arXiv subjects

Sai Swagat Mishra

Publications and source records attributed to Sai Swagat Mishra.

15 recordsLinked to original sources

arXiv2026

Inflationary Dynamics and CMB Constraints in $f(T,L_m)$ gravity

We investigate slow-roll inflation in a modified teleparallel framework with a non-minimal coupling between torsion and the matter sector. Focusing on Starobinsky-type plateau inflation, we derive the modified background dynamics and the corresponding slow-roll observables, and examine how the gravitational couplings affect the scalar spectral index, tensor-to-scalar ratio, and running of the spectral index. The model continuously recovers the standard Starobinsky scenario in the appropriate weak-coupling limit, while the matter--geometry coupling produces a non-monotonic modification of the inflationary predictions. We find that weak couplings leave the standard predictions essentially unchanged, whereas stronger couplings can significantly shift the scalar spectral index and eventually become incompatible with current cosmic microwave background constraints. The quadratic torsion correction has a comparatively mild impact within the controlled low-energy regime, primarily affecting the tensor amplitude. We confront the resulting predictions with the Planck and BICEP/Keck constraints and find that the model remains viable over a broad range of the inflationary parameter space. Our results demonstrate that the matter--geometry coupling can leave a potentially testable imprint on primordial observables while retaining the successful plateau-inflation phenomenology of the Starobinsky scenario.

physics.gen-ph↗

arXiv2026

Benchmarking Exact, GP-Emulated, and Simulation-Based Inference for Late-Time Cosmology

Forthcoming cosmological surveys require inference pipelines that are both statistically reliable and computationally scalable. In this work, we perform a systematic comparison of three complementary inference strategies for late-time $Λ$CDM cosmology: exact Markov Chain Monte Carlo (MCMC), Gaussian Process (GP)-assisted MCMC, and neural Simulation-Based Inference (SBI). Using a common analysis framework based on Cosmic Chronometers, DESI DR2 baryon acoustic oscillation measurements, and the Pantheon+ Type Ia supernova compilation, we consider two dataset combinations of increasing complexity, namely CC+DESI and CC+DESI+PP, under identical cosmological assumptions and priors. For CC+DESI, both GP emulation and SBI reproduce the exact posterior constraints on $(H_0,Ω_{m,0})$ to better than $0.3σ$. For the more constraining CC+DESI+PP combination, modest method-dependent shifts emerge, reaching at most $\sim1.5σ$ in a single parameter. Despite these differences, all methods recover a nearly identical expansion history, with percent-level agreement across the full redshift range. From a computational perspective, GP emulation accelerates model evaluations but remains limited by MCMC sampling, whereas SBI achieves order-of-magnitude reductions in total runtime through amortized posterior learning. We further investigate the convergence of SBI as a function of simulation budget and identify the number of simulations required to obtain stable posterior constraints. Overall, our results demonstrate that accelerated inference techniques can deliver reliable cosmological constraints for realistic late-time datasets considered here at a fraction of the computational cost of conventional likelihood-based analyses.

astro-ph.CO↗

arXiv2026

Kinematic Probes of Type-II MMG: Padé Cosmographic Analysis of VCDM

We study the late-time expansion history of the Universe within the VCDM model, a Type-II MMG realization that preserves the successes of General Relativity while extending beyond constant vacuum energy through a minimal Hamiltonian modification, generating a time-dependent vacuum sector without introducing additional degrees of freedom. We investigate this framework within a cosmographic approach by employing a Padé $P_{(2,1)}$ approximation for the Hubble parameter and luminosity distance, allowing the cosmographic parameters to be expressed directly in terms of the underlying VCDM model parameters and enabling a data-driven reconstruction of the expansion history. The model is constrained within a Bayesian framework using the MCMC technique, implemented via the affine-invariant ensemble sampler, with a joint analysis of cosmic chronometers, DESI BAO, and Type Ia supernova datasets (Union3, Pantheon+, and DESY5). We find that the model parameters are tightly constrained and consistent across different dataset combinations, with the jerk parameter remaining very close to its $Λ$CDM value, $j_0 \simeq 1$, indicating no significant deviation at the level of higher-order cosmography. Furthermore, the transition feature previously reported in VCDM is not observed within the Padé $P_{(2,1)}$ cosmographic reconstruction, suggesting that it is not a robust requirement of current observational data but is sensitive to the choice of parametrization. Overall, our results indicate that the VCDM model effectively mimics $Λ$CDM at the background level when constrained through a cosmographic approach, underscoring the importance of model-independent reconstructions in assessing alternative cosmological scenarios.

physics.gen-ph↗

arXiv2026

Exploring Late-Time Cosmic Acceleration in VCDM Cosmology

In this work, we have considered a minimally modified gravity theory that effectively reproduces VCDM-like behavior to investigate its cosmological implications. The model parameters are constrained using a combination of CC, RSD, DESI BAO DR2, and Union3 datasets. The model parameters are constrained using an MCMC framework, ensuring a robust estimation of credible intervals. We examine both background and perturbation-level observables, analyzing the Hubble parameter, deceleration parameter, effective equation of state, distance modulus, and the growth rate of cosmic structures through the $fσ_8(z)$ observable. Our results show that the model successfully reproduces the observed expansion history, featuring a smooth transition in the Hubble evolution around $z \simeq 0.3$, and consistent behaviour of cosmological parameters. The model outperforms $Λ$CDM in the statistical comparisons for the full dataset combination (CC+RSD+DESI DR2+Union3). These results highlight the potential of minimally modified gravity theories with VCDM-like dynamics as consistent and competitive alternatives to the standard cosmological paradigm.

astro-ph.CO↗

arXiv2026

BBN to Late-Time Acceleration in $f(T,\mathcal{L}_m)$ Gravity

We present, to our knowledge, the first systematic study of early-late cosmic evolution and acceleration in the framework of $f(T,\mathcal{L}_m)$ gravity, an extension of teleparallel theories coupling torsion with the matter Lagrangian. By incorporating the Big-Bang Nucleosynthesis (BBN) bound on the freeze-out temperature, we obtain a tight constraint on the inverse-torsion parameter, ensuring consistency with early-time physics. Employing Markov Chain Monte Carlo analyses with progressively richer observational datasets, CC, Union3, and SN22 supernovae, we constrain a well-motivated model and reconstruct key cosmological functions. The reconstructed Hubble and distance modulus functions show excellent agreement with the observations, confirming the observational viability of the model. The model successfully reproduces the observed late-time expansion history, yielding a transition from deceleration to acceleration through the deceleration parameter. The effective equation of state is found to remain negative throughout, with present values $w_0 > -1$, indicating a quintessence-like behavior rather than a cosmological constant or phantom regime. These results highlight the ability of $f(T,\mathcal{L}_m)$ gravity to mimic the concordance scenario while allowing controlled deviations in the expansion history.

gr-qc↗

arXiv2026

Cosmological Evolution of the Universe in Torsion-based Modified Gravity

General Relativity, despite its century-long success, faces conceptual and observational challenges, including singularities, incompatibility with quantum mechanics, and the need to introduce dark matter and dark energy. Precision cosmology has also revealed persistent tensions, notably the H0 and S8 discrepancies, which question the completeness of the standard Lambda-CDM model. This thesis investigates cosmological applications of teleparallel gravity and its extensions, focusing on f(T) and f(T,T) theories. We show that torsion-based modifications can shift late-time expansion and matter clustering, alleviating the H0 and S8 tensions. Using datasets including cosmic chronometers, baryon acoustic oscillations, Type Ia supernovae, Pantheon+SH0ES, Union3, DESI, and gravitational wave standard sirens, we perform Markov Chain Monte Carlo analyses to constrain model parameters. Model-independent diagnostics using cosmography demonstrate that extended teleparallel theories can be tightly constrained. Pade approximations and direct dynamical reconstructions yield consistent results, with some models outperforming Lambda-CDM using recent DESI and Union3 data. We also propose a connection between late-time acceleration and early-Universe baryogenesis, showing that torsional gravity can reproduce the observed baryon asymmetry while remaining consistent with late-time expansion. Overall, teleparallel gravity provides a robust alternative framework capable of alleviating key cosmological tensions and linking early- and late-Universe physics.

gr-qc↗

arXiv2026

Observational constraints on viscous free-$γ$ fluid in $f(Q)$ gravity

We study the late-time cosmological dynamics of a spatially flat FLRW universe in the framework of $f(Q)$ gravity, where $Q$ denotes the nonmetricity scalar. The matter sector is modeled as a bulk viscous fluid with a free equation-of-state parameter $γ$, allowing for a generalized description of cosmic matter beyond the standard dust approximation. We derive the background evolution equations and analyze the resulting expansion history. The model parameters are constrained using a combination of observational datasets, including cosmic chronometers (CC), baryon acoustic oscillations from DESI DR2, and Type~Ia supernovae (GRBs and Union3). Using the best-fit parameters, we further employ the statefinder and $\mathrm{Om}(z)$ diagnostics to distinguish the viscous $f(Q)$ scenario from the standard $Λ$CDM model. In addition, we examine the evolution of the deceleration parameter, which exhibits a transition from an early decelerated phase to the current accelerated expansion, and analyze the effective equation of state behavior. Our results show that bulk viscosity within $f(Q)$ gravity provides a viable and observationally consistent description of late-time cosmic acceleration.

gr-qc↗

arXiv2024

Chebyshev cosmography in the framework of extended symmetric teleparallel theory

Cosmography has been extensively utilized to constrain the kinematic state of the Universe using measured distances. In this work, we propose a new method to reconstruct coupling theories using the first kind of Chebyshev polynomial for two variables in which the functional form of the $f(Q,T)$ theory has been obtained. Further, the unknowns that appeared in the series are constrained using the cosmographic parameters. We find the explicit form of the luminosity distance in terms of cosmographic parameters to perform MCMC analysis using the PANTHEON+SH0ES data set. Through the distance modulus function, we observe that the result comes out to be an excellent match to the standard cosmological model and data.

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↗

arXiv2024

Can $f(T)$ models play a bridge between early and late time Universe?

The ability of Big Bang Nucleosynthesis theory to accurately predict the primordial abundances of helium and deuterium, as well as the baryon content of the Universe, is considered one of the most significant achievements in modern physics. In the present study, we consider two highly motivated hybrid $f(T)$ models and constrain them using the observations from the Big Bang Nucleosynthesis era. In addition, using late-time observations of Cosmic Chronometers and Gamma-Ray-Bursts, the ranges of the model parameters are confined which are in good agreement with early time bounds. Subsequently, the common ranges obtained from the analysis for early and late time are summarized. Further, we verify the intermediating epochs by investigating the profiles of cosmographic parameters using the model parameter values from the common range. From this study, we find the considered teleparallel models are viable candidates to explain the primordial-intermediating-present epochs.

gr-qc↗

arXiv2024

Constraining extended teleparallel gravity via cosmography: A model-independent approach

As a classical approach, the dynamics of the Universe, influenced by its dark components, are unveiled through prior modifications of Einstein's equations. Cosmography, on the other hand, is a highly efficient tool for reconstructing any modified theory in a model-independent manner. By employing kinematic variables, it offers a profound explanation for cosmic expansion. Although the cosmographical approach has been highly successful in several geometric theories in recent years, it has not been extensively explored in coupled gravities. With this in mind, we intend to constrain an extended teleparallel gravity model, $f(T,\mathcal{T})$, through cosmographic parameters. We utilize Taylor series expansion, assuming a minimally coupled form, to constrain the unknowns involved in the series. To achieve this, we conduct a Markov Chain Monte Carlo analysis (MCMC) using three different datasets (CC, BAO, and Pantheon+SH0ES). The constrained results obtained from MCMC are then compared and verified using various cosmological parameters. Finally, we compare the resulting models with \textbf{three} well-known $f(T,\mathcal{T})$ models.

gr-qc↗

arXiv2024

Probing Baryogenesis in $f(Q)$ gravity

The origin of matter domination in the Universe is one of the most exciting open puzzles in particle physics and cosmology. Despite many theoretical developments, the actual reason behind baryon-antibaryon asymmetry is still unknown. Our aim here is to examine this phenomenon in the framework of modified gravity theories, which have impressively elucidated the contemporary accelerated expansion of the universe as well as the early phase. Consequently, this letter sets its sights on the task of constraining a specific variant of modified gravity, namely, $f(Q)$ gravity, in conjunction with gravitational baryogenesis. The Power Law model and recently proposed DGP-like $f(Q)$ models are considered to find the baryon-to-entropy ratio and compare them with the observed value, that is ${n_B}/s=9.42\times 10^{-11}$. Furthermore, we impose constraints on the additional degrees of freedom introduced by this modified theory of gravity.

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

$H_0$ Tension in Torsion-based Modified Gravity

The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble parameter using two exponential Lagrangian functions of torsion $T$ and a trace of energy-momentum tensor $\mathcal{T}$ for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble, SNe Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background samples, and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant $H_0$ for our model, and the outputs align with the recent observational measurements of $H_0$. In addition, we check the deviation of our results from model-independent measurements of $H_0$ from Planck2018, S$H_0$ES, and $H_0$LiCOW experiments. In contrast, our finding partially solved the $H_0$ tension but gave a new possible direction to alleviate the $H_0$ tension.

astro-ph.CO↗

arXiv2023

Constraining $f(T,\mathcal{T})$ Gravity with Gravitational Baryogenesis

Gravitational baryogenesis is one of the mechanisms which help us to explore more about our early universe, especially baryon-anti-baryon asymmetry. As we know, modified theories of gravity are very successful in describing the present accelerated scenario of the universe. Therefore, in this letter, we aim to constrain the generalized torsion-based modified theory of gravity, namely, $f(T,\mathcal{T})$ gravity with gravitational baryogenesis, where $T$, $\mathcal{T}$ are the torsion scalar, trace of the energy-momentum tensor, respectively. For this, we examine how the various Lagrangian forms of $f(T,\mathcal{T})$ affect the baryogenesis. We also impose the constraints on the extra degrees of freedom induced by modified theory with the observational values of the baryon-to-entropy ratio. In addition, we further explore how more generalized gravitational baryogenesis can attribute in a physically viable and consistent way to the cosmologies of the modified theory of gravity.

gr-qc↗