Papers

We investigate exact static, spherically symmetric electrically charged black hole solutions in a gravitational theory with spontaneous Lorentz-symmetry breaking induced by a background Kalb–Ramond field. In contrast to previous analyses that retained only one nonminimal curvature coupling, we include the combined effects of the two independent nonminimal curvature couplings and obtain charged black hole solutions both with and without a cosmological constant. Using the Iyer–Wald covariant phase-space formalism, we derive the corrected thermodynamic quantities and analyze the Joule–Thomson expansion, including the inversion curve and the cooling/heating regions. We further apply the topological approach to black hole thermodynamics to characterize the van der Waals-like phase transition and show how the thermodynamic critical temperature and pressure are encoded in the corresponding topological defect curve. These results clarify the thermodynamic and topological signatures of electrically charged black holes in gravity with a Lorentz-violating Kalb–Ramond background.

Ultralight bosonic dark matter can accumulate around a rotating black hole, where superradiance amplifies the field until a macroscopic cloud forms. Whether such a cloud behaves as a Bose–Einstein condensate depends on the self-interaction, which earlier work has either retained on static backgrounds or dropped on the Kerr metric. Here we treat the two together. Starting from the Klein–Gordon equation with a quartic potential, we separate the linear problem into spheroidal and radial equations and solve them self-consistently, obtain the superradiant growth rate from the conserved Noether current, project the nonlinear term onto a single mode, and integrate the resulting Gross–Pitaevskii equation with a bordered Newton method at fixed particle number. Rotation modulates the self-interaction geometrically: the effective coupling carries a factor $\Delta(r)$ and therefore switches off at the horizon. Once the field is rescaled, the whole solution family depends on the single dimensionless parameter $\mathcal{N}=\lambda N$. We recover the hydrogenic spectrum of the gravitational atom and the $\alpha^{4\ell+5}$ scaling of the growth rate, and we obtain the exact relation $J_{z}=\hbar mN$, which receives no correction from the self-interaction. We find that the condensate is a torus rather than a spherical shell, with its density vanishing identically on the rotation axis, and that the cloud is modified appreciably only for $\mathcal{N}\gtrsim10^{3}$.

Dissipative equations admit distinct variational descriptions in the Bateman and Caldirola–Kanai (CK) formalisms. The classical correspondence between them is extended to a homogeneous canonical scalar field on a prescribed spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) background, where the expansion produces the time-dependent damping coefficient $3H(t)$. A multiplier action yields the Klein–Gordon equation and a complementary anti-damped equation containing the term $-3\dot{H}(t)\chi$. A first-order Bateman Lagrangian derived from the same multiplier action reproduces this physical–auxiliary pair for a general potential. Specializing to a free massive field gives the Bateman and doubled CK Lagrangians and Hamiltonians used in the canonical comparison. The factors $a^{3}(t)$ and $a^{-3}(t)$ generate the damped and anti-damped CK sectors, respectively. An explicit time-dependent canonical transformation, generated by a function linear in the Bateman momenta, maps the complete doubled CK system to the Bateman system. For this point transformation, the terms proportional to $\dot{H}(t)$ are required for Hamiltonian equivalence. In rotated variables, the Bateman scalar-field Hamiltonian takes the difference form $H_{B,\mathrm{SF}} = E_{u} - E_{v}$. It is conserved for constant $H$ and generally varies with time otherwise. For the power-law background $a(t) \propto t^{p}$, however, a correlated family at $p = 2/3$ has conserved $H_{B,\mathrm{SF}}$ despite the time dependence of $H(t)$. These results concern classical homogeneous fields on a prescribed FLRW background and exclude the gravitational phase space.

Higher-curvature gravity usually leads to very complicated field equations, which makes it hard to find analytical solutions. In this work, we obtain analytical charged black hole solutions in higher-curvature gravity by using black hole thermodynamics and the continued fraction expansion. We study the thermodynamics of static black holes using the first law of thermodynamics and the Smarr formula. We show that our results agree with those obtained by directly solving the field equations in Einstein gravity.

We revisit metastable dark energy (DE) models described by a radioactive-like decay law. We consider three scenarios: an effective, exponentially decaying DE component; decay of DE into non-baryonic dark matter (DM); and decay of DE into dark radiation (DR). We constrain the metastable DE models using DESI DR2 baryon acoustic oscillation (BAO) data, Type Ia supernovae (SNIa), cosmic microwave background (CMB) observations, and, for the first time, the current available DESI DR1 full-shape (FS) clustering measurements. The BAO+SNIa combinations show a mild preference for positive $\Gamma/H_0$, with a deviation from the $\Lambda$CDM limit at the $\gtrsim 2\sigma$ level. This corresponds to a decaying DE density and an effective quintessence-like behaviour at low redshift. Once CMB information from either Planck or P-ACT is included, however, the constraints become statistically consistent with $\Gamma/H_0=0$. The FS measurements probe the growth sector and help distinguish the interacting DM-DE behaviour of Model 2 from the effective decaying-DE response of Model 1 and the weaker DR-induced response of Model 3. For CMB+FS+DES-Dovekie, Model 1 shows a slight deviation from $\Gamma/H_0=0$ at the $\gtrsim 2\sigma$ level, while Models 2 and 3 remain consistent with the $\Lambda$CDM limit within $2\sigma$. Overall, metastable DE remains phenomenologically viable: current data allow late-time dynamics but do not provide decisive evidence for a nonzero decay rate. These results motivate extending our analysis to the upcoming DESI DR2 FS data to obtain tighter constraints on metastable dynamics.

In the framework of the Iyer-Wald formalism, the first law of black hole mechanics is examined within the context of conformal Einstein-power-Yang-Mills (CEPYM) theory. By comparing two inffnitesimally neighbouring stationary black hole solutions, we obtain the explicit analytical expression for the first law of black hole thermodynamics in CEPYM theory.

We propose a quantum dominant energy condition (QDEC) for the stress tensor in the context of quantum field theory in curved spacetimes. A rigorous proof is given for the case of Rindler wedges, and a heuristic discussion about possible generalizations to more general geometric setups, including curved spacetime, is provided. In Minkowski spacetime, we establish a connection with state recovery bounds. We illustrate the QDEC for coherent states of the free scalar field, where it turns out to be related to the ordinary DEC for the stress tensor of the classical solution that defines the coherent state.

We compute the tail-of-tail contribution to the conservative dynamics of eccentric, non-spinning compact binaries to relative 1PN order and to $\mathcal O(e_t^{12})$. Using the $1$PN quasi-Keplerian dynamics in harmonic coordinates, we derive the Delaunay-averaged Hamiltonian at $5.5$PN and $6.5$PN order and match it to the effective-one-body description, allowing us to determine the corresponding contributions to the non-geodesic EOB $Q$ potential through the $\mathcal{O}(p_r^{12})$, including the dependence on the symmetric mass ratio up to $\mathcal{O}(\nu^2)$. The terms linear in the mass ratio reproduce the available first-order self-force results, while the quadratic terms provide qualitatively new eccentric second-order self-force predictions arising from the tail-of-tail terms. We independently rederive the averaged Hamiltonian using a Fourier–Bessel decomposition of the hereditary interaction. Applying the first law at fixed orbital frequencies, we recover the known first-order self-force redshift through $\mathcal O(e^{12})$ and obtain the complete tail-of-tail contribution to the second-order inverse redshift at $5.5$PN and $6.5$PN through $\mathcal O(e^{10})$.

Cosmological collider signals are usually discussed for isolated massive particles, whose exchange produces characteristic logarithmic oscillations in primordial correlators. In this work, we study how this signal is modified when the exchanged states form a continuous mass spectrum. We first develop a spectral representation for inflationary correlators mediated by a continuum field. In the soft limit, the non-analytic part of the seed function is expressed as a Fourier–Laplace transform of the spectral weight with respect to logarithmic momentum variables. This representation shows that a continuum superposes clock signals with different frequencies and can dephase the sharp oscillation associated with a single massive particle. We then realize this mechanism in an RS2-like inflationary braneworld, where the inflaton is localized on a de Sitter brane in a five-dimensional AdS bulk. The tensor sector contains a localized massless graviton and a continuum of Kaluza-Klein gravitons starting at \(m=3H/2\). We derive the KK wavefunctions and identify the brane spectral weight fixed by the continuum wavefunction on the brane. Applying this weight to the inflaton four-point function, we find a smooth seed function rather than a persistent logarithmic clock oscillation. This behavior follows from the fact that the continuum starts at zero clock frequency, while the KK spectral weight vanishes near threshold. Our results provide a concrete higher-dimensional example in which deviations from the standard cosmological collider signal encode the structure of a continuum high-energy spectrum.

We show that the quantum entanglement between the conformal field theories (CFTs) living on the future and past boundaries in the de Sitter/conformal field theory (dS/CFT) correspondence can be described by Wick rotating to a geometry with two timelike directions. We propose a new realization of the dS/CFT correspondence in which quantum gravity on this two-time geometry is holographically dual to a CFT defined on a Lorentzian torus. We show that this duality reproduces key features of dS holography, including the dS entropy, correlation functions, and pseudoentropy. Finally, by extending the framework of path-integral optimization, we explain how dS spacetime emerges from the CFT on the Lorentzian torus.

We perform a complete qualitative analysis of the cosmological dynamics of Lovelock gravity in a spatially flat Friedmann-Robertson-Walker (FRW) universe filled with a perfect fluid obeying a barotropic equation of state $p=\omega\rho$. Starting from the generalized $N$-dimensional Friedmann equations written in terms of the independent components of the Riemann tensor, we reduce the dynamics to a single autonomous first-order equation for the Hubble parameter $H$, whose right-hand side is a ratio of polynomials in $H^2$ fixed by the order $n$ of the Lovelock polynomial, the number of dimensions $N$, and the coupling constants $\alpha_i$. The real roots of these polynomials determine the fixed points and the singular points of the system, which govern its asymptotic behaviour. We give a complete classification of the possible evolution scenarios, identify the attractive and repulsive fixed points, locate the "phantom intervals" in which the effective matter density becomes negative, and show that fixed points are reached in infinite time whereas singular points are reached in finite time. For $N>4$ and suitable negative couplings a qualitatively new scenario arises, which we call the "Big Shock": the universe starts from a state with finite density and finite scale factor but with an infinite rate of change of the Hubble parameter, replacing the standard Big Bang. The general analysis is illustrated for $n=1$, $N=4$ (general relativity), $n=2$, $N=5$ (Einstein-Gauss-Bonnet gravity), and $n=3$, $N=7$ (cubic Lovelock gravity).

Considering the mass parameters as free phenomenological parameters, we investigate inflation driven by a canonical scalar field with different potentials in LQC. We find that a smaller mass value makes it easier to obtain a sufficient number of e-folds for the quadratic potential, while for the Starobinsky potential, more e-folds are obtained for larger mass values. By estimating the probability of slow-roll inflation, we find that it remains very close to unity for all considered cases. We compute the primordial power spectrum using three approaches: the dressed metric, the hybrid, and the alternative mass function approaches for the case that the kinetic energy dominated at bounce. The resulting power spectrum for both potentials and different mass values shows the same qualitative pattern in all three approaches. It is suppressed at small $k$, amplified and oscillating over an intermediate range, and nearly scale-invariant at large $k$. The approaches mainly differ in how fast the power spectrum converges to this regime, with the hybrid approach converging the fastest, followed by the alternative mass function and dressed metric approaches. We find that the pivot scale $k_\star$ is highly sensitive to the inflaton mass, changing by more than an order of magnitude for a mass variation of only a few percent. Using the tensor power spectrum at $k_\star$, we calculate the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$, finding good agreement with current data. Finally, the scalar power spectra are fed into the CAMB code to obtain the angular power spectrum and compare with the Planck 2018 data and the best-fit $\Lambda$CDM model. All three approaches are consistent with data at high multipoles, while at low multipoles the hybrid approach gives the closest agreement and the dressed metric approach shows the largest deviation.