arXiv subjects

Biswajit Pandey

Publications and source records attributed to Biswajit Pandey.

At least 55 records · Page 3Linked to original sources

arXiv2020

Renyi entropy as a measure of cosmic homogeneity

We propose a method for testing homogeneity in three dimensional spatial distributions using Renyi entropy. We apply the proposed method to data from cosmological N-body simulations and Monte Carlo simulations of homogeneous Poisson point process. We show that the method can effectively characterize the inhomogeneities and identify any transition scale to homogeneity, if present in such distributions. The proposed method can be used to study the cosmic homogeneity in present and future generation galaxy redshift surveys.

astro-ph.CO↗

arXiv2020

Do galactic bars depend on environment?: An information theoretic analysis of Galaxy Zoo 2

We use an information theoretic framework to analyze data from the Galaxy Zoo 2 project and study if there are any statistically significant correlations between the presence of bars in spiral galaxies and their environment. We measure the mutual information between the barredness of galaxies and their environments in a volume limited sample ($M_r \leq -21$) and compare it with the same in datasets where (i) the bar/unbar classifications are randomized and (ii) the spatial distribution of galaxies are shuffled on different length scales. We assess the statistical significance of the differences in the mutual information using a t-test and find that both randomization of morphological classifications and shuffling of spatial distribution do not alter the mutual information in a statistically significant way. The non-zero mutual information between barredness and environment arises due to the finite and discrete nature of the dataset which can be entirely explained by mock Poisson distributions. We also separately compare the cumulative distribution functions of the barred and unbarred galaxies as a function of their local density. Using a Kolmogorov-Smirnov test, we find that the null hypothesis can not be rejected even at $75\%$ confidence level. Our analysis indicates that environments do not play a significant role in the formation of a bar, which is largely determined by the internal processes of the host galaxy.

astro-ph.GA↗

arXiv2020

Exploring galaxy colour in different environments of the cosmic web with SDSS

We analyze a set of volume limited samples from the SDSS to study the dependence of galaxy colour on different environments of the cosmic web. We measure the local dimension of galaxies to determine the geometry of their embedding environments and find that filaments host a higher fraction of red galaxies than sheets at each luminosity. We repeat the analysis at a fixed density and recover the same trend which shows that galaxy colours depend on geometry of environments besides local density. At a fixed luminosity, the fraction of red galaxies in filaments and sheets increases with the extent of these environments. This suggests that the bigger structures have a larger baryon reservoir favouring higher accretion and larger stellar mass. We find that the mean colour of the red and blue populations are systematically higher in the environments with smaller local dimension and increases monotonically in all the environments with luminosity. We observe that the bimodal nature of the galaxy colour distribution persists in all environments and all luminosities, which suggests that the transformation from blue to red galaxy can occur in all environments.

astro-ph.GA↗

arXiv2020

A method for classification of red, blue and green galaxies using fuzzy set theory

Red and blue galaxies are traditionally classified using some specific cuts in colour or other galaxy properties, which are supported by empirical arguments. The vagueness associated with such cuts are likely to introduce a significant contamination in these samples. Fuzzy sets are vague boundary sets which can efficiently capture the classification uncertainty in the absence of any precise boundary. We propose a method for classification of galaxies according to their colours using fuzzy set theory. We use data from the SDSS to construct a fuzzy set for red galaxies with its members having different degrees of `redness'. We show that the fuzzy sets for the blue and green galaxies can be obtained from it using different fuzzy operations. We also explore the possibility of using fuzzy relation to study the relationship between different galaxy properties and discuss its strengths and limitations.

astro-ph.GA↗

arXiv2020

Can a conditioning on stellar mass explain the mutual information between morphology and environment?

Recent studies with SDSS have shown that a statistically significant non-zero mutual information between morphology and environment persists up to several tens of Mpc, which awaits an explanation. Galaxies in different environments acquire their stellar mass through accretion and merger and the stellar mass function of galaxies is known to depend on both environment and morphology. Naturally, stellar mass can be an important link between morphology and environment which may explain the non-zero mutual information between the two. Measuring the mutual information between morphology and environment by conditioning the stellar mass would allow us to test this possibility. We employ here a volume and stellar mass limited sample from the $16^{th}$ data release (DR16) of the SDSS and find a non-zero conditional mutual information throughout the entire length scales probed. We compare the results with three different semi-analytic models implemented on the Millennium simulation and find their predictions to be in fairly good agreement with SDSS on smaller length scales ( $\lesssim 30 h^{-1}$ Mpc ), with a clear discrepancy observed at larger length scales ( $\gtrsim 30 h^{-1}$ Mpc ) where the models predict significantly lower conditional mutual information than the SDSS. Our analysis therefore suggests that only environmental and morphology dependence of stellar mass are inadequate in explaining the observed mutual information between morphology and environment and that physical processes which alters morphology may not necessarily have an impact on the stellar mass of galaxies and vice versa.

astro-ph.GA↗

arXiv2020

A study on the statistical significance of mutual information between morphology of a galaxy and its large-scale environment

A non-zero mutual information between morphology of a galaxy and its large-scale environment is known to exist in SDSS upto a few tens of Mpc. It is important to test the statistical significance of these mutual information if any. We propose three different methods to test the statistical significance of these non-zero mutual information and apply them to SDSS and Millennium Run simulation. We randomize the morphological information of SDSS galaxies without affecting their spatial distribution and compare the mutual information in the original and randomized datasets. We also divide the galaxy distribution into smaller subcubes and randomly shuffle them many times keeping the morphological information of galaxies intact. We compare the mutual information in the original SDSS data and its shuffled realizations for different shuffling lengths. Using a t-test, we find that a small but statistically significant (at 99.9% confidence level) mutual information between morphology and environment exists upto the entire length scale probed. We also conduct another experiment using mock datasets from a semi-analytic galaxy catalogue where we assign morphology to galaxies in a controlled manner based on the density at their locations. The experiment clearly demonstrate that mutual information can effectively capture the physical correlations between morphology and environment. Our analysis suggests that physical association between morphology and environment may extend to much larger length scales than currently believed and the information theoretic framework presented here, can serve as a sensitive and useful probe of the assembly bias and large-scale environmental dependence of galaxy properties.

astro-ph.GA↗

arXiv2020

Can we constrain the evolution of HI bias using configuration entropy?

We study the evolution of the configuration entropy of HI distribution in the post-reionization era assuming different time evolution of HI bias. We describe time evolution of linear bias of HI distribution using a simple form $b(a)=b_{0} a^{n}$ with different index $n$. The derivative of the configuration entropy rate is known to exhibit a peak at the scale factor corresponding to the $Λ$-matter equality in the unbiased $Λ$CDM model. We show that in the $Λ$CDM model with time-dependent linear bias, the peak shifts to smaller scale factors for negative values of $n$. This is related to the fact that the growth of structures in the HI density field can significantly slow down even before the onset of $Λ$ domination in presence of a strong time evolution of the HI bias. We find that the shift is linearly related to the index $n$. We obtain the best fit relation between these two parameters and propose that identifying the location of this peak from observations would allow us to constrain the time evolution of HI bias within the framework of the $Λ$CDM model.

astro-ph.CO↗

arXiv2019

Can we constrain the dark energy equation of state parameters using configuration entropy?

We propose a new scheme for constraining the dark energy equation of state parameter/parameters based on the study of the evolution of the configuration entropy. We analyze a set of one parameter and two parameter dynamical dark energy models and find that the derivative of the configuration entropy in all the dynamical dark energy models exhibit a minimum. The magnitude of the minimum of the entropy rate is decided by both the parametrization of the equation of state as well as the associated parameters. The location of the minimum of the entropy rate is less sensitive to the form of the parametrization but depends on the associated parameters. We determine the best fit equations for the location and magnitude of the minimum of the entropy rate in terms of the parameter/parameters of the dark energy equation of state. These relations would allow us to constrain the dark energy equation of state parameter/parameters for any given parametrization provided the evolution of the configuration entropy in the Universe is known from observations.

astro-ph.CO↗

arXiv2019

Configuration entropy of the Cosmic Web: Can voids mimic the dark energy?

We propose an alternative physical mechanism to explain the observed accelerated expansion of the Universe based on the configuration entropy of the cosmic web and its evolution. We show that the sheets, filaments and clusters in the cosmic web act as sinks whereas the voids act as the sources of information. The differential entropy of the cosmic velocity field increases with time and also acts as a source of entropy. The growth of non-linear structures and the emergence of the cosmic web may lead to a situation where the overall dissipation rate of information at the sinks are about to dominate the generation rate of information from the sources. Consequently, the Universe either requires a dispersal of the overdense non-linear structures or an accelerated expansion of the underdense voids to prevent a violation of the second law of thermodynamics. The dispersal of the sheets, filaments and clusters are not a viable option due to the attractive nature of gravity but the repulsive and outward peculiar gravitational acceleration at the voids makes it easier to stretch them at an accelerated rate. We argue that this accelerated expansion of the voids inside the cosmic web may mimic the behaviour of dark energy.

astro-ph.CO↗

arXiv2019

A new method to probe the mass density and the cosmological constant using configuration entropy

We study the evolution of the configuration entropy for different combinations of $Ω_{m0}$ and $Ω_{\Lambda0}$ in the flat $Λ$CDM universe and find that the cosmological constant plays a decisive role in controlling the dissipation of the configuration entropy. The configuration entropy dissipates at a slower rate in the models with higher value of $Ω_{\Lambda0}$. We find that the entropy rate decays to reach a minimum and then increases with time. The minimum entropy rate occurs at an earlier time for higher value of $Ω_{\Lambda0}$. We identify a prominent peak in the derivative of the entropy rate whose location closely coincides with the scale factor corresponding to the transition from matter to $Λ$ domination. We find that the peak location is insensitive to the initial conditions and only depends on the values of $Ω_{m0}$ and $Ω_{\Lambda0}$. We propose that measuring the evolution of the configuration entropy in the Universe and identifying the location of the peak in its second derivative would provide a new and robust method to probe the mass density and the cosmological constant.

astro-ph.CO↗

arXiv2019

Unravelling the Cosmic Web: An analysis of the SDSS DR14 with the Local Dimension

We analyze a volume limited galaxy sample from the SDSS to study the environments of galaxies on different length scales in the local Universe. We measure the local dimension of the SDSS galaxies on different length scales and find that the sheets or sheetlike structures are the most prevalent pattern in the cosmic web throughout the entire length scales. The abundance of sheets peaks at $30 \, h^{-1}\, {\rm Mpc}$ and they can extend upto a length scales of $90 \, h^{-1}\, {\rm Mpc}$ . Analyzing mock catalogues, we find that the sheets are non-existent beyond $30 \, h^{-1}\, {\rm Mpc}$ in the Poisson distributions. We find that the straight filaments in the SDSS galaxy distribution can extend only upto a length scale of $30 \, h^{-1}\, {\rm Mpc}$. Our results indicate that the environment of a galaxy exhibits a gradual transition towards higher local dimension with increasing length scales finally approaching a nearly homogeneous network on large scales. We compare our findings with a semi analytic galaxy catalogue from the Millennium Run simulation which are in fairly good agreement with the observations. We also test the effects of the number density of the sample and the cut-off in the goodness of fit which shows that the results are nearly independent of these factors. Finally we apply the method to a set of simulations of the segment Cox process and find that it can characterize such distributions.

astro-ph.CO↗

arXiv2018

Testing isotropy in the Universe using photometric and spectroscopic data from the SDSS

We analyze two volume limited galaxy samples from the SDSS photometric and spectroscopic data to test the isotropy in the local Universe. We use information entropy to quantify the global anisotropy in the galaxy distribution at different length scales and find that the galaxy distribution is highly anisotropic on small scales. The observed anisotropy diminishes with increasing length scales and nearly plateaus out beyond a length scale of 200 Mpc/h in both the datasets. We compare these anisotropies with those predicted by the mock catalogues from the N-body simulations of the Lambda CDM model and find a fairly good agreement with the observations. We find a small residual anisotropy on large scales which decays in a way that is consistent with the linear perturbation theory. The slopes of the observed anisotropy converge to the slopes predicted by the linear theory beyond a length scale of ~ 200 Mpc/h indicating a transition to isotropy. We separately compare the anisotropies observed across the different parts of the sky and find no evidence for a preferred direction in the galaxy distribution.

astro-ph.CO↗

arXiv2018

Configuration entropy in the $Λ$CDM and the dynamical dark energy models: Can we distinguish one from the other?

The evolution of the configuration entropy of the mass distribution in the Universe is known to be governed by the growth rate of density perturbations and the expansion rate of the Universe. We consider the $Λ$CDM model and a set of dynamical dark energy models with different parametrization of the equation of state and explore the evolution of the configuration entropy in these models. We find that the nature of evolution of the configuration entropy depends on the adopted parametrization which may allow us to discern them from each other. The configuration entropy initially decreases with time but nearly plateaus out at present in the $Λ$CDM model. The models where dark energy becomes less dominant at late times exhibit a larger decrease in the configuration entropy compared to the $Λ$CDM model after redshift $z \sim 1$. We find that the configuration entropy remains nearly unchanged in the models where dark energy becomes more dominant at late times. Our results suggest that the method presented here may be also used to constrain the initial value of the configuration entropy of the Universe.

astro-ph.CO↗

arXiv2017

Does information entropy play a role in the expansion and acceleration of the Universe?

We propose an interpretation of the expansion and acceleration of the Universe from an information theoretic viewpoint. We obtain the time evolution of the configuration entropy of the mass distribution in a static Universe and show that the process of gravitational instability leads to a rapid dissipation of configuration entropy during the growth of the density fluctuations making such a Universe entropically unfavourable. We find that in an expanding Universe, the configuration entropy rate is governed by the expansion rate of the Universe and the growth rate of density fluctuations. The configuration entropy rate becomes smaller but still remains negative in a matter dominated Universe and eventually becomes zero at some future time in a $Λ$ dominated Universe. The configuration entropy may have a connection to the dark energy and possibly plays a driving role in the current accelerating expansion of the Universe leading the Universe to its maximum entropy configuration.

gr-qc↗

arXiv2017

Can anisotropy in the galaxy distribution tell the bias?

We use information entropy to analyze the anisotropy in the mock galaxy catalogues from dark matter distribution and simulated biased galaxy distributions from $Λ$CDM N-body simulation. We show that one can recover the linear bias parameter of the simulated galaxy distributions by comparing the radial, polar and azimuthal anisotropies in the simulated galaxy distributions with that from the dark matter distribution. This method for determination of the linear bias requires only $O(N)$ operations as compared to $O(N^{2})$ or at least $O(N \log N)$ operations required for the methods based on the two-point correlation function and the power spectrum. We apply this method to determine the linear bias parameter for the galaxies in the 2MASS Redshift Survey (2MRS) and find that the 2MRS galaxies in the $K_{s}$ band have a linear bias of $\sim 1.3$.

astro-ph.CO↗

arXiv2017

Testing isotropy in the Two Micron All-Sky redshift survey with information entropy

We use information entropy to test the isotropy in the nearby galaxy distribution mapped by the Two Micron All-Sky redshift survey (2MRS). We find that the galaxy distribution is highly anisotropic on small scales. The radial anisotropy gradually decreases with increasing length scales and the observed anisotropy is consistent with that expected for an isotropic Poisson distribution beyond a length scale of $90 \, h^{-1}\, {\rm Mpc}$. Using mock catalogues from N-body simulations, we find that the galaxy distribution in the 2MRS exhibits a degree of anisotropy compatible with that of the $Λ$CDM model after accounting for the clustering bias of the 2MRS galaxies. We also quantify the polar and azimuthal anisotropies and identify two directions $(l,b)=(150^{\circ}, -15^{\circ})$, $(l,b)=(310^{\circ},-15^{\circ})$ which are significantly anisotropic compared to the other directions in the sky. We suggest that their preferential orientations on the sky may indicate a possible alignment of the Local Group with two nearby large scale structures. Despite the differences in the degree of anisotropy on small scales, we find that the galaxy distributions in both the 2MRS and the $Λ$CDM model are isotropic on a scale of $90 \, h^{-1}\, {\rm Mpc}$.

astro-ph.CO↗

arXiv2016

An information theory based search for homogeneity on the largest accessible scale

We analyze the SDSS DR12 quasar catalogue to test the large-scale smoothness in the quasar distribution. We quantify the degree of inhomogeneity in the quasar distribution using information theory based measures and find that the degree of inhomogeneity diminishes with increasing length scales which finally reach a plateau at $\sim 250 \, h^{-1}\, {\rm Mpc}$. The residual inhomogeneity at the plateau is consistent with that expected for a Poisson point process. Our results indicate that the quasar distribution is homogeneous beyond length scales of $250 \, h^{-1}\, {\rm Mpc}$.

astro-ph.CO↗

arXiv2016

How much a galaxy knows about its large-scale environment?: An information theoretic perspective

The small-scale environment characterized by the local density is known to play a crucial role in deciding the galaxy properties but the role of large-scale environment on galaxy formation and evolution still remain a less clear issue. We propose an information theoretic framework to investigate the influence of large-scale environment on galaxy properties and apply it to the data from the Galaxy Zoo project which provides the visual morphological classifications of $\sim 1$ million galaxies from the Sloan Digital Sky Survey. We find a non-zero mutual information between morphology and environment which decreases with increasing length scales but persists throughout the entire length scales probed. We estimate the conditional mutual information and the interaction information between morphology and environment by conditioning the environment on different length scales and find a synergic interaction between them which operates upto at least a length scales of $ \sim 30 \, h^{-1}\, {\rm Mpc}$. Our analysis indicates that these interactions largely arise due to the mutual information shared between the environments on different length scales.

astro-ph.CO↗