PROBING COSMOLOGICAL ISOTROPY WITH TYPE Ia SUPERNOVAE

Abstract

We investigate the validity of the Cosmological Principle by mapping the cosmological parameters H 0 and q 0 through the celestial sphere. In our analysis, performed in a low-redshift regime to follow a model-independent approach, we use two compilations of type Ia Supernovae (SNe Ia), namely the Union2.1 and the JLA datasets. Firstly, we show that the angular distributions for both SNe Ia datasets are statistically anisotropic at high confidence level (p-value < 0.0001), in particular the JLA sample. Then we find that the cosmic expansion and acceleration are mainly of dipolar type, with maximal anisotropic expansion [acceleration] for the Union2.1 and JLA data, respectively. Secondly, we use a geometrical method to test the hypothesis that the non-uniformly distributed SNe Ia events could introduce anisotropic imprints on the cosmological expansion and acceleration. For the JLA compilation, we found significant correlations between the celestial distribution of data points and the directional studies of H 0 and q 0 , suggesting that these results can be attributed to the intrinsic anisotropy of the sample. In the case of the Union2.1 data, nonetheless, these correlations are less pronounced, and we verify that the dipole asymmetry found in the H 0 analyses coincides with the well-known bulk-flow motion of our local group. From these analyses, we conclude that the directional asymmetry on the cosmological parameters maps are mainly either of local origin or due to celestial incompleteness of current SNe Ia samples.

Testing the isotropy of the Universe by using the JLA compilation of Type Ia supernovae

“…Compared to previous compilations such as Union2 (Amanullah et al 2010) and Union2.1 (Suzuki et al 2012), the number of SNe Ia in the JLA compilation is highly enlarged and the systematic uncertainties are significantly reduced. Recently, the JLA SNe Ia have been used to probe the anisotropic Hubble diagram in Bianchi type I cosmology (Schucker, Tilquin & Valent 2014) and test the cosmological principle (Bengaly, Bernui & Alcaniz 2015). However, the work (Bengaly, Bernui & Alcaniz 2015) did not consider the full covariance matrix between SNe Ia.…”

“…Recently, the JLA SNe Ia have been used to probe the anisotropic Hubble diagram in Bianchi type I cosmology (Schucker, Tilquin & Valent 2014) and test the cosmological principle (Bengaly, Bernui & Alcaniz 2015). However, the work (Bengaly, Bernui & Alcaniz 2015) did not consider the full covariance matrix between SNe Ia. In this paper, we use the JLA compilation to restudy the anisotropic Hubble diagram of the Universe.…”

Probing cosmological isotropy with Planck Sunyaev–Zeldovich galaxy clusters

“…We can achieve it by directly studying the angular distribution of cosmic objects in antipodal patches of the celestial sphere. In this sense, the second release of Planck Sunyaev-Zeldovich (PSZ2) catalogue (Ade et al 2015b,c) provides an ideal observational sample 1 The possibility of anisotropy in the cosmological expansion was also investigated with Type Ia Supernovae luminosity distance data (Schwarz & Weinhorst 2007;Antoniou & Perivolaropoulos 2010;Cai & Tuo 2012;Turnbull et al 2012;Kalus et al 2013;Feindt et al 2013;Jiménez et al 2015;Appleby et al 2015;Bengaly et al 2015;Javanmardi et al 2015). No significant result supporting an anisotropic expansion has been detected so far.…”

Asymmetry and dark energy evolution

“…Using the Loss sample, the authors of [31] found a hint for anisotropy -most pronounced in the range 0.015 < z < 0.045 -but once they take into account large scale velocity perturbations, the results shows no evidence for any anomalous deviation from the isotropic ΛCDM. In [29] the authors studied this issue using the Union 2.1 and JLA set. Although they found that both datasets are statistically anisotropic, the correlation with the anisotropic distribution found turns out to be different: using the Union 2.1 they found a small correlation but using the JLA sample they found a stronger correlation.…”

“…In this context, it is interesting to cite the case of a preferred direction of expansion, as for example, the large scale bulk-flow [20,22,21,24,23], the low multipoles alignment in the CMB angular power spectrum [25], the large scale alignment of quasar polarization [26], and studies using SNIa [27,32,33,28,34,35,30,31,29]. So the evidence points towards the necessity to a study further the effects on the cosmological parameters of a inhomogeneous local distribution of matter.…”