arXiv subjects

Bikash R. Dinda

Publications and source records attributed to Bikash R. Dinda.

At least 19 recordsLinked to original sources

arXiv2026

Beyond ΛCDM with the SKA Observatory -- II: Unveiling the Secrets of the Early Universe

The origins of the universe remain one of the biggest mysteries in modern cosmology. While the Planck satellite has provided a wealth of information about the early universe, there is still much to be discovered. The Square Kilometre Array Observatory (SKAO) offers a unique opportunity to probe the universe's infancy, going beyond the current limitations of our knowledge. By measuring the power spectrum of biased tracers of the dark matter distribution on the largest cosmological scales and exploring beyond 2-point statistics, SKAO will enable us to refine our understanding of the primordial universe, including the shape of the inflationary power spectrum and the presence of primordial non-Gaussianity. In this chapter we will review recent works looking at the potential of SKAO's surveys, and how synergies with other surveys can revolutionize our understanding of the origins of the cosmos.

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arXiv2026

Impact of lensing magnification on the power spectrum turnover

The turnover scale $k_0$ of the matter power spectrum -- and consequently of the standard galaxy power spectrum monopole -- encodes a fundamental signature of matter-radiation equality and constitutes an important standard ruler independent of baryon acoustic oscillations. In principle, we can detect the turnover at multiple redshifts and amplify the signal by stacking redshift bins. However, in spectroscopic surveys reaching high redshifts, such as the Euclid H$α$ survey and the proposed MegaMapper Lyman-break galaxy survey, the monopole of the observed galaxy power spectrum receives a scale-dependent correction from lensing magnification. This can modify the signal shape and shift the turnover scale, undermining its use as a standard ruler. Using mock surveys similar to Euclid and MegaMapper, we forecast this shift and the consequent bias in the turnover scale that is recovered from the mock data. The shift in the turnover scale grows with redshift, leading to a maximum bias of $\sim 0.4σ$ (Euclid-like) and $\sim 3.6σ$ (MegaMapper-like). To avoid a bias $>1σ$, the maximum redshift for a MegaMapper-like survey is $z\approx 2.9$. Data in the remaining range $2.9\lesssim z\le 5$ does not directly provide a reliable recovery of the intrinsic turnover. In fact, we find that the turnover vanishes in a MegaMapper-like survey for $z\gtrsim 3.7$. Our results show that the lensing correction to the monopole should be included and carefully modelled when surveys are used to measure the cosmological turnover at high redshifts.

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arXiv2026

No evidence for phantom crossing: local goodness-of-fit improvements do not persist under global Bayesian model comparison

Recent cosmological data have been interpreted as indicating deviations from $Λ$CDM within the standard $w_0w_a$ parametrization, including hints of phantom crossing and dynamical dark energy. However, such inferences can be parametrization-dependent and need not imply a statistically robust detection. We test these claims by comparing $Λ$CDM, $w_0w_a$, and thawing quintessence models, using the Deviance Information Criterion (DIC) and the Bayesian evidence $\ln \mathcal{Z}$. We find that $w_0w_a$ can provide a slightly improved local fit; however, this improvement is confined to a limited region of the parameter space. The global Bayesian evidence does not support it once the full prior volume is accounted for. In particular, cases with $Δ{\rm DIC}<0$ but $Δ\ln \mathcal{Z}<0$ indicate that these improvements are not statistically significant. We show that all models are statistically indistinguishable, and that there is no statistically consistent evidence across different datasets for either dynamical dark energy or phantom crossing.

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arXiv2026

Calibration-independent consistency test of BAO and SNIa data: update

In a recent paper, arXiv:2509.19899, we presented a new method to test the consistency between uncalibrated BAO and SNIa data through a common parameter, the Alcock-Paczynski variable. Using Gaussian Processes, we can determine if various datasets are consistent, independently of dark energy or modified gravity models, and of the sound horizon and SNIa peak magnitude. We found that the DES-Y5 SNIa data showed non-negligible tension with other datasets. However, the recent update DES-Dovekie removes this tension. We find that all uncalibrated data from DESI DR2 BAO and three SNIa datasets, Union3, Pantheon+, and DES-Dovekie, are consistent with each other within $\sim 1σ$.

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arXiv2026

Turnover detection using the power spectrum and bispectrum

The turnover at the peak of the Fourier matter power spectrum encodes a fundamental signature of matter-radiation equality in the early Universe. This delivers a potential standard ruler, independent of baryon acoustic oscillations and therefore able to break parameter degeneracies and improve precision. Furthermore, the turnover scale is independent of redshift and clustering bias, allowing for stacking of the signals from redshift bins. In practice, the very large scale of the turnover means that sample variance and systematics are serious impediments to its detection. Detections of the turnover and measurements of its scale have been made in the WiggleZ, eBOSS, Quaia, and DESI surveys. Upcoming surveys should improve the detection significance and reduce errors on the turnover scale. We use MCMC forecasts for turnover detection in a spectroscopic Euclid-like survey and a futuristic MegaMapper-like survey. In addition to the power spectrum, we include the signal from the bispectrum in equilateral configurations. These surveys are forecast to detect the turnover at $\sim\! 6σ$ (Euclid-like) and $\sim\! 15σ$ (MegaMapper-like), with precision on the turnover scale of $\sim\! 4\%$ and $\sim\! 2\%$. The inclusion of the bispectrum delivers a modest improvement of $\sim\! 10-17\%$ in the constraints on the turnover scale.

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arXiv2026

Phantom crossing or dark interaction?

Recent results from DESI BAO measurements, together with Planck CMB and Pantheon+ data, suggest that there may be a `phantom' phase ($w_{\rm de}<-1$) in the expansion of the Universe. This inference follows when the $w_0, w_a$ parametrization for the dark energy equation of state $w_{\rm de}$ is used to fit the data. Since phantom dark energy in general relativity is unphysical, we investigate the possibility that the phantom behaviour is not intrinsic, but effective -- due to a non-gravitational interaction between dark matter and non-phantom dark energy. To this end, we assume a physically motivated thawing quintessence-like form of the intrinsic dark energy equation of state $w_{\rm de}$. Then we use a $w_0, w_a$ model for the \emph{effective} equation of state of dark energy. We find that the data favours a phantom crossing for the effective dark energy, but only at low significance. The intrinsic equation of state of dark energy is non-phantom, without imposing any non-phantom priors. A nonzero interaction is favoured at more than $3σ$ at $z\sim0.3$. The energy flows from dark matter to dark energy at early times and reverses at later times.

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arXiv2025

Calibration-independent consistency test of DESI DR2 BAO and SNIa

We investigate the consistency between DESI DR2 BAO and three SNIa datasets, Pantheon+, Union3, and DES-Y5. Our consistency test is {calibration}-independent since it is independent of cosmological nuisance parameters such as the absolute peak magnitude $M_B$ and the comoving sound horizon at the baryon drag epoch $r_d$. Importantly, the test is also model-agnostic, independent of any model of dark energy or modified gravity. We define a tension parameter to quantify tension across different datasets compared to DESI DR2 BAO. The Pantheon+ and Union3 data have tension $\lesssim\! 1σ$ across their redshift ranges, whereas the DES-Y5 tension is $\gtrsim3σ$ near $z=1$. This hints that DES-Y5 data has significant offset values for redshifts close to 1, compared to the other SNIa datasets. Since this consistency test is independent of cosmological nuisance parameters, the tension is minimal: other consistency tests involving differences in nuisance parameters may show greater tension.

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arXiv2025

Improved null tests of $Λ$CDM and FLRW in light of DESI DR2

The DESI DR2 BAO data, in combination with CMB and different SNIa datasets, exclude the flat $Λ$CDM model at more than 2.5$σ$ when analyzed through the $w_0w_a$CDM parametrization for evolving dark energy. This simple parametrization may not accurately capture the behavior of the entire redshift range at late times, which may introduce bias in the results. We use null tests that probe for deviations from flat $Λ$CDM at late times, independent of any specific dark energy parametrization. We provide several diagnostics for null tests and discuss their advantages and disadvantages. In particular, we derive diagnostics that improve on previous ones, such as the popular $O_{\rm m}$ diagnostic. The diagnostics are derived from both background and perturbed quantities. Using the combination of DESI DR2 BAO and supernova data, with or without CMB data, we find that deviations from flat $Λ$CDM are at $\sim1σ$ confidence level in most of the redshift range (more than 1$σ$ for a few small redshift intervals in a few cases). When considering SDSS BAO data instead of DESI BAO data, in combination with PantheonPlus, with or without CMB data, we find even smaller deviations. Since spatial curvature can potentially modify the results, we also test for curvature in the general $Λ$CDM model and the general FLRW model. While there is slight evidence for nonzero cosmic curvature at lower redshifts in a general $Λ$CDM model, there is no statistically significant evidence in a general FLRW model.

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arXiv2025

Physical vs phantom dark energy after DESI: thawing quintessence in a curved background

Recent data from DESI, in combination with other data, provide moderate evidence of dynamical dark energy, $w\neq-1$. In the $w_0, w_a$ parametrization of $w$, there is a preference for a phantom crossing, $w<-1$, at redshift $z\sim0.5$. In general relativity, the phantom equation of state is unphysical. Thus it is important to check whether phantom crossing is present in other physically self-consistent models of dark energy that have equivalent evidence to the $w_0, w_a$ parametrization. We find that thawing quintessence with nonzero cosmic curvature can fit the recent data as well as $w_0, w_a$ in a flat background, based on both parametric and realistic scalar field evolutions. Although the realistic model does not allow $w<-1$, the parametrizations do allow it. However even if we allow $w<-1$ the data do not enforce phantom crossing. Thus, the phantom crossing is an artifact of a parametrization that is not based on a physical model.

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arXiv2025

Cosmic curvature on large-scale structures with homogeneous dark energy

This study explores the impact of cosmic curvature on structure formation through general relativistic first-order perturbation theory. We analyze continuity and Euler equations, incorporating cosmic curvature into Einstein equations. Emphasizing late-time dynamics, we investigate matter density contrast evolution in the presence of cosmic curvature, with a specific focus on sub-hubble scales. Solving the evolution equation, we conduct data analysis using cosmic chronometers, baryon acoustic oscillations from Dark Energy Spectroscopic Instrument Data Release 2 (DESI DR2) (including $r_d$ constraint from CMB), type Ia supernova observations from Pantheon+ sample, logarithmic growth rate $f$ data and $fσ_8$ data. While constraints on some parameters remain consistent, inclusion of cosmic curvature losens constraints on $Ω_{\rm m0}$ and $σ_{\rm 8,0}$ in $Λ$CDM and $w_0$CDM models. The non-phantom behavior of dark energy proves more favorable in $w_0$CDM model, whereas in CPL model, there is certain evidence for non-phantom behaviour at lower redshift and phantom behaviour at higher redshift. Interestingly, we find certain evidence for non-zero cosmic curvature in $Λ$CDM model, but no evidence for it in both $w_0$CDM and CPL models, which indirectly proves the non-degeneracy between dynamical dark energy and cosmic curvature.

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arXiv2025

A new diagnostic for the null test of dynamical dark energy in light of DESI 2024 and other BAO data

We introduce a new diagnostic for the null tests of dynamical dark energy alongside two other combined equivalent diagnostics. These diagnostics are useful, especially when we include anisotropic baryon acoustic oscillation (BAO) data in an analysis, to quantify the deviations from the standard $Λ$CDM model. We also consider another diagnostic for isotropic BAO observations. These null tests are independent of any late-time dark energy model or parametrization. With these diagnostics, we study the evidence for dynamical dark energy in light of Dark Energy Spectroscopic Instrument (DESI) 2024 data combined with cosmic microwave background (CMB) observations of the Planck 2018 mission and local $H_0$ measurements. We find no strong evidence for dynamical dark energy. The exclusion of the individual deviations at the effective redshift 0.51 of the DESI 2024 data makes the evidence even weaker. We get nearly similar results for other non-DESI BAO data. Both for DESI 2024 and other non-DESI BAO data, the evidence is almost independent of early-time physics. The evidence corresponding to the SHOES value of $H_0$ is higher than the corresponding tRGB value of $H_0$ for all combinations of data, but still not strong enough to reject the flat $Λ$CDM model.

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arXiv2025

Model-agnostic assessment of dark energy after DESI DR1 BAO

Baryon acoustic oscillation measurements by the Dark Energy Spectroscopic Instrument (Data Release 1) have revealed exciting results that show evidence for dynamical dark energy at $\sim3σ$ when combined with cosmic microwave background and type Ia supernova observations. These measurements are based on the $w_0w_a$CDM model of dark energy. The evidence is less in other dark energy models such as the $w$CDM model. In order to avoid imposing a dark energy model, we reconstruct the distance measures and the equation of the state of dark energy independent of any dark energy model and driven only by observational data. Our results show that the model-agnostic (in terms of late-time models) evidence for dynamical dark energy from DESI is not significant. Our analysis also provides model-independent constraints on cosmological parameters such as the Hubble constant and the matter-energy density parameter at present. Although we used CMB distance priors (not full CMB data) from a $Λ$CDM early-time model, our results remain largely similar for other cosmological models, provided that these models do not differ significantly from the standard model.

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arXiv2024

A comprehensive data-driven odyssey to explore the equation of state of dark energy

For the first time, we reconstruct the dark energy equation of the state parameter $w$ from the combination of background and perturbation observations, specifically combining the Hubble parameter data from cosmic chronometer observations and the logarithmic growth rate data from the growth rate observations. We do this analysis using posterior Gaussian process regression without considering any specific cosmological model or parametrization. However there are three main assumptions: (I) a flat Friedmann-Lemaître-Robertson-Walker (FLRW) metric is considered for the cosmological background, (II) there is no interaction between dark energy and matter sectors, and (III) for the growth of inhomogeneity, sub-Hubble approximation and linear perturbations are considered. This study is unique in the sense that the reconstruction of $w$ is independent of any derived parameters such as the present values of the matter-energy density parameter and Hubble parameter. From the reconstruction, we look at how the dark energy equation of state evolves between redshifts 0 and 1.5, finding a slight hint of dynamical behavior in dark energy. However, the evidence is not significant. We also find a leaning towards non-phantom behavior over phantom behavior. We observe that the $Λ$CDM model ($w=-1$) nearly touches the lower boundary of the 1$σ$ confidence region in the redshift range $0.6 \lesssim z \lesssim 0.85$. However, it comfortably resides within the 2$σ$ confidence region in the whole redshift range under investigation, $0\leq z \leq 1.5$. Consequently, the non-parametric, model-independent reconstruction of dark energy provides no compelling evidence to deviate from the $Λ$CDM model when considering cosmic chronometer and growth rate observations.

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arXiv2024

Analytical Gaussian Process Cosmography: Unveiling Insights into Matter-Energy Density Parameter at Present

In this study, we introduce a novel analytical Gaussian Process (GP) cosmography methodology, leveraging the differentiable properties of GPs to derive key cosmological quantities analytically. Our approach combines cosmic chronometer (CC) Hubble parameter data with growth rate (f) observations to constrain the $Ω_{\rm m0}$ parameter, offering insights into the underlying dynamics of the Universe. By formulating a consistency relation independent of specific cosmological models, we analyze under a flat FLRW metric and first-order Newtonian perturbation theory framework. Our analytical approach simplifies the process of Gaussian Process regression (GPR), providing a more efficient means of handling large datasets while offering deeper interpretability of results. We demonstrate the effectiveness of our methodology by deriving precise constraints on $Ω_{\rm m0}h^2$, revealing $Ω_{\rm m0}h^2=0.139\pm0.017$. Moreover, leveraging $H_0$ observations, we further constrain $Ω_{\rm m0}$, uncovering an inverse correlation between mean $H_0$ and $Ω_{\rm m0}$. Our investigation offers a proof of concept for analytical GP cosmography, highlighting the advantages of analytical methods in cosmological parameter estimation.

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arXiv2024

Constraints on the speed of sound in the k-essence model of dark energy

We consider a particular k-essence scalar field model for the late-time cosmic acceleration in which the sound speed, parametrized as $c_s$ is constant. We compute the relevant background and perturbation quantities corresponding to the observables like cosmic microwave background, type Ia supernova, cosmic chronometers, baryon acoustic oscillations, and the $fσ_8$. We put constraints on the $c_s^2$ parameter from these observations along with other parameters. We find lower values of $c_s^2$ which are close to zero are tightly constrained. Particularly, we find mean value of $\log_{\rm 10} (c_s^2)$ to be $-0.61$ and $c_s^2 \leq 10^{-3}$ is more than 3$σ$ away from this mean value. This means these observations favor a homogeneous dark energy component compared to the clustering one.

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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.

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arXiv2023

Minimal model dependent constraints on cosmological nuisance parameters and cosmic curvature from combinations of cosmological data

The study of cosmic expansion history and the late time cosmic acceleration from observational data depends on the nuisance parameters associated with the data. For example, the absolute peak magnitude of type Ia supernova associated with the type Ia supernova observations and the comoving sound horizon at the baryon drag epoch associated with baryon acoustic oscillation observations are two nuisance parameters. The nuisance parameters associated with the the gamma-ray bursts data are also considered. These nuisance parameters are constrained by combining the cosmological observations using the Gaussian process regression method with minimal model dependence. The bounds obtained in this method can be used as the prior for the data analysis while considering the observational data accordingly. Along with these nuisance parameters, the cosmic curvature density parameter is also constrained simultaneously. We find that the constraints on the cosmic curvature density parameter show no significant deviations from a flat Universe.

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arXiv2023

Constraints on cosmic curvature from cosmic chronometer and quasar observations

We consider cosmic chronometer (CC) data for the Hubble parameter, quasar (QSO) luminosities data of X-rays and ultraviolet rays emission, and the latest measurements of the present value of the Hubble parameter from 2018 Planck mission (PL18), and SH0ES observations (SHOES) to constrain the present value of cosmic curvature density parameter. We consider three kinds of dark energy models: the $Λ$CDM model, the wCDM model, and the CPL parametrization. In all these three models, we find higher values of the matter-energy density parameter, $Ω_{\rm m0}$ compared to the one obtained from the Planck 2018 mission of CMB observation. Also, we find evidence for a nonflat and closed Universe at 0.5$σ$ to 3$σ$ confidence levels. The flat Universe is almost 2 to 3$σ$, 1 to 1.5$σ$, and 0.5 to 1$σ$ away from the corresponding mean values, obtained in $Λ$CDM model, wCDM model, and CPL parametrization respectively obtained from different combinations of datasets. The evidence for nonzero cosmic curvature is lesser in dynamical dark energy models compared to the $Λ$CDM model. That means the evidence of nonzero cosmic curvature depends on the behavior of the equation of state of the dark energy. Since the values of the cosmic curvature are degenerate to the equation of state of the dark energy, we also consider a model independent analysis to constrain the cosmic curvature using the combination of Gaussian process regression analysis and artificial neural networks analysis. In the model independent analysis, we also find evidence for a closed Universe, and the flat Universe is almost 1$σ$ away. So, both the model dependent and independent analyses favor a closed Universe from the combinations of CC, QSO, and $H_0$ observations.

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