arXiv subjects

Yashi Tiwari

Publications and source records attributed to Yashi Tiwari.

6 recordsLinked to original sources

arXiv2026

Can Distance Duality Violation Save Late-time Solutions to the Hubble Tension?

The discrepancy between early- and late-Universe determinations of the Hubble constant may point to physics beyond $\Lambda$CDM or to unaccounted-for systematics. Numerous late-time modifications to the expansion history have been proposed to alleviate this discrepancy, with limited success. Recent works have shown that, when the sound-horizon and supernova calibrations are held fixed, any purely late-time resolution requires a violation of the cosmic distance duality relation (CDDR). Recasting the tension in the $r_d$-$M_B$ plane, we show explicitly that distance duality, together with BAO and uncalibrated supernova data and a fixed sound-horizon calibration, determines $H_0$ independently of the late-time expansion history. We then test the viability of the required CDDR violation by separately constraining reciprocity violation and photon number non-conservation, deriving a new constraint on reciprocity-violating distortions of angular-diameter distances from BAO and cosmic-chronometer data. Combining this result with existing photon-number-conservation constraints, we find that the level of distance-duality violation needed to resolve the tension is strongly disfavoured by current data. We therefore conclude that, for fixed sound-horizon and supernova calibrations, no modification confined to the late-time expansion history -- even one violating distance duality -- can resolve the Hubble tension, pointing instead toward early-Universe physics or unresolved local systematics.

astro-ph.CO↗

arXiv2026

Updated Constraints on Infrared Cutoff Models and Implications for Large-Scale CMB Anomalies

The nearly scale-invariant primordial power spectrum provides the standard initial conditions for cosmological perturbations. However, the largest scales remain only weakly constrained by CMB observations, leaving room for deviations such as an infrared (IR) cut-off. This possibility is further motivated by the persistence of large-scale CMB anomalies, most notably the low quadrupole power. In this work, we revisit several broad classes of phenomenologically motivated IR cut-off scenarios using parametrised functional forms of the primordial power spectrum. We confront these models with the latest CMB, BAO, and supernova data and derive updated constraints on the cut-off scale and associated features. Our results remain consistent with earlier studies, showing that although such models suppress power at low multipoles, the improvement in fit is marginal and does not overcome the associated parameter penalties. We therefore find no statistically significant evidence favouring IR cut-off models over the standard power-law spectrum with current data. We further explore the interplay between IR cut-off features and a possible increase in the reionisation optical depth, motivated by the recent CMB-BAO tension highlighted by DESI DR2 within the $Λ$CDM framework. We show that the additional freedom introduced by large-scale suppression is generally insufficient to support a substantial increase in optical depth, owing to the weak statistical preference for suppressed large-scale temperature power. Finally, we examine the implications of IR cut-off models for large-scale CMB anomalies by analysing the corresponding anomaly statistics within a Bayesian framework.

astro-ph.CO↗

arXiv2026

Revisiting Cosmic Distance Duality with Megamasers and DESI DR2 Observations: Model Independent Constraints on Early-Late Calibration

The Cosmic Distance Duality Relation (CDDR) connects the angular diameter distance ($d_A$) and the luminosity distance ($d_L$) at a given redshift. This fundamental relation holds in any metric theory of gravity, provided that photon number is conserved and light propagates along null geodesics. A deviation from this relation could indicate new physics beyond the standard cosmological model. In this work, we test the validity of the CDDR at very low redshifts ($z < 0.04$) by combining $d_A$ from the Megamaser Cosmology Project with $d_L$ from the Pantheon+ sample of Type Ia Supernovae (SNIa). We further incorporate high-redshift Baryon Acoustic Oscillation (BAO)-based $d_A$ measurements from DESI DR2 in combination with SNIa data, highlighting the critical role of the $r_d-M_b$ (early-late) calibration in testing the CDDR using these two probes. Assuming CDDR holds, we perform a Bayesian analysis to derive model-independent constraints on the calibration parameters. Using only BAO and SNIa data, we observe a strong degeneracy between $r_d$ and $M_b$. However, the inclusion of calibration-free Megamaser measurements breaks this degeneracy, enabling independent constraints without relying on a specific cosmological model or distance-ladder techniques. Additionally, we present a forecast incorporating the expected precision from future Megamaser and SNIa observations, demonstrating their potential to significantly tighten constraints on early-late calibration parameters, under the assumption of validity of CDDR.

astro-ph.CO↗

arXiv2025

Exploring cosmological imprints of phantom crossing with dynamical dark energy in Horndeski gravity

In the current era of precision cosmology, the persistence of cosmological tensions, most notably the Hubble tension and the $S_8$ tension, challenges the standard $Λ$CDM model. To reconcile these tensions via late-time modifications to expansion history, various features such as phantom crossing in the dark energy equation of state, a negative energy density at high redshifts, etc., are favoured. However, these scenarios cannot be realized within the framework of GR without introducing ghost or gradient instabilities. In this work, we investigate a dynamical dark energy scenario within the framework of Horndeski gravity, incorporating nonminimal coupling to gravity and self-interactions. We highlight that the model can exhibit novel features like phantom crossing and negative dark energy densities at high redshifts without introducing any instabilities. For this specific Horndeski model, we perform a comprehensive analysis of the background evolution along with the effects on perturbations, examining observables like growth rate, matter and CMB power spectrum. To check the consistency of the model with the observational data, we employ MCMC analysis using BAO/$fσ_8$, Supernovae, and CMB data. While the model does not outperform the standard $Λ$CDM framework in a combined likelihood analysis, there remains a preference for non-zero values of the model parameters within the data. This suggests that dynamical dark energy scenarios, particularly those with non-minimal couplings, merit further exploration as promising alternatives to GR, offering rich phenomenology that can be tested against a broader range of current and upcoming observational datasets.

astro-ph.CO↗

arXiv2023

Towards a possible solution to the Hubble tension with Horndeski gravity

The Hubble tension refers to the discrepancy in the value of the Hubble constant $H_0$ inferred from the cosmic microwave background observations, assuming the concordance $Λ$CDM model of the Universe, and that from the distance ladder and other direct measurements. In order to alleviate this tension, we construct a plausible dark energy scenario, within the framework of Horndeski gravity which is one of the most general scalar-tensor theories yielding second-order equations. In our set-up, we include the self-interactions and nonminimal coupling of the dynamical dark energy scalar field which enable very interesting dynamics leading to a phantom behaviour at low redshifts along with negative dark energy densities at high redshifts. These two features together make this model a promising scenario to alleviate the Hubble tension for appropriate choices of the model parameters. Towards a consistent model building, we show that this set-up is also free from both the gradient and ghost instabilities. Finally, we confront the predictions of the model with low redshift observations from Pantheon, SH0ES, cosmic chronometers and BAO, to obtain best fit constraints on model parameters.

astro-ph.CO↗

arXiv2023

Understanding large scale CMB anomalies with the generalized non-minimal derivative coupling during inflation

We study the observational implications of a class of inflationary models wherein the inflaton is coupled to the Einstein tensor through a generalized non-minimal derivative coupling (GNMDC). In particular, we explore whether these models can generate suitable features in the primordial spectrum of curvature perturbations as a possible explanation for the large-scale anomalies associated with the angular power spectrum of CMB temperature anisotropies. We derive model-independent constraints on the GNMDC function for such a scenario, considering both the scalar and tensor perturbations. We modify CosmoMC to accommodate our GNMDC framework and investigate different classes of inflationary models using a fully consistent numerical approach. We find that the hilltop-quartic model with a specific choice of the GNMDC function provides a considerable improvement over the best-fit reference $Λ$CDM model with a nearly scale-invariant power spectrum. While the large-scale structure observations should be able to provide independent constraints, future CMB experiments, such as CMB-S4 and CMB-Bharat, are expected to constrain further the parameter space of such beyond canonical single-field inflationary models.

astro-ph.CO↗