Studies regarding extra dimensions have been of great interest in modern theoretical physics, including their observational consequences from future gravitational wave (GW) observatories. In this direction, extreme mass-ratio inspirals (EMRIs), attracting considerable interest in GW astronomy and fundamental physics, can potentially provide a useful platform for the search of extra dimensions. In this paper, we examine a rotating braneworld black hole in the context of equatorial eccentric EMRIs and attempt to provide an order of magnitude analysis for the extra-dimensional parameter termed “tidal charge.” We estimate GW fluxes for the dominant mode and determine the impact of the tidal charge parameter on the orbital evolution. We further evaluate the prospects of detecting such a parameter through mismatch computation. We observe a significant enhancement in the mismatch as the value of orbital eccentricity or tidal charge parameter increases; the phenomenon becomes more obvious for rapidly rotating massive black holes. Thus, the study suggests that eccentric EMRIs can potentially probe the existence of extra dimensions with future low-frequency detectors such as the Laser Interferometer Space Antenna.

We report the existence of horizonless compact object solutions supported by dust in the minimal exponential measure (MEMe) model, a theory which modifies the couplings between gravity and matter without introducing dynamical degrees of freedom. For a perfect fluid source, the field equations for the MEMe model can be rewritten as the Einstein field equations sourced by a perfect fluid with a transformed equation of state, which can endow a sufficiently dense cloud of dust with an effective pressure. The resulting dust-supported horizonless compact objects can have masses below $\sim {10}^{-11}{M}_{\odot }$, making them suitable as massive compact halo objects comprising a significant mass fraction for dark matter. A necessary condition for the existence of these compact object solutions is that the single free parameter in the MEMe model is positive valued. Additionally, we find that this positive sign for the parameter can provide a mechanism for suppressing the formation of (primordial) black holes from the gravitational collapse of matter below a certain mass scale.

ModMax electrodynamics is a remarkable example of nonlinear electrodynamics that preserves both conformal and duality invariance. When coupled to General Relativity, the resulting black hole solutions introduce a tunable nonlinearity parameter that effectively screens the electromagnetic charge. We study the scattering and absorption of massless scalar waves by a ModMax black hole by applying the partial waves method. We also compute some well-known analytical approximations for the scattering and absorption cross sections and contrast them with the full numerical solution. Our results adequately reproduce those of Maxwell electrodynamics in the appropriate limit and illustrate the effect of the charge screening in ModMax electrodynamics.

Small deviations in the spacetime around black holes can lead to instabilities in the underlying quasinormal mode spectrum, potentially altering the hierarchy of its overtones. A practical way to induce such spectral instability is by introducing small modifications to the effective potential governing the dynamics of fluctuations in the black hole spacetime. While finding a physically meaningful interpretation for such ad hoc modifications in an astrophysical context can be challenging, analog black hole models provide an alternative framework to explore their effects and study the instabilities. In this work, we consider an analog black hole modeled by a draining bathtub flow and demonstrate that vorticities in the fluid introduce a small bump in the effective potential of the wave equation. This naturally realizes a physically motivated version of the elephant and the flea configuration. We analyze the spectrum using two complementary approaches: direct mode computation via two distinct frequency-domain methods and time evolution of initial perturbations. As in astrophysical black holes, the vorticities destabilizes the quasinormal mode spectrum of the analog system, possibly yielding time evolution with long-lived ringing effects, akin to those observed for massive fields in curved spacetimes.

For gravitationally lensed type II signals, the phase of the dominant (2, 2) mode and the higher-order (3, 3) mode is offset by $-\pi /12$, or roughly $-0.26\mathrm{rad}$. Using this, we develop a test for type II imagery by allowing the phases of the (2, 2) and (3, 3) modes to vary separately and introducing a new waveform parameter to represent the phase offset between the two. We use simulated, asymmetric mass ratio, precessing signals to show that the test can reproduce the $-\pi /12$ phase offset when detected by three detectors for H-L optimal $\mathrm{SNR}\gtrsim 40$ and $\mathcal{M}\le 30$. We analyze GW190412 and GW190814 using this parametrization, measuring the offset to be ${0.13}_{-0.17}^{+0.22}$ for GW190412 and $-0.05_{-0.22}^{+0.20}$ for GW190814. We also measure the Bayes factor in support of zero phase offset, ${\log }_{10}{\mathcal{B}}_{\mathrm{\Delta }\phi =0}$, to be $-0.14$ for GW190412 and 0.21 for GW190814. This implies our results are not strong enough to confidently argue if either event is a type II image and is consistent with our statistical analysis.

The kinematics of the ultradiffuse galaxy NGC1052-DF44 is primarily influenced by the presence of dark matter (DM). In this paper, we conduct a contrasting kinematic study of DF44 within the alternative modified gravity framework. In comparison to the Navarro-Frenk-White DM profile, we test three alternative gravity models viz Milgromian dynamics (MOND), characterized by a known acceleration scale, a generic $f(R)$ model, assuming an expansion of the Ricci scalar, and a quantum gravity-inspired renormalization group correction to general relativity (RGGR), which involves the running of the gravitational coupling parameter $G$ with the Universe’s energy scale. For each gravity model, we evaluate the velocity dispersion (VD) of the galaxy beyond the conventional radial isotropic assumption and extend to two anisotropy scenarios, i.e., constant and Osipkov-Merritt. Our results show that all three gravity models can provide consistent fits to the observed VD of DF44; however, only MOND and RGGR remain competitive with the Navarro-Frenk-White profile. Interestingly, the constant anisotropy scenario in all the models is also found to be competitive with the complete isotropic assumption.

We adopt a novel approach to combine path integral methods with loop quantum gravity (LQG). Our approach builds upon the recently developed coherent state path integral formulation of LQG to compute the one-loop effective action. We compare this methodology with the conventional quantum field theory (QFT) prescription for path integrals and extend the formalism to account for the dependence on boundary (coherent) states. This work aims to explore two aspects: to compare our results with the divergences observed in one-loop calculations of Einstein gravity testing UV finiteness and to initiate an exploration of the IR effective properties of LQG. We compute the effective action around flat spacetime obtaining analytical and numerical results in the long and short wavelength approximations, respectively. Due to the one-loop dynamics of the LQG area, we find a divergence-free effective action. We study the propagator and the dynamical modes and derive the quantum equation of motion at one loop. We ensure consistency with the semiclassical approximation in the long wavelength limit and, beyond this approximation, analyze numerical scaling as the lattice size increases.

We explore the origin of the preference of Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2) baryon acoustic oscillation measurements and external data from cosmic microwave background (CMB) and type Ia supernovae (SNIa) that dark energy behavior departs from that expected in the standard cosmological model with vacuum energy ($\mathrm{\Lambda }\mathrm{CDM}$). In our analysis, we allow a flexible scaling of the expansion rate with redshift that nevertheless allows reasonably tight constraints on the quantities of interest, and adopt and validate a simple yet accurate compression of the CMB data that allows us to constrain our phenomenological model of the expansion history. We find that data consistently show a preference for a 3%–4% increase in the expansion rate at $z\simeq 0.7$ relative to that predicted by the standard $\mathrm{\Lambda }\mathrm{CDM}$ model, in excellent agreement with results from the less flexible $(w_0,w_a)$ parametrization which was used in previous analyses. Even though our model allows a departure from the best-fit $\mathrm{\Lambda }\mathrm{CDM}$ model at zero redshift, we find no evidence for such a signal. We also find no evidence (at greater than $1\sigma$ significance) for a departure of the expansion rate from the $\mathrm{\Lambda }\mathrm{CDM}$ predictions at higher redshifts for any of the data combinations that we consider. Overall, our results strengthen the robustness of the findings using the combination of DESI, CMB, and SNIa data to dark-energy modeling assumptions.

We apply two machine learning methods, a CNN deep-learning model and a gradient-boosting decision tree, to estimate individual cluster optical depths from observed properties derived from multiple complementary datasets. The models are trained and tested with simulated N-body derived halo catalogs and synthetic full-sky cosmic microwave background (CMB) maps designed to mirror data from the DESI and Simons Observatory experiments. Specifically, the thermal Sunyaev-Zel’dovich (tSZ) and CMB lensing convergence along with cluster virial mass estimates are used as features to train the machine learning models. The predicted optical depths are combined with kinematic Sunyaev-Zel’dovich (kSZ) measurements to estimate individual cluster radial peculiar velocities. The method is shown to recover an unbiased estimate of the pairwise velocity statistics of the simulated cluster sample. The models are demonstrated to be effective for halos with mass range ${10}^{13}{M}_{\odot }<M_{200}<{10}^{15}{M}_{\odot }$ over a redshift range $0<z<1$, and validated in the presence of primary CMB, instrument noise, lensing convergence noise, and potential uncertainties in halo virial mass estimates. We apply the method to Atacama Cosmology Telescope (ACT) CMB data, using ACT DR4 component-separated maps for tSZ and CMB lensing and ACT DR5 maps for kSZ, in conjunction with galaxy clusters observed in the SDSS DR15 spectroscopic survey. We demonstrate that the machine learning approach is an effective one to analyze data from current and upcoming CMB experiments such as Simons Observatory and Cerro Chajnantor Atacama Telescope, and galaxy surveys, such as DESI and Roman, for which the pairwise velocity statistics can provide valuable insights into the properties of neutrinos and gravity on cosmic scales.

In the Regge-Teitelboim model, gravity is described by embedding the space-time manifold in a (usually flat) fixed higher-dimensional background, where the embedding coordinates, rather than the metric tensor, are the dynamical degrees of freedom. Stern and Xu extended the Regge-Teitelboim framework to encompass scenarios where the background embedding space is not flat, noting that when the background is a five-dimensional de Sitter space, the Robertson-Walker manifold undergoes a transition from a decelerating phase to an accelerating one. Previously, we constrained this model using only low-redshift observations. Here, we further explore the observational constraints on this scenario and report significantly more stringent constraints by including high-redshift data, specifically from the cosmic microwave background. Our results are consistent with the $\mathrm{\Lambda }$ cold dark matter model, with the putative model-specific energy component responsible for the recent acceleration being constrained to ${\mathrm{\Omega }}_{\mathrm{RT}}<0.006$ and the de Sitter curvature radius in units of the Hubble constant being constrained to $L{H}_0>1.45$, both at the 95% confidence level.

We consider dark matter self-interactions mediated by ultralight scalars. We show that effectively massless mediators lead to an enhancement of the matter power spectrum, while heavier mediators lead to a suppression, together with a feature around their Jeans scale. We derive the strongest present constraints by combining Planck and BOSS data. The recent DESI measurements of baryon acoustic oscillations exhibit a mild $2\sigma$ preference for long-range self-interactions, as strong as 4 per mille of the gravitational coupling. Full-shape analyses of forthcoming DESI and Euclid data will confirm or disprove such a hint.

Recently, the Dark Energy Survey Collaboration presented evidence that the equation of state $w$ of the dark energy is varying or $w\simeq -0.948$ if it is constant. In either case, the dark energy cannot be due to a cosmological constant alone. Here, we study an ultralight axion (or axionlike particle) with mass $m_{\varphi }\simeq 2\times {10}^{-33}\mathrm{eV}$ that has properties that can explain the new $w$ measurement. In particular, $w\ge -1$ and a negative cosmological constant $\mathrm{\Lambda }<0$ is preferred in this model. We also present a simple formula for $w$ for the model to ease data fitting.

We revisit a class of coupled dark energy models where dark energy interacts with dark matter via phenomenological energy exchange terms. We include the perturbation of the Hubble rate in the interaction term, which was absent in previous works. We also consider more recent datasets such as cosmic microwave background (CMB) anisotropies from Planck 2018, type I-a supernovae (SNIa) measurements from $\mathrm{Pantheon}+$ and data from baryon acoustic oscillations (BAO), and redshift space distortions (RSD). One of the models presents a strong incompatibility when different cosmological datasets are used. We analyze the influence of the SH0ES Cepheid host distances on the results and, although for one model the discrepancy of $H_0$ is reduced to $3.2\sigma$ when compared to the value obtained by the Planck collaboration and $3.6\sigma$ when compared to the SH0ES team, joint analysis is incompatible. Including BAO with RSD shows incompatibility with SH0ES for all models considered here. We perform a model comparison and, although there is a preference for interacting dark energy over $\mathrm{\Lambda }\mathrm{CDM}$ for all of the models for joint analysis $\mathrm{CMB}+\mathrm{BAO}+\mathrm{RSD}+\mathrm{SNIa}$, most of the 2D contours do not overlap. We conclude that the models of interactions in the dark sector considered in this paper are not flexible enough to fit all the cosmological data including values of $H_0$ from SH0ES in a statistically acceptable way. Therefore, the addition of one extra degree of freedom (i.e., the coupling between dark matter and dark matter) does not help enough to alleviate the already existing tension in the vanilla $\mathrm{\Lambda }\mathrm{CDM}$, suggesting that the models would need to be modified to include further flexibility of predictions to help elucidate this issue.

Weak gravitational lensing of the cosmic microwave background (CMB) has been established as a robust and powerful observable for precision cosmology. However, the impact of Galactic foregrounds, which has been studied less extensively than many other potential systematics, could in principle pose a problem for CMB lensing measurements. These foregrounds are inherently non-Gaussian and hence might mimic the characteristic signal that lensing estimators are designed to measure. We present an analysis that quantifies the level of contamination from Galactic dust in lensing measurements, focusing particularly on measurements with the Atacama Cosmology Telescope and the Simons Observatory. We employ a whole suite of foreground models and study the contamination of lensing measurements with both individual frequency channels and multifrequency combinations. We test the sensitivity of different estimators to the level of foreground non-Gaussianity and the dependence on sky fraction and multipole range used. We find that Galactic foregrounds do not present a problem for the Atacama Cosmology Telescope experiment (the bias in the inferred CMB lensing power spectrum amplitude remains below $0.3\sigma$). For Simons Observatory, not all foreground models remain below this threshold. Although our results are conservative upper limits, they suggest that further work on characterizing dust biases and determining the impact of mitigation methods is well motivated, especially for the largest sky fractions.

Recent cosmological observations suggest that the dark energy equation of state may have changed in the latter stages of cosmic history. We introduce a quintessence scenario, termed bounded dark energy, capable of explaining this feature in a technically natural way. Our approach is motivated from a bottom-up perspective, based on the concept of mirage cutoff, where we demonstrate the stability of the quintessence potential against large quantum corrections. At the same time, the bounded dark energy framework aligns well with top-down considerations motivated from quantum gravity arguments. We employ both human-driven insights and machine learning techniques to identify explicit realizations of bounded dark energy models. We then perform an analysis based on Markov Chain Monte Carlo method to assess their predictions against cosmic microwave background, galaxy surveys, and supernova data, showing that bounded dark energy provides a good fit to current observations. We also discuss how upcoming measurements can further test and refine our proposal.

The brightest gamma ray burst (GRB) ever observed, GRB221009A, produced a surprisingly large flux of gamma rays with multi-TeV energies, which are expected to be absorbed in interactions with extragalactic background light (EBL). If the highest energy gamma rays were produced at the source, their spectral shape would have to exhibit a nonphysical spike even for the lowest levels of EBL. We show that, for widely accepted models of EBL, the data can be explained by secondary gamma rays produced in cosmic ray interactions along the line of sight, as long as the extragalactic magnetic fields along the line of sight are ${10}^{-16}\mathrm{G}$ or smaller, assuming 1 Mpc correlation length. Our interpretation supports the widely held expectation that GRB jets can accelerate cosmic rays to energies as high as 10 EeV and above, and it has implications for understanding the magnitudes of extragalactic magnetic fields.

We studied the properties of dark matter admixed-neutron stars (DMANS), considering fermionic dark matter (DM) that interacts gravitationally with hadronic matter (HM). Using relativistic mean-field equations of state (EOSs) for both components, we solved the two-fluid Tolman-Oppenheimer-Volkoff equations to determine neutron star (NS) properties assuming that DM is confined within the stellar core. For hadronic matter, we employed realistic EOSs derived from low energy nuclear physics experiments, heavy-ion collision data, and NS observations. To constrain key dark matter parameters—such as particle mass, mass fraction, and the coupling to mass ratio—we applied Bayesian inference, incorporating various astrophysical data including mass, radii, and NICER mass-radius distributions for PSR $\mathrm{J}0740+6620$ and PSR $\mathrm{J}0030+0451$. Additionally, we explored the influence of high-density HM EOSs and examined the impact of stiffer hadronic EOSs, excluding the vector meson self-interaction term. Our findings indicate that current astrophysical observations primarily constrain the dark matter fraction, while providing limited constraints on the particle mass or coupling. However, the dark matter fraction is largely insensitive to how astrophysical observations or uncertainties in the high-density EOS are incorporated. Instead, it is predominantly determined by the stiffness of the hadronic EOS at high densities, with stiffer hadronic EOSs yielding a higher dark matter mass fraction. Therefore, we conclude that the dark matter fraction plays a crucial role in shaping the properties of DMANS. Future investigations incorporating more realistic EOSs and astrophysical observations of other compact objects may provide deeper insights into dark matter.

Pulsars are rotating neutron stars that are observed to be slowing down, implying a loss of their rotational energy. There can be several different physical mechanisms involved in their spin-down process. The properties of fast-rotating pulsars depend on the nature of the neutron star matter, which can also affect the spin-down mechanisms. In this work, we examine three different physical phenomena contributing to the spin-down: magnetic dipole radiation, gravitational mass quadrupole radiation due to the “mountain” formation, and gravitational mass current quadrupole radiation—or the r-modes—and calculate the expressions for the braking indices due to all of them. We have also considered the implications of the uncertainties of the equation of the state of neutron star matter and rapid rotation on the braking indices corresponding to the aforementioned processes and their combinations. In all cases, the rapid rotation results in a departure of the braking index from the standard values when the rotational effects are ignored. If generated with a saturation amplitude within the range of ${10}^{-4}–{10}^{-1}$, the r-mode oscillations dominate the spin-down of millisecond pulsars. Moreover, we explore the braking index in the context of millisecond magnetars. We also study the effects of different choices of baryon mass on the braking indices.

Using the parity doublet model (PDM) for hadronic matter and a modified Nambu-Jona-Lasinio (NJL) model for quark matter, we investigate the potential existence of two- and three-flavor quark matter in neutron star cores. Both models respect chiral symmetry, and a sharp first-order phase transition is implemented via Maxwell construction. We find stable neutron stars with quark cores within a specific parameter space that satisfies current astronomical observations. Typical neutron stars with masses around $1.4M_{\odot }$ may possess deconfined quark matter in their centers. The hybrid star scenario with a two-flavor quark core offers enough parameter space to allow the neutron stars with large quark cores exceeding $\sim 1M_{\odot }$ and to allow the early deconfinement position before $2{\rho }_0$, where ${\rho }_0$ is the nuclear saturation density. The observations of gravitational wave event GW170817 suggest a relatively large chiral invariant mass $m_0=600\mathrm{MeV}$ in the PDM for scenarios involving three-flavor quark matter cores. The maximum mass of the hybrid star with a quark core is found to be approximately $2.2M_{\odot }$ for both two- or three-flavor quark matter in their centers.

Cosmic-ray air shower detection with the low-frequency part of the Square Kilometre Array (SKA) radio telescope is envisioned to yield very high precision measurements of the particle composition of cosmic rays between ${10}^{16}$ and ${10}^{18}\mathrm{eV}$. This is made possible by the extreme antenna density of the core of SKA-Low, surpassing the current most dense radio air shower observatory LOFAR by over an order of magnitude. In order to make these measurements, the technical implementation of this observation mode and the development of reconstruction methods have to happen hand in hand. As a first lower limit of what is obtainable, we apply the current most precise reconstruction methods as used at LOFAR to a first complete simulation of air shower signals for the SKA-Low array. We describe this simulation setup and discuss the obtainable accuracy and resolution. A special focus is put on effects of the dynamic range of the system, beam-forming methods to lower the energy threshold, as well as the limits to the mass composition accuracy given by statistical and systematic uncertainties.