We introduce and define the phenomenological parameter $\kappa$, defined by $\mathrm{\Delta }a/g=\kappa \mathrm{\Delta }(q/m)$, to quantify potential linear coupling between electric charge and gravitational acceleration. A synthesis of existing precision equivalence principle experiments yields the first quantitative estimate of the effective sensitivity to this coupling: $|\kappa |<2.1\times {10}^{-4}\mathrm{kg}{\mathrm{C}}^{-1}$ at 95% confidence level. This constraint is approximately 11 orders of magnitude less stringent than corresponding bounds on composition-dependent violations, revealing that the electromagnetic axis remains a largely underexplored frontier in empirical gravity. We connect $\kappa$ to established frameworks—the Standard-Model Extension and the $TH\epsilon \mu$ formalism—showing that it occupies a region of parameter space untouched by existing high-precision tests. An effective field theory analysis shows that dimension-six operators that couple curvature directly to the electromagnetic field strength are suppressed by the minuscule terrestrial spacetime curvature ($G_N{\rho }_{\oplus }\sim {10}^{-55}{\mathrm{GeV}}^2$) and are therefore phenomenologically irrelevant. Consequently, a future measurement of $\kappa$ at an accessible level would not probe minimal geometric couplings but would signal physics beyond minimal gravitational EFT, such as mediation by light scalar fields as in Einstein-Maxwell-Dilaton theory. We examine the Schiff-Barnhill effect, the primary systematic background for any such measurement, and show how it can be separated from a genuine signal. We outline the necessary experimental strategy, focused on maximizing charge-to-mass ratio differences, to transform this overlooked axis into a targeted probe for new physics.

In the context of Brans-Dicke scalar-tensor theories of gravity, it has recently been shown that the post-Newtonian parameters should be generalized in the context of strongly gravitating bodies, and that its generalization—the so-called exact parameters—actually depends on the pressure and energy density of the considered celestial body. Here we develop two new methods to numerically obtain the exact parameters by means of the usual Tolman-Oppenheimer-Volkoff computation, and find that the difference from the value of standard post-Newtonian parameters can be more than 80% in some situations. We also provide the connection with the Damour-Esposito-Farèse nonperturbative parameter ${\alpha }_{DEF}$. We then apply the methodology to the case of entangled relativity, and derive these exact parameters for the Sun and the Earth, as well as for neutron stars. We argue that current and foreseeable experiments are likely able to constrain the theory under the assumption that ${\mathcal{L}}_m=-\rho$, where $\rho$ is the total energy density. If ${\mathcal{L}}_m=T$ instead, as often advocated in the literature, then there is no deviation with respect to general relativity and the prospects of testing entangled relativity become much more remote in time, as only compact objects with extreme electric or magnetic fields could lead to some deviation from general relativity.

We examine the robustness of black hole ringdown to stochastic horizon-scale structure within an effective field framework in a proof-of-principle Schwarzschild setup. Consistent with the understanding that the spectral instability of quasinormal modes does not necessarily imply observational breakdown, our results demonstrate that the macroscopic gravitational waveform remains robust. We identify the phase averaging mechanism as the physical origin of this stability, demonstrating that the spatial integration of the wave equation efficiently attenuates ultraviolet geometric details below the resolution limit of the probing wavelength. Building on the scaling law $\mathcal{M}\propto {\epsilon }^2$ and the characteristic mismatch profile with respect to $L_c$, we propose a geometric selection rule for observability: a detectable signal imposes a strict dual constraint requiring both macroscopic spatial coherence ($L_c\sim M$) and classical-level intensity ($\epsilon \gtrsim {10}^{-4}$). This criterion quantitatively rules out the observability of incoherent, high-entropy quantum foam in the present static Schwarzschild model, suggesting that any significant ringdown deviation would instead serve as evidence for macroscopically coherent horizon structures.

We construct an analytical black hole solution in the Einstein-Maxwell-Yang-Mills theory with a conformally coupled scalar field in four dimensions, which generalizes the charged Martínez-Troncoso-Zanelli (MTZ) black hole in the presence of self-gravitating non-Abelian gauge fields. The internal gauge group is determined by the horizon curvature, becoming $SU(N)$ in the case of positive curvature and $SU(N-1,1)$ when the curvature is negative. Moreover, this solution is employed as a conformal seed to obtain new meronic spacetimes dressed with all superrenormalizable contributions of the scalar field, which provides the generalization of the Anabalón-Cisterna solution when self–gravitating non-Abelian gauge fields are included. Finally, we consider the non-Noetherian extension of the conformal scalar fields, which still yields a second-order conformally invariant scalar equation, even though the action is not. In that case, we show that static black hole solutions can also be charged with Yang-Mills fields.

Possible violations of the equivalence principle are investigated within the framework of metric-affine gravity, and their connections to finite-temperature effects are highlighted. Thermal corrections to particle dynamics, originally derived in a quantum-field-theory setting, can be evaluated in a purely Riemannian framework and lead to a shift in the gravitational-to-inertial mass ratio. We show that the ensuing departure from universality of free fall can be also formulated in metric-affine gravity, where the presence of the nonmetricity tensor modifies the Newtonian law in a way that closely parallels the finite-temperature scenario. Furthermore, we introduce a generalized Fermi-Walker derivative adapted to non-Riemannian contexts, which naturally reveals that no orthonormal tetrad can be propagated along an observer worldline. Although metric-affine gravity admits a pointwise realization of the Einstein equivalence principle in its gauge-theoretic, elementary-matter form, the new operator offers a direct geometric signature that this principle, in its modern formulation, is not retained, in general. Potential tests of the analyzed effects are also discussed.

We investigate spin-induced scalarization of Kerr black holes in an Einstein-scalar-Gauss-Bonnet (EsGB) model that does not admit a linear tachyonic instability of the scalar-free solution. The scalarization mechanism is therefore genuinely nonlinear. We first analyze the decoupled scalar dynamics on fixed Kerr backgrounds and show that sufficiently rapid rotation modifies the Gauss-Bonnet invariant such that a negative near-horizon region develops near the poles. This region provides a geometric trapping mechanism for nonlinear scalar growth, which becomes effective above a threshold spin $\chi =0.5$. We then construct stationary scalarized black hole solutions with full backreaction and determine their domain of existence. We find that the solutions occupy a finite low-mass high-spin wedge in the spin-mass plane. This is in contrast to spin-induced spontaneous scalarization, where the scalarized solutions form a narrow band. In this wedge, toward the high-spin end, the scalar hair becomes stronger, and the solutions approach a near-extremal regime, while toward the low-spin boundary, the scalar field is strongly suppressed and approaches a weak-hair limit as $\chi \to 0.5$.

In this paper, we investigate a possible time variation of the gravitational constant ($G$) using a nonparametric approach. Our main cosmological probe is the gas mass fraction of galaxy clusters measured from x-ray observations. We also account for the effect of a varying $G$ on the intrinsic luminosity of type Ia supernovae (SNe Ia) through the Chandrasekhar mass–luminosity relation. We consider a specific phenomenological scenario, motivated by some scalar-tensor and screened modified-gravity frameworks, in which the standardized luminosity of SNe Ia decreases with increasing Chandrasekhar mass. Using gas mass fraction measurements jointly with luminosity distances from the Pantheon+ compilation, we reconstruct the evolution of $G$ through Gaussian processes. Our results indicate that a constant gravitational coupling remains broadly consistent with the data, although mild low-redshift departures are allowed.

Quintessential inflation provides a unified description of the early and late accelerated phases of the Universe, linking the inflationary epoch to the present-day dark energy-dominated era through a single scalar degree of freedom. In this work, we explore the implications of this unification for cosmological constraints on the sum of neutrino masses. Focusing on the $\alpha$-attractor scenario, we implement the model in a modified version of the Boltzmann solver class to compute the relevant cosmological observables and perform a Bayesian parameter estimation analysis using data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAOs), and type Ia supernovae. The model naturally breaks the degeneracy between the dark energy equation of state and the total neutrino mass, yielding tight upper bounds of $\sum m_{\nu }<0.067\mathrm{eV}$ for flat spatial geometry and $\sum m_{\nu }<0.116\mathrm{eV}$ when curvature is included. We also provide forecasts for future probes, showing that the Simons Observatory, LiteBIRD, and Euclid configurations may reduce the uncertainty on $\sum m_{\nu }$ by $\approx 9\%$, while the precision on the quintessential parameter ${\alpha }_{\mathrm{QI}}$ is improved by $\approx 72\%$. These results highlight the importance of consistently accounting for neutrino mass when assessing the viability of extensions to the standard cosmological model.

We show that scalar field fluctuations alone can drive cosmic acceleration, provided the Universe is spatially closed and the Compton wavelength of the field exceeds the radius of curvature. This mechanism may open new perspectives on inflation and dark energy, which could arise from a gas of sufficiently light bosons in a closed universe.

Black hole relics are of significant interest in cosmology and theoretical physics. In this work, we consider tiny primordial black holes (PBHs) ($M_{\mathrm{PBH}}\lesssim {10}^7\mathrm{g}$), which are generated soon after the end of inflation and evaporate and reheat the Universe before big bang nucleosynthesis (BBN), but leave their remnants due to incomplete evaporation. These PBHs remnants may contribute as part or all of the dark matter (DM) today. Assuming that there exist PBH relics, we point out that the number density of PBH today can be directly read from the peak positions of the induced gravitational waves (IGWs) due to the inhomogeneous PBH distribution. If PBH relics are of Planck mass and they form all the DM today, the PBH number density would be of ${10}^{-25}\mathrm{c}{\mathrm{m}}^{-3}$ with the peak frequency 60 Hz. The peak frequency scales as $f_{\text{relic}}^{1/3}$ where $f_{\text{relic}}$ is the fraction of the PBH relics in the total DM density. The peak amplitude carries the information of initial PBH abundance. For monochromatic-mass PBH with the current number density ${10}^{-41}–{10}^{-25}{\mathrm{cm}}^{-3}$ and initial abundance ${10}^{-13}–{10}^{-7}$, the amplitude may be large enough to be detected by planned gravitational wave experiments in the near future.

We investigate how perturbative reheating after inflation modifies the primordial gravitational wave (GW) spectrum generated by cosmological phase transitions. Within a specific inflationary setup, we show that the thermodynamic quantities that control the phase transition depend on the effective equation of state of the cosmological background, which is itself set by the form of the inflaton potential during reheating. Assuming reheating proceeds via perturbative dissipation of the inflaton condensate into boson or fermion pairs, we find that phase transitions taking place in this epoch generally produce GW signals that are systematically suppressed compared with the standard radiation-dominated scenario. We also identify characteristic spectral features that may arise in this case, which could serve as distinctive signatures of the modified expansion history during reheating.

The purpose of the present work is based on two main observations: the tensions encountered by the standard $\mathrm{\Lambda }\mathrm{CDM}$ model when confronted to precision small scale cosmological data and the finding that the matter distribution is dipolar and the expansion of the Universe is axially symmetric roughly in the direction of the CMB dipole. Therefore, we propose, as a model for the inhomogeneous local universe, an axially symmetric Szekeres solution. After describing its main properties, we are left with three metric functions to be fitted to data between the observer and the transition to homogeneity which is an intrinsic feature of Szekeres spacetimes. So as to turn a difficult functional inference problem into a classical parameter estimation problem, we propose to use Chebyshev polynomial expansions, which, as a first step, we truncate after the second order terms. We are thus left with nine constant parameters: six for the metric functions, plus the observer’s location and the cosmological constant. Here are the proper ingredients needed to implement the data fitting to the model in the future.

We investigate the effects of dynamical dark energy (DDE) on the growth of cosmic structure using a two-fluid model. This framework allows the dark energy equation of state to smoothly cross the phantom divide, in agreement with recent DESI results. In this effective description, DDE supports propagating perturbations that behave like sound waves. These perturbations induce a scale dependence in the growth of matter fluctuations and in halo bias, which can be exploited to test the dynamical nature of dark energy at the level of its fluctuations. For cluster-sized halos, the amplitude of the scale-dependent halo bias is comparable to that produced by massless neutrinos in $\mathrm{\Lambda }\mathrm{CDM}$. Using a Fisher forecast for a multitracer analysis of the power spectrum ($P$) and bispectrum ($B$) of galaxy number counts, we find that bispectrum information is essential to detect the scale dependence induced by the DDE sound mode. For a survey of volume $V\sim 10h^{-3}{\mathrm{Gpc}}^3$ at redshift $z=0.5–1$, a two-tracer $P+B$ analysis could detect this scale dependence if the sound speeds of the dark energy fluids are in the range $c_s^2\sim {10}^{-2}-{10}^{-4}$. Lower sound speeds cause halos to experience a gravitational drag force through the excitations of sound waves. This effect impacts measurements of the growth rate inferred from cluster-sized halos at the 10% level if one of the fluids has a very low sound speed $c_s^2\sim {10}^{-5}$. Larger sound speeds $c_s^2>{10}^{-2}$ could be probed with optimal weighting schemes that reduce shot noise and increase the effective bias.

The sound speed of dark energy encodes fundamental information about the microphysics underlying cosmic acceleration, yet remains essentially unconstrained by existing observations. We demonstrate that a lunar-based laser interferometer, such as the proposed Laser Interferometer Lunar Antenna (LILA), can directly probe the sound speed of dark energy by measuring the real-time evolution of horizon-scale gravitational potentials. Operating in the ultra-low-frequency gravitational band inaccessible from Earth, LILA is sensitive to scalar metric perturbations sourced by dark energy dynamics. Using both fluid and effective field theory descriptions, we develop a complete framework linking dark energy sound speed to observable strain signatures. We construct a likelihood pipeline and Fisher forecasts, showing that LILA can either detect clustering dark energy or exclude broad classes of models with unprecedented sensitivity. This establishes lunar interferometry as a novel and powerful probe of the physics driving cosmic acceleration.

Isocurvature perturbations—expected from multifield inflation models—can leave unique signatures in the early Universe, but remain weakly constrained, especially on small scales. In this work, we investigate the constraining power of one-point statistics (variance and skewness) of the 21-cm brightness temperature during cosmic dawn and the epoch of reionization, using seminumerical simulations from 21cmfast. We model both adiabatic and cold dark matter isocurvature modes, exploring their impact on the matter power spectrum, the timing of structure formation, and the evolution of neutral hydrogen. By varying astrophysical parameters as well as the isocurvature fraction and spectral index, we quantify their respective effects on the 21-cm power spectrum and on one-point statistics, using the former as a physical diagnostic and the latter as the basis of our final forecast. Our results show that while variance is highly sensitive to the timing of cosmic events and provides tight constraints on isocurvature parameters, skewness is more strongly affected by astrophysical uncertainties and observational noise. Incorporating realistic instrumental noise based on Square Kilometre Array configurations, we perform a Fisher analysis using only the redshift evolution of the one-point statistics and demonstrate that the isocurvature fraction can be constrained down to the percent level, though a strong degeneracy with the spectral index remains. We discuss the importance of complementary probes, such as the 21-cm forest and galaxy surveys, to break these parameter degeneracies. Our findings highlight the power of 21-cm one-point statistics as robust and independent tools for probing early-Universe physics beyond what is accessible with traditional power spectrum analyses.

Light scalar fields acquire isocurvature fluctuations during inflation. While these fluctuations could lead to interesting observable signatures at small scales, they are strongly constrained on large scales by cosmic microwave background observations. When the mass of the scalar is much lighter than the inflationary Hubble scale, $m\ll H_I$, the spectrum of these fluctuations is flat. Meanwhile, if $m\gg H_I$, the fluctuations are suppressed. A blue-tilted isocurvature spectrum that exhibits enhanced structure on small scales but avoids observational constraints on large scales therefore requires a coincidence of scales $m\sim H_I$ for a free massive scalar. In this work, we show that if a scalar field possesses a nontrivial potential, its inflationary dynamics naturally cause this condition to be satisfied, and so a blue-tilted spectrum is generically expected for a large class of potentials. Specifically, if its potential $V$ exhibits a region that satisfies the slow-roll condition $V^{\prime \prime }<3H_I^2$, the scalar condensate will spend most of inflation close to the boundary of this region, so that its effective mass is typically close to $H_I$. The resulting blue tilt is inversely proportional to the number of $e$-folds of inflation prior to horizon crossing. If the scalar is long-lived, this mechanism leads to an attractor prediction for its relic abundance, which is insensitive to initial conditions of the scalar. In particular, a scalar field with quartic self-interactions can achieve the correct abundance to constitute all of the dark matter for a wide range of masses. We compute the relationship between the mass and self-coupling of quartic dark matter predicted by this mechanism.

Using unitarity and causality, we derive an infinite tower of two-sided positivity bounds on the effective field theory coefficients which describe the propagation of heavy fields on de Sitter spacetime. We design this effective field theory (EFT) to describe propagators with the in-in boundary conditions that are relevant for cosmological correlators. Our positivity bounds therefore identify EFT correlators that can never emerge from a consistent underlying model of inflation. This implies nontrivial constraints on primordial non-Gaussianity; for instance the cosmological collider oscillations in the squeezed bispectrum from the exchange of a heavy scalar are tied to the shape of its EFT background.

The epoch of reionization (EoR) marks the last phase transition of hydrogen in our Universe, as it evolves from cold and neutral to hot and ionized in the intergalactic medium (IGM). While its end point and duration can be estimated from current observations, albeit with large uncertainties, there is no known avenue to constrain its onset. We propose a novel method based on the Pearson cross-correlation coefficient between 21-cm brightness temperature maps and line-intensity maps tracing star formation (e.g., [OIII], CO, [CII]). This real-space estimator evolves from negative to positive as x-ray heating progresses and saturates prior to the EoR. We predict a sharp drop from saturation during the earliest EoR stages, when the IGM ionized fraction is $x_{\mathrm{HII}}\lesssim 10\%$. We show that in standard scenarios, where IGM heating precedes reionization, the drop is a clear, model-robust signature that the EoR is still in its early stages, even when ${\overline{x}}_{\mathrm{HII}}$ cannot be measured precisely. This information is not accessible through the detection of an anticorrelation alone, which only indicates that reionization is ongoing. To assess the detectability of this feature, we provide a preliminary estimate of its signal-to-noise ratio in our fiducial scenario, assuming Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer-like and Square Kilometre Array Observatory-like noise levels, indicating that it is within reach of next-generation surveys. The detection of the Pearson drop therefore will provide a unique anchor for the EoR onset, and an upper bound on $x_{\mathrm{HII}}$, complementing existing probes and tightening constraints on early galaxy formation models.

In noncentral relativistic heavy-ion collisions, the produced quark-gluon plasma behaves approximately as a rotating fluid due to the system’s initial angular momentum. In this rotating fluid, the spins of quarks become polarized due to the coupling between spin and angular momentum, as well as random spin-spin interactions. Since the Landau-Lifshitz (LL) equation effectively describes the spin polarization of fermions in a medium with a magnetic field, we derive a phenomenological equation analogous to the LL equation for heavy quark spin dynamics in the rotating medium. The spin-angular momentum coupling and random spin-spin interactions are incorporated, leading to a detailed balance of heavy quark spin distributions. This equation provides insight into the spin dynamics of heavy quarks and quarkonium in relativistic heavy-ion collisions.

We investigate the impact of nuclear pasta on crustal structure and torsional oscillations using a Bayesian ensemble of unified neutron-star equations of state based on relativistic mean-field models constrained by nuclear experiments, empirical saturation properties, chiral effective field theory, and multimessenger observations. For each posterior sample, we compute the pasta sequence within a compressible liquid-drop model and quantify the onset density, thickness, and mass fraction of the pasta layers. We show that the appearance and extent of nuclear pasta are primarily controlled by the symmetry-energy slope parameter $L$, consistent with earlier studies and indicative of a universal trend. While spherical and rodlike pasta configurations are present for all equations of state, only a small fraction of the posterior supports slab, tube, or bubble geometries. The transition from spherical nuclei to rods is tightly constrained to occur at a density of ${\rho }_{\mathrm{sr}}=0.0588_{-0.0065}^{+0.0045}{\mathrm{fm}}^{-3}$. We further predict that the nuclear pasta layer occupies a relative radial thickness of $\mathrm{\Delta }{R}_{\text{pasta}}/\mathrm{\Delta }{R}_{\mathrm{c}}=0.140_{-0.036}^{+0.025}$ and contributes a relative mass fraction of $\mathrm{\Delta }{M}_{\text{pasta}}/\mathrm{\Delta }{M}_{\mathrm{c}}=0.475_{-0.113}^{+0.071}$. Using the resulting crust models, we present the quasiperiodic oscillations (QPOs) analysis based on a Bayesian posterior ensemble of neutron-star equations of state and systematically assess their compatibility with observed low-frequency quasiperiodic oscillations. We find that the predicted QPO frequencies are strongly correlated with the curvature of the symmetry energy evaluated at subsaturation density, $K_{\mathrm{sym}}({\rho }_0/2)$, and that uncertainties in the equation of state translate into a range of angular indices $\ell$ consistent with the observed frequencies.