arXiv subjects

Prabhakar Tiwari

Publications and source records attributed to Prabhakar Tiwari.

At least 19 recordsLinked to original sources

arXiv2026

Constraining primordial non-Gaussianity and energy injection with the thermal Sunyaev-Zeldovich effect and integrated Sachs-Wolfe effect cross-correlation

Constraining primordial non-Gaussianity (PNG) provides key insights into the physics of cosmic inflation and the initial conditions of the Universe, which remain central topics in cosmology. In this study, we use the cross-correlation between the integrated Sachs-Wolfe (ISW) effect and the thermal Sunyaev-Zeldovich (tSZ) effect derived from Ibitoye et al. (2024) to jointly constrain PNG and early-Universe energy injection, including the standard intergalactic medium contribution. For scale-independent PNG we obtain $f_{\rm NL} = -358^{+140}_{-114}$ ($68\%$~C.L.). For a scale-dependent model ($f_{\rm NL}=f_{\rm NL}^{0}(\ell/\ell_{0})^{n_{\rm NL}}$, with $\ell_{0}=200$), we find $f^{0}_{\rm NL} = -296^{+173}_{-157}$ and $n_{\rm NL} = 0.62^{+1.02}_{-0.64}$, both consistent with Gaussian initial conditions. We also constrain the early-Universe energy injection amplitude to be $\alpha_{\rm inj} = -3.93^{+1.34}_{-0.99}$, with uncertainty reduced by a factor of $\sim\!2.6$ if Planck 2018 $f_{\rm NL}$ constraint is applied as a prior. Future surveys such as Simons Observatory and Euclid will tighten these constraints further. Complementary to conventional probes, this work provides the first ISW-tSZ constraint on exotic energy injection and enables precision tests of early-Universe physics while probing late-time gravitational potential and thermal energy perturbations.

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arXiv2026

The Hydrostatic Mass Bias and the $\sigma_8$ Tension: A Multi-Probe Forecast for Stage-IV/V Surveys

The hydrostatic mass bias ($b_{\mathrm{HSE}}$) is a leading systematic uncertainty in cluster cosmology and a principal source of degeneracy with $\sigma_8$ and $\Omega_m$. We investigate the capability of Stage-IV CMB and optical surveys to calibrate $b_{\mathrm{HSE}}$ using tomographic cross-correlations between the thermal Sunyaev--Zel'dovich (tSZ) effect, galaxy clustering, and weak lensing. We perform a Fisher forecast incorporating realistic survey noise, foreground modeling for clustered CIB and radio sources, and full marginalization over cosmological and astrophysical nuisance parameters, including per-bin galaxy bias perturbations, photometric redshift shifts, intrinsic alignments, and baryonic feedback modeled with HMCode2020. With optimized tomographic binning (10 lens and 5 source bins for LSST; 6 lens and 5 source bins for CSST), we forecast marginalized constraints of $0.98\%$ for SO+LSST, $1.60\%$ for CMB-S4+LSST, and $2.40\%$ for CMB-S4+CSST. Tomography improves $b_{\mathrm{HSE}}$ precision by factors of approximately three relative to non-tomographic analyses, reflecting the role of redshift information in breaking the $b_{\mathrm{HSE}}$--$\sigma_8$ degeneracy. Optical-only probes provide no direct constraint on $b_{\mathrm{HSE}}$, whereas inclusion of tSZ-containing spectra enables percent-level calibration under realistic systematic assumptions. The results demonstrate that multi-probe tomographic analyses with Stage-IV surveys can achieve robust control of hydrostatic mass bias, strengthening cluster-based constraints on structure growth.

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arXiv2026

Constraining Quintessence Models with ISW-tSZ Cross-Correlations: A Comparative Analysis of Thawing, Tracker, and Scaling-Freezing Dynamics

We present constraints on quintessence dark energy models using the observational detection of the Integrated Sachs-Wolfe (ISW)--thermal Sunyaev-Zeldovich (tSZ) cross-correlation dataset. Our analysis compares three classes of quintessence dynamics: thawing, tracker, and scaling-freezing with the standard $Λ$CDM cosmology. Through a comprehensive likelihood analysis, we derive best-fit values and 68\% confidence intervals for key cosmological parameters, finding $Ω_{\rm m} = 0.322^{+0.027}_{-0.030}$ and $σ_8 = 0.735^{+0.045}_{-0.035}$ for $Λ$CDM, with deviations in alternative models consistent within $1σ$. For the thawing model, we consider an exponential potential with slope $λ= 0.736^{+0.270}_{-0.227}$, while for the tracker and scaling-freezing models, we use inverse axion-like and double exponential potentials, respectively. Observationally, the tracker model yields $n = 5.651^{+1.625}_{-1.604}$ and $f = 0.258^{+0.149}_{-0.096}$, and the scaling-freezing model gives $λ_1 = 0.405^{+0.293}_{-0.322}$ and $λ_2 = 23.226^{+7.975}_{-7.258}$. The dimensionless tSZ amplitude ($\widetilde{W}^{\rm SZ}$) and cosmic infrared background (CIB) parameters are tightly constrained across all models, providing additional insights into astrophysical foregrounds. Our results demonstrate the effectiveness of ISW--tSZ cross-correlations as a probe of dark energy dynamics, with the Thawing quintessence model yielding the lowest $χ^2_{\rm min}$ among the tested scenarios, and highlight the need for future high-precision measurements to distinguish between quintessence models and $Λ$CDM.

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arXiv2025

Overdispersed radio source counts and excess radio dipole detection

The source count dipole from wide-area radio continuum surveys allows us to test the cosmological standard model. Many radio sources have multiple components, which can cause an overdispersion of the source counts distribution. We account for this effect via a new Bayesian estimator, based on the negative binomial distribution. Combining the two best understood wide-area surveys, NVSS and RACS-low, and the deepest wide-area survey, LoTSS-DR2, we find that the source count dipole exceeds its expected value as the kinematic dipole amplitude from standard cosmology by a factor of $3.67 \pm 0.49$ -- a $5.4σ$ discrepancy.

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arXiv2025

Cosmology from LOFAR Two-metre Sky Survey Data Release 2: Cross-correlations with luminous red galaxies from eBOSS

We cross-correlated galaxies from the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) second data release (DR2) radio source with the extended Baryon Oscillation Spectroscopic Survey (eBOSS) luminous red galaxy (LRG) sample to extract the baryon acoustic oscillation (BAO) signal and constrain the linear clustering bias of radio sources in LoTSS DR2. In the LoTSS DR2 catalogue, employing a flux density limit of $1.5$ mJy at the central LoTSS frequency of 144 MHz and a signal-to-noise ratio (S/N) of $7.5$, additionally considering eBOSS LRGs with redshifts between 0.6 and 1, we measured both the angular LoTSS-eBOSS cross-power spectrum and the angular eBOSS auto-power spectrum. These measurements were performed across various eBOSS redshift tomographic bins with a width of $Δz=0.06$. By marginalising over the broadband shape of the angular power spectra, we searched for a BAO signal in cross-correlation with radio galaxies, and determine the linear clustering bias of LoTSS radio sources for a constant-bias and an evolving-bias model. Using the cross-correlation, we measured the isotropic BAO dilation parameter as $α=1.01\pm 0.11$ at $z_{\rm eff}=0.63$. By combining four redshift slices at $z_{\rm eff}=0.63, 0.69, 0.75$, and $0.81$, we determined a more constrained value of $α= 0.968^{+0.060}_{-0.095}$. For the entire redshift range of $z_{\rm eff}=0.715$, we measured $b_C = 2.64 \pm 0.20$ for the constant-bias model, $b(z)=b_C$, and then $b_D = 1.80 \pm 0.13$ for the evolving-bias model, $b(z) = b_D / D(z)$, with $D(z)$ denoting the growth rate of linear structures. Additionally, we measured the clustering bias for individual redshift bins.

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arXiv2025

Cosmology from LOFAR Two-metre Sky Survey Data Release 2: Counts-in-Cells Statistics

We investigate the statistical distribution of source counts-in-cells in the second data release of the LOFAR Two-Metre Sky Survey (LoTSS-DR2) and we test a computationally cheap method based on the counts-in-cells to estimate the two-point correlation function. We compare three stochastic models for the counts-in-cells which result in a Poisson distribution, a compound Poisson distribution, and a negative binomial distribution. By analysing the variance of counts-in-cells for various cell sizes, we fit the reduced normalised variance to a single power-law model representing the angular two-point correlation function. Our analysis confirms that radio sources are not Poisson distributed, which is most likely due to multiple physical components of radio sources. Employing instead a Cox process, we show that there is strong evidence in favour of the negative binomial distribution above a flux density threshold of 2 mJy. Additionally, the mean number of radio components derived from the negative binomial distribution is in good agreement with corresponding estimates based on the value-added catalogue of LoTSS-DR2. The scaling of the counts-in-cells normalised variance with cell size is in good agreement with a power-law model for the angular two-point correlation. At a flux density threshold of 2 mJy and a signal-to-noise ratio of 7.5 for individual radio sources, we find that for a range of angular scales large enough to not be affected by the multi-component nature of radio sources, the value of the exponent of the power law ranges from -0.8 to -1.05. This closely aligns with findings from previous optical, infrared, and radio surveys of the large scale structure. The scaling of the counts-in-cells statistics with cell size provides a computationally efficient method to estimate the two-point correlation properties, offering a valuable tool for future large-scale structure studies.

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arXiv2024

Impact of Large-Scale Anisotropies on Galaxy Clustering and Cosmological Constraints

We critically assess the impact of significant dipole and large-scale anisotropies on galaxy clustering signals, with a focus on radio continuum surveys. Our study reveals that these anisotropies -- resulting from intrinsic cosmological effects and/or observational systematics -- profoundly influence the two-point correlation function (2PCF) and angular power spectrum ($C_\ell$). Notably, large-scale anisotropies can obscure or simulate non-Gaussianity signals, complicating the extraction of precise cosmological information. The results emphasize that it is crucial to address systematics and rigorously mask the dipole and its surrounding multipoles to obtain accurate cosmological constraints. This approach is essential for extracting cosmological results from clustering signals, particularly for future surveys such as SKA, DESI, and LSST, to ensure the precision and reliability of cosmological analyses.

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arXiv2024

An Independent Measure of the Kinematic Dipole from SDSS

We utilize the Sloan Digital Sky Survey (SDSS) extended Baryon Oscillation Spectroscopic Survey (eBOSS) and Baryon Oscillation Spectroscopic Survey (BOSS) catalogs with precise spectroscopic redshifts to estimate the kinematic redshift dipole caused by the proper motion of the Solar system. We find that the velocity extracted from the kinematic dipole is consistent with Cosmic Microwave Background inferred values. Although the small sky coverage and limited number density of the SDSS sources constrain us from obtaining precise and robust measurements, we leverage the redshift dipole method to estimate the kinematic dipole. The velocity measurements in this study are insensitive to intrinsic clustering, associated with the source count dipole. The kinematic dipole measured in this work and its consistency with CMB values do not guarantee isotropy at large scales. The anisotropy (excess dipole) measured with the NRAO VLA Sky Survey (NVSS) and the WISE Catalog (CatWISE) could be due to the intrinsic distribution of galaxies. The results in this work focus solely on the kinematic dipole term.

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arXiv2024

Flux dependence of redshift distribution and clustering of LOFAR radio sources

In this work we study the flux density dependence of the redshift distribution of low-frequency radio sources observed in the LOFAR Two-metre Sky Survey (LoTSS) deep fields and apply it to estimate the clustering length of the large-scale structure of the Universe, examining flux density limited samples (1 mJy, 2 mJy, 4 mJy and 8 mJy) of LoTSS wide field radio sources. We utilise and combine the posterior probability distributions of photometric redshift determinations for LoTSS deep field observations from three different fields (Boötes, Lockman hole and ELAIS-N1, together about $26$ square degrees of sky), which are available for between $91\%$ to $96\%$ of all sources above the studied flux density thresholds and observed in the area covered by multi-frequency data. We estimate uncertainties by a bootstrap method. We apply the inferred redshift distribution on the LoTSS wide area radio sources from the HETDEX field (LoTSS-DR1; about $424$ square degrees) and make use of the Limber approximation and a power-law model of three dimensional clustering to measure the clustering length, $r_0$, for various models of the evolution of clustering. We find that the redshift distributions from all three LoTSS deep fields agree within expected uncertainties. We show that the radio source population probed by LoTSS at flux densities above $1$ mJy has a median redshift of at least $0.9$. At $2$ mJy, we measure the clustering length of LoTSS radio sources to be $r_0 = (10.1\pm 2.6) \ h^{-1}$Mpc in the context of the comoving clustering model. Our findings are in agreement with measurements at higher flux density thresholds at the same frequency and with measurements at higher frequencies in the context of the comoving clustering model.

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arXiv2023

Probing Cosmology beyond $Λ$CDM using the SKA

The cosmological principle states that the Universe is statistically homogeneous and isotropic at large distance scales. There currently exist many observations which indicate a departure from this principle. It has been shown that many of these observations can be explained by invoking superhorizon cosmological perturbations and may be consistent with the Big Bang paradigm. Remarkably, these modes simultaneously explain the observed Hubble tension, i.e., the discrepancy between the direct and indirect measurements of the Hubble parameter. We propose several tests of the cosmological principle using SKA. In particular, we can reliably extract the signal of dipole anisotropy in the distribution of radio galaxies. The superhorizon perturbations also predict a significant redshift dependence of the dipole signal which can be nicely tested by the study of signals of reionization and the dark ages using SKA. We also propose to study the alignment of radio galaxy axes as well as their integrated polarization vectors over distance scales ranging from a few Mpc to Gpc. We discuss data analysis techniques that can reliably extract these signals from data.

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arXiv2023

The clustering properties of AGNs/quasars in CatWISE2020 catalog

We study the clustering properties of 1,307,530 AGNs/quasars in the CatWISE2020 catalog prepared using the Wide-field Infrared Survey Explorer (WISE) and Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) survey data. For angular moments $\ell \gtrapprox 10$ ($\lessapprox 18^\circ$) down to non-linear scales, the results are in agreement with the standard $Λ$CDM cosmology, with a galaxy bias roughly matching that of the NRAO VLA Sky Survey (NVSS) AGNs. We further explore the redshift dependence of the fraction of infrared bright AGNs on stellar mass, $f_{\rm IB} \sim M_*^{α_0 + α_1 z}$, and find $α_1=1.27^{+0.25}_{-0.30}$, ruling out a non-evolution hypothesis at $\approx 4.6σ$ confidence level. The results are consistent with the measurements obtained with NVSS AGNs, though considerably more precise thanks to the significantly higher number density of objects in CatWISE2020. The excess dipole and high clustering signal above angular scale $\approx 18^\circ$ remain anomalous.

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arXiv2022

A study of Dipolar Signal in distant Quasars with various observables

We study the signal of anisotropy in AGNs/quasars of CatWISE2020 catalogue using different observables. It has been reported earlier that this data shows a strong signal of dipole anisotropy in the source number counts. We test this claim using two independent data analysis procedures and find our number count dipole consistent with the earlier results. In addition to number counts, we test for the anisotropy signal in two other observables -- mean spectral index $\barα$ and mean flux density $\bar{B}$. We find a dipole signal of considerable strength both in the mean spectral index and the mean flux density. The dipole in mean flux density points towards the galactic center and becomes very weak after imposing a flux cut to remove sources with flux greater than 1 mJy. This can be attributed to the presence of some bright sources. The signal in mean spectral index, however, is relatively stable as a function of both flux and galactic cuts. The dipole in this observable points roughly opposite to the galactic center and hence most likely arises due to galactic bias. Hence, the signal in both the mean spectral index and mean flux density appears to be consistent with isotropy.

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arXiv2022

A mechanism to explain Galaxy alignment over a range of scales

The observed large-scale alignment of polarization angles and galaxy axis have been challenging the fundamental assumption of homogeneity and isotropy in standard cosmology since more than two decades. The intergalactic magnetic field, and its correlations in real space, potentially seems as a viable candidate for explaining this phenomenon. It has been shown earlier that the large-scale intergalactic magnetic field correlations can explain the alignment signal of quasars over Gpc scale, interestingly they can also explain the radio polarization alignment observed in JVAS/CLASS data over 100 Mpc. Motivated with recent observations of galaxy axis alignment over several tens of Mpc, and Mpc scale, i.e., the cluster scale, we further explore the correlations of background magnetic field to explain these relatively small scale alignment observations. In particular, we explore two recently claimed signals of alignment in the radio sources in the FIRST catalog and in the ACO clusters. We find that the FIRST alignment signal is well explained in terms of the intergalactic magnetic field with a spectral index of $-2.62\pm 0.03$. Furthermore, the model also partially explains the very small scale alignment (alignment within clusters). Though the elementary model proposed in this work seems to have its limitations at very small scales, the large-scale magnetic field correlations potentially seem to explain the polarization and galaxy axis alignment from Gpc to Mpc scales.

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arXiv2022

Galaxy power spectrum and biasing results from the LOFAR Two-metre Sky Survey (first data release)

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing survey aiming to observe the entire Northern sky, providing an excellent opportunity to study the distribution and evolution of the large-scale structure of the Universe. The source catalogue from the public LoTSS first data release (DR1) covers 1% of the sky, and shows correlated noise or fluctuations of the flux density calibration on few degree scales. We explore the LoTSS DR1 to understand the survey systematics and data quality of this first data release. We produce catalog mocks to estimate uncertainties, and measure the angular clustering statistics of LoTSS galaxies, which fit the $Λ$CDM cosmology reasonably well. We employ a Markov chain Monte Carlo (MCMC) based Bayesian analysis to recover the best galaxy biasing scheme and multi-component source fraction for LoTSS DR1 above $1$ mJy assuming different possible redshift templates. After masking some noisy and uneven patches and with suitable flux density cuts, the LOFAR survey appears qualified for large-scale cosmological studies. The upcoming data releases from LOFAR are expected to be deeper and wider, and will therefore provide improved cosmological measurements.

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arXiv2021

Superhorizon perturbations: A possible explanation of the Hubble--Lemaître Tension and the Large Scale Anisotropy of the Universe

Current cosmological observations point to a serious discrepancy between the observed Hubble parameter obtained using direct and cosmic microwave background radiation (CMBR) measurements. Besides this, the so called Hubble--Lemaître tension, we also find considerable evidence in diverse cosmological observables that indicate violation of the cosmological principle. In this paper, we suggest that both these discrepancies are related and can be explained by invoking superhorizon perturbations in the Universe. We implement this by considering a single superhorizon mode and showing that it leads to both a dipole in large scale structures and a shift in the Hubble--Lemaître parameter. Furthermore, the shift is found to be independent of redshift up to a certain distance. This is nicely consistent with the data.

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arXiv2021

The Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: measurement of the BAO and growth rate of structure of the emission line galaxy sample from the anisotropic power spectrum between redshift 0.6 and 1.1

We analyse the large-scale clustering in Fourier space of emission line galaxies (ELG) from the Data Release 16 of the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey. The ELG sample contains 173,736 galaxies covering 1,170 square degrees in the redshift range $0.6 < z < 1.1$. We perform a BAO measurement from the post-reconstruction power spectrum monopole, and study redshift space distortions (RSD) in the first three even multipoles. Photometric variations yield fluctuations of both the angular and radial survey selection functions. Those are directly inferred from data, imposing integral constraints which we model consistently. The full data set has only a weak preference for a BAO feature ($1.4σ$). At the effective redshift $z_{\rm eff} = 0.845$ we measure $D_{\rm V}(z_{\rm eff})/r_{\rm drag} = 18.33_{-0.62}^{+0.57}$, with $D_{\rm V}$ the volume-averaged distance and $r_{\rm drag}$ the comoving sound horizon at the drag epoch. In combination with the RSD measurement, at $z_{\rm eff} = 0.85$ we find $fσ_8(z_{\rm eff}) = 0.289_{-0.096}^{+0.085}$, with $f$ the growth rate of structure and $σ_8$ the normalisation of the linear power spectrum, $D_{\rm H}(z_{\rm eff})/r_{\rm drag} = 20.0_{-2.2}^{+2.4}$ and $D_{\rm M}(z_{\rm eff})/r_{\rm drag} = 19.17 \pm 0.99$ with $D_{\rm H}$ and $D_{\rm M}$ the Hubble and comoving angular distances, respectively. These results are in agreement with those obtained in configuration space, thus allowing a consensus measurement of $fσ_8(z_{\rm eff}) = 0.315 \pm 0.095$, $D_{\rm H}(z_{\rm eff})/r_{\rm drag} = 19.6_{-2.1}^{+2.2}$ and $D_{\rm M}(z_{\rm eff})/r_{\rm drag} = 19.5 \pm 1.0$. This measurement is consistent with a flat $Λ$CDM model with Planck parameters.

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arXiv2019

The galaxy power spectrum from TGSS ADR1 and the effect of flux calibration systematics

We explore the large to moderate scale anisotropy in distant radio sources using the TGSS ADR1 catalog. We use different measures, i.e. number counts, sky brightness and flux per source, for this study. In agreement with earlier results, we report a significant excess of clustering signal above the angular scale of roughly $10$ degrees (i.e. $l\lessapprox 20-30$). We find that some survey areas have a systematically low/high flux and argue this may be the cause of the observed signal of excess power at low multipoles. With mocks we demonstrate the effect of such large scale flux systematics and recover TGSS like excess clustering signal by assuming $20\%$ flux uncertainties over $\sim 10^\circ \times 10^\circ$ size patches. We argue that that TGSS at this stage, i.e. TGSS ADR1, is not suitable for large scale clustering measurements. We find that the measure, flux per source, shows evidence of isotropy for all multipoles $l > 2$ despite the presence of systematics in the data.

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arXiv2019

Radio spectral index from NVSS and TGSS

I extract the radio spectral index, $α$, from 541,195 common sources observed in the 150 MHz TIFR GMRT Sky Survey (TGSS) and the 1.4 GHz NRAO VLA Sky Survey (NVSS). This large common source catalogue covers about $80\%$ of the sky. The flux density limits in these surveys are such that the observed galaxies are presumably hosts of active galactic nuclei (AGNs). I confirm the steepening of $α$ with increasing flux density for this large sample and provide a parametric fit between $α$ and flux density. Next, I divide the data into a low flux (LF) and a high flux (HF) density sample of roughly equal number of galaxies. The LF sample contains all galaxies below 100 mJy TGSS and 20 mJy NVSS flux density and the HF sample is all galaxies above 100 mJy TGSS and 20 mJy NVSS. I observe an increase in $α$ with source size (TGSS measured), saturating for large sizes to $0.89\pm0.22$ and $0.76\pm 0.21$ for the LF and HF sources, respectively. I discuss the observed results and possible physical mechanisms to explain observed $α$ dependence with source size for LF and HF samples.

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