arXiv subjects

S. Shankaranarayanan

Publications and source records attributed to S. Shankaranarayanan.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Evolution of Primordial Black Holes: UV/IR Decoupling and the KM3NeT 220 PeV Neutrino Anomaly

The recent observation of a 220 PeV neutrino event (KM3-230213A) by the KM3NeT observatory presents a formidable challenge to standard astrophysical source models. We investigate the hypothesis that this ultra-high-energy signature originates from the terminal evaporation burst of a Primordial Black Hole (PBH). Since PBH evolution spans cosmic history, static vacuum approximations fail to capture early-universe dynamics. Embedding the PBH in a cosmological background via the McVittie spacetime, we demonstrate that early-universe cosmological accretion and expansion-suppressed Hawking emission shift the required initial mass window for a terminal burst occurring today. We show that although the early universe environment dictates the black hole's overall lifespan, its final explosion today ($z \approx 0$) is governed by standard Schwarzschild thermodynamics. This mechanism naturally produces the intense 220 PeV local flux while suppressing early emissions, thereby satisfying diffuse isotropic background limits. Consequently, this dynamical framework alters the mapping between current ultra-high-energy neutrino observables and the primordial curvature perturbations that seeded them.

astro-ph.HE↗

arXiv2026

Anomaly quenching and dynamical cooling of Hawking evaporation in Horndeski gravity

We investigate the semiclassical Hawking evaporation of a two-dimensional Callan-Giddings-Harvey-Strominger (CGHS) black hole perturbed by first-order Horndeski scalar-tensor couplings. By evaluating the exact scalar curvature invariants, we demonstrate that coordinate singularities near the apparent horizon parametrize a physical deformation of the surface gravity while preserving a strictly regular event horizon. In the linear-order $\gamma_3 \Phi X$ coupling sector, the surface gravity and correspondingly the Hawking temperature decreases dynamically as the black hole loses mass. Extrapolating this cooling effect via a resummed temperature equation suggests that evaporation halts, potentially leaving behind a stable, macroscopic cold remnant at the boundary of perturbative control. Utilizing both the conformal trace anomaly and the Robinson-Wilczek gravitational anomaly, we establish that the asymptotic radiation flux identically reflects this geometric shift. Consequently, the fine-grained entanglement entropy of the Hawking radiation departs from linear growth and transitions to an asymptotic logarithmic regime. This indicates that quantum information may be permanently retained within the remnant, altering the standard semiclassical Page curve without requiring a breakdown of horizon regularity.

gr-qc↗

arXiv2026

Integrated cosmological memory: A dark-siren method to probe dark energy

Gravitational-wave (GW) cosmology is currently bottlenecked by the scarcity of electromagnetic counterparts for bright sirens and the systematic uncertainties of galaxy catalogs for dark sirens. We propose a purely gravitational resolution using the Integrated Cosmological Memory (ICM)-the cumulative GW strain encoded in the spacetime geometry of an expanding Universe. While the GW transient emitted by the source provides the luminosity distance, the ICM accumulates a mathematically distinct integral of the cosmic expansion history. We demonstrate that extracting both observables from a single binary merger completely breaks the distance-redshift degeneracy within the gravitational sector. This establishes a novel, catalog-free dark siren framework for third-generation GW detector networks. Crucially, the resulting constraints on late-time dark energy are only weakly sensitive to the local expansion rate, providing a robust cosmological probe that can potentially mitigate the impact of the H0 tension.

astro-ph.CO↗

arXiv2026

Resolution of Infrared Entanglement Divergences via the Extended Uncertainty Principle

The entanglement entropy of quantum systems typically exhibits both ultraviolet and infrared (IR) divergences. In the low-frequency limit, the IR divergence is intimately tied to the unbounded spatial delocalization of zero-modes, a pathological feature common to both coupled harmonic oscillators and massless scalar fields. In this work, we demonstrate that this infinite growth is naturally resolved by invoking the Extended Uncertainty Principle (EUP), which introduces large-length-scale geometric corrections to the canonical commutation relations. By exactly solving the simple harmonic oscillator under the EUP framework, we establish the existence of an intrinsic geometric confinement that enforces a strict upper bound on the position variance, limits spatial delocalization, and introduces an intrinsic localization length scale related to the background Ricci scalar. We extend this regularizing mechanism to many-body systems by evaluating the entanglement entropy and entanglement spectrum of a one-dimensional harmonic chain and a massless scalar field. We show that the EUP-induced spatial bounds prevent the accumulation of low-lying long-wavelength modes, keeping the entanglement spectrum discrete and evenly gapped even in the strictly massless limit. This non-vanishing modular gap effectively caps the local entanglement temperature of the vacuum. Consequently, the entanglement entropy saturates to a finite value, providing a robust, geometric resolution to the zero-mode IR divergence problem in quantum field theory.

gr-qc↗

arXiv2026

Primordial Black Holes: A Review of Formation and Evolution

Primordial Black Holes (PBHs) have emerged as a leading non-particulate candidate for dark matter and a unique cosmological probe, a paradigm shift accelerated by the detection of anomalous binary mergers by the LIGO-Virgo-KAGRA (LVK) collaboration. While the literature is rich with phenomenological constraints, the fundamental quantum and relativistic underpinnings governing PBH genesis and evolution often receive comparatively less emphasis. This review aims to bridge that gap by systematically detailing the physics of PBH formation and their subsequent evolutionary trajectory. We critically examine the hydrodynamic complexity of the early universe, establishing the relativistic thresholds for collapse, the non-linear \emph{race against sound} in the primordial plasma, and the rigorous mathematical utility of the compaction function. Furthermore, by incorporating the dynamic nature of FLRW backgrounds, higher curvature corrections, and quantum backreaction via the memory burden effect, we challenge the standard hawking evaporation and show that extreme-curvature environments halt evaporation entirely, leaving Planck-scale relics that evade current extragalactic bounds. Finally, we map the multimessenger observational landscape, highlighting how the imminent search for sub-solar mass inspirals by next-generation gravitational wave observatories such as the Einstein Telescope and Cosmic Explorer could yield smoking-gun evidence for the PBH paradigm, ultimately transforming these primordial relics into unparalleled laboratories for high-energy physics.

gr-qc↗

arXiv2026

The Gravitational Spectral Radio Forest: A Signature of Primordial Black Holes

We propose a novel gravitational signature to detect Primordial Black Hole (PBH) dark matter by treating interstellar hydrogen as a quantum sensor for spacetime curvature. Focusing on H II regions, we demonstrate that the Riemann tidal tensor of an \emph{asteroid-mass} PBH induces a symmetric splitting of the $2P_{3/2}$ state in bound hydrogen atoms. This relativistic effect redistributes $9.9\,\mathrm{GHz}$ absorption line into a gravitational spectral radio forest with a bandwidth $\sim 2\,\mathrm{GHz}$. By accounting for active accretion of Hydrogen atoms and the resulting density-squared emission measure within the Bondi radius, we find a relatively enhanced absorption spectrum. This feature presents a concrete, high-contrast target for upcoming radio-surveys to constrain PBH populations in the dark matter sector.

gr-qc↗

arXiv2026

Finite 4-D Gauss-Bonnet quantum corrections from matter-graviton interactions in a curved background

The Glavan-Lin proposal for 4D Einstein-Gauss-Bonnet (EGB) gravity introduces a singular dimensional scaling to bypass Lovelock's theorem, though its fundamental origin remains debated. In this work, we demonstrate that this specific dimension-dependent scaling naturally emerges from the one-loop self-energy corrections of gravitons. By employing real-space techniques to evaluate graviton interactions with minimally coupled scalar and electromagnetic fields in a de Sitter background, we show that the $1/(D-4)$ pole from dimensional regularization naturally yields a term proportional to the Gauss-Bonnet invariant. Importantly, because the Gauss-Bonnet density is a total derivative in four dimensions, this combination yields a strictly finite quantum correction analogous to the conformal anomaly, rather than a divergent counterterm. We confirm that the true ultraviolet divergences are strictly renormalized by standard quadratic curvature counterterms, specifically the Weyl-squared and Ricci-scalar-squared invariants. Our results indicate that the Glavan-Lin scaling is not an ad-hoc classical limit, but effectively captures a finite, loop-induced quantum feature of the effective action. We discuss the implications of these finite, background-dependent corrections in the early-Universe and their consequences in the strong gravity regime.

hep-th↗

arXiv2026

Emergent Hawking Radiation and Quantum Sensing in a Quenched Chiral Spin Chain

We investigate the emergence and detection of Hawking radiation (HR) in a 1D chiral spin chain model, where the gravitational collapse is simulated by a sudden quantum quench that triggers a horizon-inducing phase transition. While our previous work Jaiswal [2025] established that this model mimics BH formation conditions even when the Hoop conjecture is seemingly violated, we here focus on the resulting stationary radiation spectrum and its detectability. By mapping the spin chain dynamics to a Dirac fermion in a curved (1 + 1)-dimensional spacetime, we analyze the radiation using two complementary approaches: field-theoretic modes and operational quantum sensors. First, using localized Gaussian wave packets to model realistic detectors, we find that the radiation spectrum exhibits deviations from the ideal Planckian form, analogous to frequency-dependent greybody factors, while retaining robust Poissonian statistics that signal the loss of formation-scale information. Second, we introduce a qubit coupled to the chain as a stationary Unruh-DeWitt detector. We demonstrate that the qubit functions as a faithful quantum sensor of the Hawking temperature only in the weak-coupling regime, where its population dynamics are governed solely by the bath spectral density. In the strong-coupling limit, the probe thermalizes with the global environment, obscuring the horizon-induced thermal signature. These results provide a clear operational protocol for distinguishing genuine analog HR from environmental noise in quantum simulation platforms.

cond-mat.stat-mech↗

arXiv2026

Gravitational wave detection via photon-graviton scattering and quantum interference

We present a fully quantum field-theoretic framework for gravitational wave (GW) detection in which the interaction is described as photon-graviton scattering. In this picture, the GW acts as a coherent background that induces inelastic energy exchanges with the electromagnetic field - analogous to the Stokes and anti-Stokes shifts in Raman spectroscopy. We propose a detection scheme sensitive to this microscopic mechanism based on Hong-Ou-Mandel interference. We show that the scattering-induced phase shifts render frequency-entangled photon pairs distinguishable, spoiling their destructive quantum interference. GW signal is thus encoded in the modulation of photon coincidence rates rather than classical field intensity, offering a complementary quantum probe of the gravitational universe that recovers the standard classical response in the macroscopic limit.

gr-qc↗

arXiv2026

Quantum Cosmology as a Hydrogen atom: Discrete $\Lambda$ and cyclic Universes from Wheeler-DeWitt quantization

Building upon our recently established correspondence between quantum cosmology and the hydrogen atom [1], we investigate the specific sector of a negative cosmological constant ($\Lambda < 0$) in a flat FLRW universe with dust. While the positive $\Lambda$ sector [1] yields a continuous spectrum and a single bounce, we show here that the negative $\Lambda$ sector leads to a discrete spectrum of energy eigenvalues, effectively quantizing the cosmological constant. Within this dual description, the operator-ordering ambiguity parameter appears as the azimuthal quantum number of the hydrogen atom. A skewed Bohr correspondence emerges for the bound states, matching classical evolution at large volumes but deviating near the bounce. By constructing wave packets from these bound states, we demonstrate that the classical Big Bang and Big Crunch singularities are resolved, and the universe oscillates between quantum bounces and classical turnaround points. The expectation values of the observables indicate a cyclic universe -- with vanishing Hubble parameter at turnarounds -- undergoing quantum bounces. This exactly solvable model offers a tractable setting to explore quantum gravitational effects in cosmology. We analyze the properties of this cyclic universe, contrasting its bound-state dynamics with the scattering states of the de Sitter case.

gr-qc↗

arXiv2025

A non-local origin for massive gravity and late-time acceleration

The accelerated expansion of the universe poses a significant challenge to General Relativity. Non-local modifications to gravity have emerged as a compelling class of theories to address this dark energy puzzle. Building upon earlier proposals, we investigate a specific non-local modified gravity action incorporating terms like $R\Box^{-2}R$, $R^{\mu\nu}\Box^{-2}R_{\mu\nu}$, $R^{\mu\nu\sigma\delta}\Box^{-2}R_{\mu\nu\sigma\delta}$ and demonstrate that it provides a dynamical origin for a massive graviton by reducing to the standard and extended Fierz-Pauli action at the linearized level. A fixed-point analysis of the background cosmology reveals a stable de Sitter attractor, ensuring the model naturally drives accelerated expansion. Crucially, we investigate the cosmological perturbations and show that the theory's six propagating degrees of freedom are free from ghost instabilities. We further demonstrate that all large-scale tensor modes are dynamically stable and decay on the accelerating background. This ghost-free massive gravity extension provides distinct predictions for gravitational wave polarizations and is theoretically consistent with $\mathbf{\Lambda CDM}$ at late times, positioning it as a unique alternative to scalar-tensor models like $f(R)$ and Galileons. This robust stability at both the background and perturbative levels establishes our model as a consistent and compelling alternative to the standard $\Lambda$CDM paradigm.

gr-qc↗

arXiv2025

Exact, non-singular black holes from a phantom DBI Field as primordial dark matter

We present the first exact, non-singular black hole solution in General Relativity sourced by a Dirac-Born-Infeld (DBI) scalar field. Crucially, the solution is exclusively supported by \emph{the phantom branch of the DBI action}, dynamically replacing the central singularity with a regular core. The solution is asymptotically flat, possesses non-trivial scalar hair, and replaces the central singularity with a regular 2-sphere. The mechanism for singularity resolution is a dynamical \emph{kinetic stiffness} which also explains the evasion of classical no-hair theorems. We show these black holes evaporate to a non-singular relic with mass of the order of a gram. This provides a robust mechanism to evade standard evaporation constraints, opening a vast, previously forbidden mass window for light \emph{Primordial Black Holes} to constitute dark matter. The model is testable via distinctive gravitational-wave signatures from its scalar hair.

gr-qc↗

arXiv2025

Covariant Carrollian Electric and Magnetic Limits of General Relativity

The Carrollian limit ($c \to 0$) of General Relativity provides the geometric language for describing null hypersurfaces, such as black hole event horizons and null infinity. Motivated by the well-established electric and magnetic limits of Galilean electromagnetism, we perform a systematic analysis of the low-velocity limit of linearized gravity to derive its Carrollian counterparts. Using a 1+3 covariant decomposition, we study the transformation properties of linear tensor perturbations (gravitational waves) on a Friedmann-Lemaitre-Robertson-Walker background under Carrollian boosts. We demonstrate that, analogous to the electromagnetic case, the full set of linearized Einstein's equations is not Carrollian-invariant. Instead, the theory bifurcates into two distinct and consistent frameworks: a Carrollian Electric Limit and a Carrollian Magnetic Limit. In the electric limit, dynamics are frozen, leaving a static theory of tidal forces ($E_{ab}$) constrained by the matter distribution. In contrast, the Magnetic Limit yields a consistent dynamical theory where the magnetic part of the Weyl tensor ($H_{ab}$), which governs gravito-magnetic and radiative effects, remains well-defined and is sourced by the spacetime shear. This framework resolves ambiguities in defining Carrollian gravity and provides a robust theory for gravito-magnetic dynamics in ultra-relativistic regimes. Our results have direct implications for the study of black hole horizons, gravitational memory, and the holographic principle.

gr-qc↗

arXiv2025

Disformal interactions in the Dark Sector: From driving Early Dark Energy to confronting cosmological tensions

The $\Lambda$CDM model faces significant challenges, including an incomplete understanding of the dark sector and persistent tensions in the Hubble constant and the clustering amplitude. To address these issues, we propose a general disformal coupling between dark energy (DE) and dark matter from a field-theoretic action which can generate a rich variety of interactions including conformal and pure-momentum coupling scenarios. Our analysis reveals that a pure disformal coupling naturally produces a unique interacting Early Dark Sector, wherein the interactions with dark matter suppress the Hubble friction on the DE scalar field leading to a kinetic-driven cosmological constant-like behavior at early times followed by its dilution as $a^{-6}$ and eventually leading to a potential-driven epoch characteristic of late-time dark energy. In contrast to existing Early Dark Energy (EDE) models that rely on finely-tuned potentials, the EDE-like behavior, in our framework, is purely a consequence of the disformal coupling paired with the dilution of dark matter, offering a more fundamental and less ad hoc solution to cosmological tensions. This framework also predicts a suppression of power in the CMB temperature spectrum on large angular scales, offering a potential physical explanation for the observed low-$\ell$ anomaly. By deriving these effects from a fundamental action, our work provides a unified, testable alternative to $\Lambda$CDM that can be constrained by next-generation cosmological surveys and gravitational wave observations.

astro-ph.CO↗

arXiv2025

Axial quasi-normal modes of slowly rotating black holes in dynamical Chern-Simons gravity to second-order in spin and coupling

We compute the quasi-normal mode (QNM) frequencies of slowly rotating black holes in dynamical Chern-Simons (dCS) gravity, including corrections up to second order in the black hole's dimensionless spin parameter $\chi = J/M^2$ and second order in the dCS coupling parameter ($\alpha$). Due to the complexities of constructing a Newman-Penrose tetrad at this order, we employ a metric perturbation approach. We derive a system of coupled ordinary differential equations for the primary axial $l$-mode and the polar $l\pm 1$ modes, which is then solved numerically with appropriate ingoing and outgoing wave boundary conditions. Our numerical framework is validated in the General Relativistic limit against known Schwarzschild QNMs and highly accurate Kerr QNM results for $\chi \leq 0.15$. For the fundamental $n=0, l=m=2$ axial mode, we present detailed numerical results illustrating the dependence of QNM frequencies on both $\chi$ and $\alpha$. We observe that while rotation generally increases the damping time, increasing the dCS coupling parameter significantly reduces the damping time of the axial mode. This finding contrasts with previous analytical work on polar modes, which suggested an increase in damping time due to dCS effects, highlighting a crucial parity-dependent difference in how dCS gravity impacts black hole ringdowns. Furthermore, we provide an analytical fitting formula for this mode. These results, incorporating coupled spin and dCS effects at second order, provide more accurate theoretical predictions for testing dCS gravity with gravitational wave observations of black hole ringdowns. The refined QNM calculations are particularly relevant for lower-mass black hole merger events, such as GW230529, where dCS corrections may be more prominent and their distinct damping signatures could be observable. [Abridged Version]

gr-qc↗

arXiv2025

Beyond Entanglement: Diagnosing quantum mediator dynamics in gravitationally mediated experiments

No experimental test to date has provided conclusive evidence on the quantum nature of gravity. Recent proposals, such as the BMV experiment, suggest that generating entanglement could serve as a direct test. Motivated by these proposals, we study a system of three-harmonic oscillator system, with the mediator oscillator operating in two distinct parameter regimes: a heavy mediator regime and a light mediator regime. These regimes induce qualitatively different entanglement dynamics between the terminal oscillators. Crucially, distinguishing these regimes experimentally remains challenging when relying solely on entanglement measures. We demonstrate that the dynamical fidelity susceptibility offers a viable and sensitive probe to contrast the regimes in practice. Our results provide testable signatures for optomechanical and trapped-ion platforms simulating gravitational interactions, and provide new avenues to characterize quantum-gravity-inspired systems beyond entanglement-based protocols.

quant-ph↗

arXiv2025

Unitary quantum matter-bounce in a universe with a positive cosmological constant

We analyze the Wheeler-DeWitt quantization of a spatially flat Friedmann-Lema\^itre-Robertson-Walker universe containing pressureless dust and a positive cosmological constant ($\Lambda > 0$). Following relational time framework, we establish a direct mathematical correspondence between the cosmological Hamiltonian and the radial Schr\"odinger equation for the scattering states of the non-relativistic hydrogen atom. This exact solvability allows us to rigorously construct the physical Hilbert space and ensure the self-adjointness of the Hamiltonian. As a concrete result, we show that the wave packets unitarily evolve depicting a non-singular quantum bounce, systematically replacing the classical Big Bang singularity. Finally, we discuss the physical relevance of this exact solution within the matter-bounce scenario. We demonstrate that this framework provides a robust quantum origin for a bounce during a dust-dominated contracting phase -- a necessary prerequisite for generating a scale-invariant spectrum of primordial perturbations -- derived from the unitary dynamics of the quantized background.

gr-qc↗

arXiv2025

Quantum signatures in black hole accretion: Pair production in dynamical magnetic fields

Accretion disks around black holes host extreme conditions where general relativity and magnetohydrodynamics dominate. These disks exhibit two distinct dynamical regimes -- Standard and Normal Evolution (SANE) and Magnetically Arrested Disk (MAD). In the MAD regime, these systems exhibit magnetic fields up to $10^8$ G and variability on gravitational timescales $t_g \sim 10^{-4}$ s for stellar-mass black holes. While classical magnetohydrodynamics has been extensively applied, quantum effects in these high-energy environments remain unexplored. Here, we employ quantum field theory in background gauge fields (QFTBGF) to demonstrate that the dynamic magnetic fields of MADs drive significant pair production via the Schwinger mechanism. The resulting pairs emit non-thermal (synchrotron) radiation with a peak frequency tunable across $ \sim 1 - 3000$ MHz, depending on the magnetic field strength (peaking at higher frequencies for stronger fields). For $ B \sim 10^8 $ G, our model predicts a peak spectral flux density of $ \sim 1 - 100$ mJy, detectable with next-generation radio telescopes (e.g., SKA, ngVLA). This work provides a direct and observable signatures of quantum effects in black hole accretion disks.

astro-ph.HE↗