We study a recursive Penrose process and the corresponding energy extraction for the decay of electrically charged particles in a Reissner–Nordström black hole spacetime with anti-de Sitter (AdS) asymptotics, incorporating the backreaction on the black hole’s mass and electric charge. A recursive process requires that the decay products remain confined within a finite spatial region so that the emitted particles bounce back and undergo further decay at some location, possibly the same location as the initial decay. In asymptotically AdS spacetimes, the confining property arises naturally. Outgoing particles encounter an outer turning point and are reflected inward. Alternatively, one may impose a reflecting mirror at a finite radius but in AdS backreaction makes these two confinement methods exactly equivalent. Let $Q_n$ denote the black hole charge after $n$ decays and define the index $n_{\mathrm{c}}$ as the value for which the black hole’s charge is zero, $Q_{n_{\mathrm{c}}}=0$. For $n_{\mathrm{c}}$ integer, which can be achieved for a specific choice of initial conditions, the black hole’s charge decreases step by step and reaches exactly zero, at least momentarily, after a finite number of decays, thereby terminating the recursive process. However, the last particle emitted turns back and, encountering zero charge, falls straight into the black hole. The final state is thus a charged black hole whose charge equals the sum of the original black hole charge with the initial particle charge. There is a fine-tuned possibility in which the process finishes with an uncharged black hole accompanied by a charged particle at rest precisely at the original decay position, which is an unstable configuration. For $n_{\mathrm{c}}$ noninteger, the black hole charge decreases and can be made arbitrarily small, but it never reaches zero. The last physically allowed decay occurs at $n=n_c^{-}$, where $n=n_c^{-}$ is the greatest integer less than $n_{\mathrm{c}}$. Any further decay would invalidate the approximations; one of the particles would carry a charge comparable to the black hole mass, transforming the problem into an uncontrolled two-body interaction. The would-be subsequent decay would moreover violate cosmic censorship, preventing it entirely. Therefore the process is terminated before any inconsistency arises. In both the integer and noninteger cases, the system yields a finite energy gain. Backreaction ensures that the process has a finite duration and extracts only a finite amount of energy. No black hole bomb occurs; the system can at most work as an energy factory. In short, we show that accounting for backreaction renders the black hole bomb impossible.

Given the multifrequency nature of relativistic orbits, transient orbital resonances are expected to be ubiquitous during an extreme-mass-ratio inspiral (EMRI). At a resonance, the orbital dynamics is modified in a nontrivial way, imprinting an overall dephasing in the emitted gravitational waves and potentially impacting both the detection and parameter estimation of these sources. In this work, using a Fisher-matrix approach, we investigate the bias induced by transient orbital resonances in EMRI parameter estimation. We focus on the most dynamically significant low-order resonances, $3:2$ and $2:1$, as well as on the high-order, subdominant resonances, $3:1$ and $4:3$. We find that, for most of the orbits considered, neglecting the effect of a resonance crossing leads to significant losses in signal-to-noise ratio and induces bias in parameter recovery. Furthermore, both the sign and the amplitude of the resonance-induced modifications to the integrals of motion play a crucial role and must be modeled accurately. Our results provide further evidence that failing to model transient orbital resonances accurately can hinder EMRI detection and parameter estimation, thereby limiting their scientific potential.

Black holes are the most efficient scramblers in nature. By mapping the instantaneous mass and angular momentum of two spinless black holes in a quasicircular binary onto those of an effective Kerr black hole, we demonstrate that the final state of the merger remnant corresponds with remarkable accuracy to the configuration that renders null geodesics unstable at the highest possible rate. This suggests a deep connection between the properties of black holes resulting from binary mergers and their unstable null orbits.

The formation of cosmological large-scale structure is usually described in terms of the evolution of density fluctuations and their statistical measures, such as the power spectrum and correlation function. However, these statistics characterize the amplitude structure of density fluctuations and do not directly describe the spatial redistribution of matter that occurs during structure formation. In this work we formulate cosmological structure formation as a transport problem of mass distributions using the Wasserstein distance from optimal transport theory. The generative process from the initial linear density field to the observed galaxy catalog is treated as a hierarchical mapping from a continuous density field to a galaxy point process, and an approximate expression for the Wasserstein distance between them is derived under the small-fluctuation approximation. We show that this distance naturally decomposes into contributions associated with three physical processes: mass transport by gravitational evolution, galaxy formation bias, and shot noise arising from the discrete sampling of galaxies. The gravitational transport term is expressed as an integral of the matter power spectrum, while the galaxy formation contribution appears as a transport-weighted integral of the galaxy correlation function. The sampling term corresponds to Poisson shot noise originating from the discreteness of the galaxy catalog. A notable feature of the resulting expression is that the correlation contribution differs fundamentally from conventional volume-weighted clustering statistics and is naturally related to cumulative matter displacements and baryon acoustic oscillation smearing. These results provide a unified framework for describing cosmological large-scale structure formation from the perspective of transport geometry and suggest that the Wasserstein distance may serve as a new statistical quantity linking continuous density fields with observed galaxy catalogs.

We revisit QCD-induced little inflation in which the Universe starts with a large baryon chemical potential and undergoes a strong first-order QCD phase transition, generating an observable stochastic gravitational-wave background in the nano-Hz range relevant for pulsar timing array (PTA) observations. We point out that the conventional homogeneous transition from the quark-gluon plasma phase to the hadronic gas phase faces an unavoidable difficulty in achieving the required strength of supercooling for the observed baryon density. This motivates us to explore whether a qualitatively different phase structure at a large baryon chemical potential can alter the relation between the baryon density and the chemical potential, and thereby modify the supercooling history of the transition. Using the nucleon-meson model with isoscalar vector mesons, we determine the critical and spinodal structure of the chiral density wave (CDW) phase in the $({\mu }_B,T)$ plane. We find that the CDW phase exhibits a nontrivial structure and can remain metastable down to a low baryon density in a certain region of the parameter space. Taking into account the subsequent liquid-gas transition and phase separation, however, the released latent heat is too small to realize a viable QCD-induced little inflation scenario and its associated PTA-scale gravitational-wave signal. Our analysis sharpens the conditions under which QCD phase transitions may act as cosmological sources of nano-Hz gravitational waves, while clarifying the possible cosmological relevance of inhomogeneous QCD phases.

We investigate primordial black hole (PBH) formation in a cosmological scenario where curvature perturbations follow purely quadratic non-Gaussianity, $\zeta =A({\varphi }^2-⟨{\varphi }^2⟩)$, arising from tachyonic instability in multicomponent inflationary models. Within an extended Press-Schechter framework based on the compaction function, we derive the probability distribution of the linear compaction function and its asymptotic exponential tail, demonstrating that the PBH abundance is exponentially sensitive not only to the amplitude of perturbations but also to the correlation coefficient $\rho$ between the smoothed field and its radial gradient. We further find that, in this tachyonic amplification scenario, the spectral width of the curvature power spectrum plays a decisive role in avoiding PBH overproduction: broad spectra yield mildly negative $\rho$ and fail to suppress PBH formation, while sufficiently narrow spectra drive $\rho \to -1$, resulting in exponential suppression while maintaining a sizable gravitational-wave signal. Thermal inflation serves as a benchmark for asteroid-mass PBH dark matter and high-frequency scalar-induced gravitational waves potentially detectable by future space-based interferometers, but its typically broad spectra make it challenging to reconcile pulsar timing array observations with PBH constraints.

We investigate five-dimensional rotating primordial black holes (PBHs) as dark matter candidates within the dark dimension scenario motivated by the Swampland Program. In this framework, a micron-scale extra dimension modifies Hawking evaporation, while a phenomenological memory-burden factor $S^{-p}$ can further prolong PBH lifetimes. We compute the evaporation dynamics for rotating 5D PBHs, derive the burdened lifetime scaling, and study the fiducial $p=2$ benchmark over ${10}^{10}\mathrm{g}\lesssim M\lesssim {10}^{21}\mathrm{g}$. The curvature perturbations responsible for PBH formation also generate a stochastic gravitational wave background through second-order scalar-induced effects. Assuming a log-normal primordial power spectrum with $\sigma =1$ and $f_{\mathrm{PBH}}=1$, we calculate the present-day energy density ${\mathrm{\Omega }}_{\mathrm{GW}}{h}^2$ across this benchmark interval. The predicted signals peak at frequencies from nHz to Hz, within the sensitivity ranges of LISA and DECIGO/BBO. Fisher forecasts illustrate the prospective sensitivity to the PBH mass, dark matter fraction, spectral width, and phenomenological exponent $p$. A joint test of the gravitational-wave spectrum and the evaporation history could probe extradimensional PBHs and memory-burden phenomenology.

We propose a mechanism for gravitational wave (GW) production sourced by spectator scalar fields during inflation. These fields, while not driving cosmic expansion, generate blue-tilted isocurvature fluctuations that naturally satisfy current cosmic microwave background (CMB) constraints at large scales while producing enhanced power spectra at smaller scales accessible to GW detectors. The resulting GW spectrum spans an exceptionally broad frequency range from ${10}^{-20}$ to 1 Hz, with amplitudes ranging from ${\mathrm{\Omega }}_{\mathrm{GW}}{h}^2\sim {10}^{-19}$ to ${10}^{-14}$ depending on the reheating temperature and spectator field mass. Observable signals across multiple detector bands are possible in a restricted region of parameter space, $m_{\chi }/H_I$ within the isocurvature- and $\mathrm{\Delta }{N}_{\mathrm{eff}}$-allowed window and $T_{\mathrm{reh}}$, such that the spectator either provides all of the dark matter (stable case) or decays near matter-radiation equality (unstable case). Our analysis reveals multiple complementary constraints on spectator field parameters. For a high reheating temperature of $T_{\mathrm{reh}}>{10}^{10}\mathrm{GeV}$, GW-induced limits on the effective number of relativistic species ($\mathrm{\Delta }{N}_{\mathrm{eff}}$) require $m_{\chi }\gtrsim 0.59H_I$, stronger than CMB isocurvature bounds alone ($m_{\chi }\gtrsim 0.54H_I$). The nonobservation of primordial B-modes by Planck provides stronger constraints, $m_{\chi }\gtrsim 0.64H_I$, with projected LiteBIRD sensitivity potentially reaching $m_{\chi }\gtrsim 0.67H_I$. This mechanism enables a unique multimessenger probe of beyond the standard model physics during inflation, providing simultaneous constraints on inflationary dynamics, dark matter production, and reheating through current and next-generation GW experiments.

CMB limits on cold-dark-matter isocurvature are often interpreted as excluding the simultaneous realization of high-scale inflation and large QCD axion decay constants in preinflationary Peccei-Quinn (PQ) scenarios. This conclusion can be evaded by exploiting field-space geometry. For a minimal complex PQ scalar with a $U(1)$-symmetric potential and a nonlinear sigma-model kinetic term $d{\sigma }^2=d{R}^2+f^2(R)d{\theta }^2$, the observable axion fluctuation is $\delta \theta \sim H_{\inf }/f(R)$, so an enhanced effective decay constant $f(R)$ suppresses isocurvature without explicit PQ breaking, extreme radial displacements, or additional couplings. We specialize to a hyperbolic metric $f(R)\propto \sinh (R/L)$ with curvature scale $L$. The same geometry also induces a time-dependent $\mathcal{O}(H_{\inf })$ effective mass for the canonical axial mode during radial slow-roll, and fixing the tilt and running of isocurvature. Thus, CMB-scale isocurvature is suppressed while a characteristic blue-tilted spectrum is generated. As a result, inflationary Hubble scales as large as $H_{\inf }\sim {10}^{13}\mathrm{GeV}$ can be compatible with $f_a\sim {10}^{14}–{10}^{16}\mathrm{GeV}$, reopening parameter space usually regarded as excluded. We present observable benchmarks and a semianalytic template that relates the scale-dependence of isocurvature to the geometric lever arm $R/L$, providing a direct phenomenological probe on PQ field-space geometry.

We investigate how the transition density (${\rho }_{\mathrm{tr}}$) affects hybrid constructions of the neutron star (NS) equation of state (EOS) in which a nucleonic description at low densities is matched to a model-agnostic high-density extension based on a speed-of-sound (CS) parametrization. Using four representative nucleonic models—Taylor expansion, $\frac{n}{3}$ expansion, Skyrme, and relativistic mean field—built from identical nuclear matter parameters, we isolate the impact of the low-density EOS and the transition density on NS observables. We find that, within the present smooth-matching prescription, NS properties such as radii and tidal deformabilities retain significant sensitivity to the choice of low-density EOS for commonly adopted transition densities around ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$, even when the same high-density parametrization is employed. This residual dependence arises from differences in the matching conditions at ${\rho }_{\mathrm{tr}}$, which propagate into the high-density extension, so different low-density inputs lead to different effective high-density EOS. These findings are robust across two qualitatively distinct CS parametrizations differing in peak height, location, and width. Quantitatively, the model spread in radius and tidal deformability at $1.4M_{\odot }$ exceeds the current observational uncertainty (${\sigma }_{R_{1.4}}=0.8\mathrm{km}$ and ${\sigma }_{{\mathrm{\Lambda }}_{1.4}}=200$) by factors of $\sim 1.8$ and $\sim 1.4$ at ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$, whereas these factors reduce to $\sim 1.05$ and $\sim 0.4$ at ${\rho }_{\mathrm{tr}}={\rho }_0$. Lowering the transition density, therefore, systematically diminishes the spread among models and leads to more consistent predictions. Our results demonstrate that the widely used choice ${\rho }_{\mathrm{tr}}\approx 2{\rho }_0$ does not guarantee practical model independence within the present hybrid EOS framework and should be treated as an explicit source of systematic uncertainty when inferring dense matter properties from NS observations.

We investigate the impact of dark matter (DM) on hybrid and twin stars within a two-fluid framework, where DM and normal matter interact only through gravity. A self-interacting fermionic DM model is considered, while for the nuclear and quark matter we employ relativistic mean-field models and the constant sound–speed parameterization, respectively. Solving the two-fluid Tolman-Oppenheimer-Volkoff equations over a five-dimensional parameter space spanning the quark matter transition pressure $p_t$, energy density jump $\mathrm{\Delta }\epsilon$, sound speed $C_s^2$, DM particle mass $m_{\chi }$, and DM fraction $f_{\chi }$, we classify the resulting configurations into hybrid stars and four twin star categories and map the onset properties of the quark phase. The effect of DM on the hybrid and twin star population is regime dependent: the DM particle mass determines whether DM forms an extended halo or a compact core, while the DM fraction sets the magnitude of the effect. In the halo regime, DM enhances quark matter formation and increases the number of hybrid and twin stars; in the core regime, it suppresses them. DM also stabilizes hybrid and twin configurations that are unstable in the pure-baryonic case and systematically pushes the nucleon-to-quark transition to higher densities, with the onset pressure, mass, and radius all rising with the DM fraction, potentially hiding quark matter from current neutron star observations.

Cosmic rays scattering with neutrinos produced in supernovae induce a flux of supernova neutrinos boosted to high energies. We calculate the neutrino flux arising from this new mechanism in environments with large cosmic-ray and supernova densities, such as some active galactic nuclei. Under plausible astrophysical conditions, this flux may be detectable with high-energy neutrino telescopes, just considering the proton-neutrino scattering cross section expected in the Standard Model. Furthermore, the center of mass energy of such scatterings can reach $\sqrt{s}\sim 10–100\mathrm{TeV}$, where the proton-neutrino cross section may be enhanced by new physics such as extradimensional theories. The boosted neutrino signal benefits from such an enhancement in the cross section not only at the detection point on Earth, but also at production in astrophysical sources, which allows us to set novel constraints on the ultrahigh energy proton-neutrino cross section with neutrino telescopes.

Typical gravitational production of relics from amplification of inflationary perturbations assumes Bunch-Davies initial conditions, i.e., a vacuum with initially no particles. In this paper we investigate the impact of non Bunch-Davies initial conditions to the final abundance of relics, with particular attention to the parameter space where the total dark matter abundance is reproduced. We present a general framework for any initial condition, through which we show their nontrivial effect on both spectrum and late-time abundance. We argue that for particles whose source of conformal symmetry breaking comes only from a mass term (spin-$1/2$ fermions and conformally coupled scalars), the choice of initial conditions has little impact on the mass range relevant to dark matter. For other particles, e.g., the longitudinal mode of spin-1, we see a deviation from the standard computation. We exemplify and quantify our results with an initial thermal state and a two-stage inflation scenario, highlighting that the total dark matter can be obtained for a wide range of masses.

The one-parameter fuzzy dark matter (FDM) model has faced increasingly stringent constraints from both Lyman-$\alpha$ forest observations and local measurements of dwarf galaxies. A natural extension to mitigate these limits is the inclusion of FDM self-interactions. In this study, we derive constraints in the two-dimensional parameter space $(m_a,f_a)$ using the dark matter density profile inferred from a Jeans analysis of the stellar kinematics in the dwarf galaxy Leo II, which has previously been employed to constrain noninteracting FDM. We find that, for a fixed particle mass $m_a$, attractive (repulsive) self-interaction leads to a more concentrated (more diffuse) FDM density profile relative to the noninteracting case, thereby improving (worsening) agreement with the Jeans analysis results. Our results indicate that, for either attractive or repulsive self-interaction with strength $f_a^{-1}\lesssim {10}^{-14}{\mathrm{GeV}}^{-1}$, the 95% confidence-level lower limits on $m_a$ lies within the range $(1–10)\times {10}^{-22}\mathrm{eV}$, although the precise bounds depend to some extent on the statistical method employed. This analysis simultaneously constrains the two parameters $(m_a,f_a)$ without relying on assumptions about cosmological or galaxy evolution histories and thus offers a complementary probe to existing constraints.

The magnetorotational instability can generate strong, turbulent substructure within magnetized shear flows. The efficacy of the mechanism as a function of microphysical aspects of the fluid, such as stratification and diffusivity, has been explored extensively. One aspect that has not been studied thus far, however, is whether the instability can also operate in solids. Motivated by the possibility that solid regions within planets or degenerate stars may rotate differentially with respect to liquid or gaseous layers during some phase of their life, we examine the extent to which elasticity suppresses the instability. A simplified, plane-parallel analysis reveals that only in cases where the flow is strongly sheared, such that the magnetic tension that would result from the instability in a liquid exceeds the shear modulus of the elastic cavity, can magnetic growth occur. In the context of dynamical tides in binary neutron-star mergers, this implies that the magnetic field can be amplified in the crust prior to coalescence only if the star boasts a spin frequency of $\gtrsim 300\mathrm{Hz}$. If viscous heating weakens the crystalline structure prior to resonance, the required spin frequency is reduced.

GRB 230307A is one of the brightest long-duration gamma-ray bursts (GRBs) ever detected, yet its progenitor remains uncertain due to the variety of plausible astrophysical scenarios. In this work, we investigate four possible progenitors for GRB 230307A: a binary neutron star (BNS), a neutron-star/white-dwarf (NS-WD) system, a neutron-star/black-hole merger, and a tidal disruption event involving a white dwarf and a supermassive black hole. Additionally, we explore three distinct central engine models powering the kilonova associated with the BNS: radioactive decay of $r$-process nuclei in a two-component ejecta model, a magnetar-driven model including magnetic dipole spin down, and a combined model of magnetar spin down with ${}^{56}\mathrm{Ni}$ radioactive decay. We perform Bayesian multiwavelength light curve analyses using physically motivated models and priors and evaluate model performance through Bayes factors and leave-one-out cross-validation scores. Our results show a statistical preference for a BNS or NS-WD progenitor producing a kilonova powered by a magnetar and ${}^{56}\mathrm{Ni}$ decay, characterized by a ${}^{56}\mathrm{Ni}$ mass of $\sim 4\times {10}^{-4}{M}_{\odot }$ and an ejecta mass of $0.06M_{\odot }$. Furthermore, under the assumption of a BNS origin within this model, we infer binary component masses of $m_1=1.32_{-0.06}^{+0.05}{M}_{\odot }$ and $m_2=1.24_{-0.04}^{+0.02}{M}_{\odot }$, with a dimensionless tidal deformability of $\tilde{\mathrm{\Lambda }}=437_{-265}^{+251}$. From the component mass posteriors, we infer that the observed offset can be explained by a natal kick as long as the systemic velocity is nearly aligned with the prekick orbital motion. In this case, the required kick velocity (comoving frame) and binary separation range within $v_{\mathrm{k}}^{\prime }\sim 100–150\mathrm{km}/\mathrm{s}$ and $a_0\sim 2–2.5R_{\odot }$, respectively.

The decay out states of unstable Standard Model (SM) particles provide a natural, well-defined quantum-information probe of the SM parameter space. We use Higgs boson decays as a test case: After tracing out kinematics, we compute entanglement among final-state spins and colors across all decay channels and investigate a near-maximal entanglement-entropy criterion. This criterion turns out to be informative about fundamental parameters. Within the SM, we find that the entanglement entropy exhibits a global maximum close to the observed Higgs boson mass and the measured $W$ mass, the latter being equivalent to the $SU(2{)}_L$ gauge coupling. In a two-parameter kappa framework, applying the same criterion is consistent with an SM-like balance between Higgs boson couplings to vector bosons and to fermions, providing sensitivity to the ratio of the sector-wide rescalings. These results suggest that entanglement extremality may serve as a complementary handle on fundamental parameters.

We investigate the viability of nonthermal leptogenesis in the gauged $U(1{)}_{B-L}$ extension of the Standard Model (BLSM) with an inverse seesaw (ISS) mechanism for neutrino mass generation. In this framework, right-handed neutrinos typically have $\mathcal{O}(1)$ Yukawa couplings, which induce strong washout effects and render conventional thermal leptogenesis ineffective. We demonstrate that a successful baryogenesis scenario can nevertheless be realized through nonthermal leptogenesis, where right-handed neutrinos are produced from the decay of the heavy $B-L$ Higgs boson $\chi$. We explicitly analyze the interplay between the dilution factor $T_R/M_{\chi }$ and the washout parameter characteristic of the ISS, highlighting the tension between suppressing washout effects and maintaining sufficient reheating. We show that a viable lepton asymmetry can be generated provided the scalar mass spectrum is appropriately tuned—a condition that requires a fine-tuning of order one part in ${10}^5$ against one-loop radiative corrections—allowing for a reduced reheating temperature while keeping washout under control. The resulting lepton asymmetry is efficiently converted into the observed baryon asymmetry of the Universe via sphaleron processes. Our results establish that the inverse-seesaw $B-L$ model remains a predictive and robust framework for nonthermal leptogenesis and baryogenesis.

Dilaton effective field theory can be employed to analyze lattice data in gauge theories that lie in close proximity of the lower edge of the conformal window. Under special conditions, we show that it can be used as a diagnostic tool to distinguish near-conformal, yet confining, theories from infrared conformal ones. We demonstrate this efficacy by analyzing two sets of lattice measurements taken from the literature. For the $SU(3)$ theory coupled to $N_f=8$ Dirac fermions transforming in the fundamental representation, our analysis favors confinement. For the $SU(2)$ theory with $N_f=1$ adjoint fermion, our fits favor infrared conformal behavior. We discuss future lattice measurements, and analysis refinements, that can further test this framework.