We make use of the improved K-matrix algorithm to obtain unitarized amplitudes in $R+\alpha R^2$ gravity (the so-called Starobinsky model, of cosmological relevance). The procedure is of some complexity because infrared divergences are present and need to be properly regulated. We focus on the behavior of a bona fide scalar resonance, known to exist in this model, and compare it to an apparent resonance detected in previous studies, thus confirming that the latter seems to be an artifact due to the introduction of the infrared regulator. We analyze the existence of other dynamical resonances and dwell on the amplitudes made unitary by this procedure.

We study black holes in the modified generalized Chaplygin gas (MGCG) model, a modified gravity framework proposed to unify dark matter and dark energy. The resulting spacetime is asymptotically nonflat and features two distinct horizons, determined by the Chaplygin parameter $\alpha$. Observational constraints favor negative values of $\alpha$, which restrict the allowed parameter space. Through linear perturbation analysis, we show that this black hole remains stable under both scalar and electromagnetic perturbation. The corresponding quasinormal mode spectra depend sensitively on the MGCG parameters, indicating that this cosmological model leaves characteristic imprints on black hole oscillations. These results suggest that gravitational-wave observations of black hole ringdowns may provide a means of constraining MGCG and testing unified dark sector scenarios in the strong-field regime.

On May 29, 2023, the LIGO Livingston observatory detected the gravitational-wave signal GW230529_181500 from the merger of a neutron star with a lower mass-gap compact object. Its long inspiral signal provides a unique opportunity to test general relativity (GR) in a parameter space previously unexplored by strong-field tests. In this work, we performed parameterized inspiral tests of GR with GW230529_181500. Specifically, we search for deviations in the frequency-domain gravitational wave phase by allowing for agnostic corrections to the post-Newtonian coefficients. We performed tests with the Flexible Theory Independent and Test Infrastructure For General Relativity frameworks using several quasicircular waveform models that capture different physical effects (higher modes, spins, tides). We find that the signal is consistent with GR for all deviation parameters. Assuming the primary object is a black hole, we obtain particularly tight constraints on the dipole radiation at $-1\mathrm{PN}$ order of $|\delta {\widehat{\phi }}_{-2}|\lesssim 8\times {10}^{-5}$, which is a factor $\sim 17$ times more stringent than previous bounds from the neutron star–black hole merger GW200115_042309, as well as on the 0.5PN and 1PN deviation parameters. We discuss some challenges that arise when analyzing this signal, namely biases due to correlations with tidal effects and the degeneracy between the 0PN deviation parameter and the chirp mass. To illustrate the importance of GW230529_181500 for tests of GR, we mapped the agnostic $-1\mathrm{PN}$ results to a class of Einstein-scalar-Gauss-Bonnet (ESGB) theories of gravity. We also conducted an analysis probing the specific phase deviation expected in ESGB theory and obtain an upper bound on the Gauss-Bonnet coupling of ${\ell }_{\mathrm{GB}}\lesssim 0.51M_{\odot }$ ($\sqrt{{\alpha }_{\mathrm{GB}}}\lesssim 0.28\mathrm{km}$), which is better than any previously reported constraint.

We study the late-time cosmological dynamics of a two-field dark-energy model consisting of a canonical quintessence scalar field and a phantom scalar field in a spatially flat Friedmann-Lemaître-Robertson-Walker universe. The fields are minimally coupled to gravity and uncoupled at the level of the potential, with the quintessence sector governed by an exponential potential and the phantom sector by an inverse power-law potential. By reformulating the background equations as a five-dimensional autonomous dynamical system, we identify and analyze the fixed points and their stability properties, revealing stable late-time attractors corresponding to phantom-dominated accelerated expansion. We confront the model with observations through a Bayesian parameter estimation performed using the cobaya framework, employing several combinations of recent cosmological datasets, including Pantheon+ supernovae, compressed cosmic microwave background distance priors, DESI DR2 baryon acoustic oscillation measurements, and DES year-5 supernova data. The observational constraints favor a dynamical dark-energy sector moderately and are consistent with deviations from a cosmological constant at the present epoch. The regions of parameter space preferred by the data are compatible with the stable accelerating solutions identified in the dynamical analysis, establishing a direct connection between phase-space stability and observational viability. A notable feature of the model is that the effective dark-energy equation of state undergoes phantom divide crossing in a gradual and asymptotic manner, rather than as a sharp transition.

We present exact solutions of the Einstein–$SU(2)$ nonlinear sigma model in $3+1$ spacetime dimensions, describing bumpy black holes and black branes. Using an Ansatz for superfluid pion multivortices, the matter sector reduces to a first-order Bogomol’nyi–Prasad–Sommerfield system, while the Einstein equations reduce to a Liouville equation with a smooth source governing the horizon deformation. These solutions describe horizons of different constant curvatures, with nontrivial bumpy geometries protected by an integer topological invariant, namely the vorticity, which also controls the number of bumps and the black hole thermodynamics. Remarkably, such horizons arise in a minimal and physically motivated matter model, without invoking exotic fields or modified gravity. The physical implications of these results in holography and astrophysics are briefly described.

Pulsar timing arrays are sensitive to low-frequency gravitational waves (GWs), such as those produced by supermassive binary black holes at subparsec separations. The incoherent superposition of GWs emitted by a cosmological population of these sources produces a gravitational wave background (GWB), while some individual sources may be resolvable as deterministic signals with slowly varying GW frequencies, which are often referred to as “continuous waves” (CWs). The ${\mathcal{F}}_p$-statistic is a frequentist method of detecting these CWs. In this paper, we study how the presence of pulsar red noise and a GWB affect the ${\mathcal{F}}_p$-statistic. We compare results when marginalizing over the red noise and using the maximum-likelihood values of the red noise, and find little difference between the two. We also present results of using the ${\mathcal{F}}_p$-statistic to analyze the NANOGrav 12.5-year dataset, where we find no evidence for CWs in agreement with the previously published Bayesian results.

We present new initial data codes for constructing stationary, axisymmetric equilibrium models of differentially rotating neutron stars in full general relativity within the Frankfurt University/kadath (fuka) suite of initial data codes. fuka leverages the kadath spectral library to solve the Einstein equations under the assumption of an isentropic fluid without magnetic fields while incorporating grhayleos to support 3D tabulated equations of state in stellar collapse format. The two solvers explored in this work include one using quasi-isotropic coordinates (QIC) in spherical coordinates while the other solves the extended conformal thin sandwich (XCTS) decomposition in Cartesian coordinates, enabling the construction of equilibrium configurations with high accuracy and efficiency. In this work we adopt the Komatsu-Eriguchi-Hachisu differential rotation law, however, the code is designed to be extensible to other rotation laws, allowing for exploration of physically relevant sequences and critical rotation thresholds. Furthermore, we perform convergence tests demonstrating the exponential accuracy of the spectral approach, we validate QIC and XCTS solutions against models well-studied in the literature, and we also compare fuka solutions against the well-known rns code. Finally, we explore the impact that initial data resolution has on dynamical simulations and recover the convergence order of the evolution scheme, the dominate source of error in this study. The new fuka codes and results presented here lay the foundation for future extensions to more general configurations, including magnetic fields, removal of isentropic assumptions, and binary systems, and have been made publicly available to support community efforts in modeling differentially rotating relativistic stars.

Gravitational waves emitted after neutron star binary coalescences and the information they carry about dense matter are a high-priority target for next-generation detectors. Even though such detectors are expected to observe millions of signals, detectable postmerger emission will remain rare. In this work, we explore postmerger detectability and inference through an alternative detector readout scheme for data dominated by quantum-noise, which is the case above 1 kHz; photon-counting. In such a readout, signals and noise become quantized into discrete distributions corresponding to the detection of single photons measured in a chosen basis of modes. Through simulated data, we demonstrate that photon counting can be efficient even for weak signals. We find $\sim 1$ in 100 signals with a postmerger signal-to-noise ratio of 0.2 can result in a single photon and thus be detected. Furthermore, after $2\times {10}^4$ signals—equivalent to ${10}^{-2}$ to 1.5 years of observation—photon counting results in a twofold improvement in the measurement of the radius of a $1.6M_{\odot }$ neutron star. Constraints can be further tightened if the detector classical noise is reduced. Photon counting offers a promising alternative to traditional homodyne readout techniques for extracting information from low signal-to-noise ratio postmerger signals.

Due to wave interference, an ultralight light dark matter halo has stochastic, granular substructures which can scatter stars, leading to the heating of stellar distributions. Studies of this phenomenon have placed lower bounds on the ultralight dark matter mass. In this paper we investigate a number of relevant systematic effects, including: (1) the heating by the central soliton, (2) the self-gravity of the stars, (3) the suppression of heating in a tidally stripped halo, and (4) the tidal field suppression of heating when the stellar cluster is much smaller than the de Broglie wavelength. The first three effects are quantified by studying the dynamics of stellar particles in Schrödinger-Poisson simulations of ultralight dark matter halos, while the last effect is studied using analytic approximations.

Under a reasonable set of ab initio assumptions, we define and chart the atlas of simple gauge theories with families of fermions whose masses are forbidden by gauge invariance. We propose a compass to navigate the atlas based on counting degrees of freedom. When searching for Grand Unification Theories with three matter generations, the free energy singles out the SU(5) Georgi-Glashow model as the minimal one, closely followed by SO(10) with spinorial matter. The atlas also defines the dryland of grand-unifiable gauge extensions of the standard model. We further provide examples relevant for gauge dual completions of the standard model as well as extensions by an additional SU(N) gauge symmetry.

We propose a model-independent test of $CP$ violation in the scalar sector. We consider a heavy neutral scalar $h_2$ with tree-level couplings at the $h_{2}VV$ and $h_2{h}_{1}Z$ vertices (with $V=W^{\pm },Z$), alongside the 125 GeV SM-like Higgs boson $h_1$. At future muon colliders, we exploit vector-boson fusion production of $h_2$ followed by the decay $h_2\to Z{h}_1$. In our framework, observing the single process $VV\to h_2\to Z{h}_1$ implies both relevant couplings are nonzero, which is sufficient to establish $CP$ violation in the scalar sector. We simulate signal and backgrounds at $\sqrt{s}=3(10)\mathrm{TeV}$ with integrated luminosity $L=0.9(10){\mathrm{ab}}^{-1}$. We then present the expected discovery sensitivities across the ($c_2$ and $c_{12}$) parameter space (with the coupling parameters $c_2$ and $c_{12}$ defined in the text) for multiple $m_{h_2}$ hypotheses.

I investigate a $d$-dimensional $U(N)$ Polyakov loop model that includes the exact static determinant with $N_f$ degenerate quark flavor and depends explicitly on the quark mass and chemical potential. In the large $N,N_f$ limit mean field gives the exact solution, and the core of the Polyakov loop model is reduced to a deformed unitary matrix model, which I solve exactly. I compute the free energy, the expectation value of the Polyakov loop, and the quark condensate. The phase diagram of the model and the type of phase transition is investigated and shows it depends on the ratio $\kappa =N_f/N$.

We have performed high-statistics lattice simulations using 4HEX improved staggered fermions on ${16}^3\times 8$ lattices. We calculated the Taylor expansion coefficients of the pressure with respect to the baryochemical potential to the tenth order at zero and the fourth order at purely imaginary chemical potentials. We used this data to construct rational function approximations of the free energy. We use a rational ansatz that explicitly satisfies the charge conjugation symmetry and the Roberge-Weiss periodicity, which are exact properties of the quantum chromodynamics free energy. We use this ansatz to estimate the position of Lee-Yang zeros in the complex chemical potential plane. The temperature dependence of the imaginary part of the Lee-Yang zeros is then fitted with ansätze motivated by the universal behavior of the free energy near a 3D Ising critical point. In principle, this allows one to estimate the temperature of the critical endpoint. We consider several sources of systematic errors. On this single lattice spacing, we find that with 84% probability the chiral critical endpoint is either below 103 MeV temperature or it does not exist. We also identify some caveats of the method, which do not disappear even with the extremely high statistics of this present study. We discuss to what extent these can be eliminated by future high statistics lattice analyses.

One proposal to compute parton distributions from first principles is the large momentum effective theory (LaMET), which requires the Fourier transform of matrix elements computed nonperturbatively. Lattice quantum chromodynamics (QCD) provides calculations of these matrix elements over a finite range of Fourier harmonics that are often noisy or unreliable in the largest computed harmonics. It has been suggested that enforcing an exponential decay of the missing harmonics helps alleviate this issue. Using nonperturbative data, we show that the uncertainty introduced by this inverse problem in a realistic setup remains significant without very restrictive assumptions, and that the importance of the exact asymptotic behavior is minimal for values of $x$ where the framework is currently applicable. We show that the crux of the inverse problem lies in harmonics of the order of $\lambda =z{P}_z\sim 5–15$, where the signal in the lattice data is often barely existent in current studies, and the asymptotic behavior is not firmly established. We stress the need for more sophisticated techniques to account for this inverse problem, whether in the LaMET or related frameworks like the short-distance factorization. We also address a misconception that, with available lattice methods, the LaMET framework allows a “direct” computation of the $x$-dependence, whereas the alternative short-distance factorization only gives access to moments or fits of the $x$-dependence.

We compute the complex, long-distance two-photon-exchange amplitude which contributes to the rare $K_{\mathrm{L}}\to {\mu }^{+}{\mu }^{-}$ decay from lattice QCD. We use a ${24}^3\times 64$ physical-pion-mass gauge field ensemble at an inverse lattice spacing of 1.023 GeV and a ${\mathrm{QED}}_{\infty }$-based formalism. Our implementation strategies for all five non-SU(3)-flavor-suppressed diagram topologies are given in detail. We achieve a 25% statistical precision on the dispersive part of this long-distance amplitude. This calculation is carried out with $2+1$ quark flavors and therefore requires the addition of counterterms to compensate for the absence of the Glashow-Iliopoulos-Maiani mechanism. These counterterms are not included in the current calculation and will be the subject of a second paper. Although a direct comparison to experiment cannot yet be made because of those omitted counterterms, the present exploratory calculation allows one to identify principal sources of statistical uncertainty in this calculation. The precision of our results is limited by the reconstruction of the physical contribution of the $\eta$ intermediate state, for which various strategies are tested and compared.

A search for new physics in events featuring a single photon and missing transverse momentum is presented, using proton-proton $\sqrt{s}=13\mathrm{TeV}$ collision data corresponding to an integrated luminosity of $101{\mathrm{fb}}^{-1}$ collected by the CMS experiment at the CERN LHC between 2017 and 2018. This analysis, combined with a previous study of $36{\mathrm{fb}}^{-1}$ of 2016 data (totaling $137{\mathrm{fb}}^{-1}$), reveals no significant deviations from standard model expectations. The results are then used to establish 95% confidence level limits on parameters in theoretical models involving dark matter and large extra dimensions. Compared to the 2016-only analysis, this search achieves up to a 14% improvement in exclusion reach for mediator masses in simplified dark matter models, along with 11% and 10% enhancements in the limits on the effective field theory suppression scale and the fundamental Planck scale, respectively. These results are the most stringent constraints on these parameters to date.

The first observation of the ${\chi }_{c1}(3872)\to J/\psi {\mu }^{+}{\mu }^{-}$ decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The decay mode is observed for the first time, with a significance of $6.5\sigma$. Its branching fraction is measured relative to the ${\chi }_{c1}(3872)\to J/\psi {\pi }^{+}{\pi }^{-}$ decay mode $\frac{\mathcal{B}({\chi }_{c1}(3872)\to J/\psi {\mu }^{+}{\mu }^{-})}{\mathcal{B}({\chi }_{c1}(3872)\to J/\psi {\pi }^{+}{\pi }^{-})}=(1.68\pm 0.32\pm 0.05)\times {10}^{-3}$, where the first uncertainty includes both statistical contributions and systematic contributions which are uncorrelated between data-taking periods, and the second represents the systematic contributions that are correlated between data-taking periods.

We study Einstein-Maxwell (non-null) source-free configurations that can be extended to any conformally invariant nonlinear electrodynamics (CINLE) by a constant rescaling of the electromagnetic field. We first obtain a criterion which characterizes such extendable solutions in terms either of the electromagnetic invariants, or (equivalently) of the canonical Newman-Penrose form of the self-dual Maxwell field. This is then used to argue that all static configurations are extendable (more generally, all configurations admitting a non-null twist-free Killing vector field). One can thus draw from the extensive literature to straightforwardly extend to CINLE various known exact solutions, whereby the duality invariance of the Einstein-Maxwell theory allows for dyonic solutions even in more general theories. This is illustrated by a few explicit examples, including the homogeneous $\mathrm{\Lambda }<0$ universe of Ozsváth, a black hole in the universe of Levi-Civita, Bertotti, and Robinson, a generalization of the charged $C$-metric, and nonexpanding gravitational waves in the Bonnor-Melvin background.

In this work, we revisit a recently reported generalization of the Bañados-Teitelboim-Zanelli black hole arising in new massive gravity sourced by the quantum fluctuations of scalar matter, now examined through the lens of a purely classical framework. We show that the same geometry, distinguished by its logarithmic asymptotic structure, emerges as the unique static solution of Einstein gravity coupled to an exponential nonlinear electrodynamics. We trace the origin of this correspondence and prove that this geometry belongs to a unique class of metrics constituting the intersection of the moduli spaces of the static and circularly symmetric sectors of the two theories, thereby revealing a dynamical equivalence between them. An explicit mapping is established between the global charges of the nonlinearly charged black holes and the parameters governing the quantum backreaction in new massive gravity, allowing for a natural reinterpretation of the quantum imprints in terms of classical charges. A detailed analysis of the horizon structure of these spacetimes is presented. In addition, the full thermodynamics of the more general configurations is constructed using the Iyer-Wald formalism, from which we derive the first law and the associated Smarr relation. Altogether, our results provide a classical realization of a semiclassical spacetime and point toward a broader correspondence between higher-curvature corrections in quantum gravity and nonlinear effects in self-gravitating electrodynamics in three dimensions.

The effective dynamics of the Janis-Newman-Winicour (JNW) spacetime inspired by loop quantum gravity is studied. Two different schemes are considered to regularize the Hamiltonian constraint for the quantum dynamics. In the ${\mu }_0$ scheme in which the quantum parameters are treated as constants, the equations of motion generated by the effective Hamiltonian are solved analytically. The resulting quantum-corrected effective spacetime obviously extends the effective spacetime previously obtained in the literature. In the new effective spacetime, the naked singularity and the central singularity presented in the classical JNW spacetime are resolved by a series of quantum bounces. In the scheme of choosing the quantum parameters as Dirac observables, the effective dynamics are also solved in the light of the solution in the ${\mu }_0$ scheme. It turns out that the resulting effective spacetime has singularities due to the appearance of the zero points of the time reparametrization functions. Hence, the effective theory in this scheme does not remain valid throughout the full spacetime.