arXiv subjects

Pravabati Chingangbam

Publications and source records attributed to Pravabati Chingangbam.

At least 19 recordsLinked to original sources

arXiv2026

High-Redshift Signatures from the Cosmic Dawn and the Epoch of Reionization

In this chapter, we provide a comprehensive overview of the astrophysical and cosmological processes that shape the 21-cm signal during Cosmic Dawn and the Epoch of Reionization. We investigate both standard and exotic signatures potentially observable with SKA-Low. Standard signatures are those expected within the $\Lambda$CDM framework, including contributions from the first stars, galaxies, and black holes. Exotic signatures are more speculative indicating new physics, such as primordial black holes, modifications to the dark matter sector, non-standard primordial fluctuations, or strongly emitting radio galaxies. The effects of these different sources or scenarios are evaluated in the context of the expected sensitivity of SKA-Low, considering the AA* and AA4 configurations. The chapter aims to provide an overview of the theoretical landscape of 21-cm signatures and to highlight how the forthcoming SKA-Low observations will improve our understanding of astrophysical processes at early times and may open the door towards new physics beyond the $\Lambda$CDM framework.

astro-ph.CO↗

arXiv2026

Understanding the non-Gaussian nature of Galactic foreground emissions towards small scales

We present a unified, multi-scale study of non-Gaussianity of Galactic foreground emissions using Minkowski Functionals and generalized skewness-kurtosis parameters, focusing on the characterization of small-scale non-Gaussianity and its underlying physical origin. We find that all foreground components studied exhibit a remarkably universal non-Gaussian nature dominated by excess kurtosis, whose shape remains stable across angular scales despite large differences in emission physics. Focusing on thermal dust, we perform a detailed comparison between observed maps (GNILC and Planck 545 GHz) and dust model realizations (PySM and filament-based models) to assess the performance of state-of-the-art models in reproducing the observed non-Gaussian properties. At the global level, GNILC and PySM display closely matched kurtosis behavior over the angular scales where the GNILC reconstruction is reliable, while the filament-based model produces substantially weaker skewness and kurtosis signals. For PySM, however, a patch-based analysis reveals statistically significant regional variations, indicating that while the model reproduces the overall non-Gaussian amplitude and scale dependence, it does not fully capture the spatial variability of the observed kurtosis signal. Using simple PDF-based toy models, we demonstrate that the universal kurtosis signature arises from the combination of heavy-tailed one-point statistics and steep large-scale spatial correlations, while its detailed amplitude and scale dependence depend on the underlying foreground physics. These results identify excess kurtosis as a robust statistical fingerprint of Galactic foregrounds and provide a practical framework for validating small-scale foreground models for future CMB analyses.

astro-ph.CO↗

arXiv2026

Morphological complexity of NGC 628 - a multiwavelength multiscale analysis using the ordinal pattern framework

As statistical systems, galaxies exhibit a rich interplay between organized structure and stochastic fluctuations across a broad range of spatial scales. This duality motivates the need for quantitative frameworks capable of capturing their morphological complexity. The ordinal patterns framework, along with its associated statistical measures: permutation entropy ($H$), disequilibrium ($D_E$), statistical complexity ($C$), and ordinal network node entropy, has recently emerged as a powerful tool for analyzing such complexity in physical systems. We apply this framework in a multiwavelength, multiscale analysis of the galaxy NGC 628, utilizing observations in the near-ultraviolet, near-infrared, mid-infrared, and millimeter bands. Our results reveal a characteristic spatial scale of approximately 200 parsecs, marking the transition from small-scale structures influenced by star formation and stellar feedback to larger-scale morphology governed by the galaxy's dynamics. Furthermore, we find that the $C$ vs. $H$ trajectories for all wavelengths converge toward a common attractor curve, consistent with the behavior of isotropic Gaussian random fields. This convergence suggests a universal statistical behavior in galactic structure at large scales, despite the differing physical processes traced by each wavelength.

astro-ph.GA↗

arXiv2026

Morphological Signatures of Gravitational Evolution, Redshift-Space Distortions, and Massive Neutrinos in Large-Scale Structure

We investigate the morphological properties of large-scale structure in the Universe and the physical processes that modify the excursion-set morphology of the three-dimensional matter density field. Using the Quijote N-body simulation suite, we study how an initially Gaussian random matter density field is altered by non-linear gravitational evolution, redshift-space distortions, and massive neutrino free-streaming. To quantify these effects, we employ a comprehensive set of morphological descriptors, including Minkowski Functionals, Betti numbers, Minkowski Tensors, and local measures of the size and shape of connected components and cavities. We find that gravitational evolution, on quasi-linear scales $R_G \sim 10 h^{-1} \mathrm{Mpc}$, strongly skews the one-point distribution and slightly smooths the field via the merging of critical points, with a more pronounced effect for minima and wall saddle points than for peaks. Redshift-space distortions produce the strongest morphological signal, generating pronounced anisotropies that are robustly captured by Minkowski Tensors and local shape measures, arising from both coherent large-scale flows and non-linear Finger-of-God effects. In contrast, massive neutrinos induce an approximately isotropic suppression of small-scale structure, slightly reducing the amplitudes of the Minkowski Functionals while leaving individual shape measures largely unchanged. We further explore the sensitivity of these statistics to variations in cosmological parameters $\Omega_m$, $n_s$, and $\sigma_8$, finding that they probe strongly degenerate combinations of $\Omega_m$ and $n_s$, while also exhibiting sensitivity to $\sigma_8$ through the non-Gaussianity of the evolved density field.

astro-ph.CO↗

arXiv2026

Minkowski Functionals of the 21 cm Signal as a Probe of Primordial Features

The redshifted 21 cm signal from the cosmic dawn and Epoch of Reionization (EoR) encodes important information about both astrophysical processes and primordial physics, such as inflation. In this work, we use morphological statistics to explore the sensitivity of the 21 cm signal to inflationary features and EoR dynamics simultaneously. Focusing on primordial features from particle production during inflation we generate semi-numerical simulations of the 21 cm signal across redshifts 5 < z < 35, incorporating these features. Using Minkowski Functionals (MFs), we analyze the morphology of 21 cm fields: density, neutral hydrogen fraction, spin temperature, and brightness temperature. We demonstrate that MFs are highly sensitive to both the amplitude and scale of primordial features, capturing rich morphological information. In particular, we show that MFs can robustly identify inflationary features and distinguish them from the standard model. We further explore various EoR scenarios, and demonstrate that combining MFs across redshifts can disentangle the signatures of primordial features from EoR effects. This approach opens new avenues for probing inflation with upcoming 21 cm surveys.

astro-ph.CO↗

arXiv2025

The CatWISE2020 Quasar dipole: A Reassessment of the Cosmic Dipole Anomaly

The Ellis-Baldwin test probes the cosmological principle by comparing the kinematic Cosmic Microwave Background dipole with the Doppler-driven dipole in the number counts of extragalactic radio sources. Recent analysis of the CatWISE2020 quasar catalog reported a number-count dipole amplitude exceeding the kinematic expectation at $4.9\sigma$ significance. We present a comprehensive reassessment of this test using the same dataset, incorporating major sources of uncertainty in the statistical inference. We employ a simulation framework based on the FLASK package, using lognormal realizations of the large-scale structure, quasar clustering bias, the survey's radial selection function, and its exact sky coverage. Our simulations account for the kinematic dipole, the intrinsic clustering dipole, shot noise, and survey geometry effects. The analysis yields a revised significance of $3.63\sigma$ in the absence of a clustering dipole, and $3.44\sigma$ with a randomly oriented clustering dipole. When the clustering dipole is aligned with the kinematic dipole, the significance decreases further to $3.27\sigma$. Although the anomaly is reduced, it cannot be explained solely by the clustering dipole or mode coupling from the survey mask. We further assess dipole measurement robustness by fitting models with successively higher-order multipoles up to $\ell = 4$. Partial sky coverage induces mode coupling, shifting the dipole estimate to higher values when the octopole is included and inflating its variance as additional modes are incorporated, reflected in the increasing condition number of the estimator. This behavior highlights a bias-variance trade-off inherent in multipole fitting on partial-sky data.

astro-ph.CO↗

arXiv2025

Non-Gaussian Expansion of Minkowski Tensors in Redshift Space

This paper focuses on extending the use of Minkowski Tensors to analyze anisotropic signals in cosmological data, focusing on those introduced by redshift space distortion. We derive the ensemble average of the two translation-invariant, rank-2 Minkowski Tensors ($W_1^{0,2}$ and $W_2^{0,2}$) for a matter density field that is perturbatively non-Gaussian in redshift space. This is achieved through the Edgeworth expansion of the joint probability density function of the field and its derivatives, expressing the ensemble averages in terms of cumulants up to cubic order. Our goal is to connect these theoretical predictions to the underlying cosmological parameters, allowing for parameter estimation by measuring them from galaxy surveys. The work builds on previous analyses of Minkowski Functionals in both real and redshift space and addresses the effects of Finger-of-God velocity dispersion and shot noise. We validate our predictions by matching them to measurements of the Minkowski Tensors from dark matter simulation data, finding that perturbation theory is a qualified success. Non-perturbative Finger-of-God effects remain significant at relatively large scales $R_G \lesssim 20 \, h^{-1} \, {\rm Mpc}$ and are particularly pronounced in the components parallel to the line of sight.

astro-ph.CO↗

arXiv2025

On the statistical nature of Betti numbers and Euler characteristic of smooth random fields

We represent excursion sets of smooth random fields as unions of a topological basis consisting of a sequence of simply and multiply connected compact subsets of the underlying manifold. The associated coefficients, which are non-negative discrete random variables, reflect the randomness of the field. Betti numbers of the excursion sets can be expressed as summations over the coefficients, and the Euler characteristic and the sum of Betti numbers can also be expressed as their (alternating) sum. This enables understanding their statistical properties as sums (or differences) of discrete random variables. We examine the conditions under which each topological statistic can be asymptotically Gaussian as the size of the manifold and the resolution increase. The coefficients of the basis elements are then modeled as Binomial variables, and the statistical natures of Betti numbers, Euler character and sum of Betti numbers follow from this fundamental property. We test the validity of the modeling using numerical calculations, and identify threshold regimes where the topological statistics can be approximated as Gaussian variables. The new representation of excursion sets thus maps the properties of topological statistics to combinatorial structures, thereby providing mathematical clarity on their use for physical inference, particularly in cosmology.

math.ST↗

arXiv2025

Local patch analysis of ACT DR6 convergence map using morphological statistics

We carry out a comprehensive hierarchical multi-scale morphological analysis to search for anomalous behaviour in the large scale matter distribution using convergence map provided by the Atacama Cosmology Telescope (ACT) Data Release 6. We use a suite of morphological statistics consisting of Minkowski functionals, contour Minkowski tensor and Betti numbers for the analysis, and compute their deviations from the ensemble expectations and median values obtained from isotropic $\Lambda$CDM simulations provided by ACT. To assess the statistical significance of these deviations, we devise a general methodology based on the persistence of the deviations across threshold ranges and spatial resolutions, while taking into account correlations among the statistics. From the analysis of the full dataset, and hemispherical regions, we find consistency with isotropic $\Lambda$CDM simulations provided by ACT. Since deviations in smaller sky regions tend to get washed out when averaged over larger regions, we further analyze smaller sky patches. This localized analysis reveals some patches that exhibit statistically significant deviations which we refer to as 'anomalous'. We find that near the CMB cold spot, both the positive and negative density fluctuations are anomalous, at 99% CL and 95% CL respectively. This region also encompasses an anomalous southern spot previously identified in Planck CMB temperature data. We also carry out a comparison of anomalous patches identified here for ACT data with a previous analysis of the convergence map from Planck. We do not find common patches between the two datasets, which suggest that the anomalous behavior of the Planck data arises from noise in the map. Further investigation of the atypical patches using large scale structure surveys is warranted to determine their physical origin.

astro-ph.CO↗

arXiv2025

Global 21 cm signal: a promising probe of primordial features

Inflationary models that involve bursts of particle production generate bump-like features in the primordial power spectrum of density perturbations. These features influence the evolution of density fluctuations, leaving their unique signatures in cosmological observations. A detailed investigation of such signatures would help constrain physical processes during inflation. With this motivation, the goal of this paper is two-fold. First, we conduct a detailed analysis of the effects of bump-like primordial features on the sky-averaged 21 cm signal. Using semi-numerical simulations, we demonstrate that the primordial features can significantly alter the ionization history and the global 21 cm profile, making them a promising probe of inflationary models. We found a special scale (namely, the turnover wavenumber, $k^{\rm turn}$) at which the effect of primordial bump-like features on the global 21 cm profile vanishes. Also, we found that the behaviour of the primordial features on the global profile and ionization history are quite opposite for $k > k^{\rm turn}$ and $k < k^{\rm turn}$. We trace the root cause of these behaviours to the effects of primordial features on the halo mass function at high redshifts. Furthermore, we discuss the degeneracy between the astrophysical parameters and the primordial features in detail. Secondly, for a fixed set of astrophysical parameters, we derive upper limits on the amplitude of bump-like features in the range $10^{-1} < k\,[{\rm Mpc}^{-1}] < 10^2$ using current limits on optical depth to reionization from CMB data by Planck.

astro-ph.CO↗

arXiv2023

Minkowski Functionals for composite smooth random fields

Minkowski functionals quantify the morphology of smooth random fields. They are widely used to probe statistical properties of cosmological fields. Analytic formulae for ensemble expectations of Minkowski functionals are well known for Gaussian and mildly non-Gaussian fields. In this paper we extend the formulae to composite fields which are sums of two fields and explicitly derive the expressions for the sum of uncorrelated mildly non-Gaussian and Gaussian fields. These formulae are applicable to observed data which is usually a sum of the true signal and one or more secondary fields that can be either noise, or some residual contaminating signal. Our formulae provide explicit quantification of the effect of the secondary field on the morphology and statistical nature of the true signal. As examples, we apply the formulae to determine how the presence of Gaussian noise can bias the morphological properties and statistical nature of Gaussian and non-Gaussian CMB temperature maps.

astro-ph.CO↗

arXiv2023

Particle production during inflation: A Bayesian analysis with CMB data from Planck 2018

A class of inflationary models that involve rapid bursts of particle productions predict observational signatures, such as bump-like features in the primordial scalar power spectrum. In this work, we analyze such models by comparing their predictions with the latest CMB data from Planck 2018. We consider two scenarios of particle production. The first one is a simple scenario consisting of a single burst of particle production during observable inflation. The second one consists of multiple bursts of particle production that lead to a series of bump-like features in the primordial power spectrum. We find that the second scenario of the multi-bump model gives better fit to the CMB data compared to the concordance $Λ$CDM model. We carried out model comparisons using Bayesian evidences. From the observational constraints on the amplitude of primordial features of the multi-bump model, we find that the coupling parameter $g$ responsible for particle production is bound to be $g< 0.05$.

astro-ph.CO↗

arXiv2023

Particle production during inflation: constraints expected from redshifted 21 cm observations from the epoch of reionization

We examine a type of features in the primordial scalar power spectrum, namely, the bump-like feature(s) that arise as a result of burst(s) of particle production during inflation. The latest CMB observations by Planck 2018 can accommodate such features. In the near future, observations of redshifted 21 cm signal from the epoch of reionization can put additional constraints on inflation models by exploiting the expected tomographic information across a wide range of co-moving wave-numbers. In this work, we study the potential of upcoming observational data from SKA-Low to constrain the parameters of the primordial power spectrum with bump-like features. We use simulated mock data expected from SKA-Low, with uncertainties estimated from different foreground removal models, and constrain the parameters of primordial features within a Bayesian framework. We study two scenarios: in the first scenario, where the astrophysical parameters relevant to the evolution of the 21 cm signal are known, we find that 21 cm power spectra do have the potential to probe the primordial bump-like features. As the input amplitude of the bump is decreased below roughly 10% of the amplitude of the primordial power spectrum without the bump, the uncertainties in the recovered values for both amplitude and location of the bump are found to increase, and the recovered values of the location of the bump also get increasingly more biased towards higher values. Hence, it becomes harder to constrain these parameters. In the second scenario, we analyze the achievable constraints on primordial features when two of the astrophysical parameters, namely, minimum halo mass and ionizing efficiency are uncertain. We find that the effect of the bump on the shape and the amplitude of the 21 cm power spectrum is distinct from the impact of varying the astrophysical parameters, and hence they may potentially be distinguished.

astro-ph.CO↗

arXiv2023

Is the Observable Universe Consistent with the Cosmological Principle?

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $Λ$-Cold-Dark-Matter ($Λ$CDM) model. Yet, tensions have emerged within the $Λ$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $Λ$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.

astro-ph.CO↗

arXiv2022

Statistical properties of Galactic synchrotron temperature and polarization maps -- a multi-frequency comparison

Understanding the statistical properties of synchrotron emission from our Galaxy is valuable from the perspective of observations targeting signals of cosmological origin, as well as for understanding physical processes in our Galaxy. In this work, we extend the analysis of arXiv:2104.00419v2 to -- (a) all-sky observed maps of total foreground emissions at different frequencies provided by WMAP, Planck and Stockert-Villa, (b) component separated synchrotron temperature maps provided by WMAP, Planck and BeyondPlanck, and (c) component separated polarization maps provided by WMAP and Planck. The tools we use are Minkowski functionals and tensors. Our main goals are twofold. First, we determine the variation of morphological properties of the total foreground maps with observing frequency and compare with simulations. This elucidates how the morphology varies due to the relative dominance of different foreground components at different frequencies. Next, we analyze the nature of non-Gaussianity and statistical isotropy of synchrotron fluctuations on smaller scales using various component separated synchrotron temperature and polarization maps. We find that all maps exhibit kurtosis non-Gaussianity, in agreement with the Haslam map. This result can be an important input for the modelling of small-scale synchrotron fluctuations for component separation pipelines. This also suggests that residual synchrotron contamination in CMB will manifest as kurtosis. Studying different component separated maps, we find that BeyondPlanck and WMAP MCMC-e agree well with Haslam at all scales. The other maps show differences of varying statistical significance. Our analysis suggests a combination of residual AME and/or free-free emissions and point sources as contributing to these differences, and underscores the need for further improvement of the pipelines.

astro-ph.CO↗

arXiv2022

Minkowski Tensors in Redshift Space -- Beyond the Plane Parallel Approximation

The Minkowski tensors (MTs) can be used to probe anisotropic signals in a field, and are well suited for measuring the redshift space distortion (RSD) signal in large scale structure catalogs. We consider how the linear RSD signal can be extracted from a field without resorting to the plane parallel approximation. A spherically redshift space distorted field is both anisotropic and inhomogeneous. We derive expressions for the two point correlation functions that elucidate the inhomogeneity, and then explain how the breakdown of homogeneity impacts the volume and ensemble averages of the tensor Minkowski functionals. We construct the ensemble average of these quantities in curvilinear coordinates and show that the ensemble and volume averages can be approximately equated, but this depends on our choice of definition of the volume average of a tensor and the radial distance between the observer and field. We then extract the tensor Minkowski functionals from spherically redshift space distorted, Gaussian random fields and gravitationally evolved dark matter density fields at $z=0$ to test if we can successfully measure the Kaiser RSD signal. For the dark matter field we find a significant, $\sim 10\%$ anomalous signal in the MT component parallel to the line of sight that is present even on large scales $R_{\rm G} \gtrsim 15 \, {\rm Mpc}$, in addition to the Kaiser effect. This is due to the line of sight component of the MT being significantly contaminated by the Finger of God effect, which can be approximately modelled by an additional damping term in the cumulants.

astro-ph.CO↗

arXiv2022

Analysis of the structural complexity of Crab Nebula observed at radio frequency using a multifractal approach

The Crab Nebula is an astrophysical system that exhibits complex morphological patterns at different observing frequencies. We carry out a systematic investigation of the structural complexity of the nebula using publicly available imaging data at radio frequency. For the analysis, we use the well-known multifractal detrended fluctuation analysis in two dimensions. We find that radio data exhibit long-range correlations, as expected from the underlying physics of the supernova explosion and evolution. The correlations follow a power-law scaling with length scales. The structural complexity is found to be multifractal in nature, as evidenced by the dependence of the generalized Hurst exponent on the order of the moments of the detrended fluctuation function. By repeating the analysis on shuffled data, we further probe the origin of the multifractality in the radio imaging data. For the radio data, we find that the probability density function is close to a Gaussian form. Hence, the multifractal behavior is due to the differing nature of long-range correlations of the large and small detrended fluctuation field values. We investigate the multifractal parameters across different partitions of the radio image and find that the structures across the image are highly heterogeneous, making the Crab Nebula a structurally complex astrophysical system. Our analysis thus provides a fresh perspective on the morphology of the Crab Nebula from a complexity science viewpoint.

nlin.AO↗

arXiv2021

The geometrical meaning of statistical isotropy of smooth random fields in two dimensions

We revisit the geometrical meaning of statistical isotropy that is manifest in excursion sets of smooth random fields in two dimensions. Using the contour Minkowski tensor, $\W_1$, as our basic tool we first examine geometrical properties of single structures. For simple closed curves in two dimensions we show that $\W_1$ is proportional to the identity matrix if the curve has $m$-fold symmetry, with $m\ge 3$. Then we elaborate on how $\W_1$ maps any arbitrary shaped simple closed curve to an ellipse that is unique up to translations of its centroid. We also carry out a comparison of the shape parameters, $α$ and $β$, defined using $\W_1$, with the filamentarity parameter defined using two scalar Minkowski functionals - area and contour length. We show that they contain complementary shape information, with $\W_1$ containing additional information of orientation of structures. Next, we apply our method to boundaries of excursion sets of random fields and examine what statistical isotropy means for the geometry of the excursion sets. Focusing on Gaussian isotropic fields, and using a semi-numerical approach we quantify the effect of finite sampling of the field on the geometry of the excursion sets. In doing so we obtain an analytic expression for $α$ which takes into account the effect of finite sampling. Finally we derive an analytic expression for the ensemble expectation of $\W_1$ for Gaussian anisotropic random fields. Our results provide insights that are useful for designing tests of statistical isotropy using cosmological data.

astro-ph.CO↗