General Relativity and Quantum Cosmology Papers

ALT We simulate the gravitational lensing of gravitational waves (GWs) by a Kerr black hole using a (1+1)-dimensional time-domain mode-sum approach. To enable efficient explicit time evolution, we implement a localized hyperboloidal foliation combined with a static field redefinition, ensuring bounded characteristic speeds across an extended, uniform tortoise-coordinate grid. Incident GW initial data are constructed by projecting a plane-wave Weyl tensor onto the Kinnersley tetrad. Through local spin-dyad rotations and Wigner D-matrix transformations, our framework accommodates both idealized wave packets and realistic binary coalescence signals at arbitrary incidence and polarization angles. To accurately resolve the lensed waveforms, we evolve over 68,000 coupled (j,m) modes. Our results reveal that a Kerr black hole acts as a diffraction-limited thick lens, focusing GWs into a forward beam. For idealized wave packets, the primary prompt pulse exhibits a spin-independent transverse Gauss

ALT We develop a double-confluent Heun (HeunD) method that extends our construction of complete quasinormal-mode (QNM) spectra to extremal Kerr black holes. Matching the horizon-ingoing and infinity-outgoing radial solutions together with angular regularity yields the frequencies directly at extremality, without extrapolation from nonextremal spins. For gravitational perturbations, we obtain an expanded spectrum on both conventional and unconventional Riemann sheets, including additional QNMs revealed by analytic continuation across frequency branch cuts. Continuation in spin connects the extremal modes to our complete Schwarzschild spectrum, including its unconventional branches. We show for the first time that unconventional Class I sequences with m<0, continued from unconventional Schwarzschild modes, approach negative superradiant bounds with vanishing damping in the extremal limit. We also identify finite-damping modes interspersed among sequences approaching zero damping, showing tha

ALT Black hole ringdowns are modelled as a superposition of quasinormal modes (QNMs), although the branch cut of the Green's function produces a nonoscillatory tail that may contaminate early-time ringdown analyses. Within linear perturbation theory, we quantify this branch-cut contribution for the scattering of localised Gaussian wave packets by a Kerr black hole with mass M. Defining the onset of ringdown as the earliest time at which the QNM and branch-cut contributions reconstruct the waveform, we evaluate the branch-cut amplitude relative to the QNM amplitude. For wave packets with a characteristic frequency ω₀∼ 1/M, the branch-cut amplitude is typically of order 10% of the QNM amplitude at ringdown onset with only a mild dependence on spin. We further find that the tail waveform before convolving with a source is almost independent of spin. The spin dependence of the relative amplitude therefore resides primarily in the excitation of the QNMs. Our results suggest that the branch-cut

ALT Gravitational-wave observations of compact binaries offer a direct way to test whether gravity admits polarizations beyond the two tensor modes of general relativity. We perform a forecast to assess how well future multi-messenger events will constrain the presence of scalar polarizations in gravitational waves. Performing binary neutron star injections, we forecast next-generation constraints on three modified gravity parameters in the PPE formalism: α (tensor amplitude), α_B (scalar amplitude) and β (tensor and scalar phase). Cosmic Explorer (CE) and Einstein Telescope (ET) each tighten every parameter by 1–2 orders of magnitude over an O5 LIGO-Virgo network, with ET's triangular geometry giving a persistent ≈2× improvement over 2×CE on the scalar-mode amplitude α_B in both a GW170817-like and a second, representative BNS. We show that, if the sky position and luminosity distance are known from EM observations, adding an EM-based prior on the orbital inclination ι narrows the α_B con

ALT Black-hole interiors with an inner (Cauchy) horizon are unstable through the Poisson-Israel-Ori mass-inflation mechanism, governed by the Dray-'t Hooft-Redmount relation and widely regarded as a dynamical realization of strong cosmic censorship. We show that static, spherically symmetric anisotropic interiors supported by a marginally divergent but integrable density ρ∼ A/r², with A>1/(8π) and mild screening at finite radius, are incompatible with this mechanism. The compactness q=m/r decreases monotonically from a super-critical central value to 1/2 at the unique event horizon, producing a strictly positive compactness gap throughout the trapped region and excluding any second zero of the lapse. This gap enters the dynamics quantitatively. (i) Under the null energy condition and a dynamical screening condition, 2m/r is non-decreasing along both future null directions of a trapped domain, so the gap propagates under nonlinear spherically symmetric evolution. (ii) For ingoing radiation

ALT We establish the general-relativistic limit of f(R) gravity coupled to a massless scalar field for small, asymptotically Euclidean, well-prepared Cauchy data near Minkowski spacetime. We prove global existence and future causal geodesic completeness, with uniform estimates for the metric, matter, and rescaled effective field as an admissible family of modified Lagrangians f(R) approaches the Einstein-Hilbert Lagrangian R. The physical and conformal metrics and the matter field converge to the limiting Einstein-massless-scalar development, uniformly on the entire future spacetime, together with finitely many space and time derivatives. The effective field, defined by a nonlinear reparametrization of the physical scalar curvature, converges to the scalar curvature of the limiting Einstein development. The main analytical difficulty is that the effective-field mass tends to infinity, while its evolution equation becomes an algebraic trace relation. Small field amplitudes may coexist with

ALT Over the past decade, the Advanced LIGO, Advanced Virgo, and KAGRA detectors have detected nearly 400 compact binary mergers, most of which are stellar-mass binary black-hole (BBH) mergers. Nevertheless, most mergers in the Universe are too weak to be individually detected at current sensitivity levels. Since a BBH merger in band is much shorter than the average time between successive mergers, the population of these subthreshold events gives rise to an intermittent background of gravitational waves (GWs). This GW background is guaranteed to exist, but it has not yet been detected with current stochastic or deterministic searches. Here we propose a fast and accurate likelihood-based search for this background, which unifies the standard model-agnostic cross-correlation search and a fully deterministic, template-based search in a single likelihood. Our search is based on a truncated likelihood ratio, which is fully marginalized over the extrinsic parameters of the individual mergers, a

ALT The stochastic gravitational-wave background from stellar-mass compact binaries is non-Gaussian since it is made up of a finite number of individual mergers. Over the years, several methods have been developed to search for this background beyond the standard cross-correlation statistic. We reformulate the data analysis problem for such astrophysical backgrounds in terms of the characteristic function of the data in multiple detectors, and show that the existing searches arise as different limiting cases. We also show that new searches can be constructed through different approximations of the characteristic function, which gives a concrete way to engineer suitable approximate sufficient statistics. To illustrate this framework in the context of a simple example, we use the formalism proposed here to analyze an exactly solvable toy model.

ALT In the massless Damour-Esposito-Farèse scalar-tensor theory of gravity, we numerically construct static, spherically symmetric star models composed of a mixture of fermionic and bosonic matter, interacting purely gravitationally. The fermionic sector is modeled as a perfect fluid, while the bosonic sector is described by a massive, free, complex scalar field. We demonstrate that mixed fermion-boson stars can undergo spontaneous scalarization, and we systematically investigate the role played by the bosonic field across different regimes of bosonic mass, central fermionic pressure, and central bosonic amplitude. Our analysis reveals a complex interplay among the parameters, giving rise to a variety of scalarized configurations, ranging from fermionic matter embedded within extended bosonic halos to bosonic cores confined within fermionic matter. We examine sequences of solutions, in particular mass-radius surfaces, and we determine the critical coupling parameter for the onset of scalar

ALT We examine the observational viability of scalarized fermion-boson stars (FBSs) in massless Damour-Esposito-Farèse gravity, using the MPA1 nuclear equation of state and a self-interacting complex scalar field playing the role of an ultralight bosonic dark matter model. Constraining stellar models within a multi-messenger-motivated mass-radius window, we obtain three main results: First, bosonic dark matter acts as a catalyst for spontaneous scalarization. Second, energetically favored configurations are scalarized FBSs featuring bosonic dark matter cores that reconcile strongly coupled regimes with radius bounds. Third, dark matter cores tend to suppress scalar charges, which mimics the effect of a massive gravitational scalar field and could contribute to evade observational constraints. Additionally, the exclusive prediction of scalarized FBSs at high masses suggests a way to test this theoretical scenario.

ALT We study odd-parity perturbations of Reissner-Nordström black holes in dynamical Chern-Simons gravity. The dCS action is parity invariant because ϑ *RR is the product of two pseudoscalars; nevertheless, the pseudoscalar perturbation has odd parity and enters only the axial sector at linear order and so lifts the axial–polar isospectrality of general relativity. We derive the finite-charge couplings between the pseudoscalar and the axial gravitational and electromagnetic perturbations directly from the linearized field equations. Both directions are governed by the radial kernel (Mr-Q²)Δ/r⁸, and at nonzero charge the pseudoscalar couples to Ψ+χΨₑ, with χ=2Q/[r√((l-1)(l+2))]. After diagonalizing the Einstein–Maxwell sector, we compute branch-resolved quasinormal-mode shifts with exterior complex scaling for the photon-sphere modes and Frobenius shooting for the near-extremal zero-damping modes. For the Schwarzschild l=2 fundamental mode we obtain β δω=-0.7733-0.3946 i (units M=α=1). Off

ALT The conservation law for the orbital plus spin angular momentum of a free Dirac particle in curved spacetime requires that the affine connection has the antisymmetric part: the torsion tensor, which extends general relativity to the Einstein-Cartan theory of gravity. In spacetime with torsion, the Dirac equation becomes a nonlinear, cubic equation in the spinor wave function. Under charge conjugation, the nonlinear term changes sign relative to the mass term. A Dirac spinor and its charge conjugate therefore satisfy different field equations. We use the spinor connection formalism to derive the Dirac equation for a spherical spinor in its centrally symmetric gravitational field with torsion. We determine the energy-momentum tensor for this spinor and show that the energy contribution from torsion is negative for matter particles and positive for antimatter particles. Consequently, the ground state of a fermion has a lower energy than the ground state of the corresponding antifermion. W

ALT General Relativity predicts that black holes lack an internal structure and that their tidal Love numbers vanish. However, if astrophysical black holes were instead ultra-compact objects (UCOs) with internal structure, their tidal Love numbers would no longer vanish. Then, during a binary inspiral, their tidal deformation could transfer orbital energy to the UCO, increasing the orbital period and leaving an observable imprint on the phase of the emitted gravitational waves. As we discussed previously, the effect is enhanced if one binary companion is resonantly excited. Here we use the point-particle effective field theory framework to calculate this enhancement. We consider circular orbits and focus on excitations of the internal electric quadrupole, modelling the UCO spectrum as a harmonic oscillator spectrum. However, our methods could be straightforwardly extended to more complicated situations. Integrating out the internal quadrupole induces a series of time-dependent finite-size

ALT We give an independent closed-form construction of rotating five-dimensional Ellis–Bronnikov wormholes with equal angular momenta and analyze the radial instability of the reflection-symmetric family. The nontrivial coupled metric equations reduce exactly to an open A₂ Toda system. The general two-ended solution is expressed in terms of elementary Toda tau functions; the symmetric subclass reduces to a particularly simple one-parameter family interpolating between the static wormhole and the extremal equal-spin Myers–Perry geometry. On this exact background we revisit the cohomogeneity-one perturbation problem. Besides the ordinary unstable mode of the static wormhole, rotation renders the descendant of its non-normalizable zero mode normalizable. The two purely imaginary unstable frequencies coalesce at |J|/A=0.100628, where their eigenvectors coalesce as well, identifying a second-order exceptional point. Beyond it the pair continues as ω±=±ω_R+mathrm iω_I with ω_I>0: the apparent lo

ALT Quasinormal modes (QNMs) provide a powerful probe of strong-field gravity. We study fundamental scalar quasinormal modes of an asymptotically flat rotating black hole in the low-energy sector of Hořava gravity, whose exact scalar equation is nonseparable due to rotation-induced Lorentz violation. We develop two complementary approximations that both preserve the exact Kerr sector: Method I applies a slow-rotation approximation only to the nonseparable Lorentz-violating part, whereas Method II employs a small-η biorthogonal spheroidal projection, with η the Lorentz-violating parameter. Using residual-controlled continuation and symmetric finite differences, validated by a Kerr continued-fraction benchmark and an independent radial spectral method, we compute the linear frequency response. Axisymmetric modes provide an exact Kerr null test. For the corotating modes studied, the real-frequency response is positive and increases with spin. At a/M=0.9, the two methods reveal a roughly 2% an

ALT We present a systematic Hamiltonian analysis of the five-parameter class of theories known as Newer General Relativity. Quadratic nonmetricity theories of this type can be divided into nine types according to the structure of their primary constraints. After reviewing the framework of symmetric teleparallel gravity, we derive the ADM formulation of Newer General Relativity, discuss the existing classification and perform the Hamiltonian formulation of the theory. For each type, we compute the conjugate momenta, identify the primary constraints, and construct both the canonical and primary Hamiltonians. We then determine the constraint structure and analyze the resulting physical degrees of freedom in each type.

ALT We investigate how a fixed, spatially uniform axion angle modifies the rotational and static tidal properties of self-bound quark stars in general relativity, using an axion-extended Polyakov-loop Nambu–Jona-Lasinio equation of state. We vary θ from 0 to π while holding the remaining model parameters fixed, and compute the leading-order moment of inertia and the second-order spin-induced quadrupole moment and mass shift. The calculation includes finite-surface-density contributions to the rotational mass shift and static tidal response. Across this range, the static maximum mass increases from 1.68 to 1.95 M⊙, remaining below the lower edge of the quoted 1σ timing interval for PSR J0740+6620. At M=1.338 M⊙, the moment of inertia increases by 13% and the dimensionless tidal deformability rises from 342 to 547. Over the common mass range examined, the differences in the moment of inertia between adjacent sampled angles exceed the tested interpolation and table-resolution sensitivities. D

ALT The present study investigates the feasibility of gravitational baryogenesis in a Gauss-Bonnet (GB) modified Randall-Sundrum (RS) type II braneworld cosmology. The modified expansion rate of the Universe in this scenario evolves through three distinct expansion regimes, the GB regime at early times where the expansion rate H ∼ ρ¹/³ falls more slowly than the standard expansion law H∼ ρ¹/² of GR, then the RS type era with an enhanced expansion H ∼ ρ, before finally reducing to the standard expansion law in the low energy limit. Gravitational baryogenesis depends on Ṙ and requires Ṙ <0. This condition is satisfied for the GB and GR regimes. However, Ṙ>0 for the RS regime and successful baryogenesis requires either that the decoupling temperature occurs during the GB regime or that the Universe makes a direct transition from the GB regime to the GR regime. The GB baryon asymmetry η(T) is proportional to μ(1+w)(1-w)(1+1/β), where μ is the energy scale μ≡ 1/l associated with the bulk curvat

ALT General relativity, a theory that is already over a century old, is still the best theory that we have to describe the universe at energy scales where only the gravitational force is important. It has stood tall to any observational tests thrown at it since its inception, and it has remarkable predictive power when it is utilized to study and model astrophysical problems. It also continues to surprise us with its ability to generate new problems and insights that render themselves useful in the study of modern physics. This is especially highlighted by the transition of modern observational astronomy away from an electromagnetic radiation epoch, and into a new gravitational wave epoch. This thesis devotes its study to phenomena that can be analyzed within the framework of classical general relativity, but whose results may contain interesting implications for cosmological and black hole observations. (Full abstract can be found in document).

ALT We develop a systematic method to compute the high overtone expansion of Kerr quasinormal modes on normal angular branches to arbitrary order. We first obtain the expansion of the angular eigenvalue at large frequencies by a simple polynomial recursion. Using this result in an alternative form of the radial Teukolsky equation, we find that all the WKB corrections are defined on a fixed genus-one spectral curve, and that the quantum periods at the second and higher orders are expressed in terms of only two elliptic periods. This allows us to determine the expansion coefficients order by order. We confirm that the truncated expansions reproduce quasinormal frequencies obtained by solving the angular and radial equations directly, for scalar, electromagnetic and gravitational fields. We also compare the scalar results with the exact WKB analysis of extremal Kerr black holes.