arXiv subjects

Purba Mukherjee

Publications and source records attributed to Purba Mukherjee.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting Metastable Dark Energy in Light of DESI DR2 BAO and DESI DR1 Full-Shape Measurements

We revisit metastable dark energy (DE) models described by a radioactive-like decay law. We consider three scenarios: an effective, exponentially decaying DE component; decay of DE into non-baryonic dark matter (DM); and decay of DE into dark radiation (DR). We constrain the metastable DE models using DESI DR2 baryon acoustic oscillation (BAO) data, Type Ia supernovae (SNIa), cosmic microwave background (CMB) observations, and, for the first time, the current available DESI DR1 full-shape (FS) clustering measurements. The BAO+SNIa combinations show a mild preference for positive $\Gamma/H_0$, with a deviation from the $\Lambda$CDM limit at the $\gtrsim 2\sigma$ level. This corresponds to a decaying DE density and an effective quintessence-like behaviour at low redshift. Once CMB information from either Planck or P-ACT is included, however, the constraints become statistically consistent with $\Gamma/H_0=0$. The FS measurements probe the growth sector and help distinguish the interacting DM-DE behaviour of Model 2 from the effective decaying-DE response of Model 1 and the weaker DR-induced response of Model 3. For CMB+FS+DES-Dovekie, Model 1 shows a slight deviation from $\Gamma/H_0=0$ at the $\gtrsim 2\sigma$ level, while Models 2 and 3 remain consistent with the $\Lambda$CDM limit within $2\sigma$. Overall, metastable DE remains phenomenologically viable: current data allow late-time dynamics but do not provide decisive evidence for a nonzero decay rate. These results motivate extending our analysis to the upcoming DESI DR2 FS data to obtain tighter constraints on metastable dynamics.

astro-ph.CO↗

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 $\Lambda$CDM model, we identify a localized deviation over the redshift interval $0.3\lesssim z\lesssim0.6$, reaching a maximum significance of approximately $3.5\sigma$ 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

Early- and Late-Time Modifications to $\Lambda$CDM: Implications for the Hubble Tension

We investigate an extension of $\Lambda$CDM in which a fraction of cold Dark Matter (DM) decays into invisible dark radiation (DR) around the radiation-matter equality epoch, together with a non-standard dark energy (DE) equation of state characterized by $w_0$. The decaying DM component modifies the early expansion history and reduces the sound horizon at baryon drag, while the DE alters the expansion rate at the late times. A comprehensive analysis combining \texttt{Planck 2018+ACT DR6+DESI DR2+CMB lensing} datasets has been carried out to explore the viability of this framework in addressing the $H_0$ tension. This model yields a Hubble constant of $H_0 = 69.83 \pm 0.98~\mathrm{km\,s^{-1}\,Mpc^{-1}}$, reducing the discrepancy with SH0ES measurement to ${\sim}2.2\sigma$ and local distance network measurement (H0DN) to ${\sim}2.9\sigma$. Further, considering \texttt{SH0ES} and \texttt{Pantheon+}, the inferred value of the Hubble constant becomes $H_0 = 70.20 \pm 0.66~\mathrm{km\,s^{-1}\,Mpc^{-1}}$. The Bayesian evidence suggests that this framework offers a fit to the relevant cosmological datasets at a statistically similar level as $\Lambda$CDM. It is observed that correlated early- and late-time modifications to the cosmological expansion history provide a more effective route to reducing the $H_0$ tension than either class of modification alone.

astro-ph.CO↗

arXiv2026

Investigating the $H_0$ Tension and Expansion-History Mismatch with Diverse Dark Energy Parametrization Frameworks

The $\Lambda$CDM model successfully explains a wide range of cosmological observations; however, persistent discrepancies most notably the $H_0$ tension between early and late time measurements challenge its completeness. No proposed extension has yet resolved this tension while retaining the overall success of $\Lambda$CDM. In this work, we investigate whether the $H_0$ tension can be associated with a specific epoch in the cosmic expansion history and identify the redshift range most relevant for understanding its origin. In addition to the cosmological constant, we consider three phenomenological models based on general parametrizations of key quantities governing cosmic expansion: the dark energy (DE) equation of state, the DE pressure density, and the scale factor. Using early time Planck data and late time Pantheon+ (with and without SH0ES calibration) and DESI measurements, we constrain model parameters and examine the evolution of the Hubble parameter $H(z)$. We find that $\Lambda$CDM exhibits discrepancies across all redshifts, whereas the other models shift the dominant deviations toward low redshifts. Among the models considered, the pressure density parametrization alleviates the $H_0$ tension, reducing it to $\sim 2.7\sigma$, while the other models do not provide significant improvement. A detailed analysis of DESI DR2 data further reveals notable deviations in $H(z)$ at $z=0.51$ and 0.706, whereas higher redshift measurements remain consistent within $1\sigma$. These results suggest that late-time modifications primarily reshape the redshift dependence of the mismatch in $H(z)$ rather than fully resolve it, in the absence of systematic effects. Furthermore, the reconstructed DE dynamics exhibit qualitatively distinct behaviors across parametrizations, highlighting a persistent inconsistency between early and late Universe probes in describing the nature of DE.

astro-ph.CO↗

arXiv2026

Probing Interacting Dark Sectors with upcoming Post-Reionization and Galaxy Surveys

We investigate the constraining power of future post-reionization and galaxy surveys on possible interactions between dynamical dark energy and dark matter. The analysis focuses on the interaction strength and the dark energy equation of state parameters, in addition to the six standard cosmological parameters. Using fiducial values obtained from the current observational bounds (Planck 2018 + DESI DR2 + Pantheon+), mock datasets for upcoming 21-cm intensity mapping, galaxy clustering and cosmic shear observations from the SKA-mid, and for the upcoming large-scale survey from the Euclid mission, were generated. Subsequently, Markov chain Monte Carlo analyses combining current cosmological data with these mock datasets were performed to forecast parameter constraints. The results indicate that both SKA-mid and Euclid observations can significantly improve constraints on interacting dark sector parameters. In particular, the interaction strength and dark energy equation of state parameters can be constrained considerably tighter than current combined constraints from Planck 2018, DESI DR2 and Pantheon+. Comparing different probe combinations and survey configurations, it is found that SKA2 provides the tightest projected constraints, particularly on the interaction strength, while Euclid achieves a precision broadly comparable to that of SKA1. The results highlight the potential of these upcoming surveys to probe interactions within the dark sector.

astro-ph.CO↗

arXiv2025

Exploring the Impact of Systematic Bias in Type Ia Supernova Cosmology Across Diverse Dark Energy Parametrizations

We investigate the impact of instrumental and astrophysical systematics on dark energy (DE) constraints from Type Ia supernova (SN-Ia) observations. Using simulated datasets consistent with current SN-Ia measurements, we examine how photometric calibration, intergalactic dust, progenitor evolution in luminosity and light-curve stretch, intrinsic color scatter, and matter density mismatch affect the inferred DE equation of state (EoS) parameters $(w_0,w_a)$. We test the Generalised Scale Factor (GEN) parametrization against three time-evolving DE models: Chevallier-Polarski-Linder (CPL), Jassal-Bagla-Padmanabhan (JBP), and Logarithmic (LOG). Calibration and progenitor-related effects emerge as the dominant sources of bias. {In particular, a calibration offset of $\Delta M_B=0.02$ can shift the inferred parameters by up to $\Delta w_0 \simeq -0.12$ and $\Delta w_a \simeq +0.60$ in JBP, while the corresponding shift in GEN is much smaller, with $\Delta w_0 \simeq -0.02$ and $\Delta w_a \simeq -0.04$. Progenitor-stretch evolution also induces substantial shifts, whereas intergalactic dust and color-scatter systematics produce only minor deviations for the fiducial amplitudes adopted here. Overall, JBP is the most sensitive to injected systematics, CPL and LOG show intermediate sensitivity, and GEN remains the most stable. We also quantify the deviation from the fiducial $\Lambda$CDM ($w_0=-1,\, w_a=0$) for the injected-systematic cases, and find that the second set of systematic injections supports the same qualitative hierarchy. These results highlight the need for sub-percent calibration precision and improved astrophysical modelling for robust DE inference from present and future SN-Ia cosmology experiments. More broadly, our results motivate model-independent tests of late-time physics, with phenomenological $(w_0,w_a)$ parametrizations used as summary statistics.

astro-ph.CO↗

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-Paczy\'nski 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 $\Lambda$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

Revisiting Gaussian Process Reconstruction for Cosmological Inference: The Generalised GP (Gen GP) Framework

We investigate uncertainties in the estimation of the Hubble constant ($H_0$) arising from Gaussian Process (GP) reconstruction, demonstrating that the choice of kernel introduces systematic variations comparable to those arising from different cosmological models. To address this limitation, we introduce the Generalized Gaussian Process (Gen GP) framework, in which the Mat\'ern smoothness parameter $\nu$ is treated as a free parameter, allowing for data-driven kernel optimization. Using the cosmic chronometer Hubble data, we find that while standard GP with $\Lambda$CDM mean function exhibits noticeable reconstruction differences between optimized and marginalized approaches, particularly at $z > 1$, Gen GP maintains methodological consistency. In Gen GP, slight increases in $\chi^2$ per degree of freedom relative to standard GP, for both the zero-mean and $\Lambda$CDM prior mean cases, reflect added flexibility rather than performance degradation. Our results emphasize that robust cosmological inference requires treating kernel parameters as free variables and implementing full Bayesian marginalization to avoid artificial precision from fixed hyperparameters. As machine learning becomes central to cosmological discovery, the Gen GP framework provides a principled approach to model-independent inference that properly accounts for methodological uncertainties while maintaining necessary flexibility for reliable parameter estimation.

astro-ph.CO↗

arXiv2025

Can an Anti-de Sitter Vacuum in the Dark Energy Sector Explain JWST High-Redshift Galaxy and Reionization Observations?

The unexpectedly large abundance of UV-bright galaxies at $z>10$ discovered by the James Webb Space Telescope poses a significant challenge to the standard $\Lambda$CDM cosmology. This work tests whether modifying the cosmological background, and thereby the growth of structures, can resolve this tension without invoking significant evolution in the astrophysical properties of early galaxies. We investigate an alternative framework featuring an anti-de Sitter vacuum in the dark energy sector, which naturally arises in quantum gravity models like string theory and can enhance early structure formation. Using a self-consistent semi-analytical model that couples galaxy evolution with reionization, we confront this scenario with a wide range of observations. We show that while a model tailored to fit the high-$z$ UV luminosity functions (UVLFs) appears promising, it is in strong tension with cosmological constraints from the CMB and other low-redshift probes. Conversely, models within this framework that satisfy these constraints provide only a modest boost to structure formation and fail to reproduce the observed galaxy abundances at $z>10$. Although these models remain consistent with the cosmic reionization history, we find that this class of cosmological modifications is insufficient on its own to explain the galaxy excess. Our study underscores the importance of holistic testing for any beyond-$\Lambda$CDM proposal; apparent success in one observational regime does not guarantee overall viability. By demonstrating the limitations of a purely cosmological solution, our results strengthen the case that evolving astrophysical properties are a necessary ingredient for solving the challenge of early galaxy formation.

astro-ph.CO↗

arXiv2025

New Expansion Rate Anomalies at Characteristic Redshifts Geometrically Determined using DESI-DR2 BAO and DES-SN5YR Observations

We perform a model-independent reconstruction of the cosmic distances using the Multi-Task Gaussian Process (MTGP) framework as well as knot-based spline techniques with DESI-DR2 BAO and DES-SN5YR datasets. We calibrate the comoving sound horizon at the baryon drag epoch $r_d$ to the Planck value, ensuring consistency with early-universe physics. With the reconstructed cosmic distances and their derivatives, we obtain seven characteristic redshifts in the range $0.3 \leq z \leq 1.7$. We derive the normalized expansion rate of the Universe $E(z)$ at these redshifts. Our findings reveal significant deviations of approximately $4$ to $5\sigma$ from the Planck 2018 $\Lambda$CDM predictions, particularly pronounced in the redshift range $z \sim 0.35-0.55$. These anomalies are consistently observed across both reconstruction methods and combined datasets, indicating robust late-time tensions in the expansion rate of the Universe and which are distinct from the existing "Hubble Tension". This could signal new physics beyond the standard cosmological framework at this redshift range. Our findings underscore the role of characteristic redshifts as sensitive indicators of expansion rate anomalies and motivate further scrutiny with forthcoming datasets from DESI-5YR BAO, Euclid, and LSST. These future surveys will tighten constraints and will confirm whether these late-time anomalies arise from new fundamental physics or unresolved systematics in the data.

astro-ph.CO↗

arXiv2025

Interacting Dark Sectors in light of DESI DR2

Possible interaction between dark energy and dark matter has previously shown promise in alleviating the clustering tension, without exacerbating the Hubble tension, when Baryon Acoustic Oscillations (BAO) data from the Sloan Digital Sky Survey (SDSS) DR16 is combined with Cosmic Microwave Background (CMB) and Type-Ia Supernovae (SNIa) data sets. With the recent Dark Energy Spectroscopic Instrument (DESI) BAO DR2, there is now a compelling need to re-evaluate this scenario. We combine DESI DR2 with Planck 2018 and Pantheon+ SNIa data sets to constrain interacting dark matter dark energy models, accounting for interaction effects in both the background and perturbation sectors. Our results exhibit similar trends to those observed with SDSS, albeit with improved precision, reinforcing the consistency between the two BAO data sets. In addition to offering a resolution to the $S_8$ tension, in the phantom-limit, the dark energy equation of state exhibits an early-phantom behaviour, aligning with DESI DR2 findings, before transitioning to $w\sim-1$ at lower redshifts, regardless of the DE parametrization. However, the statistical significance of excluding $w=-1$ is reduced compared to their non-interacting counterparts.

astro-ph.CO↗

arXiv2025

A New $\sim 5\sigma$ Tension at Characteristic Redshift from DESI-DR1 BAO and DES-SN5YR Observations

We perform a model-independent reconstruction of the angular diameter distance ($D_{A}$) using the Multi-Task Gaussian Process (MTGP) framework with DESI-DR1 BAO and DES-SN5YR datasets. We calibrate the comoving sound horizon at the baryon drag epoch $r_d$ to the Planck best-fit value, ensuring consistency with early-universe physics. With the reconstructed $D_A$ at two key redshifts, $z\sim 1.63$ (where $D_{A}^{\prime} =0$) and at $z\sim 0.512$ (where $D_{A}^{\prime} = D_{A}$), we derive the expansion rate of the Universe $H(z)$ at these redshifts. Our findings reveal that at $z\sim 1.63$, the $H(z)$ is fully consistent with the Planck-2018 $\Lambda$CDM prediction, confirming no new physics at that redshift. However, at $z \sim 0.512$, the derived $H(z)$ shows a more than $5\sigma$ discrepancy with the Planck-2018 $\Lambda$CDM prediction, suggesting a possible breakdown of the $\Lambda$CDM model as constrained by Planck-2018 at this lower redshift. This emerging $\sim 5\sigma$ tension at $z\sim 0.512$, distinct from the existing ``Hubble Tension'', may signal the first strong evidence for new physics at low redshifts.

astro-ph.CO↗

arXiv2025

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

We investigate deviations from $\Lambda$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 $A=\Omega_{m0}$ and $B=2/3$ in the $\Lambda$CDM limit, while deviations in the growth of structures are captured through a fitting function for $f\sigma_8$ involving the growth index $\gamma$. 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 $\Lambda$CDM in the background expansion whereas there is no finite evidence for deviations from $\Lambda$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 $\Lambda$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↗

arXiv2025

Quintessential Implications of the presence of AdS in the Dark Energy sector

We explore the implications for an Anti-de Sitter (AdS) vacuum, equivalently a negative cosmological constant (nCC), in the dark energy (DE) sector using current cosmological observations. Our joint analysis uses DESI BAO, DESY5 supernovae, and P-ACT CMB (ACT-DR6 + Planck) measurements. We also use the KiDS weak-lensing measurement to cross-check consistency with the inferred value of $S_{8}$. Within the Chevallier--Polarski--Linder parametrization for the evolving component of the DE, the inclusion of an AdS term provides a theoretically motivated mechanism that opens up a finite non-phantom region in the CPL parameter space while remaining compatible with current observations. A negative cosmological constant also implies a finite cosmic lifetime, thereby linking observational DE phenomenology to broader questions in quantum gravity and string theory.

astro-ph.CO↗

arXiv2024

Deep Learning Based Recalibration of SDSS and DESI BAO Alleviates Hubble and Clustering Tensions

Conventional calibration of Baryon Acoustic Oscillations (BAO) data relies on estimation of the sound horizon at drag epoch $r_d$ from early universe observations by assuming a cosmological model. We present a recalibration of two independent BAO datasets, SDSS and DESI, by employing deep learning techniques for model-independent estimation of $r_d$, and explore the impacts on $\Lambda$CDM cosmological parameters. Significant reductions in both Hubble ($H_0$) and clustering ($S_8$) tensions are observed for both the recalibrated datasets. Moderate shifts in some other parameters hint towards further exploration of such data-driven approaches.

astro-ph.CO↗

arXiv2024

Model-Agnostic Cosmological Inference with SDSS-IV eBOSS: Simultaneous Probing for Background and Perturbed Universe

Here we explore certain subtle features imprinted in data from the completed Sloan Digital Sky Survey IV (SDSS-IV) extended Baryon Oscillation Spectroscopic Survey (eBOSS) as a combined probe for the background and perturbed Universe. We reconstruct the baryon Acoustic Oscillation (BAO) and Redshift Space Distortion (RSD) observables as functions of redshift, using measurements from SDSS alone. We apply the Multi-Task Gaussian Process (MTGP) framework to model the interdependencies of cosmological observables $D_M(z)/r_d$, $D_H(z)/r_d$, and $f\sigma_8(z)$, and track their evolution across different redshifts. Subsequently, we obtain constrained three-dimensional phase space containing $D_M(z)/r_d$, $D_H(z)/r_d$, and $f\sigma_8(z)$ at different redshifts probed by the SDSS-IV eBOSS survey. Furthermore, assuming the $\Lambda$CDM model, we obtain constraints on model parameters $\Omega_{m}$, $H_{0}r_{d}$, $\sigma_{8}$ and $S_{8}$ at each redshift probed by SDSS-IV eBOSS. This indicates redshift-dependent trends in $H_0$, $\Omega_m$, $\sigma_8$ and $S_8$ in the $\Lambda$CDM model, suggesting a possible inconsistency in the $\Lambda$CDM model. Ours is a template for model-independent extraction of information for both background and perturbed Universe using a single galaxy survey taking into account all the existing correlations between background and perturbed observables and this can be easily extended to future DESI-3YR as well as Euclid results.

astro-ph.CO↗

arXiv2024

Model-independent calibration of Gamma-Ray Bursts with neural networks

The $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cosmological model has been highly successful in predicting cosmic structure and evolution, yet recent precision measurements have highlighted discrepancies, especially in the Hubble constant inferred from local and early-Universe data. Gamma-ray bursts (GRBs) present a promising alternative for cosmological measurements, capable of reaching higher redshifts than traditional distance indicators. This work leverages GRBs to refine cosmological parameters independently of the $\Lambda$CDM framework. Using the Platinum compilation of long GRBs, we calibrate the Dainotti relations-empirical correlations among GRB luminosity properties-as standard candles through artificial neural networks (ANNs). We analyze both the 2D and 3D Dainotti calibration relations, leveraging an ANN-driven Markov Chain Monte Carlo approach to minimize scatter in the calibration parameters, thereby achieving a stable Hubble diagram. This ANN-based calibration approach offers advantages over Gaussian processes, avoiding issues such as kernel function dependence and overfitting. Our results emphasize the need for model-independent calibration approaches to address systematic challenges in GRB luminosity variability, ultimately extending the cosmic distance ladder in a robust way. By addressing redshift evolution and reducing systematic uncertainties, GRBs can serve as reliable high-redshift distance indicators, offering critical insights into current cosmological tensions.

astro-ph.CO↗

arXiv2024

Non-parametric reconstruction of cosmological observables using Gaussian Processes Regression

The current accelerated expansion of the Universe remains ones of the most intriguing topics in modern cosmology, driving the search for innovative statistical techniques. Recent advancements in machine learning have significantly enhanced its application across various scientific fields, including physics, and particularly cosmology, where data analysis plays a crucial role in problem-solving. In this work, a non-parametric regression method with Gaussian processes is presented along with several applications to reconstruct some cosmological observables, such as the deceleration parameter and the dark energy equation of state, in order to contribute with some information that helps to clarify the behavior of the Universe. It was found that the results are consistent with $\Lambda$CDM and the predicted value of the Hubble parameter at redshift zero is $H_{0}=68.798\pm 6.340(1\sigma) \text{ km}\text{ s}^{-1}\text{ Mpc}^{-1}$.

astro-ph.CO↗