In this article, we study scalar wave perturbations of arbitrary frequency to the five-dimensional (5D) Schwarzschild-Tangherlini black hole (STBH) within general relativity. For the first time, we derive a closed formula for the 5D partial wave gravitational Raman scattering amplitude applicable to a broad class of boundary conditions, expressed in terms of the Nekrasov-Shatashvili (NS) function for the reduced confluent Heun problem. Furthermore, up to $\mathcal{O}(G^2)$ we compute the dynamical $\ell =0$, and the static $\ell =1$, scalar tidal Love numbers of the STBH by matching an effective field theory description for a scalar wave scattering off the black hole, to our novel ultraviolet-NS solutions. The matched Love numbers do not vanish and present renormalization group running behavior.

We revisit the quasinormal mode (QNM) problem in Schwarzschild–de Sitter spacetimes providing a unified infrastructure tailored for studying limiting configurations. Geometrically, we employ the hyperboloidal framework to explicitly implement Geroch’s rigorous limiting procedures for families of spacetimes. This enables a controlled transition between Schwarzschild, de Sitter, and Nariai geometries. Numerically, we introduce the analytical mesh refinement technique into quasinormal mode calculations, successfully recovering—within the appropriate limiting scenarios—both known families of quasinormal modes: complex light ring modes and purely imaginary de Sitter modes. We interpret these results in terms of spectral instability, where the notions of stable and unstable modes depends on the specific spacetime limit under consideration. In the Schwarzschild limit, de Sitter modes appear as a destabilizing effect on the continuous branch cut at $\omega =0$. Conversely, the branch cut can be understood as emerging from an infinite accumulation of discrete modes at $\omega =0$ in the transitional regime. We propose a heuristic measure of QNM density to characterize this accumulation and highlight the need for a more rigorous study of potential branch cut instabilities—especially relevant in the context of late-time gravitational wave signals. The proposed infrastructure provides a general and extensible framework for investigations in more complex spacetimes, such as Reissner–Nordström–de Sitter or Kerr–Newman–de Sitter.

Starting from the Einstein equations in Schwarzschild–de Sitter (SdS) spacetime and imposing Friedmann-Robertson-Walker coordinates at large distances, we find two coordinate systems with time-dependent metrics that are smooth across both the black hole and cosmological horizons. These coordinates require positive cosmological constant for them being regular, and thus they are not de Sitter extensions of the Kruskal–Szekeres or Israel coordinates. One of the coordinate systems was only found in 1999 (Abbassi coordinates), and it has led to conflicting interpretations in the literature; while the other was briefly commented and promptly dismissed as unphysical or incompatible with SdS. We derive that the second solution is equivalent to the first one, and that both are indeed equivalent descriptions of SdS spacetime. We also derive explicit coordinate transformations linking these coordinate systems to the Kottler coordinates and the maximally extended Lake–Israel coordinates. Among other applications, these results, which extend the largely used cosmological and local coordinates, should be useful for further developments on understanding the exact interplay between black holes and the cosmological background, which has been the focus of a number of recent works.

A set of tidal dissipation numbers (TDNs) quantifies the absorption of the tidal force exerted by a companion during an inspiralling phase of a binary compact object. This tidal dissipation generally affects the gravitational waveform, and measuring the TDNs of a black hole (BH) allows us to test the nature of gravity in the strong-field regime. In this paper, we develop a parametrized formalism for calculating the TDNs of static and spherically symmetric BH backgrounds using the Mano-Suzuki-Takasugi method, which connects the underlying perturbative equations with observable quantities in gravitational-wave observations in a theory-agnostic manner. Our formalism applies to any system where the master equation has the form of the Regge-Wheeler/Zerilli equation with a small correction to the effective potential. As an application of our formalism, we consider three examples: the effective field theory of BH perturbations with timelike scalar profile, the Einstein-Maxwell system, and a higher-curvature extension of general relativity. We also discuss the absence of logarithmic running for the TDNs.

The Laser Interferometer Space Antenna (LISA) mission poses a difficult parameter estimation challenge: the sources will be so dense in both time and frequency that they all must be fit simultaneously in a ‘global fit’. Successful tests of global fit efforts on synthetic datasets have been recently reported, recovering extragalactic black hole mergers and galactic binaries, including the GLASS pipeline in Littenberg and Cornish [Prototype global analysis of LISA data with multiple source types, Phys. Rev. D 107, 063004 (2023).]. Injected stochastic sources, however, have so far been absent in these datasets. In this work we report our development of a stochastic search pipeline ready for inclusion in future tests of the global fit, capable of detecting or placing limits on a wide variety of possible cosmologically- and astrophysically-inspired stochastic backgrounds (SGWBs). The code uses short-time Fourier transforms to allow for inference despite the nonstationarity of the noise. We quote results using both purely synthetic confusion noise and two GLASS residuals, and quantify the impact of the residuals’ non-Gaussianity on injected signal recovery and on setting upper limits. We find that, if not properly mitigated, non-Gaussianities can preclude setting accurate SGWB upper limits and lead to false detections. We also stress that the narrow-band non-Gaussianities we find do not affect all sources equally, and many narrower-band, cosmologically-inspired SGWBs are more sensitive to non-Gaussianity than others.

Smooth hybrid inflation is a hybrid inflation model that is free from topological defects and predicts the density perturbation with the spectral index of about 0.97. We show that the prediction on the spectral index is robust regardless the power of nonrenormalizable terms and meets with the latest results reported by Atacama Cosmology Telescope. Viable scenarios for baryogenesis and dark matter are also discussed.

Scalar dark matter is a viable alternative to particle dark matter models such as weakly interacting massive particles. This is particularly the case for scalars with a low mass $m\gtrsim {10}^{-21}\mathrm{eV}$ as required to make quantum effects macroscopic on galactic scales. We point out that by synchronizing the measurements of arrival times of pairs of pulsars, pulsar timing arrays could probe ultralight dark matter (ULDM) scenarios with a mass ${10}^{-23}\mathrm{eV}\lesssim m\lesssim {10}^{-19}\mathrm{eV}$ that is greater than the one reached in standard analysis. The upper limit on the mass $m$ is set by the time lag $\mathrm{\Delta }t$ between the observations of the two pulsars and could be pushed above ${10}^{-19}\mathrm{eV}$ for $\mathrm{\Delta }t$ smaller than one hour. However, for these high scalar masses only very high density dark matter clouds could be detected and the capture rate of neutron stars is too low to provide sufficient statistics. Significant detection probabilities would thus require direct dark-matter-baryon interactions that favor the formation of neutron stars within such dark matter clouds, or the discovery of black hole/pulsar binary systems, taking advantage of the dark matter spike generated by the black hole.

A variety of processes in the very early Universe can give rise to a population of primordial black holes (PBHs) with an extended mass spectrum. For certain mass spectra of this sort, it has been shown that the evaporation of these PBHs into radiation can drive the universe toward an epoch of cosmological stasis which can persist for a significant number of $e$-folds of cosmological expansion. However, in general, the initial mass spectrum which characterizes a population of PBHs at the time of production can subsequently be distorted by processes such as mergers and accretion. In this paper, we examine the effects that these processes have on the spectra that lead to a PBH-induced stasis. Within such stasis models, we find that mergers have only a negligible effect on these spectra within the regime of interest for stasis. We likewise find that the effect of accretion is negligible in many cases of interest. However, we find that the effect of accretion on the PBH mass spectrum is non-negligible in situations in which this spectrum is particularly broad. In such situations, the stasis epoch is abridged or, in extreme cases, does not occur at all. Thus accretion plays a nontrivial role in constraining the emergence of stasis within scenarios which lead to extended PBH mass spectra.

A variety of scenarios for early-universe cosmology give rise to a population of primordial black holes (PBHs) with a broad spectrum of masses. The evaporation of PBHs in such scenarios has the potential to place the Universe into an extended period of “stasis” during which the abundances of matter and radiation—i.e., their energy densities divided by the critical density—remain absolutely constant despite cosmological expansion. This surprising phenomenon can give rise to new possibilities for early-universe dynamics and lead to distinctive signatures of the evaporation of such PBHs. In this paper, we discuss how this stasis epoch arises and explore a number of its phenomenological consequences, including implications for inflationary observables, the stochastic gravitational-wave background, baryogenesis, and the production of dark matter and dark radiation.

We construct a realistic supersymmetric model for superheavy metastable cosmic strings (CSs) that can be investigated in the current pulsar timing array (PTA) experiments. It is based on shifted $\mu$-hybrid inflation in which the symmetry breaking $SU(4{)}_c\times SU(2{)}_L\times U(1{)}_R\to SU(3{)}_c\times SU(2{)}_L\times U(1{)}_{B-L}\times U(1{)}_R$ proceeds along an inflationary trajectory such that the topologically unstable primordial monopoles are inflated away. The breaking of $U(1{)}_{B-L}\times U(1{)}_R\to U(1{)}_Y$ after inflation ends yields the superheavy metastable CSs carrying the right handed neutrino zero modes that generate the stochastic gravitational wave background, which we show is consistent with the current PTA dataset and the LIGO O3 run. The scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$ are compatible with the Planck 2018 and Atacama Cosmology Telescope measurements. Both reheating and leptogenesis are briefly discussed.

We consider models where the dilaton, seen as the pseudo-Goldstone boson of broken scale invariance, plays the role of dark energy. We revisit Weinberg’s theorem and show that quantum corrections induced by the graviton lead to the screening of the dilaton locally. We also discuss the time evolution of the equation of state and find that phantom crossing is a natural feature of these models. The time variation of the equation of state and its deviation from $-1$ is limited by screening locally and can only be relaxed when the dilaton is allowed to have a mass of the order of the Hubble rate cosmologically, thus going beyond single-field screened dark-energy models. This obstruction extends to all single-field screened models of the chameleon type where the large mass of the scalar on cosmological scales leads to a negligible variation of the equation of state at low redshift.

Because of nonzero neutrino rest masses, we expect the energy density today in nonrelativistic matter, ${\omega }_{\mathrm{m}}$, to be greater than the sum of baryon and cold dark matter densities, ${\omega }_{\mathrm{cb}}$. We also expect the amplitude of deflections of cosmic microwave background (CMB) photons due to gravitational lensing to be suppressed relative to expectations assuming massless neutrinos. The combination of CMB and baryon acoustic oscillation (BAO) data, however, appears to be defying both of these expectations. Here we review how the neutrino rest mass is determined from cosmological observations, and emphasize the complementary roles played by BAO and lensing data in this process. We then use a phenomenological model to find that the preference from CMB and BAO data for a matter density that is below expectations from the CMB alone is at the $2.3\sigma$ level. We also show that, if a fraction of the dark matter decays to dark radiation, the preference for ${\omega }_{\mathrm{m}}>{\omega }_{\mathrm{cb}}$ can be restored, but with a small increase to the CMB lensing excess.

The sensitivity of gravitational wave interferometers is fundamentally limited by quantum noise, as dictated by the Heisenberg uncertainty principle, due to their continuous position measurement of the end mirrors. Speedmeter configurations, which measure mirror velocity rather than position, have been proposed as a means to suppress quantum backaction noise, but practical implementations remain at an early stage. In this work, we present a table-top realization of the Einstein-Podolsky-Rosen (EPR) speedmeter concept, employing an optical readout scheme based on two orthogonal polarization modes that probe the interferometer with different effective bandwidths. Using a triangular cavity, we demonstrate that the differential optical response between the linear p- and s-polarized modes exhibits a speedlike frequency dependence: vanishing at DC and increasing linearly with signal frequency, up to the bandwidth of the slower mode. With this we show that an optical system equivalent to the EPR speedmeter indeed performs a velocity readout of the end mirror.

This work presents the first demonstration of nonlinear noise regression in the Virgo detector using deep learning techniques. We use DeepClean, a convolutional autoencoder previously shown to be effective in denoising LIGO data, as our tool for modeling and subtracting environmental and technical noise in Virgo. The method uses auxiliary witness channels to learn correlated noise features and remove them from the strain data. For this study, we apply DeepClean to Virgo O3b data, using 225 witness channels selected across 13 targeted frequency bands. Our analysis confirms the presence of nonlinear couplings in the subtracted noise, highlighting the importance of DeepClean-like tools in capturing such effects. We observe up to a 1.3 Mpc improvement in the binary neutron star inspiral range ($\sim 2.5\%$ gain), and an average increase of 1.7% in the recovered signal-to-noise ratio for injected binary black hole signals. Parameter estimation studies further confirm that DeepClean does not introduce bias in the recovery of source parameters. These results demonstrate the robustness of DeepClean on Virgo data and support its adoption in real-time noise subtraction frameworks for future observing runs.

Many well-motivated dark matter (DM) particle candidates can decay into detectable x-ray photons. We analyze the Final Equatorial Depth Survey data from eROSITA early data release to search for unidentified x-ray lines that could indicate DM signals. Having discovered no anomalous signal, we set limits on DM decay rate in the mass range between 1.8 and 18 keV and constrain the parameter space of two DM particles: sterile neutrino and axionlike particle. Finally, we also study the projected sensitivity of eROSITA full sky search, showing that the eROSITA all-sky survey is expected to set the most stringent limits in the soft x-ray band.

Continuous gravitational waves (CWs) from nonaxisymmetric neutron stars (NSs) are key targets for the Advanced LIGO-Virgo-KAGRA detectors. While no CW signals have been detected so far, stringent upper limits on the CW strain amplitude have been established. Detecting CWs is challenging due to their weak amplitude and high computational demands, especially with poorly constrained source parameters. Stochastic gravitational-wave background (SGWB) searches using cross-correlation techniques can identify unresolved astrophysical sources, including CWs, at lower computational cost, albeit with reduced sensitivity. This motivates a hybrid approach where SGWB algorithms act as a first-pass filter to identify CW candidates for follow-up with dedicated CW pipelines. We evaluated the discovery potential of the SGWB analysis tool pystoch for detecting CWs, using simulated signals from spinning down NSs. We then applied the method to data from the third LIGO-Virgo-KAGRA observing run (O3), covering the (20–1726) Hz frequency band, and targeting four supernova remnants: Vela Jr., G347.3-0.5, Cassiopeia A, and the NSs associated with the 1987A supernova remnant. If necessary, significant candidates are followed up using the five-vector resampling and band-sampled data frequency-Hough techniques. However, since no interesting candidates were identified in the real O3 analysis, we set 95% confidence-level upper limits on the CW strain amplitude $h_0$. The most stringent limit was obtained for Cassiopeia A, and is $h_0=1.13\times {10}^{-25}$ at 201.57 Hz with a frequency resolution of $1/32\mathrm{Hz}$. As for the other targets, the best upper limits have been set with the same frequency resolution, and correspond to $h_0=1.20\times {10}^{-25}$ at 202.16 Hz for G347.3-0.5, $1.20\times {10}^{-25}$ at 217.81 Hz for Vela Jr., and $1.47\times {10}^{-25}$ at 186.41 Hz for the NS in the 1987A supernova remnant.

Dark objects streaming into the solar system can be probed using gravitational wave (GW) experiments through the perturbations that they would induce on the detector test masses. In this work, we study the detectability of the resulting gravitational signal for a number of current and future GW observatories. Dark matter in the form of clumps or primordial black holes with masses in the range ${10}^7–{10}^{11}\mathrm{g}$ can be detected with the proposed DECIGO experiment.

We discuss a search for the electric dipole moment (EDM) of a light beta-radioactive ion using a compact ion trap by adapting the “frozen-spin” method. The measurement will be done on ions stripped of their valence electrons, thereby bypassing the significant Schiff screening that hinders the application of successful contemporary EDM searches using heavy neutral atoms and molecules to light nuclei. We identified ${}^8\mathrm{Li}$ as the most promising candidate for a proof-of-concept EDM search and we estimate that the current indirect proton EDM limit of a few ${10}^{-25}e·\mathrm{cm}$ set by ${}^{199}\mathrm{Hg}$ measurements can be surpassed with a week of measurement time at existing facilities.

We study linear relations between color-ordered all-plus amplitudes at one loop in Yang-Mills theory. We show that on general grounds, there are $(n-1)!/2-2$ relations for $n\ge 5$, leaving only two independent color-ordered amplitudes. We present two complementary approaches to finding such relations; one using numerical linear algebra and the other using syzygies in computational algebraic geometry. We obtain explicit forms for all relations through $n=7$. We also study relations for the tree-level maximally helicity violating amplitudes through $n=8$. The latter relations include the well-known color and Bern-Carrasco-Johansson identities.

Inspired by the promising prospect of the $B_c(2S)$ meson at the coming HL-LHC experiments, the nonleptonic $B_c(2S)$ meson weak decays induced by both the $b$ and $c$ decays are investigated with the quantum chromodynamics (QCD) factorization approach. It is found that branching ratios for the color- and Cabibbo-Kobayashi-Maskawa (CKM)-favored $B_c(2S)\to B_s\rho$, $B_s\pi$ decays can reach up to $\mathcal{O}({10}^{-9})$, which might be measurable. The $B_c(2S)$ decays into the final states containing one charmonium are highly suppressed by the CKM factors, and have significantly small branching ratios, $\mathcal{O}({10}^{-12})$ and less, which might be outside the future measurement capability. This paper provides a ready reference for the future experimental study on the hadronic $B_c(2S)$ weak decays.