arXiv subjects

Shibendu Gupta Choudhury

Publications and source records attributed to Shibendu Gupta Choudhury.

13 recordsLinked to original sources

arXiv2026

Model-Independent Indication for a Localized Anomaly in the Late-Time Expansion History

We investigate the late-time expansion history of the Universe using a model-independent spline reconstruction of cosmological distances based on the latest DESI DR2 baryon acoustic oscillation (BAO) measurements and the DES Dovekie Type Ia supernova compilation. Comparing the reconstructed expansion history with the prediction of the Planck 2018 $Λ$CDM model, we identify a localized deviation over the redshift interval $0.3\lesssim z\lesssim0.6$, reaching a maximum significance of approximately $3.5σ$ at $z\simeq0.47$. We demonstrate that this feature persists under substantial variations of the reconstruction methodology, dataset composition and sound-horizon calibration. Mock analyses further show that the reconstruction is unbiased and that the observed anomaly is unlikely to arise from reconstruction bias or miscalibrated uncertainties. If confirmed by future observations, this localized feature could point to previously unrecognized late-time physics or reveal subtle inconsistencies between early and late Universe cosmological probes.

astro-ph.CO↗

arXiv2026

Beyond $Λ$CDM with a Logistic RG-like Flow of the Low Redshift Cosmic Evolution

Recent observations hint at possible late-time deviations from $Λ$CDM. We introduce a minimal phenomenological framework in which the total equation of state $w_{\rm T}(z)$ follows a logistic evolution motivated by renormalization-group-like flow between cosmological fixed points. Using DESI-DR2 BAO, DES supernova data, and CMB distance priors, we find that this parametrization provides an improved description of the expansion history relative to $Λ$CDM. The reconstructed evolution, though model specific, also shows statistically strong low-redshift deviations from $Λ$CDM.

astro-ph.CO↗

arXiv2026

Scalaron dark matter dynamics: effects of Higgs non-minimal coupling to gravity

One of the key features of the $R^2$-gravity is the embedding of a scalar field, scalaron, into the gravity sector. The scalaron interacts with the Standard Model (SM) matter fields through Planck-suppressed couplings. If the scalaron serves as a viable dark matter (DM) candidate, it can account for the lack of evidence of DM interactions beyond gravity in experimental and observational probes to date. The realization of the scalaron, as a cold DM candidate, depends on an induced trilinear interaction with the SM Higgs via its quartic self coupling. Here, we introduce a Higgs non-minimal coupling to gravity that additionally contributes to the induced trilinear interaction with its existing competing part, originated from the $R^2$-gravity. We study the interplay between these two contributions in the early universe, which determines both the initial conditions and evolution of the scalaron, leading to cold DM behavior at a later epoch. The trilinear interaction vanishes at the leading order for certain combinations of the Higgs non-minimal coupling ($ξ$) and the scalaron mass ($m$), thereby setting the scalaron density through misalignment mechanism, as in axions. In this case, the scalaron DM mass is obtained as, $2.7 ~{\rm meV} \lesssim m \lesssim 0.7 ~\rm{MeV}$. The lower limit on the mass is set by the fifth force constraints, whereas the upper bound arises from INTEGRAL/SPI limits on the excess gamma-ray flux due to scalaron decaying into two photons. On the other hand, when the trilinear interaction is non-zero and dominated by the Higgs quartic self coupling, the DM relic density is satisfied with $m \simeq 3.6$ meV. When the Higgs non-minimal coupling dominates, the mass lies within 10-770 meV. We also obtain, for the first time within the scalaron-Higgs mixed model, an upper bound on $|ξm|$ of $1.5\times 10^{17}$ GeV, from Higgs mass measurement at the LHC.

hep-ph↗

arXiv2025

Anchoring the Universe with Characteristic Redshifts using Raychaudhuri Equation Informed Reconstruction Algorithm (REIRA)

We study the robustness and physical implications of a set of characteristic redshifts that capture key features of the late-time Universe. Using both model-independent reconstructions as well as different dark energy (DE) parameterizations, we show that these redshifts remain stable across cosmological models and reconstruction algorithm, making them reliable geometric anchors of the expansion history. Moreover, the Alcock-Paczyński corrections at these redshift anchors are found to be unity with high statistical significance, making them natural isotropy points in the comoving distance-redshift relation. We also find that certain redshifts anchors $(z < 1)$ coincide with epochs where strong deviations from the Planck $Λ$CDM baseline are apparent irrespective of DE parametrisation like CPL or reconstruction algorithm, indicating their potential as probes of new physics in cosmological evolution. Finally, we demonstrate, for the first time, that a Raychaudhuri Equation Informed Reconstruction Algorithm, substantially enhances the precision of the inferred distance measures and the Hubble expansion rate as well as results tighter constraints in the DE parameter space. These results demonstrate that combining geometric reconstruction with physics-informed kinematic information offers a powerful and consistent algorithm to probe new physics in the late-time dynamics of our Universe.

astro-ph.CO↗

arXiv2025

Towards a Composite Framework for Simultaneous Exploration of New Physics in Background and Perturbed Universe

We investigate deviations from $Λ$CDM by independently parameterizing modifications in the background evolution and the growth of structures. The background is characterized by two parameters, $A$ and $B$, which reduce to $A=Ω_{m0}$ and $B=2/3$ in the $Λ$CDM limit, while deviations in the growth of structures are captured through a fitting function for $fσ_8$ involving the growth index $γ$. Using recent observational datasets involving background expansion and growth of structures (related to observations involving redshift space distortions), we find significant evidence for departures from $Λ$CDM in the background expansion whereas there is no finite evidence for deviations from $Λ$CDM behaviour in the growth of structures. This suggests that with the current precision in observational data involving background and perturbed Universe, a deviation from $Λ$CDM behaviour is confirmed (as shown in the recent DESI-DR2 results). But whether this deviation is due to an evolving Dark Energy or due to the modification of gravity at cosmological scales is still an open question, largely due to less precise data from perturbed Universe. We further demonstrate how the future high-precision growth data (from Euclid, for example) can answer such question using a forecast study.

astro-ph.CO↗

arXiv2024

Torsional four-fermion interaction and the Raychaudhuri equation

Intrinsic spin of fermions can generate torsion in spacetime. This torsion is a non-propagating field that can be integrated out, leaving an effective non-universal four-fermion interaction. This geometrical interaction affects fermions inside a matter distribution and can be expected to become stronger as the density grows. In this work, we investigate the role of this interaction in a gravitationally collapsing fermionic distribution. Our specific aim is to explore if this interaction can provide a repulsive contribution and prevent the final singularity formation. We consider a collapsing distribution of massive fermions, ignoring other interactions. Using simplified yet reasonable assumptions, we establish that a repulsive contribution can arise depending on how torsion couples with different chiralities. Also, the interaction starts to dominate as the collapse proceeds, accelerating or decelerating the collapse depending on the relative signs of the geometrical interaction between different species of fermions.

gr-qc↗

arXiv2023

The fate of a Quantum-Corrected Collapsing Star in General Relativity

We incorporate some corrections inspired by loop quantum gravity into the concept of gravitational collapse and propose a complete model of the dynamic process. The model carries the essence of a mass-independent upper bound on the curvature scalars originally found as a crucial feature of black holes in loop quantum gravity. The quantum-inspired interior is immersed in a geometry filled with null radiation and they are matched at a distinct boundary hypersurface. The ultimate fate of the process depends on inhomogeneities of the metric tensor cofficients. We find a critical parameter $λ$ embedded in the inhomogeneity of the conformal factor of the interior metric. Examples with $λ< 0$ enforce an eventual collapse to singularity and $λ> 0$ cases produce a non-singular collapse resulting in a loop-quantum-corrected Schwarzschild geometry modulo a conformal factor. Interestingly, for $λ< 0$ as well, there exist situations where the quantum effects are able to cause a bounce but fall short of preventing the ultimate formation of singularity. The trapped surface formation condition is studied for $λ<0$ case to infer about the visibility of the final singularity. Interestingly, we find a possibility of formation of three horizons during the course of the collapse. Eventually all of them merge into one single horizon which envelopes the final singularity. For the non-singular case, there is a possibility that the sphere can evolve into a wormhole throat whose radius is found to be inversely proportional to the critical parameter $λ$. Depending on the nature of evolution and the shell regions, the collapsing shells violate some standard energy conditions which can be associated with the quantum inspired corrections.

gr-qc↗

arXiv2023

Application of the Raychaudhuri Equation in Gravitational Systems

The works reported in this thesis primarily address the application of the Raychaudhuri equation in two intriguing problems in gravitational physics. These problems still lack universally accepted explanations. The first problem is related to the existence of spacetime singularities. We aim to find possible escape routes from these problematic singularities at the classical level. The second problem is associated with the late time accelerated expansion of the Universe. In this context, our goal is to find a possible explanation of this phenomenon without assuming the presence of any exotic contribution to the stress-energy tensor.

gr-qc↗

arXiv2022

Role of a magnetic field in the context of inhomogeneous gravitational collapse

Magnetic fields have been found to have an inherent capability of acting against gravity. An important question posed in the literature is whether presence of a magnetic field can alter the dynamics of a gravitational collapse and prevent the final formation of a singularity. Inhomogeneous models of collapse have not been explored significantly in this context. In the present work we investigate the role of magnetic fields in the evolution of inhomogeneous cylindrically symmetric models. We use an approach based on the Raychaudhuri equation for such an analysis. We show that it is quite possible for the magnetic field to avert the gravitational collapse in these models.

gr-qc↗

arXiv2022

Raychaudhuri equation in scalar-tensor theory

Implications of the Raychaudhuri equation in focusing of geodesic congruences are studied in the framework of scalar--tensor theory of gravity. Specifically, we investigate the Brans--Dicke theory and Bekenstein's scalar field theory. In both of these theories, we deal with a static spherically symmetric distribution and a spatially homogeneous and isotropic cosmological model as specific examples. We find that it is possible to violate the convergence condition under reasonable physical assumptions. This leads to the possibility of avoiding a singularity.

gr-qc↗

arXiv2021

The Raychaudhuri equation for a quantized timelike geodesic congruence

A recent attempt to arrive at a quantum version of Raychaudhuri's equation is looked at critically. It is shown that the method, and even the idea, has some inherent problems. The issues are pointed out here. We have also shown that it is possible to salvage the method in some limited domain of applicability. Although no generality can be claimed, a quantum version of the equation should be useful in the context of ascertaining the existence of a singularity in the quantum regime. The equation presented in the present work holds for arbitrary $n + 1$ dimensions. An important feature of the Hamiltonian in the operator form is that it admits a self-adjoint extension quite generally. Thus, the conservation of probability is ensured.

gr-qc↗

arXiv2019

Self Similar Collapse and the Raychaudhuri equation

The role of the Raychaudhuri equation in studying gravitational collapse is discussed. A self-similar distribution of a scalar field along with an imperfect fluid in a conformally flat spacetime is considered for the purpose. The general focusing condition is found out and verified against the available exact solutions. The connection between the Raychaudhuri equation and the critical phenomena is also explored.

gr-qc↗

arXiv2019

Reconstruction of $f(R)$ gravity models for an accelerated universe using Raychaudhuri equation

A new strategy for the reconstruction of $f(R)$ gravity models have been attempted using Raychaudhuri equation. Two examples, one for an eternally accelerating universe and the other for one that mimics a $Λ$CDM expansion history have been worked out. For both the cases, the relevant $f=f(R)$ could be found out analytically. In the first case, $f(R)$ is found to be a combination of power-law terms and in the expression for the second case involves hypergeometric functions. The evolution history of the universe, given as specific values of the kinematical quantities like the jerk or the deceleration parameter, serve as the input. It is found that the corresponding $f(R)$ gravity models, in both the examples, are not viable options.

gr-qc↗