arXiv subjects

Anjan A Sen

Publications and source records attributed to Anjan A Sen.

At least 19 recordsLinked to original sources

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 $Λ$CDM: Implications for the Hubble Tension

We investigate an extension of $Λ$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 ${\sim}2.2σ$ and local distance network measurement (H0DN) to ${\sim}2.9σ$. 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 $Λ$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

Beyond $\Lambda$CDM with a Logistic RG-like Flow of the Low Redshift Cosmic Evolution

Recent observations hint at possible late-time deviations from $\Lambda$CDM. We introduce a minimal phenomenological framework in which the total equation of state $w_{\rm T}(z)$ follows a logistic evolution motivated by renormalization-group-like flow between cosmological fixed points. Using DESI-DR2 BAO, DES supernova data, and CMB distance priors, we find that this parametrization provides an improved description of the expansion history relative to $\Lambda$CDM. The reconstructed evolution, though model specific, also shows statistically strong low-redshift deviations from $\Lambda$CDM.

astro-ph.CO↗

arXiv2026

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↗

arXiv2026

The dynamics of background evolution and structure formation in phase space: a semi-cosmographic reconstruction

The Baryon Acoustic Oscillation (BAO) feature, imprinted in the transverse and radial clustering of dark matter tracers, enables the simultaneous measurement of the angular diameter distance $D_A(z)$ and the Hubble parameter $H(z)$ at a given redshift. Further, measuring the redshift space anisotropy (RSD) allows us to measure the combination $f_8(z)\equiv fσ_8(z)$. Motivated by this, we simultaneously study the dynamics of background evolution and structure formation in an abstract phase space of dynamical quantities: $ x = H_0 D_A/c$, $p = dx/dz$, and $f_8$. We adopt a semi-cosmographic approach, whereby we do not pre-assume any specific dark energy model to integrate the dynamical system. The Luminosity distance is expanded as a Padé rational approximation in the variable $(1+z)^{1/2}$. The dynamical system is solved by using a semi-cosmographic equation of state, which incorporates the dark matter density parameter along with the parameters of the Padé expansion. The semi-cosmographic $D_A(z), H(z)$ and $fσ_8(z)$, thus obtained, are fitted with BAO and RSD data from the SDSS IV. The reconstructed phase trajectories in the $3D$ $(x,p,f_8)$ space are used to reconstruct some diagnostics of background cosmology and structure formation. At low redshifts, a discernible departure from the $Λ$CDM model is observed. The geometry of the phase trajectories in the projected spaces allows us to identify three key redshifts where future observations may be directed for a better understanding of cosmic tensions and anomalies.

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

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 $5σ$ from the Planck 2018 $Λ$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

Disentangling the growth rate of perturbations from the HI bias using only clustering data from galaxy surveys

This work serves two-fold purpose. Firstly, we provide an alternative to the traditional method of determining the growth rate of density perturbations $f(z)$. In usual practice, $f(z)$ can not be directly measured from tracer clustering at some redshift without knowledge of the bias. While the bayron acoustic oscillation (BAO) imprint allows the determination of $(D_A(z), H(z))$, redshift space anisotropy (RSD) allows the measurement of a quantity $f_8(z) = f(z) σ_{8,0} D_{+}(z)$. To extract $f(z)$ from $f_8(z)$, one usually requires some other data set. We show that precise BAO and RSD measurements in and around some key redshifts themselves can solely reconstruct $f(z)$ without requiring any other data sets. Secondly, we extend this approach to another tracer, namely the post-reionization 21-cm brightness temperature intensity maps. We demonstrate that the measured $f(z)$ from purely redshift space clustering allows us to measure the 21-cm bias, which is a largely unknown quantity. This may help interpret the observed intensity mapping signal in the future.

astro-ph.CO↗

arXiv2025

A semi-cosmographic approach to study cosmological evolution in phase space

The signature of Baryon Acoustic Oscillation in the clustering of dark-matter tracers allows us to measure $(D_A(z), H(z))$ independently. Treating these as conjugate variables, we are motivated to study cosmological evolution in the phase space of dimensionless variables $x = H_0 D_A/c$ and $p = dx/dz$. The dynamical variables $(x(z),p(z))$ can be integrated for a known set of equation of state parameters for different matter/energy components. However, to avoid any preference for specific dark energy models, we adopt a cosmographic approach. We consider two scenarios where the Luminosity distance is expanded as Padé rational approximants using expansion in terms of $z$ and $(1+z)^{1/2}$ respectively. However, instead of directly using the Padé ratios to fit kinematic quantities with data, we adopt an alternative approach where the evolution of the cold dark matter sector is incorporated in our analysis through a semi-cosmographic equation of state, which is then, used to solve the dynamical problem in the phase space. The semi-cosmographic $(D_A(z), H(z))$, thus obtained, is fitted with BAO data from DESI DR1, cosmic chronometer (CC) data and SNIa data from Pantheon+ respectively. We also consider a futuristic 21-cm intensity mapping experiment for error projections. We further use the semi-cosmographic fitting to reconstruct some diagnostics of background cosmology and compare our results for the two scenarios of Padé expansions.

astro-ph.CO↗

arXiv2025

A New $\sim 5σ$ 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 $Λ$CDM prediction, confirming no new physics at that redshift. However, at $z \sim 0.512$, the derived $H(z)$ shows a more than $5σ$ discrepancy with the Planck-2018 $Λ$CDM prediction, suggesting a possible breakdown of the $Λ$CDM model as constrained by Planck-2018 at this lower redshift. This emerging $\sim 5σ$ 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

Inferring dark energy properties from the scale factor parametrisation

We propose and implement a novel test to assess deviations from well-established concordance $Λ$CDM cosmology while inferring dark energy properties. In contrast to the commonly implemented parametric forms of the dark energy equation-of-state (EoS), we test the validity of the cosmological constant on the more fundamental scale factor [$a(t)$] which determines the expansion rate of the Universe. We constrain our extended `general model' using the late-time observables. The posterior of the dark energy EoS is mainly constrained to be quintessence-like naturally excluding physically unviable regions such as phantom crossings or exponential growth.

astro-ph.CO↗

arXiv2023

Traversable wormholes in bi-metric gravity

The ghost-free bi-metric gravity theory is a viable theory of gravity that explores the interaction between a massless and a massive graviton and can be described in terms of two dynamical metrics. In this paper, we present an exact static, spherically symmetric vacuum solution within this theory. The solution is spatially Schwarzschild-de Sitter, with the value of the cosmological constant determined by the graviton mass and the interaction parameters of the theory. Notably, for specific parameter ranges, the solution represents a traversable Lorentzian wormhole that violates the weak energy condition near its throat. Furthermore, we have investigated the evolution of scalar and electromagnetic fields in this wormhole spacetime and observed the presence of arbitrarily long-lived quasi-resonant modes in the quasinormal spectrum.

gr-qc↗

arXiv2019

Cosmology With Low-Redshift Observations: No Signal For New Physics

We analyse various low-redshift cosmological data from Type-Ia Supernova, Baryon Acoustic Oscillations, Time-Delay measurements using Strong-Lensing, $H(z)$ measurements using Cosmic Chronometers and growth measurements from large scale structure observations for $Λ$CDM and some different dark energy models. By calculating the Bayesian Evidence for different dark energy models, we find out that the $Λ$CDM still gives the best fit to the data with $H_{0}=70.3^{+1.36}_{-1.35}$ Km/s/Mpc (at $1σ$). This value is in $2σ$ or less tension with various low and high redshift measurements for $H_{0}$ including SH0ES, Planck-2018 and the recent results from H0LiCOW-XIII. The derived constraint on $S_{8}=σ_{8}\sqrt{Ω_{m0}/{0.3}}$ from our analysis is $S_{8} = 0.76^{+0.03}_{-0.03}$, fully consistent with direct measurement of $S_{8}$ by KiDS+VIKING-450+DES1 survey. We hence conclude that the $Λ$CDM model with parameter constraints obtained in this work is consistent with different early and late Universe observations within $2σ$. We therefore, do not find any compelling reason to go beyond concordance $Λ$CDM model.

astro-ph.CO↗

arXiv2019

Model independent constraints on dark energy evolution from low-redshift observations

Knowing the late time evolution of the Universe and finding out the causes for this evolution are the important challenges of modern cosmology. In this work, we adopt a model-independent cosmographic approach and approximate the Hubble parameter considering the Pade approximation which works better than the standard Taylor series approximation for $z>1$. With this, we constrain the late time evolution of the Universe considering low-redshift observations coming from SNIa, BAO, $H(z)$, $H_{0}$ , strong-lensing time-delay as well as the Megamaser observations for angular diameter distances. We confirm the tensions with $Λ$CDM model for low-redshifts observations. The present value of the equation of state for the dark energy has to be phantom-like and for other redshifts, it has to be either phantom or should have a phantom crossing. For lower values of $Ω_{m0}$, multiple phantom crossings are expected. This poses serious challenges for single, non-interacting scalar field models for dark energy. We derive constraints on the {\it statefinders} $(r,s)$ and these constraints show that a single dark energy model cannot fit data for the whole redshift range $0\leq z\leq 2$: in other words, we need multiple dark energy behaviors for different redshift ranges. Moreover, the constraint on sound speed for the total fluid of the Universe, and for the dark energy fluid (assuming them being barotropic), rules out the possibility of a barotropic fluid model for unified dark sector and barotropic fluid model for dark energy, as fluctuations in these fluids are unstable as $c_{s}^2 < 0$ due to constraints from low-redshift observations.

astro-ph.CO↗

arXiv2018

Dark energy constraints from the 21~cm intensity mapping surveys with SKA1

Understanding the nature of dark energy is one of the most outstanding problems in cosmology at present. In last twenty years, cosmological observations related to SNIa, Cosmic Microwave Background Radiation, Baryon Acoustic Oscillations etc, have put stringent constraints on the the dark energy evolution, still there is enough uncertainty in our knowledge about dark energy that demands new generation of cosmological observations. Post-reionization neutral hydrogen 21 cm intensity mapping surveys are one of the most promising future cosmological observations that have the potential to map the cosmological evolution from dark ages till present time with unprecedented accuracy and Square Kilometer Array (SKA) is one of the most sensitive instruments to measure the post-reionization 21 cm signal. In this work, we study the future dark energy constraints using post-reionization 21 cm intensity mapping power spectra with SKA1-mid specifications. We use three different parametrizations for dark energy equation of state (EoS) including the widely used CPL one. To generate simulated data, we use to two fiducial models: the concordance $Λ$CDM and the best fit CPL model for Planck+SNIa+BAO+HST. Our study shows that SKA1-mid alone has the potential to reach the present accuracy for combined Planck+SNIa+BAO+HST to constrain the dark energy behaviour. Whether dark energy is phantom or non-phantom or whether it exhibits phantom crossing, we may potentially address such questions with SKA1-mid. We also show that it is crucial to choose the correct parametrization for dark energy equation of state as some parametrizations are better than others to constrain the dark energy behaviour. Specifically, as observed in this study, the widely used CPL parametrization may not give the best constraint for dark energy behaviour.

astro-ph.CO↗

arXiv2017

Bayesian Evidences for Dark Energy models in light of current obsevational data

We do a comprehensive study of the Bayesian evidences for a large number of dark energy models using a combination of latest cosmological data from SNIa, CMB, BAO, Strong lensing time delay, Growth measurements, measurements of Hubble parameter at different redshifts and measurements of angular diameter distance by Megamaser Cosmology Project . We consider a variety of scalar field models with different potentials as well as different parametrisations for the dark energy equation of state. Among 21 models that we consider in our study, we do not find strong evidences in favour of any evolving dark energy model compared to $Λ$CDM. For the evolving dark energy models, we show that purely non-phantom models have much better evidences compared to those models that allow both phantom and non-phantom behaviours. Canonical scalar field with exponential and tachyon field with square potential have highest evidences among all the models considered in this work. We also show that a combination of low redshift measurements decisively favours an accelerating $Λ$CDM model compared to a non-accelerating power law model.

astro-ph.CO↗

arXiv2017

Imprint of thawing scalar fields on large scale galaxy overdensity

We investigate the observed galaxy power spectrum for the thawing class of scalar field models taking into account various general relativistic corrections that occur on very large scales. We consider the full general relativistic perturbation equations for the matter as well as the dark energy fluid. We form a single autonomous system of equations containing both the background and perturbed equations of motion which we subsequently solve for different scalar field potentials. First we study the percentage deviation from $Λ$CDM model for different cosmological parameters as well as in the observed galaxy power spectra on different scales in scalar field models for various choices of scalar field potentials. Interestingly the difference in background expansion results enhancement of power from $Λ$CDM on small scales whereas the inclusion of GR corrections results the suppression of power from $Λ$CDM on large scales. This can be useful to distinguish scalar field models from $Λ$CDM with future optical/radio surveys. We also compare the observed galaxy power spectra for tracking and thawing types of scalar field using some particular choices for the scalar field potentials. We show that thawing and tracking models can have large differences in observed galaxy power spectra on large scales and for smaller redshifts due to different GR effects. But on smaller scales and for larger redshifts, the difference is small and is mainly due to difference in background expansion.

astro-ph.CO↗

arXiv2017

The Price of Shifting the Hubble Constant

An anisotropic measurement of the baryon acoustic oscillation (BAO) feature fixes the product of the Hubble constant and the acoustic scale $H_0 r_d$. Therefore, regardless of the dark energy dynamics, to accommodate a higher value of $H_0$ one needs a lower $r_d$ and so necessarily a modification of early time cosmology. One must either reduce the age of the Universe at the drag epoch or else the speed of sound in the primordial plasma. The first can be achieved, for example, with dark radiation or very early dark energy, automatically preserving the angular size of the acoustic scale in the Cosmic Microwave Background (CMB) with no modifications to post-recombination dark energy. However it is known that the simplest such modifications fall afoul of CMB constraints at higher multipoles. As an example, we combine anisotropic BAO with geometric measurements from strong lensing time delays from H0LiCOW and megamasers from the Megamaser Cosmology Project to measure $r_d$, with and without the local distance ladder measurement of $H_0$. We find that the best fit value of $r_d$ is indeed quite insensitive to the dark energy model, and is also hardly affected by the inclusion of the local distance ladder data.

astro-ph.CO↗