24 research outputs found
Testing the Cosmological Principle in the radio sky
The Cosmological Principle states that the Universe is statistically
isotropic and homogeneous on large scales. In particular, this implies
statistical isotropy in the galaxy distribution, after removal of a dipole
anisotropy due to the observer's motion. We test this hypothesis with number
count maps from the NVSS radio catalogue. We use a local variance estimator
based on patches of different angular radii across the sky and compare the
source count variance between and within these patches. In order to assess the
statistical significance of our results, we simulate radio maps with the NVSS
specifications and mask. We conclude that the NVSS data is consistent with
statistical isotropy.Comment: 7 pages, 3 figures. To appear in JCA
Revisiting the statistical isotropy of GRB sky distribution
The assumption of homogeneity and isotropy on large scales is one of the main
hypotheses of the standard cosmology. In this paper, we test the hypothesis of
isotropy from the two-point angular correlation function of 2626 gamma-ray
bursts (GRB) of the FERMI GRB catalogue. We show that the uncertainties in the
GRB positions induce spurious anisotropic signals in their sky distribution.
However, when such uncertainties are taken into account no significant evidence
against the large-scale statistical isotropy is found. This result remains
valid even for the sky distribution of short-lived GRB, contrarily to previous
reports.Comment: 9 pages, 10 figures, 2 tables, match accepted versio
Examining the validity of the minimal varying speed of light model through cosmological observations: relaxing the null curvature constraint
We revisit a consistency test for the speed of light variability, using the
latest cosmological observations. This exercise can serve as a new diagnostics
for the standard cosmological model and distinguish between the minimal varying
speed of light in the Friedmann-Lema\^{i}tre-Robertson-Walker universe. We
deploy Gaussian processes to reconstruct cosmic distances and ages in the
redshift range utilizing the Pantheon compilation of type-Ia supernova
luminosity distances (SN), cosmic chronometers from differential galaxy ages
(CC), and measurements of both radial and transverse modes of baryon acoustic
oscillations (-BAO and -BAO) respectively. Such a test has the advantage
of being independent of any non-zero cosmic curvature assumption - which can be
degenerated with some variable speed of light models - as well as any dark
energy model. We also examine the impact of cosmological priors on our
analysis, such as the Hubble constant, supernova absolute magnitude, and the
sound horizon scale. We find null evidence for the speed of light variability
hypothesis for most choices of priors and data-set combinations. However, mild
deviations are seen at confidence level for redshifts with
some specific prior choices when -BAO data is employed, and at with a
particular reconstruction kernel when -BAO data are included. Still, we
ascribe no statistical significance to this result bearing in mind the
degeneracy between the associated priors for combined analysis, and
incompleteness of the -BAO data set at higher .Comment: 15 pages, 8 sets of figures, revised version, including a change in
title, to appear in Phys. Dark Uni
Is the local Hubble flow consistent with concordance cosmology?
Yes. In a perturbed Friedmann model, the difference of the Hubble constants
measured in two rest-frames is independent of the source peculiar velocity and
depends only on the relative velocity of the observers, to lowest order in
velocity. Therefore this difference should be zero when averaging over
sufficient sources, which are at large enough distances to suppress local
nonlinear inhomogeneity. We use a linear perturbative analysis to predict the
Doppler effects on redshifts and distances. Since the observed redshifts encode
the effect of local bulk flow due to nonlinear structure, our linear analysis
is able to capture aspects of the nonlinear behaviour. Using the largest
available distance compilation from CosmicFlows-3, we find that the data is
consistent with simulations based on the concordance model, for sources at
Mpc.Comment: 10 pages, 3 figures. Version accepted by JCA
Evidence for cosmic acceleration with next-generation surveys: A model-independent approach
We quantify the evidence for cosmic acceleration using simulations of H(z) measurements from SKA- and Euclid-like surveys. We perform a non-parametric reconstruction of the Hubble parameters and its derivative to obtain the deceleration parameter q(z) using the Gaussian Processes method. This is a completely model-independent approach, so we can determine whether the Universe is undergoing accelerated expansion regardless of any assumption of a dark energy model
Measuring our velocity from fluctuations in number counts
Our velocity relative to the cosmic microwave background (CMB) generates a
dipole from the CMB monopole, which was accurately measured by COBE. The
relative velocity also modulates and aberrates the CMB fluctuations, generating
a small signature of statistical isotropy violation in the covariance matrix.
This signature was first measured by Planck 2013. Galaxy surveys are similarly
affected by a Doppler boost. The dipole generated from the number count
monopole has been extensively discussed, and measured (at very low accuracy) in
the NVSS and TGSS radio continuum surveys. For the first time, we present an
analysis of the Doppler imprint on the number count fluctuations, using the
bipolar spherical harmonic formalism to quantify these effects. Next-generation
wide-area surveys with a high redshift range are needed to detect the small
Doppler signature in number count fluctuations. We show that radio continuum
surveys with the SKA should enable a detection at in Phase
2, with marginal detection possible in Phase 1.Comment: Version accepted by JCA
Machine Learning the Hubble Constant
Local measurements of the Hubble constant () based on Cepheids e Type Ia
supernova differ by from the estimated value of from
Planck CMB observations under CDM assumptions. In order to better
understand this tension, the comparison of different methods of analysis
will be fundamental to interpret the data sets provided by the next generation
of surveys. In this paper, we deploy machine learning algorithms to measure the
through a regression analysis on synthetic data of the expansion rate
assuming different values of redshift and different levels of uncertainty. We
compare the performance of different algorithms as Extra-Trees, Artificial
Neural Network, Extreme Gradient Boosting, Support Vector Machines, and we find
that the Support Vector Machine exhibits the best performance in terms of
bias-variance tradeoff, showing itself a competitive cross-check to
non-supervised regression methods such as Gaussian Processes.Comment: 13 pages, 3 figures. Comments welcome. Scripts available at
https://github.com/astrobengaly/machine_learning_H
Is there evidence for a hotter Universe?
The measurement of present-day temperature of the Cosmic Microwave Background
(CMB), K (1), made by the Far-InfraRed
Absolute Spectrophotometer (FIRAS), is one of the most precise measurements
ever made in Cosmology. On the other hand, estimates of the Hubble Constant,
, obtained from measurements of the CMB temperature fluctuations assuming
the standard CDM model exhibit a large () tension when
compared with low-redshift, model-independent observations. Recently, some
authors argued that a slightly change in could alleviate or solve the
-tension problem. Here, we investigate evidence for a hotter or colder
universe by performing an independent analysis from currently available
temperature-redshift measurements. Our analysis (parametric and
non-parametric) shows a good agreement with the FIRAS measurement and a
discrepancy of from the values required to solve the
tension. This result reinforces the idea that a solution of the
-tension problem in fact requires either a better understanding of the
systematic errors on the measurements or new physics.Comment: 4 pages, 2 figures, 1 table. Accepted for publication in European
Physical Journal