideration while the image of Sgr A* favors the Kerr-Sen scenario and allows general relativity when errors in the mass and distance are taken into account.

We prove there is a unique vacuum solution in split-signature spacetimes with Kleinian $SO(2,1)$ spherical symmetry. We extend our analysis to accommodate a positive or negative cosmological constant and we prove the Kleinian spherically symmetric solutions to Einstein’s equation are locally isomorphic to the split-signature analogs of Schwarzschild–(anti)-de Sitter or Nariai spacetimes. Our analysis provides a Kleinian extension of Birkhoff’s theorem to metrics with split signature. Axisymmetric vacuum solutions are also considered, including (2,2) signature formulations of the Kerr and Taub-Newman-Unti-Tamburino metrics.

In this paper, we study the tidal effects in the gravitationally bound two-body system at next-to-next-to leading post-Newtonian order for spinless sources in massless scalar-tensor theories. We compute the conservative dynamics, using both a Fokker Lagrangian approach and effective field theory with the post-Newtonian effective field theory formalism. We also compute the ten conserved quantities at the same next-to-next-to leading order. Finally, we extend our results from simple scalar-tensor theories to Einstein-scalar-Gauss-Bonnet gravity. Such results are important in preparation of the science case of the next generation of gravitational wave detectors.

The nonisometric holographic model of the black hole interior stands out as a potential resolution of the long-standing black hole information puzzle since it remedies the friction between the effective calculation and the microscopic description. In this study, combining the final-state projection model, the nonisometric model of black hole interior, and Hayden-Preskill thought experiment, we investigate the information recovery from decoding Hawking radiation and demonstrate the emergence of the Page time in this setup. We incorporate the effective modes into the scrambling inside the horizon, which are usually disregarded in Hayden-Preskill protocols, and show that the Page time can be identified as the transition of information transmission channels from the EPR projection to the local projections. This offers a new perspective on the Page time. We compute the decoupling condition under which retrieving information is feasible and show that this model computes the black hole entropy consistent with the quantum extremal surface calculation. Assuming the full knowledge of the dynamics of the black hole interior, we show how Yoshida-Kitaev decoding strategy can be employed in the modified Hayden-Preskill protocol. Furthermore, we perform experimental tests of both probabilistic and Grover’s search decoding strategies on the seven-qubit IBM quantum processors to validate our analytical findings and confirm the feasibility of retrieving information in the nonisometric model. This study would stimulate more interest to explore black hole information problem on the quantum processors.

The era of precision cosmology allows us to test the composition of the dark matter. Mixed ultralight or fuzzy dark matter (FDM) is a cosmological model with dark matter composed of a combination of particles of mass $m\le {10}^{-20}\mathrm{eV}$, with an astrophysical de Broglie wavelength, and particles with a negligible wavelength sharing the properties of cold dark matter (CDM). In this work, we simulate cosmological volumes with a dark matter wave function for the ultralight component coupled gravitationally to CDM particles. We investigate the impact of a mixture of CDM and FDM in various proportions (0%, 1%, 10%, 50%, 100%) and for ultralight particle masses ranging over five orders of magnitude ($2.5\times {10}^{-25}\mathrm{eV}–2.5\times {10}^{-21}\mathrm{eV}$). To track the evolution of density perturbations in the nonlinear regime, we adapt the simulation code axionyx to solve the CDM dynamics coupled to a FDM wave function obeying the Schrödinger-Poisson equations. We obtain the nonlinear power spectrum and study the impact of the wave effects on the growth of structure on different scales. We confirm that the steady-state solution of the Schrödinger-Poisson system holds at the center of halos in the presence of a CDM component when it composes 50% or less of the dark matter but find no stable density core when the FDM accounts for 10% or less of the dark matter. We implement a modified friends-of-friends halo finder and find good agreement between the observed halo abundance and the predictions from the adapted halo model axionhmcode.

We study the cosmological implications of gravity models which break diffeomorphisms (Diff) invariance down to transverse diffeomorphisms (TDiff). We start from the most general gravitational action involving up to quadratic terms in derivatives of the metric tensor and identify TDiff models as the only stable theories consistent with local gravity tests. These models propagate an additional scalar graviton and although they are indistinguishable from GR at the post-Newtonian level, their cosmological dynamics exhibits a rich phenomenology. Thus we show that the model includes standard $\mathrm{\Lambda }\mathrm{CDM}$ as a solution when the extra scalar mode is not excited, but different cosmological evolutions driven by the new term are possible. In particular, we show that for a soft Diff breaking, the new contribution always behaves as a cosmological constant at late times. When the extra contribution is not negligible, generically its evolution either behaves as dark energy or tracks the dominant background component. Depending on the initial conditions, solutions in which the universe evolves from an expanding to a contracting phase, eventually recollapsing, are also possible.

We investigate the imprints of accretion and dynamical friction on the gravitational-wave signals emitted by binary black holes embedded in a scalar dark matter cloud. As a key feature in this work, we focus on scalar fields with a repulsive self-interaction that balances against the self-gravity of the cloud. To a first approximation, the phase of the gravitational-wave signal receives extra correction terms at $-3\mathrm{PN}$, $-4\mathrm{PN}$, and $-5.5\mathrm{PN}$ orders, relative to the prediction of vacuum general relativity, due to cloud gravity, accretion and dynamical friction. Future observations by LISA and DECIGO have the potential to detect these effects for a large range of scalar masses $m_{\mathrm{DM}}$ and self-interaction couplings ${\lambda }_4$. This would correspond to scenarios with dark matter clouds smaller than 0.1 pc, which would be difficult to detect by other probes.

The existing matched filtering method for gravitational wave (GW) search relies on a template bank. The computational efficiency of this method scales with the sizes of the templates within the bank. Higher-order modes and eccentricity will play an important role when third-generation detectors operate in the future. In this case, traditional GW search methods will hit computational limits. To speed up the computational efficiency of GW searches, we propose the utilization of a deep learning (DL) model bank as a substitute for the template bank. This model bank predicts the latent templates embedded in the strain data. Combining an envelope extraction network and an astrophysical origin discrimination network, we realize a novel GW search framework. The framework can predict the GW signal’s matched filtering signal-to-noise ratio (SNR). Unlike the end-to-end DL-based GW search method, our statistical SNR holds stronger physical interpretability than the $p_{\text{score}}$ metric. Moreover, the intermediate results generated by our approach, including the predicted template, offer valuable assistance in subsequent GW data processing tasks such as parameter estimation and source localization. Compared to the traditional matched filtering method, the proposed method can realize real-time analysis.

We study the role of perturbative unitarity in the resonant annihilation of two dark matter particles into the standard model bath. Systematically including all kinematically allowed holomorphic cuts of the corresponding forward-scattering diagram, cancellation of the singularities occurs, resulting in a fixed-order correction to the narrow-width approximation for the annihilation cross section. Unlike the standard approach based on including the finite width of the mediator, no double counting of intermediate states occurs.

The origin and nature of ultrahigh-energy cosmic rays remains a mystery despite the great progress that has been made in recent years mainly due to the observations performed by the Pierre Auger Observatory and Telescope Array. In particular, the composition information of the cosmic rays as a function of the energy appears to be crucial for the understanding of their origin. The best observables for primary mass composition are the muon content of extensive air showers and the atmospheric depth of the shower maximum development. In this work we present a novel method which allows us to use any type of classification method to perform mass composition analyses based on the number of muons. The method also provides the information of the mean values of this observable corresponding to different primary particles. The analyses are based on numerical simulations of the showers but including experimental uncertainties in the reconstruction of the energy and in the measurement of the number of muons. We also study the impact of the use of different high-energy hadronic interaction models in the composition analyses performed. The biases introduced by the use of different high-energy hadronic interaction models to analyze the data obtained with the presented method are considerably reduced compared with those obtained with classic statistical methods that use a single physical observable.

Starting with on-shell amplitudes compatible with the scattering of Kerr black holes up to Compton-amplitude contact terms, we produce the gravitational waveform and memory effect including spin at their leading post-Minkowskian orders to all orders in the spins of both scattering objects. For the memory effect, we present results at next-to-leading order as well, finding a closed form for all spin orders when the spins are anti-aligned and equal in magnitude. Considering instead generically oriented spins, we produce the next-to-leading-order memory to sixth order in spin. Compton-amplitude contact terms up to sixth order in spin are included throughout our analysis.

First-order relativistic conformal hydrodynamics in a general (hydrodynamic) frame is characterized by a shear viscosity coefficient and two UV-regulator parameters. Within a certain range of these parameters, the equilibrium is stable and propagation is causal. In this work we study the correlation functions of fluctuations in this theory. We first compute hydrodynamic correlation functions in the linear response regime. Then we use the linear response results to explore the analytical structure of response functions beyond the linear response. A method is developed to numerically calculate the branch-cut structure from the well-known Landau equations. We apply our method to the shear channel and find the branch cuts of a certain response function, without computing the response function itself. We then solve the Landau equations analytically and find the threshold singularities of the same response function. Using these results, we achieve the leading singularity in momentum space, by which, we find the long-time tail of the correlation function. The results turn out to be in complete agreement with the loop calculations in effective field theory.

We propose a relativistic theory for spin density matrices of vector mesons based on Kadanoff-Baym equations in the closed-time-path formalism. The theory puts the calculation of spin observables such as the spin density matrix element ${\rho }_{00}$ for vector mesons on a solid ground. Within the theory we formulate ${\rho }_{00}$ for $\varphi$ mesons into a factorization form in separation of momentum and spacetime variables. We argue that the main contribution to ${\rho }_{00}$ at lower energies should be from the $\varphi$ fields that can polarize the strange quark and antiquark in the same way as electromagnetic fields. The key observation is that there is correlation inside the $\varphi$ meson wave function between the $\varphi$ field that polarizes the strange quark and that polarizes the strange antiquark. This is reflected by the fact that the contributions to ${\rho }_{00}$ are all in squares of fields that are nonvanishing even if the fields may strongly fluctuate in spacetime. The fluctuation of strong force fields can be extracted from ${\rho }_{00}$ of unflavored vector mesons as links to fundamental properties of quantum chromodynamics.

In this work, the transition form factors of the semileptonic decays of $B_{(s)}$ to the light scalar mesons with masses close to 1.5 GeV such as $K_0^{*}(1430),a_0(1450)$, and $f_0(1500)$ are calculated in the framework of the light-cone sum rules. For this purpose, the two- and three-particle $B$-meson distribution amplitudes (DAs) are used. Note that it is possible to use the $B$-meson DAs for the $B_s$ meson in the $\mathrm{SU}(3{)}_{\mathrm{F}}$ symmetry limit. The transition form factors are obtained in terms of the two-particle DAs up to twist-five accuracy, and the three-particle up to twist-six level. We apply two classes of the phenomenological models for the DAs of the $B$ meson. The longitudinal lepton polarization asymmetries and branching fractions for the semileptonic decays of $B_{(s)}$ to the light scalar mesons are estimated with the help of these form factors.

Confinement in $SU(N_{\mathrm{DC}})$ Yang-Mills theories is known to proceed through first-order phase transition. The wall velocity is bounded by $v_w\lesssim {10}^{-6}$ due to the needed time for the substantial latent heat released during the phase transition to dissipate through Hubble expansion. Quarks which are much heavier than the confinement scale can be introduced without changing the confinement dynamics. After they freeze-out, heavy quarks are squeezed into pockets of the deconfined phase until they completely annihilate with antiquarks. We calculate the dark baryon abundance surviving annihilation, due to bound-state formation occurring both in the bulk and—for the first time—at the boundary. We find that dark baryons can be dark matter with a mass up to ${10}^3\mathrm{TeV}$. We study indirect and direct detection, cosmic microwave background and big bang nucleosynthesis probes, assuming portals to Higgs and neutrinos.

We collect spectra extracted in the $I=\ell =1$ $\pi \pi$ sector provided by various lattice QCD collaborations and study the $m_{\pi }$ dependence of $\rho$-meson properties using Hamiltonian effective field theory (HEFT). In this unified analysis, the coupling constant and cutoff mass, characterizing the $\rho -\pi \pi$ vertex, are both found to be weakly dependent on $m_{\pi }$, while the mass of the bare $\rho$, associated with a simple quark-model state, shows a linear dependence on $m_{\pi }^2$. Both the lattice results and experimental data can be described well. Drawing on HEFT’s ability to describe the pion mass dependence of resonances in a single formalism, we map the dependence of the phase shift as a function of $m_{\pi }$ and expose interesting discrepancies in contemporary lattice QCD results.

We provide the first determination of the mass of the lightest flavor-singlet pseudoscalar and scalar bound states (mesons) in the Sp(4) Yang-Mills theory coupled to two flavors of fundamental fermions, using lattice methods. This theory has applications both to composite Higgs and strongly interacting dark matter scenarios. We find the singlets to have masses comparable to those of the light flavored states, which might have important implications for phenomenological models. We focus on regions of parameter space corresponding to a moderately heavy mass regime for the fermions. We compare the spectra we computed to existing and new results for SU(2) and SU(3) theories, uncovering an intriguing degree of commonality. As a by-product, in order to perform the aforementioned measurements, we implemented and tested, in the context of symplectic lattice gauge theories, several strategies for the treatment of disconnected-diagram contributions to two-point correlation functions. These technical advances set the stage for future studies of the singlet sector in broader portions of parameter space of this and other lattice theories with a symplectic gauge group.