arXiv subjects

Kanabar Jay

Publications and source records attributed to Kanabar Jay.

4 recordsLinked to original sources

arXiv2026

The cosmological inflation inside the cyclic model of the universe

Inflationary cosmology explains the homogeneity and large-scale structure of the universe through a brief epoch of accelerated expansion following the Big Bang. Cyclic cosmologies, in contrast, describe a universe undergoing successive phases of expansion and contraction and can generate primordial perturbations through alternative mechanisms while extending cosmic history beyond a singular beginning. Because these paradigms address different aspects of cosmic evolution, they are often treated separately. Here we explore the possibility that they may instead arise together. In a cosmological scenario involving two scalar fields, one field governs the cyclic evolution of the universe while the other drives an inflationary phase after each cosmological bounce, suggesting that inflation may be a recurring feature of a cyclic universe.

gr-qc↗

arXiv2026

Obstructions to global visibility of singularities in asymptotically flat spacetimes

Consider an $(N+1)$-dimensional asymptotically flat spacetime and a future-directed, affinely parametrized outgoing null generator $\gamma$ of an achronal boundary $\partial J^+(S_\varepsilon)$, where $\{S_\varepsilon\}$ is a nested family of smooth compact codimension $2$ surfaces approaching a singular boundary set $S$ in the past. In the twist-free case and under the null energy condition, the Raychaudhuri equation on the $m:=N-1$ dimensional screen bundle reads, $$ \theta'=-\frac1m\theta^2-\|\sigma\|^2-\mathrm{Ric}(k,k), $$ where $k$ is the tangent to $\gamma$. This equation linearizes, via the rescaling $u:=A^{1/m}$ with $A := |\det D|$ the Jacobi-map $m$-volume, to the Sturm-type ODE $$ u''+\frac1m f\,u=0,\qquad f:=\|\sigma\|^2+\mathrm{Ric}(k,k)\ge 0. $$ We develop two purely generator-wise criteria forcing a first zero of $u$: (i) an exact Volterra identity combined with concavity leads to a barrier-weighted integral inequality, and (ii) Sturm comparison and a Pr\"ufer-angle estimate yields failure of disconjugacy whenever $\int_c^d \sqrt{f/m}\,d\lambda>\pi$ on a subinterval. We prove that $u(\lambda_\ast)=0$ is equivalent to the existence of a focal (conjugate) point and implies $\theta= m u'/u\to-\infty$ at $\lambda_\ast$. Using the standard structure of achronal boundaries, this yields a geodesic-wise obstruction: if every generator that could reach $\mathscr I^+$ satisfies one of the above conditions in the regular spacetime region, then $J^+(S_\varepsilon)\cap \mathscr I^+=\emptyset$, and hence $S$ is not globally visible. As an application, we illustrate one of these criteria in the Einstein-massless scalar field collapse model of Christodoulou.

gr-qc↗

arXiv2025

Particle production and Higgs reheating

Reheating is essential for transforming the cold, vacuum dominated Universe at the end of inflation into the hot thermal bath required by the Standard Model. In many well motivated inflationary models, however, the inflaton has no direct couplings to other fields, raising the question of how the Universe becomes repopulated with particles. We address this question within the framework of geometric reheating, where energy transfer occurs purely through gravitational effects. Focusing on a Higgs inflationary scenario with a non-minimal curvature coupling $\xi \phi^2 R$, we derive the post-inflationary dynamics and compute particle production using the Bogoliubov formalism. We show that the rapid, oscillatory evolution of the curvature scalar after inflaton acts as a time dependent gravitational pump, creating scalar spectator particles even in the absence of explicit interactions. This curvature driven production mechanism provides a natural and efficient route to reheating, demonstrating that gravity alone can initiate the standard thermal history and bridge inflation with radiation domination in minimal, coupling free models of the early Universe.

astro-ph.CO↗

arXiv2025

The final fate of anisotropic-dissipative gravitational collapse

The final fate of a collapsing star depends not only on how much matter it contains but also on how that matter resists gravity in different directions. In this work, we investigate the final fate of highly magnetized radiation-dominated spherically symmetric dissipative stellar configurations. We study the dynamics of collapse by introducing a dimensionless geometric factor $f(\theta, \phi)$ defined by the angular dependence of the radiative opacity. Using the field equations, we derive direction-sensitive threshold conditions that determine whether collapse initiates, halts, or reverses. The resulting inequalities unify black hole formation, bounce behavior, and delayed trapping of geodesics into a single geometrically controlled framework. This theoretical analysis would help analyze the collapse through pre-computed opacity tables for different magnetic field, temperature, and density profiles, along with other data available for such considered profiles.

gr-qc↗