Black hole ringdowns in extensions of general relativity (GR) generically exhibit two distinct signatures: i) theory-dependent shifts in the standard black-hole quasinormal modes, and ii) additional modes arising from extra fundamental fields—such as scalar, vector, or tensor degrees of freedom—that can also contribute to the gravitational-wave signal. As recently argued, in general both effects are present simultaneously, and accurately modeling them is essential for robust tests of GR in the ringdown regime. In this work, we investigate the impact of extra field-induced modes, which are often neglected in standard ringdown analyses, on the interpretation of gravitational-wave signals. To provide some concrete examples, we focus on dynamical Chern-Simons and Einstein-scalar-Gauss-Bonnet theories, well-motivated extensions of GR, characterized, respectively, by a parity-odd and a parity-even coupling between a dynamical scalar field and quadratic curvature invariants. We show that including extra field-induced modes improves the bounds on these theories compared to standard spectroscopy and also allows for equally constraining complementary tests not based on quasinormal mode shifts. Our analysis highlights the relevance of incorporating extra field-induced modes in ringdown templates and assesses their potential to either bias or enhance constraints on GR deviations.

We present a framework for detecting gravitational-wave signals lensed by cosmic strings (CSs), addressing a key gap in current searches. CSs, whose detection would provide a unique probe of high-energy physics and the early Universe, possess distinct topological and geometric features that require a dedicated search strategy. Our approach employs a full-wave transmission factor, expressed analytically via Fresnel integrals, which captures the characteristic diffraction and interference effects of the conical spacetime around a straight CS. We contrast CS lensing with the well-studied point mass lens (PML) model, highlighting their fundamental differences: CS lensing depends on cosmological distances, string tension $\mathrm{\Delta }$, and wavelength $\lambda$, and produces two nonamplified images set by the global conical geometry. In contrast, PML lensing is governed by the distance-independent ratio $\sim M_{Lz}/\lambda$, where $M_{Lz}$ represents the redshifted mass of the lens, with image properties derived from the lens equation. For binary black hole mergers lensed by CSs, we show that the waveforms exhibit a characteristic beating pattern or time-separated exact replicas. We derive a detectability bound on the string tension and, using Bayesian model selection, demonstrate that CS lensing is distinguishable from both unlensed and PML-lensed signals across a wide region of parameter space.

We present an alternative formulation of generalized unimodular gravity (GUMG), a class of modifications of general relativity characterized by a special partial breaking of general coordinate covariance. The action for this formulation is derived constructively through a sequence of equivalent representations, beginning with the original GUMG setup and extending the configuration space and gauge structure by introducing time parametrization. Our approach, based on a canonical formalism, parallels the method used by Henneaux and Teitelboim to covariantize the action of unimodular gravity (UMG), which was generalized to consistently accommodate a parametrization via local fields for the entire GUMG family. For completeness, we explore the dynamical structure of the theory and provide a detailed account of its gauge properties. A notable consequence of the consistent parametrization is the emergence of explicit spatial delocalization in the action, manifestations of which we carefully examine. Within the GUMG family, we identify a subfamily of models described by a local action: preserving the essential structural features, these models avoid the complications of spatial nonlocality allowing for a clearer analysis. Unimodular gravity possesses dynamical and constraint structures distinct from those of the GUMG family models; however, UMG appears as an exceptional special case within the family, providing valuable comparative insights.

We propose an alternative description of generalized unimodular gravity, extending the Henneaux-Teitelboim approach to unimodular gravity. The central feature of this formulation is the consistent incorporation of time reparametrization, which enhances the gauge structure and reveals a spatial nonlocality hidden in the dynamics of the original formulation. We examine the resulting dynamics, emphasizing the effects of spatial nonlocality, and outline the constraint structure. We show that the gauge symmetry in the gravitational sector is extended by a functionally incomplete symmetry, as occurs in the unimodular gravity. However, in contrast to the latter, the resulting action is not fully diffeomorphism invariant.

We examine the possible characterization of ringdown waves in a dynamical Vaidya spacetime using the Penrose limit geometry around the dynamical photon sphere. In the case of a static spherically symmetric black hole spacetime, it is known that the quasinormal frequency in the eikonal limit can be characterized by the angular velocity and the Lyapunov exponent for the null geodesic congruence on the orbit of the unstable circular null geodesic. This correspondence can be further backed up by the analysis of the Penrose limit geometry around the unstable circular null geodesic orbit. We try to extend this analysis to a Vaidya spacetime, focusing on the dynamical photon sphere in it. Then we discuss to what extent the Penrose limit geometry can be relevant to the ringdown waves in the Vaidya spacetime, comparing the results with the numerically calculated waveform in the Vaidya spacetime.

In this paper we investigate quark deconfinement in neutrons stars and their mergers, focusing on the effects of higher orders for the phase transition between hadronic and quark matter. The different descriptions we use to describe matter microscopically contain varying particle degrees of freedom, including nucleons, hyperons, Delta baryons, and light and strange quarks. We use tabulated equations of state from the CompOSE database in which the quark deconfinement phase transition is described as being first order, and then smooth it out by introducing a percolation, replacing the single first-order phase transition with two transitions of second or third order. We then perform binary neutron-star merger simulations using these new equations of state, focusing on groups of binaries with the same single-star mass, radius, and tidal deformability, but different equations of state. We go on to discuss differences in their evolution, and the ramifications for interpreting future gravitational wave observations and the potential to learn about dense matter.

Neutron stars (NSs) are excellent laboratories for testing gravity theories as they are strongly self-gravitating bodies and have rich observational phenomena. However, strong-field gravity effects in NSs could be degenerate with their equation of state (EOS) which is largely unknown. Fortunately, there exist the so-called universal relations among the NS macroscopic quantities that are found to be insensitive to the underlying EOS. Studying the origin of these relations can lead to a better understanding of NSs and the gravitational interaction. We develop a new perspective of view to analyze the $I\text{−}C$ and $I$-Love universal relations for NSs. At the linear order, we separate the deviation of the universal relations into two factors that are multiplied together. One is the EOS difference, while the other factor only depends on the background star structure. The smallness of the second factor that does not depend on the EOS difference then indicates the origin of the universality. We discuss the validity of our linear approximation when considering the difference among realistic EOS. Our study can be regarded as a new frame for quantitative representation of the universality and may provide new insights to the universal relations of NSs.

In this work we consider a generalization of Starobinsky inflation obtained by combining power law ($R^{\beta }$) and $\alpha$-Starobinsky inflation ($E$ model). The Einstein frame potential for this model is that of power law Starobinsky inflation modified by a parameter $\alpha$ in the exponential. After computing power spectra for scalar and tensor perturbations numerically, we perform Markov chain Monte Carlo analysis to put constraints on the potential parameters $\alpha$, $\beta$ and $M$, and the number of e-foldings $N_{\text{pivot}}$ during inflation, using Planck 2018, BICEP/Keck, DES, and baryon acoustic oscillation (BAO) observations. We find ${\log }_{10}\alpha =0.37_{-0.85}^{+0.82}$, $\beta =1.969_{-0.023}^{+0.020}$, $M=(3.54_{-1.73}^{+2.62})\times {10}^{-5}$, and $N_{\text{pivot}}=47\pm 10$. With these mean values of the potential parameters $\alpha$ and $\beta$, and varying $N_{\text{pivot}}$ between 40 and 55, we also find that the $r-n_s$ predictions of our model lie well within the $1\sigma$ bounds of joint constraints from combined analysis of ACT, Planck 2018, BICEP, and BAO observations. We compute the Bayesian evidences for our proposed model, power law Starobinsky inflation, $\alpha$-Starobinsky inflation, and Starobinsky inflation. Considering the Starobinsky model as the base model, we calculate the Bayes factor and find that our proposed model is mildly favored by the cosmic microwave background and large-scale structure observations.

We study the cosmological dynamics and predictions in the theory of warped massive gravity. This setup postulates a five-dimensional ghost-free massive graviton with a brane-localized four-dimensional massive gravity potential, and has the virtue of raising the strong-coupling scale of the 4D theory. We identify two classes of models that lead to decoupled equations for the scale factor on the brane; one characterized by a particular choice of boundary conditions for the Stückelberg fields and one characterized by a special tuning between the coefficients of the 5D and 4D potentials. In the first case, we find interesting solutions including a cosmological bounce without the need of exotic matter. The second case leads to a modified Friedmann equation, and comparison with data shows the potential of the model to alleviate the Hubble tension.

We propose a new mechanism for generating cosmological relics—black holes, gravitational waves (GWs), and possibly dark matter (DM)—in a bouncing Universe. Relics arise through two channels: (i) compact objects and GWs produced during prebounce collapse that remain superhorizon and re-enter after the bounce, and (ii) dark matter halos formed during collapse that exit the horizon and collapse into black holes upon re-entry. Unlike inflationary primordial black holes, these relic black holes originate from nonlinear structure formation during collapse. We derive the particle-horizon and horizon-crossing conditions in bouncing cosmology and show that perturbations or compact objects larger than $\sim 90\mathrm{m}$ survive the bounce. The resulting population of relic black holes and GWs spans a wide mass range, offering a unified origin for dark matter, gravitational-wave backgrounds, and the early growth of supermassive black holes and galaxies.

In this work, we consider the possibility that the dynamical dark energy hinted at by recent Dark Energy Spectroscopic Instrument data may be mimicked by the effects of additional components in the universe, potentially arising from topological defects. We find that the data does not show a particular preference for the existence of cosmic strings. However, a domain wall contribution at the percent level can improve the fit, yielding a $\mathrm{\Delta }{\chi }^2=-1.72$ and $\mathrm{\Delta }\mathrm{DIC}=-0.94$ compared to the lambda-cold dark matter model ($\mathrm{\Lambda }\mathrm{CDM}$). The improvement indicates that topological defects remain a viable and interesting extension to $\mathrm{\Lambda }\mathrm{CDM}$, meriting further investigation with future cosmological data.

The 21-cm line of neutral hydrogen is a promising probe of the epoch of reionization but suffers from contamination by spectrally smooth foregrounds, which obscure the large-scale signal along the line of sight. We explore the possibility of indirectly extracting the 21-cm signal in the presence of foregrounds by taking advantage of nonlinear couplings between large and small scales, both at the level of the density field and a biased tracer of that field. When combined with effective field theory techniques, bias expansions allow for a systematic treatment of nonlinear mode couplings in the map between the underlying density field and the 21-cm field. We apply an effective bias expansion to density information generated with 21 cmFAST using a variety of assumptions about which information can be recovered by different surveys and density estimation techniques. We find that the 21-cm signal produced by our effective bias expansion, combined with our least optimistic foreground assumptions, produces $\sim 30\%$ cross-correlations with the true 21-cm field. Providing complementary density information from high-redshift galaxy surveys yields a cross-correlation of 50–70%. The techniques presented here may be combined with foreground mitigation strategies in order to improve the recovery of the large-scale 21-cm signal.

A recent determination of the growth index indicates a value significantly higher than the $\mathrm{\Lambda }$ cold dark matter (CDM) prediction, suggesting that alternative scenarios to $\mathrm{\Lambda }\mathrm{CDM}$ may be required. In this work, we investigate whether a time-varying Newton’s constant, $G_N$, can account for such a high growth index, $\gamma =0.633_{-0.025}^{+0.024}$. Adopting a phenomenological approach, we study two parametrizations of the effective gravitational coupling, $G_{\mathrm{eff}}$, one based on a Taylor expansion and another linked to the energy density parameter of dark energy. We constrain the models with cosmic chronometers, Dark Energy Spectroscopic Instrument baryon acoustic oscillations (Data Release 2), cosmic microwave background priors, and a Gaussian likelihood for the growth index. We show that the constant $\gamma$ approximation is accurate for the parametrization linked to the energy density parameter of dark energy, but presents a non-negligible error for the other case, which we treat as a systematic error in the analysis. We find a $2.4\sigma -3.4\sigma$ tension level with constant $G_{\mathrm{eff}}$, depending on the parametrization. The results indicate that $G_{\mathrm{eff}}<G_N$ around the period of accelerated expansion, corresponding to a weaker effective gravitational interaction on cosmological scales, which leads to a suppression of the growth of cosmological structures.

Despite the assumption that an ideal Friedmann–Lemaître–Robertson–Walker observer is not dependent on the local environment, observations are biased by the positions of the observers due to the matter correlations in the large-scale structure (LSS) of the universe. The variation of the mass distribution of the LSS of the universe implies that observers residing in different locations may suffer bias in their measurements when they look at the images of distant galaxies. Here, we assess the influence of the local environment on weak gravitational lensing (WL) shear statistics in the context of relativistic $N$-body code, gevolution. We derive numerical constraints on the cosmological parameters from the WL shear angular power spectrum and comment on the local environment’s influence on WL shear. We find tighter constraints on the parameter ${\mathrm{\Omega }}_{\mathrm{m}}$ above redshift $z=0.2$, which implies over this redshift the local environment’s impact is minor. We also investigate the bispectrum and conclude that on average the impact of the local environment on $f_{\mathrm{NL}}$ (a measure of non-Gaussianities) is minimal and consistent with zero effect. However, we find that within the assembly of all possible observers/locations, there will also be a few that could infer the parameter $f_{\mathrm{NL}}$ of the order 10. These results could thus be used to estimate the uncertainty in the inference of cosmological parameters such as $f_{\mathrm{NL}}$ based on WL shear bispectrum and thus may have implications for future surveys requiring precision at the percent level.

We prove a no-go theorem for the mass-radius relation of localized and stable field configurations, known as solitons. Defining the mass-radius index by $\mathrm{\Gamma }\equiv \frac{\mathrm{d}\ln M}{\mathrm{d}\ln R}$, for real scalar field theories in $d$ spatial dimensions, we show that typical nontopological, nonrelativistic, and spherically symmetric solitons cannot have $\mathrm{\Gamma }$ in the range $[0,d]$. The forces considered originate from gradient energy, self-interaction, and gravitation, with the typicality assumption excluding the fine-tuned region of the parameter space where all three forces have comparable strength. Importantly, the theorem works for an arbitrary self-interaction that, in the relativistic theory, is allowed to be non-power-law in the field, be nonanalytic around the classical vacuum (where the field amplitude vanishes), or to include derivative couplings. Additionally, the theorem makes no assumptions about the explicit form of the soliton’s density profile or the behavior of $\mathrm{\Gamma }$ as a function of $R$. We also argue that the same exclusion applies to compact objects formed from self-gravitating, nonrelativistic, barotropic fluids with arbitrary equations of state. As a consequence for cosmology, it is worth noting that observations favor a core in the centers of dark matter halos with $\mathrm{\Gamma }\simeq 1.7$, which (for $d=3$) lies approximately in the middle of the excluded range. Therefore, proposals such as ultralight or fluidlike dark matter models are essentially ruled out as natural explanations for halo cores, provided other astrophysical effects are negligible.

Baryon acoustic oscillations (BAO) provide a robust standard ruler for observational cosmology, enabling precise constraints on the expansion history of the Universe. We present a weakly model-dependent measurement of the BAO angular scale in the low-redshift Universe using the blue galaxies from the Southern Photometric Local Universe Survey (S-PLUS). Our analysis is based on the 2-point angular correlation function applied to a selected photometric sample of 5977 galaxies with redshifts $0.03\le z\le 0.1$. To account for photometric redshift uncertainties, we implement a resampling technique using the probability distribution function of each galaxy. Angular correlations are computed using the Landy-Szalay estimator; the uncertainties are quantified using a set of 1000 log-normal mock catalogs. Our 2-point angular correlation analyses reveal a prominent BAO signal that after a shift correction, due to the projection effect caused by the finite thickness of the redshift bin, provides the transversal BAO measurement: ${\theta }_{\mathrm{BAO}}=20.41°\pm 2.14°$, at $z_{\mathrm{eff}}=0.075$, detected with a statistical significance of $1.50\sigma$. In addition, we performed consistency tests that support the robustness of our result. Our measurement constitutes the first robust detection of the transversal BAO scale: at the lowest-redshift in the Universe and using multiband ($\mathrm{narrow}+\mathrm{wide}$) photometry data from the S-PLUS.

Simulation-based inference (SBI) has become an important tool in cosmology for extracting additional information from observational data using simulations. However, all cosmological simulations are approximations of the actual universe, and SBI methods can be sensitive to model misspecification—particularly when the observational data lie outside the support of the training distribution. We present a method to improve the robustness of cosmological analyses under such conditions. Our approach first identifies and discards components of the summary statistics that exhibit inconsistency across related simulators, then learns a transformation that brings the observation back within the support of the training distribution. We apply our method in the context of a recent simbig SBI galaxy clustering analysis using the wavelet scattering transform (WST) summary statistic. The original analysis struggled to produce robust constraints for certain subsets of WST coefficients, where the observational data appeared out of distribution (OOD) relative to the training data. We show that our method enables robust cosmological inference and resolves OOD issues, while preserving most of the constraining power. In particular, the improved simbig WST analysis yields $\mathrm{\Lambda }\mathrm{CDM}$ constraints of ${\mathrm{\Omega }}_m=0.32_{-0.02}^{+0.02}$ and ${\sigma }_8=0.80_{-0.02}^{+0.02}$, which are respectively $1.4\times$ and $3.1\times$ tighter than those from a standard perturbation-theory-based power spectrum analysis, confirming the significant information gain of WST summary statistics. The proposed method is easily applicable to other cosmological SBI contexts and represents a step toward more robust SBI pipelines.

Improved observations of relativistic jets have underscored the need for tractable theoretical models. Here we develop a semianalytical hybrid jet model that combines black hole-driven and disk-driven components within steady, axisymmetric, ideal general relativistic magnetohydrodynamics framework. We derive a criterion for the launching sites of cold outflows, introducing a new constraint on the magnetic field configuration threading a thin disk. Using the Bernoulli equation and critical-point analysis under Michel’s minimal-energy ansatz, we obtain flow solutions along different classes of field lines. The hybrid model shows that discontinuities in field-line angular velocity produce pronounced velocity shear and density jumps at the interface between the two jet components, together with localized enhancements that may account for observed limb brightening.

We investigate how resonant excitation near exceptional points manifests in Kerr black hole ringdown waveforms and examine its extraction. Using waveforms generated by localized initial data, where quasinormal mode amplitudes are given solely by excitation factors, we establish a controlled benchmark for overtone extraction. Applying an iterative fitting method with mirror modes, we analyze a mild resonance in the $(l,m)=(2,2)$ multipole and a sharp resonance in the (3,1) multipole occurring as part of a sequence of successive resonances. For (2,2), we extract the fundamental mode, the first three overtones, and the fundamental mirror mode with relative errors below 10%, and show that residual waveforms exhibit the expected damped sinusoids together with distinctive resonance signatures. For (3,1), we demonstrate that resonances can not only amplify but also reduce quasinormal mode excitations, reshaping the overtone hierarchy and rendering the sharp resonance more pronounced in ringdown. Our results clarify the imprint of resonance in Kerr ringdown and highlight both the robustness and limitations of current extraction techniques, providing a foundation for more reliable extraction of higher overtones and for applications to observational data analysis.

We propose a new method to determine the dark matter density profile in the vicinity of distant supermassive black holes (SMBHs) using reverberation mapping (RM) measurements of active galactic nuclei (AGN). The mapping of multiple emission lines allows the measurement of the enclosed mass within different radii from the central SMBH, which can be used to infer or constrain the dark matter density profile on subparsec scales. We apply a toy model based on this method to a sample of 14 AGN to test its feasibility based on current measurements. We find that, for five objects, the observed enclosed mass does grow with radii, hinting toward the presence of a dark matter component at the $1-2\sigma$ level. For these sources, we find global evidence for a universal dark matter profile with a preferred radial steepness of index $\gamma \sim 1.6$, consistent with the scenario expected for a dark matter spike mildly relaxed by stellar heating processes. The enclosed dark matter mass, however, is found to be significantly larger than expected. We show that the current RM-based mass measurements suffer from large systematic uncertainties that limit the effectiveness of our method. Our work emphasizes the importance of applying the recent developments in mass determination techniques to target multiple emission lines with future RM and interferometry campaigns. This provides the most direct way of constraining the dark matter density in the subparsec regions around extragalactic SMBHs, which is crucial to our understanding of the dynamics and nature of dark matter.