arXiv subjects

Ameya Kolhatkar

Publications and source records attributed to Ameya Kolhatkar.

6 recordsLinked to original sources

arXiv2026

The Amplitude-Growth Degeneracy and Implied $A_s$ Diagnostic for Background-Inert Modified Gravity

We prove that any background-inert perturbative coupling $ λ$ in coincident $ f(Q) $ gravity exhibits a degeneracy with the clustering amplitude $ σ_{80} $, when using compressed CMB distance priors. This degeneracy is, in fact, a direct materialization of a deeper $ A_s-D_0(λ) $ degeneracy between the primordial amplitude $ A_s $ and the present day growth factor $ D_0(λ) $. We outline a consistency check scheme, whose logic extends to any model in which the coupling is background inert, by computing $ A_s $ per posterior sample, needed to reproduce the $ σ_{80} $ preferred by the sampler. We perform our analysis with two dataset pipelines, based on the coupled/decoupled $ fσ_8(z) $ data. To ensure theoretical diversity, we include $ Λ$CDM and the Hybrid model in the $ f(Q) $ framework. Our results illustrate that adding the $ λ_0\sqrt{QQ_0} $ correction to the models inflates $ σ_{80} $ by $ 5\%-8\% $ as compared to its vanilla variant, while the Bayesian evidence disfavors every alternative considered - $Δ\log\mathcal{Z} = -0.5$ to $-2.6$ against $Λ$CDM on the same pipeline. Propagating this inflated $ σ_{80} $ through a per-sample computation of the implied primordial amplitude accounting for both the transfer function and the modified growth factor yields $\ln(10^{10}A_s)$ in $1.5σ-2.6σ$ tension with Planck 2018. Imposing the $ \ln(10^{10}A_s) $ constraint from Planck 2018 as an additional prior removes this inflation, pulling $ σ_{80} $ to the Planck value and $ λ_0 $ to values consistent with $ 0 $, with the implied amplitude recovering to within $ 0.2σ$ of Planck in every case. We find no model-dataset combination preferred over $ Λ$CDM.

astro-ph.CO↗

arXiv2026

Beyond the Cosmological Constant: Breaking the Geometric Degeneracy of $ f(Q) $ cosmology via Redshift-Space Distortions

We present a rigorous theoretical and observational analysis of the Hybrid $ f(Q) $ class of models by including the late-time modifying $ 1/Q $ term. After deriving strict viability conditions from the analytical expansion history, we show that preserving early-universe structure formation dictates that the linear coupling be exactly unity. This fixes the background of the Hybrid model into a geometric degeneracy with $ Λ$CDM which is confirmed explicitly through MCMC analysis with the latest background-only probes. The physical novelty of this model is manifest in the perturbation sector, where the geometric coupling breaks the background degeneracy and induces a late-time suppression of the effective gravitational constant $ G_{eff} < G_N $. Consequently, the inclusion of RSD data reveals an amplitude compensation mechanism, by which the matching of the signature $ fσ_8 $ of the data causes the clustering amplitude $ σ_8 $ to inflate under weaker gravity. Statistical model comparison through AIC/DIC demonstrates that incorporating growth data yields a moderate to weak preference for the Hybrid model keeping the background cosmology intact. This provides a physically bounded alternative to $ Λ$CDM with a falsifiable signature in the large scale structure, directly testable by the next generation of galaxy surveys.

gr-qc↗

arXiv2024

Investigating early and late-time epochs in $ f(Q) $ gravity

In the following work, a new hybrid model of the form $ f(Q)=Q(1+a)+b\frac{Q_0^2}{Q} $ has been proposed and confronted using both early as well as late-time constraints. We first use conditions from the era of Big Bang Nucleosynthesis (BBN) in order to constrain the models which are further used to study the evolution of the Universe through the deceleration parameter. This methodology is employed for the hybrid model as well as a simple model of the form $ α_1 Q+α_2 Q_0 $ which is found to reduce to $Λ$CDM. The error bar plot for the Cosmic Chronometer (CC) and Pantheon+SH0ES datasets which includes the comparison with $Λ$CDM, has been studied for the constrained hybrid model. Additionally, we perform a Monte Carlo Markov Chain (MCMC) sampling of the model against three datasets -- CC, Pantheon+SH0ES, and Baryon Acoustic Oscillations (BAO) to find the best-fit ranges of the free parameters. It is found that the constraint range of the model parameter ($a$) from the BBN study has a region of overlap with the ranges obtained from the MCMC analysis. Finally, we perform a statistical comparison between our model and the $Λ$CDM model using AIC and BIC method.

gr-qc↗

arXiv2023

Big Bang Nucleosynthesis constraints on $f(T, \mathcal{T})$ gravity

Big Bang Nucleosynthesis provides us with an observational insight into the very early Universe. Since this mechanism of light element synthesis comes out of the standard model of particle cosmology which follows directly from General Relativity, it is expected that any modifications to GR will result in deviations in the predicted observable parameters which are mainly, the neutron-to-proton ratio and the baryon-to-photon ratio. We use the measured neutron-to-proton ratio and compare the theoretically obtained expressions to constrain two models in the framework of $ f(T,\mathcal{T}) $ gravity. The theoretically constrained models are then tested against observational data from the Hubble dataset and the $ Λ$CDM model to explain the accelerated expansion of the Universe.

astro-ph.CO↗

arXiv2023

Correction to Lagrangian for Bouncing Cosmologies in $f(Q)$ Gravity

Symmetric teleparallel gravity offers to reformulate the gravitational formalism without the presence of curvature and torsion with the help of non-metricity tensors. Interestingly, Symmetric teleparallel gravity can be formulated equivalently to teleparallel gravity or general relativity for an appropriate setup. In this study, our aim lies in exploring the bouncing cosmologies as an alternative to the initial singularity of the Universe in the background of modified symmetric teleparallel gravity. To explore this, we adopt the reconstruction technique to present the possible reconstructed Lagrangian for various cosmological bouncing solutions in a flat Friedmann-Lemaître-Robertson-Walker spacetime with a perfect fluid matter distribution. We study the reconstructed gravitational Lagrangians, which are capable of reproducing analytical solutions for \textit{symmetric bounce}, \textit{super-bounce}, \textit{oscillatory bounce}, \textit{matter bounce}, and \textit{exponential bouncing} model settings. Further, we examine the dark energy profiles of the models using reconstructed Lagrangians. In addition, we found that an additional term arises in each reconstructed Lagrangian compared to general relativity (GR). That extra term corrected the background GR to present bouncing cosmology in modified gravity. These newly motivated cosmological models may have an effect on gravitational phenomena at other cosmological scales.

gr-qc↗

arXiv2022

Gravitational Collapse of Massless Vector Field with Positive Cosmological Constant

We investigate the dynamics of homogeneous gravitational collapse of a massless vector field in the presence of a positive cosmological constant $Λ$. The corresponding density function $ρ(a)$ obtained for the massless vector field is inversely proportional to the fourth power of the scale factor $a (t)$. The variation of the scale factor shows that for $0\, \leΛ< 1$, we obtain the gravitational collapse of the vector fields leading singularity formation in a {\it finite} comoving time resulting in a {\it Blackhole} such that with increasing $Λ$, the singularity formation time, $t_s$ increases. For $Λ= 1$, we obtain $a(t) = 0$, thus limiting the maximum value of $Λ$, (w.r.t the initial density $ρ_0$) for which the system could collapse under gravity.

gr-qc↗