We apply the criterion of finite naturalness to the limiting case of a generic heavy sector decoupled from the Standard Model. The sole and unavoidable exception to this decoupling arises from gravitational interactions. We demonstrate that gravity can couple the Higgs to the heavy scale significantly earlier than the well-known three-loop top-quark-mediated diagrams discussed in previous literature. As an application, we show that finite naturalness disfavors large-field inflationary models involving super-Planckian field excursions. In contrast, in the small-field regime, achieving successful inflation requires substantial fine-tuning of the initial conditions, in agreement with previous results. Recent data from the Atacama Cosmology Telescope further amplify the tension between naturalness and fine-tuning, challenging the theoretical robustness of single-field inflation as a compelling explanation for the origin of the Universe.

We perform a detailed study of the gravitational tidal Love numbers of extremal zero-temperature Kerr black holes. These coefficients are finite and exhibit the dissipative nature of these maximally spinning black holes. Upon considering the dynamical behavior of the tidal deformations of the extremal Kerr black holes, we provide explicit expressions of the Love numbers at low frequencies. Their calculation is simplified to specific formulas, which are directly derived using the Leaver–Mano-Suzuki-Takasugi method.

Inspired by the novel idea proposed by Padmanabhan [Phys. Rev. D 100, 045024 (2019)], we develop a method to uncover the hidden thermal properties of the inertial Feynman propagator in Minkowski spacetime in a causally consistent manner. This, in turn, enables a coherent interpretation based on future-directed propagation. In our approach, the Fourier transform is implemented following the convention used in the analysis of vacuum fluctuations. As a result, future-directed propagation across causal horizons can be consistently interpreted, from the perspective of an observer confined to a causally disconnected region, as the emission of scalar quanta at the past horizon and their absorption at the future horizon. Moreover, we find that the ratio between emission and absorption processes reproduces the characteristic Boltzmann factor of a thermal ensemble. We first apply this analysis to a causal diamond of length $2\alpha$, performing a detailed study of the near-horizon geometry and thereby obtaining the temperature associated with the thermal behavior of the Minkowski vacuum as perceived by an observer with finite lifetime $2\alpha$. For completeness, we also apply the method to the right Rindler wedge, recovering the well-known Unruh temperature, $T=a/(2\pi )$. Our results demonstrate that thermality can emerge directly from causal structure, independently of acceleration or gravity, with causal diamonds encoding intrinsic thermodynamic behavior in quantum field theory.

We consider wave packets of a massless scalar field that have well-localized Rindler energy, and examine how their energy appears to a Minkowski observer to study how the classical gravitational redshift formula is modified quantum mechanically. We derive, by using the saddle point approximation, an analytic expression for the Minkowski momentum distribution of such Rindler wave packets. We find a universal lower bound on the uncertainty in the Minkowski momentum; the momentum distribution can never become arbitrarily sharp.

The interplay of relativistic fluid dynamics and spacetime geometry is discussed in the regime of small wave numbers and frequencies. A combination of gravitational Ward identities and fluid dynamic equations of motion in the Mueller-Israel-Stewart formulation is used to explicitly determine the retarded linear response of the energy-momentum tensor to metric perturbations. We also discuss applications to gravitational wave production and the damping of gravitational waves in a relativistic fluid.

Regular black holes (BHs) which are singularity-free alternatives to the standard black hole paradigm in general relativity (GR), offer effective models for probing the interface between classical and quantum gravity. They serve as promising candidates for exploring the nature of strong gravity and potential extensions of GR by providing testing grounds to understand how quantum corrections might manifest in astrophysical black holes. In the present work, we investigate the regular BH scenario described by the Simpson-Visser (SV) spacetime and explore its imprints on the high-frequency quasiperiodic oscillations (HFQPOs) observed in the black hole power spectrum. The Simpson-Visser spacetime represent the simplest, globally regular extensions of the Schwarzschild scenario, through the presence of a regularizing parameter. We explore the imprints of the regularizing parameter on the orbital and epicyclic frequencies associated with the motion of test particles in the rotating SV spacetime. Models aimed to explain the observed HFQPOs often invoke these fundamental frequencies and hence can potentially constrain the regularizing parameter from the available HFQPO data. We test eleven well-established HFQPO models against available observations from six black hole sources, obtaining spin constraints that, when compared with previous independent estimates, help identify the observationally favored models for each source. Based on the present data, we report that the observationally favored models cannot discriminate between the Kerr and the Simpson-Visser scenario. This when coupled with the large discrepancy in previous spin estimates of these sources, may plausibly indicate some deviation from GR in the strong gravity regime near BHs which requires further investigation.

While most of the binary configurations in modified theories of gravity are studied under quasicircular orbit limit, eccentricity effects could play a significant role in future gravitational wave detections. We derive the gravitational radiation reaction force for nonspinning eccentric orbits within the effective-one-body (EOB) description for the massless scalar-tensor theories up to 1.5 post-Newtonian (PN) order. The effects in such theories start at $1/c^3$ and interestingly, the 1.5PN order effect is due to radiation reaction square effects, which is a conservative effect. The results derived here can be implemented in the quasicircular EOB-based waveform models to construct waveform templates for generic orbit binaries within massless scalar-tensor theories of gravity.

In this paper, we investigate the interferometric signatures of hairy Schwarzschild black holes (hSBHs) that have either single or double photon spheres. Our interest mainly stems from two considerations: (i) the photon ring structure in black hole images produces strong and universal interferometric signatures on long baselines, enabling precision measurements of black hole parameters and testing gravitational theory; (ii) the hSBH describes the deformation of standard Schwarzschild black hole induced by additional sources, and they can feature double photon spheres within certain parameter regimes. Using both analytical and numerical methods, we find that for a hSBH with a single photon sphere the complex visibility amplitude of the image exhibits damped oscillations. Similar behavior appears in the double photon sphere case when the inner photon sphere has lower effective potential than the outer one, as the photons near the inner photon sphere remain trapped by gravity. However, when the inner potential is higher, a beat pattern rises. Our findings reveal that the complex visibility amplitude can encode the signature of the photon sphere structure of the central black hole.

In earlier companion papers, we showed that nonsingular primordial black holes (PBHs) could account for all the dark matter (DM) over a significantly wider mass range compared to Schwarzschild PBHs. Those studies, mostly based on phenomenological metrics, are now extended by considering the quantum-corrected space-time recently proposed by Zhang, Lewandowski, Ma, and Yang (ZLMY), derived from an effective canonical (loop) quantum gravity approach explicitly enforcing general covariance. Unlike the BHs considered earlier, ZLMY BHs are free from Cauchy horizons and are hotter than their Schwarzschild counterparts. We show that this higher temperature boosts the evaporation spectra of ZLMY PBHs, tightening limits on their abundance relative to Schwarzschild PBHs and shrinking the asteroid mass window where they can constitute all the DM, a result which reverses the earlier trend, but rests on firmer theoretical ground. While stressing the potential key role of quantum gravity effects in addressing the singularity and DM problems, our study shows that working within a consistent theoretical framework can strongly affect observational predictions.

We propose tests of the weak equivalence principle (WEP) using a torsion balance, in which superposition of energy eigenstates are created in a controllable way for the test masses. After general considerations on the significance of tests of the WEP using quantum states and the need for considering inertial and gravitational masses as operators, we develop a model to derive the matrix elements of the free-fall operator, showing that the variance of the acceleration operator, in addition to its mean, enables estimation of violations of the WEP due to quantum coherence in a way that is robust with respect to shot-to-shot fluctuations. Building on this analysis, we demonstrate how the validity of the WEP may be tested in a torsion balance setup, by accessing the mean and variance of a torque operator we introduce and quantize. Due to the long acquisition times of the signal as compared to the timescale on which coherent superposition states may survive, we further propose a dynamical setting, where the torsion balance is subject to a time-dependent gravitational field, and measurements of angular acceleration encode possible violations of the WEP.

We develop a relativistically accurate formalism to model the interaction between stellar mass compact objects embedded in thin accretion disks around a nonspinning supermassive black hole, using tools from self-force theory and Hamiltonian perturbation theory. We then apply this formalism to analyze the evolution of a compact object on a nearly circular and equatorial orbit interacting with a thin equatorial disk. We provide analytic and relativistically accurate expressions for the rates of energy and angular momentum exchanged during interactions due to Lindblad and corotation resonances. Our results show that relativistic corrections can enhance the magnitude of the torque by 1–2 orders of magnitude compared to purely Newtonian expressions when the orbit of the compact object is smaller than ten Schwarzschild radii of the supermassive black hole. We also demonstrate that strong relativistic effects shifts the inner Lindblad resonances closer to the compact object than the outer Lindblad resonances when the compact object is closer than four Schwarzschild radii to the supermassive black hole, potentially leading to a reversal in the direction of the torque acting on the compact object. Finally, we provide a dephasing estimate and show that using the relativistic torque formula is crucial to obtain reliable estimates for extreme mass-ratio inspirals in orbits closer than five Schwarzschild radii to the supermassive black hole. Our results highlight the importance of using relativistically accurate models of environmental interactions in extreme mass-ratio inspirals close to a supermassive black hole.

Stochastic background gravitational waves have not yet been detected by ground-based laser interferometric detectors, but recent improvements in detector sensitivity have raised considerable expectations for their eventual detection. Previous studies have introduced methods for exploring anisotropic background gravitational waves using Bayesian statistics. These studies represent a groundbreaking approach by offering physically motivated anisotropy mapping that is distinct from the Singular Value Decomposition regularization of the Fisher Information Matrix. However, they are limited by the use of a single model, which can introduce potential bias when dealing with complex data that may consist of a mixture of multiple models. Here, we demonstrate the bias introduced by a single-component model approach in the parametric interpretation of anisotropic stochastic gravitational-wave backgrounds, and we confirm that using multiple-component models can mitigate this bias.

We explore resonant production of sterile neutrino dark matter via the Shi-Fuller (SF) mechanism, revisiting its cosmological viability in light of recent results demonstrating that lepton-number asymmetries $L_{\alpha }\gtrsim 1$ at temperatures $\mathrm{T}>20\mathrm{MeV}$ are consistent with big bang nucleosynthesis (BBN). Using a quasiclassical Boltzmann transport calculation of the dark matter production, we compute the nonthermal phase space distributions of sterile neutrinos across a broad range of particle mass $m_s$ and mixing angle ${\sin }^2(2\theta )$ parameter space. We then evolve the resulting distributions through linear structure formation using CLASS and fit the resulting matter power spectra to thermal warm dark matter (WDM) transfer functions, enabling a direct mapping between SF models and equivalent thermal WDM particle masses $m_{\mathrm{th}}$. This allows us to reinterpret existing structure formation limits and Lyman-$\alpha$ forest preferences in the context of SF production. We find that lepton asymmetries $L\gtrsim 0.5$ at high temperatures open significant viable parameter space in the $m_s\gtrsim 10\mathrm{keV}$ and ${\sin }^2(2\theta )\lesssim {10}^{-14}$ regime, compatible with both x-ray constraints from NuSTAR and INTEGRAL/SPI and recent Lyman-$\alpha$ inferences of $m_{\mathrm{th}}\approx 4.1\mathrm{keV}$. Following lepton number evolution below 20 MeV, we also specifically show that this lepton asymmetry parameter space is compatible with BBN and cosmic microwave background constraints. We present updated constraints, a refined $m_{\mathrm{th}}$ fitting function, and power-law approximations for $L$ across the parameter space. Our results motivate future x-ray observations targeting the $\sim 20\mathrm{keV}$ photon regime and testing of the $m_{\mathrm{th}}\gtrsim 10\mathrm{keV}$ WDM region.

We present new constraints on the halo masses and matter density profiles of DESI galaxy groups by cross-correlating samples of luminous red galaxies (LRGs) and bright galaxy survey (BGS) galaxies with the publicly available CMB lensing convergence map from ACT DR6. This provides an independent, lensing-based calibration of halo masses, complementary to methods relying on clustering or dynamics. We derive constraints on the mean halo mass for three DESI-selected samples, finding $\log (M_{\mathrm{halo}}/(M_{\odot }/h))\approx 13.18$, 13.03 and 13.02 for the main LRG, extended LRG, and BGS samples, respectively. Using a halo model approach, we also compare the projected galaxy-matter density profiles with previously reported gas profiles inferred from measurements of the kinematic Sunyaev-Zel’dovich (kSZ) effect. This work addresses one of the key uncertainties in interpreting kSZ signals—the unknown host halo mass distribution—by providing an independent and consistent mass calibration. The agreement between the gas and total mass profiles at large aperture suggests that sufficiently far from the group center (2–3 virial radii), we recover all the baryons, offering a resolution to the missing baryon problem. We further study the cumulative gas fractions for all galaxies as well as for the most massive galaxy groups in the sample [$\log (M_{\mathrm{halo}}/(M_{\odot }/h))\approx 13.5$], finding values that are physically sensible and in agreement with previous findings using kSZ and x-ray data: compared to the TNG300 simulation, the observed gas fractions are systematically lower at fixed radius by $\gtrsim 4\sigma$, providing compelling, independent evidence for stronger baryonic feedback in the real Universe. These findings highlight the power of combining CMB lensing with galaxy surveys to probe the interplay between baryons and dark matter in group-sized halos.

Discrete symmetry-breaking phase transitions in the early Universe may have caused the formation of networks of sheet-like topological defects, usually referred to as domain walls, which separate regions that have settled into different vacuum states. Field theory simulations predict the successive collapse of increasingly larger domains, which could potentially leave observable imprints in present-day large-scale structures. We use a nonparametric analytical model to provide an estimate of the final decay energy of these walls and their associated collapse rate, as a function of redshift. The energy released by collapsing walls can act as a seed for density perturbations in the background matter field, influencing structure formation. We estimate the dependence of the current mass of the resulting nonlinear objects on the collapse redshift and wall tension, showing that domain walls can contribute to the formation of objects as massive as present-day galaxy clusters. Still, we confirm that the contribution of standard domain walls to structure formation is subdominant. In contrast, biased domain walls—originating in models with an approximate (or biased) discrete symmetry breaking—generally face much less stringent constraints on their tension, which allows for significantly higher collapse energies. Based on our analysis, we are able to show that the collapse of such biased wall networks can provide a significant contribution to structure formation, and in particular, a mass excess at $z\gtrsim 7$, as suggested by JWST data.

Collisionless self-gravitating systems such as cold dark matter halos are known to harbor universal density profiles despite the intricate nonlinear physics of hierarchical structure formation in the $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) paradigm. The origin of such states has been a persistent mystery, particularly because the physics of collisionless relaxation has remained poorly understood. To solve this long-standing problem, we develop a self-consistent quasilinear theory in action-angle space for the collisionless relaxation of inhomogeneous, self-gravitating systems by perturbing the governing Vlasov-Poisson equations. We obtain a quasilinear diffusion equation that describes the secular evolution of the mean coarse-grained distribution function $f_0$ of accreted matter in the fluctuating force field of a spherical isotropic halo. The diffusion coefficient not only depends on the fluctuation power spectrum but also on the evolving potential of the system, which reflects the self-consistency of the problem. Diffusive heating in the preassembled halo develops an $r^{-\gamma }$ cusp ($r$ is the halocentric radius) in the density profile of the accreted material. Accretion and relaxation in this $r^{-\gamma }$ inner cusp develops an $r^{-\beta }$ outer fall-off with $\beta \approx 5-2\gamma$ in the quasisteady state. Spherical collapse theory dictates that a quasi-steady outer halo must settle to $\beta \approx 3$ since then the mass enclosed within a radially moving shell barely changes with time. This implies that the quasisteady $\gamma$ must be approximately 1, which is possible in the quasilinear framework only if (i) the preassembled halo harbors an $r^{-{\gamma }_{\mathrm{P}}}$ profile with ${\gamma }_{\mathrm{P}}\gtrsim 0.5$, (ii) its fluctuations are sufficiently correlated in time (red noise), and (iii) the initial value of $\gamma$ is smaller than 1, implying that the $r^{-1}$ cusp is a neutral equilibrium. Self-consistent quasilinear relaxation therefore establishes the Navarro-Frenk-White (NFW) profile. We demonstrate for the first time how this profile emerges as a quasisteady state of collisionless relaxation.

High-energy muon beam dump experiments are powerful probes of new physics models beyond the Standard Model, particularly those involving flavor-dependent interactions. We demonstrate the potential of muon beam dump by placing strong constraints on three new physics models utilizing data from the recent $\mathrm{NA}64\mu$ experiment: (1) axions coupling to photons, (2) axions coupling to muons, and (3) a dark sector mediated by a massless $U(1{)}_{L_{\mu }-L_{\tau }}$ gauge boson. The new particles can be identified from the significant missing energy as the invisible channel, or the distinct energy deposition signature as the visible channel. We find that the current $\mathrm{NA}64\mu$ data do not yet probe new parameter region on axion-photon coupling, while excluding new parameter space for the axion-muon coupling $g_{a\mu \mu }\gtrsim 4\times {10}^{-3}{\mathrm{GeV}}^{-1}$ and the axion mass $m_a\lesssim 0.2\mathrm{GeV}$. For the dark sector, the current data provide stringent constraints that surpass existing ones by nearly one order of magnitude. The data from the 2023 $\mathrm{NA}64\mu$ run, once available, will be capable of excluding new axion-photon coupling parameter space and probing the flavor structure of new physics, with more sensitivity advancement expected in near future runs.