It has been shown, via specific examples and a pseudospectrum analysis, that the black hole quasinormal spectra are unstable. The implication of such a result for gravitational-wave physics and of our understanding of black holes is, still, unclear. The purpose of this work is twofold: (i) we show that some of the setups leading to instabilities are unphysical and triggered by exotic matter or extreme spacetimes; (ii) nevertheless, we also show simple examples of compelling physical scenarios leading to spectral instabilities. Our results highlight the importance of understanding the overtone content of time-domain waveforms, and their detectability.

Using the exact solution that describes multicentered rotating black holes, recently found by Teo and Wan, we investigate the innermost stable circular orbit (ISCO) for massive particles and the circular orbit for massless particles moving around a spinning black hole binary. We assume equal masses $M_1=M_2=m$ and equal spin angular momenta $|J_1|=|J_2|$ for both black holes. Firstly, we examine the case where two black holes are spinning in the same direction ($J_1=J_2$). We clarify that for particles rotating in the same direction as (opposite directions to) black holes’ spin, the greater the spin angular momenta of the black holes, the more the radii of the ISCO for massive particles, and the circular orbit for massless particles decreases (increases). We show that distinct ISCO transitions occur for particles rotating in the same direction as the black holes in three ranges of spin angular momenta: $0<J_1/m^2=J_2/m^2<0.395\dots$, $0.395\dots <J_1/m^2=J_2/m^2<0.483\dots$, and $0.483\dots <J_1/m^2=J_2/m^2<0.5$. Conversely, particles rotating in the opposite direction to the black holes exhibit a consistent transition pattern for the case $0<J_1/m^2=J_2/m^2<0.5$. Secondly, we study the situation where binary black holes are spinning in opposite directions ($J_1=-J_2$). We clarify that for large (small) separations between black holes, the ISCO appears near the black hole that is spinning in the same (opposite) direction as particles’ rotation. Additionally, we show that different ISCO transitions occur in the three angular momentum ranges: $0<J_1/m^2=-J_2/m^2<0.160\dots$, $0.160\dots <J_1/m^2=-J_2/m^2<0.467\dots$, and $0.467\dots <J_1/m^2=-J_2/m^2<0.5$.

The accurate observations of neutron stars have deepened our knowledge of both general relativity and the properties of nuclear physics at large densities. Relating observations to the microphysics that govern these stars can sometimes be aided by approximate universal relations. One such relation connects the ratio of the central pressure to the central energy density and the compactness of the star, and it has been found to be insensitive to realistic models for the equation of state to a $\sim 10\%$ level. In this paper, we clarify the meaning of the microscopic quantity appearing in this relation, which is reinterpreted as the average of the speed of sound squared in the interior of a star, $⟨c_s^2⟩$. The physical origin of the quasiuniversality of the $⟨c_s^2⟩-C$ relation is then investigated. Using post-Minkowskian expansions, we find it to be linked to the Newtonian limit of the structure equations and the fact that the equations of state that describe neutron stars are relatively stiff. The same post-Minkowskian approach is also applied to the relations between $⟨c_s^2⟩$, the moment of inertia, and the tidal deformability of a neutron star, in which cases the same degree of universality is found across post-Minkowskian orders.

Recently massless test scalar field perturbations of the holonomy-corrected black holes [Z. S. Moreira et al., Quasinormal modes of a holonomy corrected Schwarzschild black hole, Phys. Rev. D 107, 104016 (2023).] were studied in order to estimate quantum corrections to the quasinormal spectrum of a black hole. Here we study both the fundamental mode and overtones of scalar, electromagnetic and Dirac fields with the help of the Leaver method and higher-order WKB formula with Padé approximants. We observe that the overtones depend on the geometry near the event horizon, while the fundamental mode is localized near the peak of the potential barrier, what agrees with previous studies. We showed that unlike a massless field, the massive one possesses arbitrarily long-lived modes. We also obtain the analytical eikonal formula for quasinormal modes and its extension beyond eikonal approximation as a series in powers of $1/\ell$, where $\ell$ is the multipole number.

In fluid dynamics, the Magnus effect is the force perpendicular to the motion of a spinning object as it moves through a medium. In general relativity, an analogous effect exists for a spinning compact object moving through matter, purely as a result of gravitational interactions. In this work, we consider a Kerr black hole moving at relativistic velocities through scalar dark matter that is at rest. We simulate the system numerically and extract the total spin-curvature force on the black hole perpendicular to its motion. We confirm that the force scales linearly with the dimensionless spin parameter $a/M$ of the black hole up to $a/M=0.99$ and measure its dependence on the speed $v$ of the black hole in the range $0.1\le v\le 0.55$ for a fixed spin. Compared to previous analytic work applicable at small $v$, higher-order corrections in the velocity are found to be important: the total force is nonzero, and the dependence is not linear in $v$. We find that in all cases the total force is in the opposite direction to the hydrodynamical analog, although at low speeds, it appears to approach the expectation that the Weyl and Magnus components cancel. Spin-curvature effects may leave an imprint on gravitational wave signals from extreme mass-ratio inspirals, where the secondary black hole has a non-negligible spin and moves in the presence of a dark matter cloud. We hope that our simulations can be used to support and extend the limits of analytic results, which are necessary to better quantify such effects in the relativistic regime.

Causal set theory is perhaps the most minimalistic approach to quantum gravity, in the sense that it makes next to zero assumptions about the structure of spacetime below the Planck scale. Yet even with this minimalism, the continuum limit is still a major challenge in causal sets. One aspect of this challenge is the measurement of distances in causal sets. While the definition and estimation of timelike distances is relatively straightforward, dealing with spacelike distances is much more problematic. Here we introduce an approach to measure distances between spacelike separated events based on their causal overlap. We show that the distance estimation errors in this approach vanish in the continuum limit even for the smallest distances of the order of the Planck length. These results are expected to inform the causal set geometrogenesis in general, and in particular the development of evolving causal set models in which space emerges from causal dynamics.

Large scale structures provide valuable information of the primordial perturbations that encode the secrets of the origin of the Universe. It is an essential step to map between observables and their initial coordinates, called Lagrangian space, from which primordial perturbations transfer their information to structures via linear theory. By using numerical simulations and state-of-the-art reconstruction techniques, we report the accuracy of estimating the Lagrangian coordinates of galaxies and galaxy clusters, represented by dark matter halos in various ranges of mass, and study the accuracy of this remapping on the angular momentum (spin) reconstruction. Our work shows that galaxy groups and clusters, represented by halos with mass $\gtrsim {10}^{13}{M}_{\odot }$, can be accurately remapped to Lagrangian space, and their spin reconstruction errors are dominated by the reconstructed initial gravitational potential. For all mass ranges, the errors of Lagrangian remapping, as well as redshift space distortions, play subdominant roles in estimating their angular momenta. This study explains the low correlation level between observed galaxy spins and reconstructed cosmic initial conditions and illustrates the potential of using angular momenta of cosmic structures to improve the reconstruction of primordial perturbations.

We study dark matter production from mediator decays in scenarios with an epoch of early matter domination. Particles that mediate interactions between dark matter and the standard model particles are kinematically accessible to the thermal bath as long as their mass is below the reheating temperature of the Universe after inflation. Decay of on-shell mediators can then lead to copious production of dark matter during early matter domination or a preceding radiation-dominated phase. In particular, for mediators that are charged under the standard model, it can exceed the standard freeze-in channel due to inverse annihilations at much lower temperatures (often by many orders of magnitude). The requirement to obtain the correct relic abundance severely constrains the parameter space for dark matter masses above a few TeV.

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multicomponent primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment $\mu \gtrsim 1.6\times {10}^{-11}{\mu }_B$, while future cosmological experiments will be able to probe nonthermal production of RH neutrinos, becoming competitive with stellar limits.

We study the performance of six $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) models, with four of them allowing for nonflat spatial hypersurfaces (nonzero current value of the spatial curvature density parameter ${\mathrm{\Omega }}_k$) and three of them allowing for a nonunity value of the lensing consistency parameter $A_L$. We also study a set of six XCDM models where the nonevolving cosmological constant $\mathrm{\Lambda }$ dark energy density is replaced by a dynamical dark energy density X-fluid parametrized by a nonevolving equation of state parameter $w$. For the nonflat models we consider two different primordial power spectra, Planck $P(q)$, used by the Planck collaboration, and new $P(q)$, resulting from quantum fluctuations in a not-necessarily-very-slow-roll nonflat inflation model. These models are constrained by and tested against: Planck 2018 CMB temperature and polarization power spectra data (P18); Planck 2018 CMB lensing potential power spectrum data (lensing); and, an updated compilation of baryon acoustic oscillation, type Ia supernova, Hubble parameter [$H(z)$], and growth factor [$f{\sigma }_8$] data points [collectively denoted by non-CMB (new) data], individually and jointly. P18 data favor ${\mathrm{\Omega }}_k<0$ (closed spatial geometry) for the $\mathrm{\Lambda }\mathrm{CDM}$ and XCDM models and $w<-1$ (phantomlike dynamical dark energy) for the XCDM models while non-CMB (new) data favor ${\mathrm{\Omega }}_k>0$ (open geometry) in the case of the $\mathrm{\Lambda }\mathrm{CDM}$ models and ${\mathrm{\Omega }}_k<0$ (closed geometry) and $w>-1$ (quintessencelike dynamical dark energy) for the XCDM models. When P18 and non-CMB (new) data are jointly analyzed there is weak evidence in favor of open spatial geometry and moderate evidence in favor of quintessencelike dynamical dark energy. On the other hand, regardless of data considered, $A_L>1$ is always favored, with different degrees of evidence, even for $\mathrm{P}18+\mathrm{lensing}+\mathrm{non}\text{−}\mathrm{CMB}$ (new) data. According to Akaike and deviance information criterion results, $A_L$-varying models are positively favored over the flat $\mathrm{\Lambda }\mathrm{CDM}$ model for $\mathrm{P}18+\mathrm{lensing}+\mathrm{non}\text{−}\mathrm{CMB}$ (new) data. The XCDM model cosmological parameter constraints obtained from P18 or $\mathrm{P}18+\mathrm{lensing}$ data and from non-CMB (new) data are incompatible at $>3\sigma$, ruling out the three $A_L=1$ XCDM models at $>3\sigma$. In the nine models not ruled out by $>3\sigma$ incompatibilities between parameter values determined from different datasets, for the $\mathrm{P}18+\mathrm{lensing}+\mathrm{non}$-CMB (new) dataset we find little deviation from flat geometry and moderate deviation from a cosmological constant. In all six nonflat models that are not ruled out at $>3\sigma$, open geometry is mildly favored (by at most $0.8\sigma$), and in all three $\mathrm{XCDM}+A_L$ models (that are not ruled out at $>3\sigma$) quintessencelike dynamical dark energy is moderately favored (by at most $1.6\sigma$). In the $A_L=1$ nonflat $\mathrm{\Lambda }\mathrm{CDM}$ cases, we find for $\mathrm{P}18+\mathrm{lensing}+\mathrm{non}\text{−}\mathrm{CMB}$ (new) data ${\mathrm{\Omega }}_k=0.0009\pm 0.0017$ [$0.0008\pm 0.0017$] for the Planck [new] $P(q)$ model, favoring open geometry at $0.53\sigma$ [$0.47\sigma$]. Given these results, the flat $\mathrm{\Lambda }\mathrm{CDM}$ model remains the simplest (largely) observationally-consistent cosmological model. Our cosmological parameter constraints obtained for the flat $\mathrm{\Lambda }\mathrm{CDM}$ model (and other models), when $\mathrm{P}18+\mathrm{lensing}+\mathrm{non}\text{−}\mathrm{CMB}$ (new) data are considered, are the most restrictive results to date.

In this work we study the power spectrum of the stochastic gravitational wave background produced by standard and biased domain wall networks, using the velocity-dependent one-scale model to compute the cosmological evolution of their characteristic scale and root-mean-squared velocity. We consider a standard $\text{radiation}+\mathrm{\Lambda }\mathrm{CDM}$ background and assume that a constant fraction of the energy of collapsing domain walls is emitted in the form of gravitational waves. We show that, in an expanding background, the total energy density in gravitational radiation decreases with cosmic time (after a short initial period of quick growth). We also propose a two parameter model for the scale-dependence of the frequency distribution of the gravitational waves emitted by collapsing domain walls. We determine the corresponding power spectrum of the stochastic gravitational wave background generated by domain walls, showing that it is a monotonic decreasing function of the frequency for frequencies larger than that of the peak generated by the walls that have decayed most recently. We also develop an analytical approximation to this spectrum, assuming perfect linear scaling during both the radiation and matter eras, in order to characterize the dependence of the amplitude, peak frequency and slope of the power spectrum on the model parameters.

A new generation of neutrino observatories will search for PeV-EeV neutrinos interacting in the ice by detecting radio pulses. Extended air showers propagating into the ice will form an important background and could be a valuable calibration signal. We present results from a Monte-Carlo simulation framework developed to fully simulate radio emission from cosmic-ray particle cascades as observed by in-ice radio detectors in the polar regions. The framework involves a modified version of coreas (a module of corsika7) to simulate in-air radio emission and a geant4-based framework for simulating in-ice radio emission from cosmic-ray showers as observed by in-ice antennas. The particles that reach the surface of the polar ice sheet at the end of the corsika7 simulation are injected into the geant4-based shower simulation code that takes the particles and propagates them further into the ice sheet, using an exponential density profile for the ice. The framework takes into account curved ray paths caused by the exponential refractive index profiles of air and ice. We present the framework and discuss some key features of the radio signal and radio shower footprint for in-ice observers.

We use publicly available data to perform a search for correlations of high energy neutrino candidate events detected by IceCube and high-energy photons seen by the HAWC Collaboration. Our search is focused on unveiling such correlations outside of the Galactic plane. This search is sensitive to correlations in the neutrino candidate and photon skymaps which would arise from a population of unidentified point sources. We find no evidence for such a correlation, but suggest strategies for improvements with new datasets.

The next generation of ground-based gravitational-wave detectors will look much deeper into the Universe and have unprecedented sensitivities and low-frequency capabilities. Especially alluring is the possibility of detecting an early-Universe cosmological stochastic background that could provide important insights into the beginnings of our Universe and fundamental physics at extremely high energies. However, even if next-generation detectors are sensitive to cosmological stochastic backgrounds, they will be masked by more dominant astrophysical backgrounds, namely the residual background from the imperfect subtraction of resolvable compact binary coalescences (CBCs) as well as the CBC background from individually unresolvable CBCs. Using our latest knowledge of masses, rates, and delay time distributions, we present a data-driven estimate of the unresolvable CBC background that will be seen by next-generation detectors. Accounting for statistical errors and astrophysical systematics, this estimate quantifies an important piece in the CBC noise budget for next-generation detectors and can help inform detector design and subtraction algorithms. Importantly, we find that the neutron-star black-hole binary contribution to the unresolved CBC background is further comparable to the binary neutron star contribution. We compare our results with predictions for backgrounds from several cosmological sources in the literature, finding that the unresolvable background will likely be a significant impediment for many models. This motivates the need for simultaneous inference methods or other statistical techniques to detect early-Universe cosmological backgrounds.

Diffusion tensor coefficients play a central role in describing cosmic-ray transport in various astrophysical environments permeated with magnetic fields, which are usually modeled as a fluctuating field on top of a mean field. In this article, a formal derivation of these coefficients is presented by means of the calculation of velocity decorrelation functions of particles. It relies mainly on expanding the two-point correlation function of the (fluctuating) magnetic field experienced by the particles between two successive times in the form of an infinite Dyson series and retaining a class of terms that converge to a physical solution. Subsequently, the velocity decorrelation functions, themselves expressed as Dyson series, are deduced from an iteration procedure that improves on the partial summation scheme. The results are shown to provide approximate solutions compared to those obtained by Monte Carlo simulations as long as the Larmor radius of the particles is larger than at least one tenth of the largest scale of the turbulence.

We constrain the neutrino-dark matter cross section using properties of the dark matter density profiles of Milky Way dwarf spheroidal galaxies. The constraint arises from core-collapse supernova neutrinos scattering on dark matter as a form of energy injection, allowing the transformation of the dark matter density profile from a cusped profile to a flatter profile. We assume a standard cosmology of dark energy and cold, collisionless, and non-self-interacting dark matter. By requiring that the dark matter cores do not lose too much mass or overshoot constraints from stellar kinematics, we place an upper limit on the cross section of ${\sigma }_{\nu -\mathrm{DM}}(E_{\nu }=15\mathrm{MeV},m_{\chi }\lesssim 130\mathrm{GeV})\approx 3.4\times {10}^{-23}{\mathrm{cm}}^2$ and ${\sigma }_{\nu -\mathrm{DM}}(E_{\nu }=15\mathrm{MeV},m_{\chi }\gtrsim 130\mathrm{GeV})\approx 3.2\times {10}^{-27}{(\frac{m_{\chi }}{1\mathrm{GeV}})}^2{\mathrm{cm}}^2$, which is stronger than previous bounds for these energies. Consideration of baryonic feedback or host galaxy effects on the dark matter profile can strengthen this constraint.

We illustrate and discuss the phenomenology of a model featuring a two-Higgs doublet sector augmented by two $SU(2)$ singlet scalars. The gauge symmetry group is extended as well with a $U(1{)}_{L_{\mu }-L_{\tau }}$ component whose spontaneous breaking leads to the gauge boson which has an important effect in the muon ($g-2$). A global PQ symmetry is introduced upon its breaking; we have the axion particle which is also dark matter (DM) in our work. In particular, we have focused on type-X and type-II 2HDM models and found out that ($g-2$) cannot be explained only by the scalar sector for type-II 2HDM mainly due to stringent constraint from $b\to s\gamma$ resulted in $M_{H^{\pm }}>800\mathrm{GeV}$. For type-II 2HDM, we can have axion coupling with the gluons which generates the axion potential and possible explanation for the strong $CP$ problem. The proposed model accommodates neutrino masses via the type-I seesaw mechanism with an upper bound on the right-handed neutrino mass 1 GeV (1 TeV) for type-II (type-X) 2HDM due to the presence of Planck scale suppressed operators. Moreover, we also have additional scalars which affect the oblique parameters and hence the W-boson mass which leads us to explain the W-boson mass observed at the CDF-II detector. The most stringent constraints on the masses and quartic couplings come from the perturbativity and potential bound from the below conditions which leads to fine-tuning among the parameters in part of the parameter space. Finally, we discuss the possible detection prospects of the axion DM and the additional gauge boson.

In the Standard Model effective field theory (SMEFT), the $SU(2{)}_L\times U(1{)}_Y$ symmetry of the Standard Model is linearly realized. However, it is possible that more general effective field theories, such as the Higgs effective field theory (HEFT) where this symmetry is realized nonlinearly, are needed to describe the data. Identifying physics beyond SMEFT could shed light on the nature of Higgs and the realization of the electroweak symmetry. We explore the possibility of such an identification by studying the effects of scalar and vector new physics operators on the angular distribution of ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c(\to \mathrm{\Lambda }\pi )\tau {\overline{\nu }}_{\tau }$. This decay is sensitive to the six-dimensional effective operator $O_V^{LR}\equiv (\overline{\tau }{\gamma }^{\mu }{P}_L{\nu }_{\tau })(\overline{c}{\gamma }_{\mu }{P}_{R}b)$, which is present in HEFT but suppressed in SMEFT. We identify the angular observables that can have significant contributions from $O_V^{LR}$ and hence would be useful for probing not only beyond the Standard Model physics but also physics beyond SMEFT. We further find that constraining the branching ratio of $B_c\to \tau {\overline{\nu }}_{\tau }$ would be crucial for performing this task.

In flavor models the vacuum alignment of flavons is typically achieved via the $F$-terms of certain fields in the supersymmetric limit. We propose a method for preserving such alignments, up to a rescaling of the vacuum expectation values, even after softly breaking supersymmetry (and the flavor symmetry). This facilitates the vacuum alignment in models which are nonsupersymmetric at low energies. Examples of models with different flavor groups, namely, $A_4$, $T_7$, $S_4$, and $\mathrm{\Delta }(27)$, are discussed.

The nature of dark matter (DM) and its interaction with the Standard Model (SM) is one of the biggest open questions in physics nowadays. The vast majority of theoretically motivated ultralight-DM (ULDM) models predict that ULDM couples dominantly to the SM strong/nuclear sector. This coupling leads to oscillations of nuclear parameters that are detectable by comparing clocks with different sensitivities to these nature’s constants. Vibrational transitions of molecular clocks are more sensitive to a change in the nuclear parameters than the electronic transitions of atomic clocks. Here, we propose the iodine molecular ion, ${\mathrm{I}}_2^{+}$, as a sensitive detector for such a class of ULDM models. The iodine’s dense spectrum allows us to match its transition frequency to that of an optical atomic clock (${\mathrm{Ca}}^{+}$) and perform correlation spectroscopy between the two clock species. With this technique, we project a few-orders-of-magnitude improvement over the most sensitive clock comparisons performed to date. We also briefly consider the robustness of the corresponding “Earth-bound” under modifications of the ${\mathbb{Z}}_{\mathcal{N}}$-QCD axion model.