arXiv subjects

Swapnil Kumar Singh

Publications and source records attributed to Swapnil Kumar Singh.

9 recordsLinked to original sources

arXiv2026

Quintessential Inflation in Light of ACT DR6

We analyze the canonical generalized-exponential quintessential-inflation potential in light of ACT DR6 scalar-tilt constraints and the distinct demands of its post-inflationary and late-time evolution. The potential slow-roll hierarchy is derived through fourth order and checked against a numerical scalar and tensor mode calculation. For a benchmark $n=15$, $λ=10^{-9}$ at a reference $60$ remaining e-folds, the numerical spectrum gives $n_s\simeq0.96396$, $r\simeq7.71\times10^{-5}$, and a negative running of running of order $10^{-5}$. The exact Hubble-flow endpoint gives a full inflationary excursion exceeding $M_{\rm Pl}$ even though the ten-e-fold observable interval is much smaller. We characterize a phenomenological radiation-injection family followed through kination, radiation and matter domination, and calculate leading-order gravitational-wave transfer spectra with an integrated radiation-density test. A shallow exponential term permits a calibrated late-time quintessence phase, whose homogeneous equation of state and expansion predictions are evaluated numerically. The result is a quantitative map of the coupled inflation, reheating, gravitational-wave and dark-energy requirements, with the limitations of the assumed transfer and thermal history stated explicitly.

physics.gen-ph↗

arXiv2026

Quantum-Deformed Phase-Space Geometry and Emergent Inflation in Effective Four-Dimensional Spacetime

We develop a phase-space approach to quantum-deformed gravity. Reducing it to an effective four-dimensional spacetime structure facilitates reanalyzing cosmic inflation and quantum gravity dynamics. Initiated on a cotangent bundle, the gravitational Hamiltonian is then deformed by a zero-homogeneous scalar determined by projective momentum directions and quantum phase-space properties, forming an anisotropic Hamilton geometry on a non-null conic domain. Through section-pullback procedures, an effective spacetime metric is derived, which in the homogeneous and isotropic sector reduces to a conformally deformed FLRW geometry governed by a scalar deformation field. The corresponding modified Einstein, Klein-Gordon, geodesic-deviation, and Raychaudhuri equations are derived and then utilized to construct inflationary background dynamics, slow-roll regimes, e-folds, and perturbation spectra. The framework shows that leading inflationary corrections stem from phase-space deformation and its time dependence, whereas canonical quantization of cosmological perturbations remains standard following background redefinitions. Ultimately, this model establishes a covariant link between quantum-deformed phase-space geometry and effective four-dimensional inflationary dynamics, demonstrating that quantum gravity effects can be encoded as projective phase-space deformations while preserving the classical limit and standard perturbative structure.

gr-qc↗

arXiv2026

Symmetry-Protected $α$-Attractor Hybrid Inflation in Supergravity and Constraints from ACT DR6 and DESI DR2

We present a symmetry-protected supergravity effective realization of hybrid $α$-attractor inflation with a sequestered Stückelberg uplift. The model contains four chiral multiplets, an inflaton modulus $T$, a stabilizer field $S$, and a pair of oppositely charged waterfall multiplets $(Ψ,\barΨ)$, together with a hidden Stückelberg $U(1)_D$ sector that generates an approximately constant positive contribution to the scalar potential. Along the real inflationary trajectory, the F-term potential realizes the E-model plateau \[ U(ϕ)=V_0\left(1-e^{-\sqrt{2/(3α)}\,ϕ}\right)^2 , \] while the sequestered uplift shifts the total energy density without directly modifying the tree-level inflaton slope. We derive the background evolution, the analytic $N$--$ϕ$ relation, the waterfall stability condition, and the leading predictions for the scalar spectral index and tensor-to-scalar ratio. The model retains the characteristic red-tilted $α$-attractor behavior, with $n_s\simeq1-2/N_*$ and tensor amplitudes controlled primarily by the curvature parameter $α$, while the uplift and the hybrid end point generate only subleading corrections at fixed $N_*$. We also discuss the assumptions required for an effectively single-field trajectory, including the treatment of the light angular component of $T$, and examine the sensitivity of the sequestered uplift to Planck-suppressed cross-couplings. The resulting construction provides a controlled supergravity embedding of hybrid $α$-attractor inflation compatible with current CMB constraints, including Planck, ACT DR6, and DESI DR2, while remaining testable by future $B$-mode searches such as LiteBIRD and CMB-S4.

hep-ph↗

arXiv2026

The Kähler Stability Wall in Perturbative Brane--Antibrane Inflation

Perturbative brane--antibrane inflation requires the physical Kähler volume to remain stabilized while the anti-D3 sector supplies the inflationary energy. This paper shows that this requirement imposes an exact analytic boundary on the allowed anti-D3 tension. In the leading nilpotent effective theory, the Kähler minimum and the decompactification barrier disappear through a saddle-node degeneration determined by $U_σ=U_{σσ}=0$. The positive-tension branch of this boundary is solved in closed form and uniformized by a single real parameter. The same solution fixes the local scaling of the Kähler mass, barrier height, and canonical distance to the endpoint. Integrating out the volume modulus reveals a Schur-complement singularity, showing that the adiabatic single-field description fails at the same boundary unless the inflaton--volume mixing is suppressed. The result is then translated into necessary flux-arithmetic filters involving the warped throat, D3 tadpole, conifold control, scalar normalization, and the full Kähler-sector mass matrix. Numerical tests verify the critical locus, the saddle-node scaling laws, and the resulting sparse throat-flux filtration. These conditions are necessary for compact realizations; no compact vacuum is asserted without the corresponding global and Wilsonian control checks.

hep-th↗

arXiv2026

What new physics can we extract from inflation using the ACT DR6 and DESI DR2 Observations?

We present a comprehensive analysis of inflationary models in light of projected sensitivities from forthcoming CMB and gravitational wave experiments, incorporating data from recent ACT DR6, DESI DR2, CMB-S4, LiteBIRD, and SPHEREx. Focusing on precise predictions in the $(n_s, α_s, β_s)$ parameter space, we evaluate a broad class of inflationary scenarios -- including canonical single-field models, non-minimally coupled theories, and string-inspired constructions such as Starobinsky, Higgs, Hilltop, $α$-attractors, and D-brane models. Our results show that next-generation observations will sharply constrain the scale dependence of the scalar power spectrum, elevating $α_s$ and $β_s$ as key discriminants between large-field and small-field dynamics. Strikingly, several widely studied models -- such as quartic Hilltop inflation and specific DBI variants -- are forecast to be excluded at high significance. We further demonstrate that the combined measurement of $β_s$ and the field excursion $Δϕ$ offers a novel diagnostic of kinetic structure and UV sensitivity. These findings underscore the power of upcoming precision cosmology to probe the microphysical origin of inflation and decisively test broad classes of theoretical models.

astro-ph.CO↗

arXiv2026

Oscillatory Freeze from Inertial Holographic Dark Energy

We study a generalized holographic dark energy model in which the infrared cutoff depends on the Hubble parameter and its first two time derivatives. The inclusion of the $\ddot H$ term introduces a finite relaxation timescale for the horizon degrees of freedom, which can be interpreted as an effective entropic inertia of the holographic vacuum energy. The resulting background dynamics admit late--time solutions in which the cosmic expansion gradually halts. In the underdamped regime, the Hubble parameter undergoes exponentially damped oscillations and asymptotically approaches $H\to0$. The scale factor grows monotonically but by a finite amount, while curvature invariants decay exponentially, leading to an asymptotically Minkowski spacetime without future singularities. We confront the full nonlinear background evolution with cosmic chronometer measurements of the Hubble parameter and find good agreement with current late--time expansion data, with a reduced chi--squared $χ^2/ν\simeq0.52$. At observable redshifts, oscillatory features are strongly suppressed and remain consistent with existing constraints.

gr-qc↗

arXiv2025

Negative non-Gaussianity as a salvager for PBHs with PTAs in bounce

Non-Gaussianity in the primordial curvature perturbation is a crucial element of the early universe due to its significant impact on the primordial black hole (PBH) production. In this work, we focus on the effects of negative non-Gaussianity on PBH abundance through the lens of the compaction function criterion for PBH formation. Our setup utilizes an effective field theory of non-singular bounce, including the standard slow-roll inflation with an ultra-slow roll phase for amplifying the curvature perturbations to form PBHs. We investigate with two separate values of the non-Gaussianity parameter, $f_{\rm NL}=(-39.95,-35/8)$, found within the ekpyrotic contraction and the matter bounce scenarios, respectively, and show that a negatively large amount of $f_{\rm NL}$ can provide sizeable abundance, $10^{-3}\leq f_{\rm PBH}\leq 1$, and completely mitigates the PBH overproduction issue. We also highlight that the case with the effective sound speed $c_{s}=1$, coupled with $f_{\rm NL}=-39.95$, provides an agreement under $1σ$ for the scalar-induced gravitational wave explanation of the latest PTA (NANOGrav15 and EPTA) signal. Lastly, we extract an upper bound on the most negative value of, $f_{\rm NL}\sim -60$, below which we show breaching of the underlying perturbativity constraints on the power spectrum amplitude.

astro-ph.CO↗

arXiv2025

Self-Interacting Gravitational Exciton Condensates from Extra-Dimensional Stabilization

We present a comprehensive study of gravitational exciton dynamics arising from higher-dimensional theories, with a focus on establishing a robust effective framework that incorporates self-interactions, higher-derivative corrections, and quantum effects. Starting from a $ D $-dimensional Einstein--Hilbert action on a warped product manifold, we perform a systematic dimensional reduction to obtain a four-dimensional effective action in the Einstein frame. Our formulation extends the classical potential by including self-interacting moduli fields and RG-improved parameters via the Coleman--Weinberg mechanism, thereby accounting for both matter and graviton loop corrections. The resulting renormalization group flow modifies the effective mass and coupling constants, which in turn plays a critical role in moduli stabilization and the low-energy phenomenology. Taking the non-relativistic limit, we derive a Gross-Pitaevskii equation that governs the dynamics of the gravitational exciton condensate, and couple it self-consistently with Poisson's equation to capture gravitational backreaction. Through both variational and numerical analyses, we obtain stationary solutions in spherically symmetric, rotating, and anisotropic configurations, and perform a linear stability analysis using the Bogoliubov-de Gennes formalism. Our results reveal a Bogoliubov dispersion relation that exhibits a phonon-like linear regime at low momenta, transitioning to quadratic free-particle behavior at high momenta.

gr-qc↗

arXiv2024

Confinement Phenomena in Topological Stars

Quantum Chromodynamics (QCD) is the fundamental theory describing the strong nuclear force and the interactions among quarks and gluons. Topological stars, characterized by extreme density conditions, offer a unique environment where QCD phenomena play a crucial role due to the confinement of fundamental particles. Understanding these phenomena is essential for unraveling the behavior and properties of these celestial bodies. In this study, we explore the implications of QCD within extreme density regimes, focusing on its contribution to the energy-momentum tensor ($T^{μν}_{\text{QCD}}$) within the framework of Quantum Chromodynamics. Our analysis sheds light on how these QCD effects influence the fabric of spacetime in the vicinity of topological stars, providing valuable insights into their underlying physics.

hep-th↗