Constraints on anisotropic cosmic expansion from supernovae

Abstract

Aims. We test the isotropy of the expansion of the Universe by estimating the hemispherical anisotropy of supernova type Ia (SN Ia) Hubble diagrams at low redshifts (z < 0.2). Methods. We compare the best fit Hubble diagrams in pairs of hemispheres and search for the maximal asymmetric orientation. For an isotropic Universe, we expect only a small asymmetry due to noise and the presence of nearby structures. This test does not depend on the assumed content of the Universe, the assumed model of gravity, or the spatial curvature of the Universe. The expectation for possible fluctuations due to large scale structure is evaluated for the Λ cold dark matter (ΛCDM) model and is compared to the supernova data from the Constitution set for four different light curve fitters, thus allowing a study of the systematic effects. Results. The expected order of magnitude of the hemispherical asymmetry of the Hubble expansion agrees with the observed one. The direction of the Hubble asymmetry is established at 95% confidence level (C.L.) using both, the MLCS2k2 and the SALT II light curve fitter. The highest expansion rate is found towards ( , b) ≈ (−35 • , −19 • ), which agrees with directions reported by other studies. Its amplitude is not in contradiction to expectations from the ΛCDM model. The measured Hubble anisotropy is ΔH/H ∼ 0.026. With 95% C.L. the expansion asymmetry is ΔH/H < 0.038.

Directional dependence of CMB parity asymmetry

“…Although there is still some debate [34][35][36], it has been claimed that these preferred directions are also aligned with the CMB kinematic dipole and the preferred direction of CMB quadrupole and octopole [33]. If all the directional preferences would be confirmed in the future, it would imply that the underlying physical or systematic reasons of all the cosmological anomalies should be connected.…”

Cosmic variance of the local Hubble flow in large-scale cosmological simulations

“…It is crucial then to theoretically predict these effects at the level of precision required by upcoming observations. Current theoretical works rely on analytical calculations based on either linear perturbation theory or its modifications that include some nonlinear effects (Wang et al 1998;Cooray & Caldwell 2006;Li & Schwarz 2008;Wiegand & Schwarz 2012;Kalus et al 2013;Marra et al 2013). Although this approach can give valid initial insights, it is insufficient for taking into account all non-linear effects and therefore to provide accurate predictions for the next generation measurements.…”

Comparison between hemisphere comparison method and dipole-fitting method in tracing the anisotropic expansion of the Universe use the Union2 data set

“…An alternative method is to analyze the SNe Ia data in a model-independent way (Antoniou & Perivolaropoulos 2010;Mariano & Perivolaropoulos 2012;Cai & Tuo 2012;Cai et al 2013;Kalus et al 2013;Zhang et al 2013;Yang et al 2014). The advantage of this method is that it does not rely on the particular cosmological model.…”