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K. K. Venkataratnam

Publications and source records attributed to K. K. Venkataratnam.

10 recordsLinked to original sources
arXiv2026

Relativistic Signatures of Dark Matter Equations of State in Static Spherically Symmetric Spacetimes

We study how three physically motivated dark matter equations of state alter the spacetime geometry of static, spherically symmetric black holes in General Relativity. The models considered are anisotropic perfect fluid dark matter (PFDM), isotropic constant-$ω$ dark matter, and Bose-Einstein condensate (BEC) dark matter with a polytropic equation of state (EoS). For each model, we derive the Einstein field equations and solve for the metric function analytically in the PFDM and constant-$ω$ cases, and numerically via the Tolman-Oppenheimer-Volkoff equations for BEC dark matter. We then compute the key strong gravity observables: event horizon radius, photon sphere radius, black hole shadow radius, innermost stable circular orbit (ISCO), and circular orbital velocity profiles. The shadow radii obtained for each model are compared against the Event Horizon Telescope constraints to place bounds on the dark matter parameters. Physical viability is assessed through the null, weak, dominant, and strong energy conditions, along with causality requirements on the sound speed. Our results show that PFDM produces near Schwarzschild geometry with mild anisotropic corrections, the constant-$ω$ model is strongly restricted by causality to $0 \leq ω\leq 1$, and BEC dark matter generates smooth, bounded deviations governed by the bosonic self-interaction parameter K. Taken together, these findings show that strong gravity observables can serve as practical tools for distinguishing between competing dark matter models through their geometric imprints on black hole space times.

gr-qc↗
arXiv2026

Role of Domain Walls in the Early Universe in the Context of Mode Matching

We study the role of domain walls and their relics in the very early Universe within the framework of the mode-matching technique. Domain walls formed during the spontaneous breaking of discrete symmetries are modelled as a short lived contribution to the background energy density, leading to a controlled deviation from standard slow-roll inflation. Solving the modified Friedmann equation, we obtain a smooth, time-dependent Hubble parameter that asymptotically approaches the standard inflationary value. We analyze the evolution of scalar perturbations using the gauge-invariant Mukhanov variable and perform mode matching across the transition between the domain wall affected phase and standard inflation. We find that only modes exiting the horizon near the transition experience a modified evolution, while modes far from the transition remain unaffected. As a result, the primordial scalar power spectrum exhibits localized, scale-dependent deviations from scale invariance, while preserving the overall success of inflation. These results demonstrate that unstable domain-wall relics can leave subtle imprints on primordial fluctuations, potentially providing a probe of early-Universe phase transitions.

astro-ph.CO↗
arXiv2025

Black Hole Evaporation Driven by Non-Thermal Squeezing Through SNS and CSNS Dynamics

In this work, we present a comprehensive semiclassical analysis of black hole radiation in a spatially flat FRW Universe for two fundamental nonclassical states: the Squeezed Number State (SNS) and the Coherent Squeezed Number State (CSNS). Unlike thermally modified earlier studies, SNS and CSNS constitute fully non-thermal, number-state-dependent quantum configurations. By embedding these states within the framework of semiclassical theory of gravity, we derive state-resolved expressions for the Hawking temperature, entropy variation, and corresponding mass loss of an evaporating black hole. The influence of the squeezing parameter $ρ$ and number state parameter $n$ on Hawking emission is examined through a series of analytical results supported by twelve detailed plots. The analysis reveals that the Hawking temperature exhibits monotonic growth with increasing $ρ$ and $n$, thereby elevating the effective temperature experienced at the black hole horizon. The entropy variations $Δ\mathbb{S}_{\mathrm{SNS}}$ and $Δ\mathbb{S}_{\mathrm{CSNS}}$ show strong nonlinear enhancement, especially at moderate and large squeezing values. Overall, the study extends earlier thermal squeezed-state approaches to a fully number-state-resolved framework, highlighting the sensitivity of Hawking emission to nonclassical quantum configurations. These findings contribute a new perspective on gravitational particle creation in cosmological settings.

gr-qc↗
arXiv2025

Reheating study of Mexican-Hat-type Potentials

We study reheating in Mexican Hat type potentials, emphasizing the role of the post-inflationary equation-of-state parameter($\overline{ω}_{\text{re}}$) in shaping observable predictions. By exploring the allowed range of $\overline{ω}_{\text{re}}$, we derive reheating temperature, e-fold counts, and inflationary observables, showing that the conventional Mexican Hat model satisfies Planck18+BK18+BAO constraints on $n_s$ and r. The analysis underscores reheating as a critical link between theoretical potentials and CMB data. In addition, the holographic Mexican Hat realization is examined as a benchmark, with our results mapping its phenomenological boundaries. This work illustrates how reheating studies sharpen constraints and guide refinements of unified inflationary scenarios.

astro-ph.CO↗
arXiv2025

Investigation of Super-Poissonian Nonclassical Nature of Inflaton Field in Flat FRW Universe through Cosmological Mandels $Q$ Parameter

This study investigates the nonclassical properties of the inflaton field within the framework of semiclassical gravity by analyzing Cosmological Mandels $Q$ parameter for Squeezed Number States (SNS) and Coherent Squeezed Number States (CSNS). Mandels $Q$ parameter serves as a critical tool for identifying nonclassical states by differentiating between sub-Poissonian and super-Poissonian statistics. The values of Cosmological Mandels $Q$ are positive shows the super-Poissonian non-classical nature of inflaton for Squeezed Number States (SNS) and Coherent Squeezed Number States (CSNS). These results provide deeper insights into the statistical properties of quantum states in early-universe cosmology and emphasize the relevance of SNS and CSNS in understanding quantum effects on cosmic inflation and particle production.

gr-qc↗
arXiv2024

Particle Production and Density Fluctuations of Non-classical Inflaton in Coherent Squeezed Vacuum State of Flat FRW Universe

We study non-classical inflaton, which is minimally coupled to the semiclassical gravity in FRW universe in Coherent Squeezed Vacuum State (CSVS). We determined Oscillatory phase of inflaton, power-law expansion, scale factor, density fluctuations, quantum fluctuations and particle production for CSVS. We obtained an estimated leading solution of scale factor in CSVS proportional to $t^{2/3}$ follow similar diversification as demonstrated by Semiclassical Einstein Equation (SCEE) of gravity in matter dominated universe. We also studied the validity of SCEE in CSVS. By determining the quantum fluctuation for CSVS validity of uncertainty relation for FRW Universe also computed. The results shows that Quantum fluctuations doesn't depend on coherent parameter $Υ$ as uncertainty relation doesn't effected by the displacement of $Υ$ in phase space. We study the production of particles in CSVS for oscillating massive inflaton in flat FRW universe.

astro-ph.CO↗
arXiv2024

Density fluctuations for Squeezed Number State and Coherent Squeezed Number State in Flat FRW Universe

We study the density fluctuations for Coherent Squeezed Number State (CSNS) and Squeezed Number States (SNS) formalism in Semiclassical theory of grav ity in flat FRW universe. We used Number state evolution of oscillatory phase of inflaton for coherent squeezed number state and squeezed number states for malisms. We analyzed that density fluctuations for SNS depends upon squeezing parameter and number state while for CSNS density fluctuations depends upon squeezing parameter, number state and coherent state parameter. These param eters plays an important role for quantum consideration of SNS and CSNS. The results of the analysis shows that increase in density fluctuations for both SNS and CSNS, demonstrate quantum behavior of SCEE as well as production of various kind of particles in these states.

gr-qc↗
arXiv2024

Reheating constraints on mutated hilltop inflation

Future research studies of cosmic microwave background polarization seems likely to provide a more improved upper bound of $r \le 0.03$ on the tensor-to-scalar ratio(r). In our work, we have done the reheating study of mutated hilltop inflation(MHI), a model falling in the broad category of small field inflation. We have parameterized reheating in terms of various parameters like reheating duration $N_{\text{rh}}$, reheating temperature $T_{\text{rh}}$ and effective equation of state $\overline{ω}_{\text{rh}}$ using observationally viable values of scalar power spectrum amplitude $A_{\text{s}}$ and scalar spectral index $n_{\text{s}}$. In our study, working over a range of $\overline{ω}_{\text{rh}}$, we found that the MHI potential is well consistent with combined Planck and BK18 observations for $\overline{ω}_{\text{rh}} > 0$ within a particular range of model's parameter space and the lower values of the model parameter in MHI generate considerably smaller r compared to normal hilltop potential without any incompatibility of $n_s$ with observational data, making MHI a better choice in accordance to recent and future studies.

astro-ph.CO↗
arXiv2007

Density Fluctuations in the Oscillatory Phase of Nonclassical Inflaton in FRW Universe

Using coherent and squeezed state formalisms of quantum optics for a minimally coupled non-classical inflaton in the FRW mertic is studied, in semiclassical theory of gravity. The leading order solution for the semiclassical Einstein equations in the coherent, squeezed and squeezed vacuum states are obtained perturbatively and are exhibit powerlaw expansion behaviour. The validity of the semiclassical theory is examined in the squeezed vacuum state in the oscillatory phase of the inflaton. The semiclassical theory in the oscillatory phase of the non-classical inflaton holds only if the associated squeezing parameter is much less compared to unity. Quantum fluctuations of the inflaton is also examined in coherent and squeezed state formalisms.

gr-qc↗