arXiv subjects

Vipin Kumar Sharma

Publications and source records attributed to Vipin Kumar Sharma.

10 recordsLinked to original sources

arXiv2026

Exploring the Observational Constraints and Cosmological Dynamics in f(Q,L_m) Gravity

We explore two scenarios of $f(Q,\mathcal{L}_m)$ gravity: linear and non-linear gravity models. The dynamical system analysis identifies two critical points for each of the proposed linear and nonlinear matter--geometry coupling models. These equilibrium points correspond to distinct phases of cosmic evolution. Depending on the model parameters, the resulting critical points successfully reproduce the observed sequence of cosmic evolution, from a decelerated matter-dominated Universe to the present epoch of accelerated expansion. The effective equation of state parameter ($\omega_{\text{eff}}$) and the deceleration parameter ($q$) exhibit smooth transitions from decelerated to accelerated expansion, with transition epochs around $N_{\text{tr}} \approx -0.27$ for linear Model and $N_{\text{tr}} \approx -0.32$ for non-linear Model, consistent with late-time cosmic acceleration. Statistical constraints derived from CC+BAO, DESI DR II, and Pantheon$^+$ datasets provide best-fit values for the model parameters ($\alpha, \beta, \gamma, H_0$), showing compatibility with current cosmological observations. The analysis employs Akaike (AIC) and Bayesian (BIC) information criteria to evaluate model performance. Our results demonstrate that $f(Q,\mathcal{L}_m)$ gravity provides a viable alternative framework for explaining late-time acceleration, with rich dynamical features that merit further exploration in view of upcoming high-precision surveys.

physics.gen-ph↗

arXiv2026

No preference for generalized emergent dark energy from current cosmological data

In this work, we revisit the generalized emergent dark energy model by confronting it with DESI DR2 baryon acoustic oscillation measurements, in combination with joint CMB data from ACT, SPT, and Planck, as well as Type Ia supernova samples including Pantheon$^+$, DES-Dovekie, and Union3. We find that the GEDE model remains compatible with the $\Lambda$CDM paradigm, with no statistically significant preference for deviations when all datasets are combined. In particular, the key model parameter $\Delta$ is consistent with the $\Lambda$CDM value $\Delta = 0$ within $2\sigma$ once SNe Ia data are included. Despite this overall agreement, the GEDE model does not exhibit the phantom crossing suggested by DESI DR2. Instead, the evolution of the dark energy equation of state $w(z)$ indicates that the model behaves either as a full phantom ($w < -1$) or quintessence ($w > -1$), depending on the dataset combination, without a clear transition across $w = -1$. When Pantheon$^+$ data are included, the model converges closely to $\Lambda$CDM with $w \simeq -1$. The GEDE model does not alleviate the existing cosmological tensions. The inferred values of $H_0$ remain in the range $67.9$-$69.8\ \mathrm{km,s^{-1},Mpc^{-1}}$, while the sound horizon $r_d$ and clustering parameter $S_8$ remain consistent with $\Lambda$CDM, failing to resolve the $H_0$ and $S_8$ tensions. Finally, both Gaussian significance levels ($<2\sigma$) and Bayesian evidence ($\ln B_{i,j} < 1$) indicate no statistically significant preference for GEDE over $\Lambda$CDM. We conclude that, although DESI DR2 hints at dynamical dark energy, the GEDE model remains observationally indistinguishable from $\Lambda$CDM and does not support the Quintom-B-type behavior suggested by DESI DR2.

astro-ph.CO↗

arXiv2026

Evidence for evolving dark energy from DESI DR2 BAO and Pantheon$^+$, DES-Dovekie, and Union3

Evidences for evolving dark energy are shown using baryon acoustic oscillation measurements from the recent Dark Energy Spectroscopic Instrument Data Release 2 , combined with different Type Ia supernova datasets (Pantheon$^+$, DES-Dovekie, and Union3) and the CMB compressed likelihood. We examine several dark energy parameterizations, including the Logarithmic, Exponential, CPL, BA, JBP, Thawing, Mirage, and GEDE models. Analyzing the DESI DR2 measurements alone, we find that evidence for evolving dark energy is primarily driven by the LRG1-2 tracers, as their inclusion yields a preferred value of $w_0 > -1$. However, as each tracer provides only limited observables, this preference can result in an underconstrained and potentially unstable inference. Further, we find that each dark energy model predicts values in the $w_0 > -1$, $w_a < 0$ quadrant, a region characterized by the Quintom-B type dark energy scenario. The logarithmic bayes factor shows that, among all models, the Mirage model shows the inconclusive-to-moderate evidence across all dataset combinations. Consistently, the statistical significance remains modest, with $Nσ\sim 1.1$-$2.3$, and no model showing a robust preference for dynamical dark energy using late-time datasets alone. The evolution of $w(z)$ shows a phantom crossing around $z \sim 0.5$ in most dynamical dark energy models, and the evolution of $f_{\mathrm{DE}}(z)$ converges to $f_{\mathrm{DE}}(0) = 1$ in all dark energy models.

astro-ph.CO↗

arXiv2026

Probing departures from $Λ$CDM by late-time datasets

Observational data play a pivotal role in identifying cosmological models that are both theoretically consistent and empirically viable. In this work, we investigate the level of preference for dynamical dark energy over a cosmological constant using current late-time observational datasets, including Cosmic Chronometers , Baryon Acoustic Oscillations from DESI DR2, and different Type Ia supernova catalogs (Pantheon$^+$, DES-Dovekie, Union3). We analyze various dynamical dark energy models, including $ω$CDM, o$ω$CDM, $ω_0ω_a$CDM, Logarithmic, Exponential, JBP, BA, and GEDE. In most cases, the o$Λ$CDM and o$ω$CDM models favor an open Universe. For the o$ω$CDM, the inclusion of DES-Dovekie or Union3 data together with CC and DESI DR2 favors a nearly flat geometry. Using the CC + DESI DR2 dataset, the preference for dynamical dark energy lies between the $1$-$2σ$ level. When different supernova catalogs (DES-Dovekie or Union3) are included, the deviation from $Λ$CDM in the $ω$CDM, $ω_0ω_a$CDM, Logarithmic, JBP, BA, and GEDE models increases to the $2$-$2.74σ$ level, while the Pantheon$^{+}$ sample yields deviations below the $2σ$ level. We find consistent evidence for $ω_0 > -1$ and $ω_a < 0$ across all dark energy models, indicating a preference for dynamical dark energy characterized by a Quintom-B type scenario. The $Λ$CDM paradigm has long served as the standard framework of modern cosmology; however recent DESI DR2 results have exposed emerging tensions with the cosmological constant $Λ$, hinting at possible new physics in the dark energy sector. Even so, the currently available data are still not strong enough to definitively rule out the $Λ$CDM model.

astro-ph.CO↗

arXiv2026

$Λ(t)$CDM Model: Cosmological Implications and Dynamical System Analysis

We investigated a time-varying cosmological constant model using recent BAO measurements from DESI DR2, combined with Type Ia supernova samples (Pantheon$^{+}$, DES-Dovekie, and Union3) and CMB shift parameters, to constrain the $Λ(t)$CDM model parameters via Markov Chain Monte Carlo analysis. We find that the interaction term $Q(z)$ shows a sign change for all dataset combinations by crossing $Q(z)=0$, depending on the choice of the dataset: at low redshift $Q(z)<0$, indicating vacuum energy decaying into dark matter, while at high redshift $Q(z)>0$, corresponding to dark matter decaying into vacuum energy. The dynamical system analysis found three critical points, namely $P_1,P_2$, and $P_3$ respectively. The resulting critical points, determined by the underlying cosmological parameters, correspond to distinct epochs in cosmic evolution. Depending on the parameter combinations, these points characterize various cosmological phases, ranging from an accelerated stiff matter-dominated era to late-time accelerated expansion. The stability of each critical point is analyzed using linear stability theory, with the relevant physical constraints on the cosmological parameters duly incorporated throughout the analysis. For each dataset combinations, the $Λ(t)$CDM model predicts that $ω_0 > -1$, showing a preference for dynamical dark energy over the cosmological constant scenario with $ω_0 = -1$. Consequently, the model exhibits a transition phase in the range $N \equiv \log a(t) \approx -0.51$ to $-0.48$ and predicts $q_0$ in the range $-0.54$ to $-0.52$, with the precise transition point depending on the choice of dataset. Finally, the Bayesian evidence shows strong support for the $Λ(t)$CDM model over $Λ$CDM

gr-qc↗

arXiv2025

Does DESI DR2 challenge $Λ$CDM paradigm ?

Although debate on DESI DR1 systematics remains, DESI DR2 is consistent with DR1 and strengthens its trends. In our analysis, the LRG1 point at $z_{\mathrm{eff}}=0.510$ and the LRG3+ELG1 point at $z_{\mathrm{eff}}=0.934$ are in tension with the $Λ$CDM-anchored $Ω_m$ inferred from Planck and SNe Ia (Pantheon$^{+}$, Union3, DES-SN5YR): for LRG1 the tensions are $2.42σ$, $1.91σ$, $2.19σ$, and $2.99σ$; for LRG3+ELG1 they are $2.60σ$, $2.24σ$, $2.51σ$, and $2.96σ$. Across redshift bins DR2 shows improved agreement relative to DR1, with the $Ω_m$ tension dropping from $2.20σ$ to $1.84σ$. Nevertheless, DR2 alone is not decisive against $Λ$CDM, and the apparent deviation is driven mainly by LRG1 and LRG2. In a $ω_0ω_a$CDM fit using all tracers we find a posterior mean with $w_0>-1$, consistent with dynamical dark energy and nominally challenging $Λ$CDM. Removing LRG1 and/or LRG2 restores $Λ$CDM concordance ($ω_0\to-1$); moreover, $ω_0^{\mathrm{(LRG2)}}>w_0^{\mathrm{(LRG1)}}$, indicating that LRG2 drives the trend more strongly. Model selection via the natural-log Bayes factor $\ln\mathrm{BF}\equiv\ln(Z_{Λ\mathrm{CDM}}/Z_{ω_0ω_a\mathrm{CDM}})$ yields weak evidence for $Λ$CDM when LRG1, LRG2, or both are removed, and is inconclusive for the full sample. Hence the data do not require the extra $ω_a$ freedom, and the apparent $ω_0>-1$ preference should be interpreted cautiously as a reflection of the $ω_0$$ω_a$ degeneracy with limited per-tracer information.

astro-ph.CO↗

arXiv2023

Probing massive gravitons in $f(R)$ with lensed gravitational waves

We investigate the novel features of gravitational wave solutions in $f(R)$ gravity under proper gauge considerations in the shifted Ricci scalar background curvature ($R^{1+ε}$). The solution is further explored to study the modified dispersion relations for massive modes at local scales and to derive constraints on $ε$. Our analysis yields new insights as we scrutinize these dispersion effects on the polarization (modified Newman-Penrose content) and lensing properties of gravitational waves. It is discovered that the existing longitudinal scalar mode, and transverse breathing scalar mode are both independent of the mass parameter for $ε<<1$. Further, by analysing the lensing amplification factor for the point mass lens model, we show that lensing of gravitational wave is highly sensitive to these dispersion effects in the milli-Hertz frequency (wave optics regime). It is expected that ultra-light modes, having mass about $\mathcal{O} (10^{-15})$ eV for $ε<<1 (\approx 10^{-7})$ lensed by ($10^3\leq M_{Lens}\leq 10^6$)$M_\odot$ compact objects are likely to be detected by the advanced gravitational wave space-borne detectors, particularly within LISA's (The Laser Interferometer Space Antenna) sensitivity band.

gr-qc↗

arXiv2022

Unified $f(R)$ gravity at local scales

We explore the shifted $f(R) (\propto R^{1+δ})$ model with $δ$ as a distinguishing physical parameter for the study of constraints at local scales. The corresponding dynamics confronted with different geodesics (null and non-null) along with its conformal analogue is investigated. For null geodesics, we discuss the light deflection angle, whereas for non-null geodesics under the weak field limit, we investigate the perihelion advance of the Mercury orbit in $f(R)$ Schwarzschild background, respectively. The extent of an additional force, appearing for non-null geodesics, depends on $δ$. Such phenomenological investigations allow us to strictly constrain $δ$ to be approximately $\mathcal{O}(10^{-6})$ with a difference of unity in orders at galactic and planetary scales and seems to provide a unique $f(R)$ at local scales. Further, at late cosmic time, we analyse the constraint on $δ$ via the bare scalar self-interaction Einstein frame potential to provide a null test of dark energy. We constrain the deviation parameter, $\midδ\mid$ to $(\approx 0.6)$ which is in a close agreement with the results obtained through various observations in the Jordan frame by several authors. Our results suggest that the present form of model is suitable for the alternate explanation of dark matter-like effects at local scales, whereas at large scales the deviations grow higher and must be addressed in terms of the accelerated background.

astro-ph.CO↗

arXiv2020

Light deflection angle through velocity profile of galaxies in $f(R)$ model

We explore a new realisation of the galactic scale dynamics via gravitational lensing phenomenon in power-law $f(R)$ gravity theory of the type $f(R)\propto R^{1+δ}$ with $δ<<1$ for interpreting the clustered dark matter effects. We utilize the single effective point like potential (Newtonian potential + $f(R)$ background potential) obtained under the weak field limit to study the combined observations of galaxy rotation curve beyond the optical disk size and their lensing profile in $f(R)$ frame work. We calculate the magnitude of light deflection angle with the characteristic length scale (because of Noether symmetry in $f(R)$ theories) appearing in the effective $f(R)$ rotational velocity profile of a typical galaxy with the model parameter $δ\approx O(10^{-6})$ constrained in previous work. For instance, we work with the two nearby controversial galaxies NGC 5533 and NGC 4138 and explore their galactic features by analysing the lensing angle profiles in $f(R)$ background. We also contrast the magnitudes of $f(R)$ lensing angle profiles and the relevant parameters of such galaxies with the generalised pseudo-isothermal galaxy halo model and find consistency.

astro-ph.CO↗

arXiv2019

Extended galactic rotational velocity profiles in $f(R)$ gravity background

An attempt has been made to explore the galactic dynamics via the rotational velocity beyond the Einstein's geometric theory of gravity. It is inspired from the geometric relation obtained in the power law $f(R)$ gravity model in vacuum. We analyse the action with a small positive deviation from the Einstein-Hilbert action (taking $R$ as $f(R)\propto R^{1+δ}$) at the galactic scales for the explanation of cosmological dark matter problem and obtain the contribution of dynamical $f(R)$ background geometry in accelerating the test mass. In the weak field limits, we obtain the effective acceleration of the test mass due to a massive spherically symmetric source in $f(R)$ background and develop an equation for the rotational velocity. We test the viability of the model by tracing the motion of test mass outside the typical galactic visible boundaries without considering any dark matter halo profile. We obtain a nice agreement in the outer regions (up to few tens of kpc beyond the visible boundary) of the typical galaxy by using the known galaxy data.\\ We further explore the galactic dynamics for a galaxy NGC 1052 of which the dark matter deficient galaxies, i.e., DF2 and DF4 are a part (satellite galaxies) and discuss plots of the dynamical feature of rotation curves in $f(R)$ background for the model parameter $δ<<1$ and interpret the results for its satellite galaxies.

gr-qc↗