arXiv subjects

Anjan A. Sen

Publications and source records attributed to Anjan A. Sen.

At least 19 recordsLinked to original sources

arXiv2026

Updated Constraints on Omnipotent Dark Energy: A Comprehensive Analysis with CMB and BAO Data

In this work, we present updated observational constraints on the parameter space of the DMS20 dark energy model, a member of the omnipotent dark energy (ODE) class. Our analysis combines multiple CMB datasets - including measurements from the Planck satellite (PL18), the South Pole Telescope (SPT), and the Wilkinson Microwave Anisotropy Probe (WMAP) - with Type Ia supernova data from the Pantheon$+$ catalog (PP), and baryon acoustic oscillation (BAO) measurements from the DESI and SDSS surveys. We find that certain data combinations, such as SPT+WMAP+BAO and PL18+BAO, can reduce the significance of the $H_0$ tension below $1σ$, but with considerably large uncertainties. However, the inclusion of PP data restores the tension in $H_0$. To provide a comprehensive view of the ODE phenomenology, we also investigate the evolution of its energy density, emphasizing its dynamical behavior at low redshifts. Our results generically exhibit multiple phantom divide line crossings in a single expansion history; if confirmed, this points beyond the simplest minimally coupled canonical single-field quintessence/phantom descriptions and motivates more general dark-sector realizations.

gr-qc↗

arXiv2025

Pressure Parametrization of Dark Energy: First and Second-Order Constraints with Latest Cosmological Data

We explore an extension of the $Λ$CDM model in which the pressure $p$ of the dark energy (DE) fluid evolves with the expansion of the Universe, expressed as a function of the scale factor $a$. The corresponding energy density $ρ$ is derived from the continuity equation, resulting in a dynamical equation-of-state parameter $w \equiv p/ρ$ during the late-time expansion of the Universe. The pressure is modeled using a Taylor expansion around the present epoch ($a = 1$), introducing deviations from a cosmological constant within the dynamical dark energy (DDE) framework. At first order, a single new parameter $Ω_1$ captures linear deviations, while a second-order parameter, $Ω_2$, accounts for quadratic evolution in the pressure. We constrain the first- and second-order DDE models using multiple observational datasets and compare their performance against $Λ$CDM and the CPL parameterization. A joint analysis of Planck CMB, DESI, and DESY5 data yields the strongest evidence for DDE, with a $2.7σ$ deviation in the first-order model and over $4σ$ in the second-order model, providing strong statistical support for a departure from a cosmological constant. The reconstructed DE evolution in the second-order case reveals a distinctive non-monotonic behavior in both energy density and $w_{\rm DE}(a)$, including clear phantom-crossing phenomena. Notably, the late-time evolution of $w_{\rm DE}(a)$ remains consistent across datasets and shows strong agreement with the CPL parameterization, underscoring the robustness of the pressure-based approach.

astro-ph.CO↗

arXiv2025

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 $fσ_8(z)$, 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 $fσ_8(z)$ at different redshifts probed by the SDSS-IV eBOSS survey. Furthermore, assuming the $Λ$CDM model, we obtain constraints on model parameters $Ω_{m}$, $H_{0}r_{d}$, $σ_{8}$ and $S_{8}$ at each redshift probed by SDSS-IV eBOSS. This indicates redshift-dependent trends in $H_0$, $Ω_m$, $σ_8$ and $S_8$ in the $Λ$CDM model, suggesting a possible inconsistency in the $Λ$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

Further support for $S_8$ increasing with effective redshift

In Adil et al. 2023, we reported an increasing trend in $S_8$ with effective redshift $z_{\textrm{eff}}$ based on $f σ_8(z)$ constraints over the redshift range $0 \lesssim z \lesssim 2$, and predicted that this trend would be observable in independent datasets. Recently, the studies by Artis et al. and the ACT+DESI collaboration appeared, presenting data that aligns with the expected trends. In this letter, we quantify the statistical significance of the increasing $S_8$ trends in recent studies by fitting a linear model to estimate the slope $Δ\,S_8/Δ\, z_{\textrm{eff}}$, and comparing the results to mock simulations. We find probabilities of $p = 0.0163$ and $p = 0.01893$, corresponding to approximately $2.1σ$ for each dataset. Using Fisher's method to combine the independent probabilities, we obtained $p=0.0027$ ($2.8 σ$). When we incorporate our earlier findings, the combined statistical significance reaches between $3σ$ and $3.7σ$. Finally, we discuss how ``scatter" in $σ_8$/$S_8$ constraints from recent DESI full-shape galaxy clustering fits this picture at low statistical significance. This letter continues a series of studies initiated in 2020 that explore redshift-dependent $Λ$CDM parameters as an indication of a breakdown in the standard cosmological model.

astro-ph.CO↗

arXiv2024

$Λ$CDM Tensions: Localising Missing Physics through Consistency Checks

$Λ$CDM tensions are by definition model dependent; one sees anomalies through the prism of $Λ$CDM. Thus, progress towards tension resolution necessitates checking the consistency of the $Λ$CDM model to localise missing physics either in redshift or scale. Since the Universe is dynamical and redshift is a proxy for time, it is imperative to first perform consistency checks involving redshift, then consistency checks involving scale, as the next steps to settle the ``systematics versus new physics" debate and foster informed model building. We present a review of the hierarchy of assumptions underlying the $Λ$CDM cosmological model and comment on whether relaxing them can address the tensions. We focus on the lowest lying fruit of identifying missing physics through the identification of redshift dependent $Λ$CDM model fitting parameters. We highlight recent progress made on ${S_8:= σ_8 \sqrt{Ω_{\rm m}/0.3}}$ tension and elucidate how similar progress can be made on $H_0$ tension. Our discussions indicate that $H_0$ tension, equivalently a redshift dependent $H_0$, and a redshift dependent $S_8$ imply a problem with background $Λ$CDM cosmology.

astro-ph.CO↗

arXiv2024

Revisiting the Concordance $Λ$CDM model using Gamma-Ray Bursts together with Supernovae Ia and Planck data

The Hubble constant, $H_0$, tension is the tension among the local probes, Supernovae Ia, and the Cosmic Microwave Background Radiation. It has been almost a decade, and this tension still puzzles the community. Here, we add intermediate redshift probes, such as Gamma-Ray Bursts (GRB) and Quasars (QS0s), to check if and to what extent these higher redshift probes can reduce this tension. We use the three-dimensional fundamental plane relation among the prompt peak luminosity, the luminosity at the end of the plateau emission, and its rest frame duration. We find similar trend in GRB intrinsic parameters as previously seen in Pantheon-Plus intrinsic parameters. We find an apparent $3.14σ$ tension for the GRB intrinsic parameter $b$. Indeed, this tension disappears and the parameters are actually compatible within $2.26σ$. Another interesting point is that the 3D relation plays an important role in conjunction with Supernovae data with Pantheon Plus and that this apparent discrepancy show how it is important the correction for selection biases and redshift evolution. The incorporation of redshift evolution correction results in a reduction of the GRB tension to $2.26σ$ when adjusting correction parameters. We envision that with more data this indication of tension will possibly disappear when the evolutionary parameters of GRBs are computed with increased precision.

astro-ph.HE↗

arXiv2024

Negative cosmological constant in the dark energy sector: tests from JWST photometric and spectroscopic observations of high-redshift galaxies

Early observations with the James Webb Space Telescope (JWST) have revealed the existence of an unexpectedly large abundance of extremely massive galaxies at redshifts $z \gtrsim 5$: these observations are in tension with the predictions not only of the standard $Λ$CDM cosmology, but also with those of a wide class of dynamical dark energy (DE) models, and are generally in better agreement with models characterized by a phantom behaviour. Here we consider a model, inspired by string theory and the ubiquity of anti-de Sitter vacua therein, featuring an evolving DE component with positive energy density on top of a negative cosmological constant, argued in an earlier exploratory analysis to potentially be able to explain the JWST observations. We perform a robust comparison of this model against JWST data, considering both photometric observations from the CEERS program, and spectroscopic observations from the FRESCO survey. We show that the model is able to accommodate the JWST observations, with a consistency probability of up to $98\%$, even in the presence of an evolving component with a quintessence-like behaviour (easier to accommodate theoretically compared to phantom DE), while remaining consistent with standard low-redshift probes. Our results showcase the tremendous potential of measurements of high-redshift galaxy abundances in tests of fundamental physics, and their valuable complementarity with standard cosmological probes.

astro-ph.CO↗

arXiv2024

Exploring Alternative Cosmologies with the LSST: Simulated Forecasts and Current Observational Constraints

In recent years, the Lambda Cold Dark Matter (LCDM) model, which has been pivotal in cosmological studies, has faced significant challenges due to emerging observational and theoretical inconsistencies. This paper explores alternative cosmological models to address these discrepancies, using simulated three years photometric Supernovae Ia data from the Legacy Survey of Space and Time (LSST), supplemented with additional Pantheon+, Union, and the recently released Dark Energy Survey 5 Years (DESY5) supernova compilations and Baryon Acoustic Oscillation (BAO) measurements. We assess the constraining power of these datasets on various dynamic dark energy models, including CPL, BA, JBP, SCPL, and GCG. Our analysis demonstrates that the LSST with its high precision data, can provide tighter constraints on dark energy parameters compared to other datasets. Additionally, the inclusion of BAO measurements significantly improves parameter constraints across all models. Except for Pantheon+, we find that across all the cosmological datasets, and the dark energy models considered in this work, there is a consistent deviation from the LCDM model that exceeds a 2-sigma significance level. Our findings underscore the necessity of exploring dynamic dark energy models, which offer more consistent frameworks with fundamental physics and observational data, potentially resolving tensions within the LCDM paradigm. Furthermore, the use of simulated LSST data highlights the survey's potential in offering significant advantages for exploring alternative cosmologies, suggesting that future LSST observations would play a crucial role.

astro-ph.CO↗

arXiv2024

Omnipotent dark energy: A phenomenological answer to the Hubble tension

This paper introduces the class of omnipotent dark energy (DE) models characterized by nonmonotonic energy densities that are capable of attaining negative values with corresponding equation of state parameters featuring phantom divide line (PDL) crossings and singularities. These nontrivial features are phenomenologically motivated by findings of previous studies that reconstruct cosmological functions from observations, and the success of extensions of $Λ$CDM, whose actual or effective DE density is omnipotent, in alleviating the observational discordance within $Λ$CDM. As an example, we focus on one embodiment of omnipotent DE, viz., the DE parametrization introduced in Di Valentino et al. [Dark energy with phantom crossing and the H0 tension, Entropy 23, 404 (2021)] (DMS20). By updating and extending the datasets used in the original paper where it was introduced, we confirm the effectiveness of DMS20 in alleviating the observational discrepancies. Additionally, we uncover that its negative DE density feature, importance of which was not previously investigated, plays a crucial role in alleviating the tensions, along with the PDL crossing feature that the parametrization presupposes. In particular, we find that there is a positive correlation between the $H_0$ parameter and the scale ($a_p$) at which DE density transitions from negative to positive, in agreement with previous studies that incorporate this transition feature. For our full dataset, the model yields $H_0=70.05 \pm 0.64$ (68% CL) relaxing the $H_0$ tension with a preference of crossing to negative DE densities ($a_p>0$ at 99% CL), along with the constraint $a_m=0.922^{+0.041}_{-0.035}$ on the scale of the presupposed PDL crossing.

astro-ph.CO↗

arXiv2024

Coupled Multi Scalar Field Dark Energy

The main aim of this paper is to present the multi scalar field components as candidates to be the dark energy of the universe and their observational constraints. We start with the canonical Quintessence and Phantom fields with quadratic potentials and show that a more complex model should bear in mind to satisfy current cosmological observations. Then we present some implications for a combination of two fields, named as Quintom models. We consider two types of models, one as the sum of the quintessence and phantom potentials and other including an interacting term between fields. We find that adding one extra degree of freedom, by the interacting term, the dynamics enriches considerably and could lead to an improvement in the fit of $-2\lnΔ\Like_{\rm max}= 5.19$, compared to $Λ$CDM. The resultant effective equation of state is now able to cross the phantom divide line, and in several cases present an oscillatory or discontinuous behavior, depending on the interaction value. The parameter constraints of the scalar field models (quintessence, phantom, quintom and interacting quintom) were performed using Cosmic Chronometers, Supernovae Ia and Baryon Acoustic Oscillations data; and the Log-Bayes factors were computed to compare the performance of the models. We show that single scalar fields may face serious troubles and hence the necessity of a more complex models, i.e. multiple fields.

astro-ph.CO↗

arXiv2023

Late-time phenomenology required to solve the $H_0$ tension in view of the cosmic ladders and the anisotropic and angular BAO data sets

The $\sim 5σ$ mismatch between the value of the Hubble parameter measured by SH0ES and the one inferred from the inverse distance ladder (IDL) constitutes the biggest tension afflicting the standard model of cosmology, which could be pointing to the need of physics beyond $Λ$CDM. In this paper we study the background history required to solve the $H_0$ tension if we consider standard prerecombination physics, paying special attention to the role played by the data on baryon acoustic oscillations (BAO) employed to build the IDL. We show that the anisotropic BAO data favor an ultra-late-time (phantom-like) enhancement of $H(z)$ at $z\lesssim 0.2$, accompanied by a transition in the absolute magnitude of supernovae of Type Ia $M(z)$ in the same redshift range. This agrees with previous findings in the literature. The effective dark energy (DE) density must be smaller than in the standard model at higher redshifts. Instead, when angular BAO data (claimed to be less subject to model dependencies) is employed in the analysis, we find that the increase of $H(z)$ starts at much higher redshifts, typically in the range $z\sim 0.5-0.8$. In this case, $M(z)$ could experience also a transition (although much smoother) and the effective DE density becomes negative at $z\gtrsim 2$. Both scenarios require a violation of the weak energy condition (WEC), but leave an imprint on completely different redshift ranges and might also have a different impact on the perturbed observables. They allow for the effective crossing of the phantom divide. Finally, we employ two alternative methods to show that current data from cosmic chronometers do not exclude the violation of the WEC, but do not add any strong evidence in its favor neither. Our work puts the accent on the utmost importance of the choice of the BAO data set in the study of the possible solutions to the $H_0$ tension.

astro-ph.CO↗

arXiv2023

Post-reionization HI 21-cm signal: A probe of negative cosmological constant

In this study, we investigate a cosmological model involving a negative cosmological constant (AdS vacua in the dark energy sector). We consider a quintessence field on top of a negative cosmological constant and study its impact on cosmological evolution and structure formation. We use the power spectrum of the redshifted HI 21 cm brightness temperature maps from the post-reionization epoch as a cosmological probe. The signature of baryon acoustic oscillations (BAO) on the multipoles of the power spectrum is used to extract measurements of the angular diameter distance $D_A(z)$ and the Hubble parameter $H(z)$. The projected errors on these are then subsequently employed to forecast the constraints on the model parameters ($Ω_Λ, w_0, w_a$) using Markov Chain Monte Carlo techniques. We find that a negative cosmological constant with a phantom dark energy equation of state (EoS) and a higher value of $H_0$ is viable from BAO distance measurements data derived from galaxy samples. We also find that BAO imprints on the 21cm power spectrum obtained from a futuristic SKA-mid like experiment yield a $1-σ$ error on a negative cosmological constant and the quintessence dark energy EoS parameters to be $Ω_Λ=-1.030^{0.589}_{-1.712}$ and $w_0=-1.023^{0.043}_{-0.060}$, $w_a=-0.141^{0.478}_{-0.409}$ respectively.

astro-ph.CO↗

arXiv2023

$S_8$ increases with effective redshift in $Λ$CDM cosmology

Hubble constant $H_0$ and weighted amplitude of matter fluctuations $S_8$ determinations are biased to higher and lower values, respectively, in the late Universe with respect to early Universe values inferred by the Planck collaboration within flat $Λ$CDM cosmology. If these anomalies are physical, i.e. not due to systematics, they naively suggest that $H_0$ decreases and $S_8$ increases with effective redshift. Here, subjecting matter density today $Ω_{m}$ to a prior, corresponding to a combination of Planck CMB and BAO data, we perform a consistency test of the Planck-$Λ$CDM cosmology and show that $S_8$ determinations from $f σ_8(z)$ constraints increase with effective redshift. Due to the redshift evolution, a $\sim 3 σ$ tension in the $S_8$ parameter with Planck at lower redshifts remarkably becomes consistent with Planck within $1 σ$ at high redshifts. This provides corroborating support for an $S_8$ discrepancy that is physical in origin. We further confirm that the flat $Λ$CDM model is preferred over a theoretically ad hoc model with a jump in $S_8$ at a given redshift. In the absence of the CMB+BAO $Ω_m$ prior, we find that $> 3 σ$ tensions with Planck in low redshift data are ameliorated by shifts in the parameters in high redshift data. Results here and elsewhere suggest that the $Λ$CDM cosmological parameters are redshift dependent. Fitting parameters that evolve with redshift is a recognisable hallmark of model breakdown.

astro-ph.CO↗

arXiv2023

Dark energy in light of the early JWST observations: case for a negative cosmological constant?

Early data from the James Webb Space Telescope (JWST) has uncovered the existence of a surprisingly abundant population of very massive galaxies at extremely high redshift, which are hard to accommodate within the standard $Λ$CDM cosmology. We explore whether the JWST observations may be pointing towards more complex dynamics in the dark energy (DE) sector. Motivated by the ubiquity of anti-de Sitter vacua in string theory, we consider a string-inspired scenario where the DE sector consists of a negative cosmological constant (nCC) and a evolving component with positive energy density on top, whose equation of state is allowed to cross the phantom divide. We show that such a scenario can drastically alter the growth of structure compared to $Λ$CDM, and accommodate the otherwise puzzling JWST observations if the dynamical component evolves from the quintessence-like regime in the past to the phantom regime today: in particular, we demonstrate that the presence of a nCC (which requires a higher density for the evolving component) plays a crucial role in enhancing the predicted cumulative comoving stellar mass density. Our work reinforces the enormous potential held by observations of the abundance of high-$z$ galaxies in probing cosmological models and new fundamental physics, including string-inspired ingredients.

astro-ph.CO↗

arXiv2023

21 cm Power Spectrum for Bimetric Gravity and its Detectability with SKA1-Mid Telescope

We consider a modified gravity theory through a special kind of ghost-free bimetric gravity, where one massive spin-2 field interacts with a massless spin-2 field. In this bimetric gravity, the late time cosmic acceleration is achievable. Alongside the background expansion of the Universe, we also study the first-order cosmological perturbations and probe the signature of the bimetric gravity on large cosmological scales. One possible probe is to study the observational signatures of the bimetric gravity through the 21 cm power spectrum. We consider upcoming SKA1-mid antenna telescope specifications to show the prospects of the detectability of the ghost-free bimetric gravity through the 21 cm power spectrum. Depending on the values of the model parameter, there is a possibility to distinguish the ghost-free bimetric gravity from the standard $Λ$CDM model with the upcoming SKA1-mid telescope specifications.

astro-ph.CO↗

arXiv2023

Cosmological Evolution in Bimetric Gravity: Observational Constraints and LSS Signatures

Bimetric gravity is an interesting alternative to standard GR given its potential to provide a concrete theoretical framework for a ghost-free massive gravity theory. Here we investigate a class of Bimetric gravity models for their cosmological implications. We study the background expansion as well as the growth of matter perturbations at linear and second order. We use low-redshift observations from SnIa (Pantheon+ and SH0ES), Baryon Acoustic Oscillations (BAO), the growth ($fσ_{8}$) measurements and the measurement from Megamaser Cosmology Project to constrain the Bimetric model. We find that the Bimetric models are consistent with the present data alongside the $Λ$CDM model. We reconstructed the `` effective dark energy equation of state" ($ω_{de}$) and "Skewness" ($S_{3}$) parameters for the Bimetric model from the observational constraints and show that the current low-redshift data allow significant deviations in $ω_{de}$ and $S_{3}$ parameters with respect to the $Λ$CDM behaviour. We also look at the ISW effect via galaxy-temperature correlations and find that the best fit Bimetric model behaves similarly to $Λ$CDM in this regard.

astro-ph.CO↗

arXiv2022

Do cosmological observations allow a negative $Λ$?

In view of the recent measurement of $H_{0}$ from HST and SH0ES team, we explore the possibility of existence of a negative cosmological constant (AdS vacua in the dark energy sector) in the Universe. In this regard, we consider quintessence fields on top of a negative cosmological constant and compare such construction with $Λ$CDM model using a different combination of CMB, SnIa, BAO and $H_{0}$ data. Various model comparison estimators show that quintessence models with a negative $Λ$ is either preferred over $Λ$CDM or performs equally as $Λ$CDM model. This suggests that the presence of a negative $Λ$ (AdS ground state) in our Universe, which can naturally arise in string theory, is consistent with cosmological observations.

astro-ph.CO↗

arXiv2022

21 cm power spectrum in interacting cubic Galileon model

We show the detectability of interacting and non-interacting cubic Galileon models from the $Λ$CDM model through the 21 cm power spectrum. We show that the interferometric observations like the upcoming SKA1-mid can detect both the interacting and the non-interacting cubic Galileon model from the $Λ$CDM model depending on the parameter values.

astro-ph.CO↗