arXiv subjects

Saroj Adhikari

Publications and source records attributed to Saroj Adhikari.

11 recordsLinked to original sources

arXiv2022

The Hubble tension in the non-flat Super-$Λ{\rm CDM}$ model

We investigate the Hubble tension in the non-flat Super-$Λ{\rm CDM}$ model. The non-flat Super-$Λ{\rm CDM}$ model extends the Super-$Λ{\rm CDM}$ model by including the spatial curvature as a free parameter. The Super-$Λ{\rm CDM}$ model extends the standard $Λ{\rm CDM}$ model of cosmology through additional parameters accounting for the possible effect of a trispectrum in the primordial fluctuations. In the cosmic microwave background data, this effect can be parameterized using parameters that change the observed angular power spectrum from the theoretical power spectrum due to a trispectrum that couples long and short wavelength modes. In this work, we perform Markov Chain Monte Carlo (MCMC) data analysis on the recent Planck 2018 temperature and polarization fluctuations data and the local Hubble constant measurements from supernovae data assuming a non-flat Super-$Λ{\rm CDM}$ model. We find that there is a preference for non-zero values of the spatial curvature parameter $Ω_k$ and the Super-$Λ{\rm CDM}$ parameter $A_0$ at a level of $Δχ^2$ improvement of approximately 23.

astro-ph.CO↗

arXiv2020

Super-CMB fluctuations and the Hubble tension

We study the covariance in the angular power spectrum estimates of CMB fluctuations when the primordial fluctuations are non-Gaussian. The non-Gaussian covariance comes from a nonzero connected four-point correlation function -- or the trispectrum in Fourier space -- and can be large when long-wavelength (super-CMB) modes are strongly coupled to short-wavelength modes. The effect of such non-Gaussian covariance can be modeled through additional freedom in the theoretical CMB angular power spectrum and can lead to different inferred values of the standard cosmological parameters relative to those in $Λ$CDM. Taking the collapsed limit of the primordial trispectrum in the quasi-single field inflation model as an example, we study how the six standard $Λ$CDM parameters shift when two additional parameters describing the trispectrum are allowed. The reduced statistical significance of the Hubble tension in the extended model allows us to combine the {\it Planck} temperature data and the type Ia supernovae data from Panstarrs with the distance-ladder measurement of the Hubble constant. This combination of data shows strong evidence for a primordial trispectrum-induced non-Gaussian covariance, with a likelihood improvement of $Δχ^2 \approx -15$ (with two additional parameters) relative to $Λ$CDM.

astro-ph.CO↗

arXiv2018

Covariance of CMB anomalies

Several unexpected features are observed at large angular scales in the CMB temperature anisotropy measurements by both WMAP and Planck. These include the lack of both variance and correlation, alignment of the lowest multipole moments with one another, hemispherical power asymmetry, and an odd-to-even parity excess. In this work, we study the statistics of eight representative large-angle CMB features in order to evaluate their covariance in LCDM. We do so using two sets of simulated CMB temperature maps; an ensemble of 100,000 simple Gaussian simulations, and 1000 Full Focal Plane (FFP) simulations provided by the Planck collaboration. In measuring feature probabilities, we pay particular attention to analysis choices, making sure that we can reproduce previous results in the literature, and explain differences where appropriate. The covariance structure we find is consistent with expectations given that many of the features studied are functions of the angular power spectrum. Notably, we find significant differences in the covariance entries associated with the quadrupole-octopole alignments derived from the Gaussian and FFP simulations. We additionally perform a principal component analysis to quantitatively gauge what combinations of features capture the most information about how simulation measurements vary, and to provide an alternative assessment of the ways in which the real sky is anomalous. The first four principal components explain about 90\% of the simulations' variance, with the first two roughly quantifying the lack of large-angle correlations, and the next two quantifying the phase-dependent anomalies (multipole alignments and power asymmetry). Though the results of this analysis are fairly unsurprising, its comprehensive approach serves to tie together a number of previous results, and will therefore provide context for future studies of large-angle anomalies.

astro-ph.CO↗

arXiv2018

Statistical anisotropies in temperature and polarization fluctuations from a scale-dependent trispectrum

We study statistical anisotropies generated in the observed two-point function of the cosmic microwave background (CMB) fluctuations if the primordial statistics are non-Gaussian. Focusing on the dipole modulations of the anisotropies, we find that the hemispherical power asymmetry observed in the CMB temperature fluctuations can be modeled by a local-type trispectrum with amplitude $τ_{\rm NL}(k_p=0.05~{\rm Mpc}^{-1}) \approx 2 \times 10^4$ and a large red tilt $n\approx -0.68$. We numerically evaluate the non-Gaussian covariance of the modulation estimators for both temperature and E-mode polarization fluctuations and discuss the prospects of constraining the model using Planck satellite data. We then discuss other effects of the scale-dependent trispectrum that could be used to distinguish this scenario from other explanations of the power asymmetry: higher-order modulations of the two-point function and the non-Gaussian angular power spectrum covariance. As an important consequence of the non-Gaussian power spectrum covariance, we discuss how the CMB-inferred spectral index of primordial scalar fluctuations can be significantly biased in the presence of a scale-dependent local-type trispectrum.

astro-ph.CO↗

arXiv2018

A new measure of tension between experiments

Tensions between cosmological measurements by different surveys or probes have always been important --- and are presently much discussed --- as they may lead to evidence of new physics. Several tests have been devised to probe the consistency of datasets given a cosmological model, but they often have undesired features such as dependence on the prior volume, or burdensome requirements such as that of near-Gaussian posterior distributions. We propose a new quantity, defined in a similar way as the Bayesian evidence ratio, in which these undesired properties are absent. We test the quantity on simple models with Gaussian and non-Gaussian likelihoods. We then apply it to data from the Planck satellite: we investigate the consistency of $Λ$CDM model parameters obtained from TT and EE angular power spectrum measurements, as well as the mutual consistency of cosmological parameters obtained from large scale (multipoles, $\ell<1000$) and small scale ($\ell \geq 1000$) portions of each measurement and find no significant discrepancy in the six-dimensional $Λ$CDM parameter space.

astro-ph.CO↗

arXiv2018

The Quest for the Inflationary Spectral Runnings in the Presence of Systematic Errors

Cosmological inflation predicts that the scalar spectral index "runs" with scale. Constraints on the values of the spectral runnings, $α_s\equiv \textrm{d} n_s/\textrm{d}\ln k$ and $β_s\equiv \textrm{d}α_s/\textrm{d}\ln k$, therefore provide a fundamental test of the physics of inflation. Here we study the feasibility of measuring the runnings when information from upcoming large-volume galaxy surveys is used to supplement the information provided by a CMB-S4 experiment, particularly focusing on the effect of including high-$k$, nonlinear scales. Since these measurements will be sensitive to modeling uncertainties for the nonlinear power spectrum, we examine how three different ways of parameterizing those systematics---introducing zero, two, or several hundred nuisance parameters---affect constraints and protect against parameter biases. Considering statistical errors alone, we find that including strongly nonlinear scales can substantially tighten constraints. However, these constraints weaken to levels not much better than those from a CMB-S4 experiment alone when we limit our analysis to scales where estimates are not strongly affected by systematic biases. Given these considerations, near-future large-scale structure surveys are unlikely to add much information to the CMB-S4 measurement of the first running $α_s$. There is more potential for improvement for the second running, $β_s$, for which large-scale structure information will allow constraints to be improved by a factor of 3--4 relative to using the CMB alone. Though these constraints are still above the value predicted by slow roll inflation, they do probe regions of parameter space relevant to nonstandard inflationary models with large runnings, for example those that can generate an appreciable abundance of primordial black holes.

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arXiv2016

Constraining primordial and gravitational mode coupling with the position-dependent bispectrum of the large-scale structure

We develop and study the position-dependent bispectrum. It is a generalization of the recently proposed position-dependent power spectrum method of measuring the squeezed-limit bispectrum. The position-dependent bispectrum can similarly be used to measure the squeezed-limit trispectrum in which one of the wavelengths is much longer than the other three. In this work, we will mainly consider the case in which the three smaller wavelengths are nearly the same (the equilateral configuration). We use the Fisher information matrix to forecast constraints on bias parameters and the amplitude of primordial trispectra from the position-dependent bispectrum method. We find that the method can constrain the local-type $g_{\rm NL}$ at a level of $σ(g_{\rm NL}^{\rm local}) \approx 3 \times 10^5$ for a large volume SPHEREx-like survey; improvements can be expected by including all the triangular configurations of the bispectra rather than just the equilateral configuration. However, the same measurement would also constrain a much larger family of trispectra than local $g_{\rm NL}$ model. We discuss the implications of the forecasted reach of future surveys in terms of super cosmic variance uncertainties from primordial non-Gaussianities.

astro-ph.CO↗

arXiv2016

Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity

We derive a general expression for the probability of observing deviations from statistical isotropy in the cosmic microwave background (CMB) if the primordial fluctuations are non-Gaussian and extend to superhorizon scales. The primary motivation is to properly characterize the monopole and dipole modulations of the primordial power spectrum that are generated by the coupling between superhorizon and subhorizon perturbations. Unlike previous proposals for generating the hemispherical power asymmetry, we do not assume that the power asymmetry results from a single large superhorizon mode. Instead, we extrapolate the observed power spectrum to superhorizon scales and compute the power asymmetry that would result from a specific realization of non-Gaussian perturbations on scales larger than the observable universe. Our study encompasses many of the scenarios that have been put forward as possible explanations for the CMB hemispherical power asymmetry. We confirm our analytic predictions for the probability of a given power asymmetry by comparing them to numerical realizations of CMB maps. We find that non-local models of non-Gaussianity and scale-dependent local non-Gaussianity produce scale-dependent modulations of the power spectrum, thereby potentially producing both a monopolar and a dipolar power modulation on large scales. We then provide simple examples of finding the posterior distributions for the parameters of the bispectrum from the observed monopole and dipole modulations.

astro-ph.CO↗

arXiv2014

Weighing the Giants IV: Cosmology and Neutrino Mass

We employ robust weak gravitational lensing measurements to improve cosmological constraints from measurements of the galaxy cluster mass function and its evolution, using X-ray selected clusters detected in the ROSAT All-Sky Survey. Our lensing analysis constrains the absolute mass scale of such clusters at the 8 per cent level, including both statistical and systematic uncertainties. Combining it with the survey data and X-ray follow-up observations, we find a tight constraint on a combination of the mean matter density and late-time normalization of the matter power spectrum, $σ_8(Ω_m/0.3)^{0.17}=0.81\pm0.03$, with marginalized, one-dimensional constraints of $Ω_m=0.26\pm0.03$ and $σ_8=0.83\pm0.04$. For these two parameters, this represents a factor of two improvement in precision with respect to previous work, primarily due to the reduced systematic uncertainty in the absolute mass calibration provided by the lensing analysis. Our new results are in good agreement with constraints from cosmic microwave background (CMB) data, both WMAP and Planck (plus WMAP polarization), under the assumption of a flat $Λ$CDM cosmology with minimal neutrino mass. Consequently, we find no evidence for non-minimal neutrino mass from the combination of cluster data with CMB, supernova and baryon acoustic oscillation measurements, regardless of which all-sky CMB data set is used (and independent of the recent claimed detection of B-modes on degree scales). We also present improved constraints on models of dark energy (both constant and evolving), modifications of gravity, and primordial non-Gaussianity. Assuming flatness, the constraints for a constant dark energy equation of state from the cluster data alone are at the 15 per cent level, improving to $\sim 6$ per cent when the cluster data are combined with other leading probes.

astro-ph.CO↗

arXiv2014

Local variance asymmetries in Planck temperature anisotropy maps

Recently, it was shown that local variance maps of temperature anisotropy are simple and useful tools for the study of large scale hemispherical power asymmetry. This was done by studying the distribution of dipoles of the local variance maps. In this work, we extend the study of the dipolar asymmetry in local variance maps using foreground cleaned Planck 143 GHz and 217 GHz data to smaller scales. In doing so, we include the effect of the CMB Doppler dipole. Further, we show that it is possible to use local variance maps to measure the Doppler dipole in these Planck channel maps, after removing large scale features (up to $l=600$), at a significance of about $3 σ$. At these small scales, we do not find any power asymmetry in the direction of the anomalous large scale power asymmetry beyond that expected from cosmic variance. At large scales, we verify previous results i.e. the presence of hemispherical power asymmetry at a significance of at least $3.3 σ$.

astro-ph.CO↗

arXiv2014

Higher moments of primordial non-Gaussianity and N-body simulations

We perform cosmological N-body simulations with non-Gaussian initial conditions generated from two independent fields. The dominant contribution to the perturbations comes from a purely Gaussian field, but we allow the second field to have local non-Gaussianity that need not be weak. This scenario allows us to adjust the relative importance of non-Gaussian contributions beyond the skewness, producing a scaling of the higher moments different from (and stronger than) the scaling in the usual single field local ansatz. We compare semi-analytic prescriptions for the non-Gaussian mass function, large scale halo bias, and stochastic bias against the simulation results. We discuss applications of this work to large scale structure measurements that can test a wider range of models for the primordial fluctuations than is usually explored.

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