Examining the cosmic acceleration with the latest Union2 supernova data

Abstract

In this Letter, by reconstructing the $Om$ diagnostic and the deceleration parameter $q$ from the latest Union2 Type Ia supernova sample with and without the systematic error along with the baryon acoustic oscillation (BAO) and the cosmic microwave background (CMB), we study the cosmic expanding history, using the Chevallier-Polarski-Linder (CPL) parametrization. We obtain that Union2+BAO favor an expansion with a decreasing of the acceleration at $z<0.3$. However, once the CMB data is added in the analysis, the cosmic acceleration is found to be still increasing, indicating a tension between low redshift data and high redshift one. In order to reduce this tension significantly, two different methods are considered and thus two different subsamples of Union2 are selected. We then find that two different subsamples+BAO+CMB give completely different results on the cosmic expanding history when the systematic error is ignored, with one suggesting a decreasing cosmic acceleration, the other just the opposite, although both of them alone with BAO support that the cosmic acceleration is slowing down. However, once the systematic error is considered, two different subsamples of Union2 along with BAO and CMB all favor an increasing of the present cosmic acceleration. Therefore a clear-cut answer on whether the cosmic acceleration is slowing down calls for more consistent data and more reliable methods to analyze them.Comment: 17 pages, 6 figures; PLB in pres

Asymmetry and dark energy evolution

“…Altogether, the apparent detection of a local under density can be considered as a possible cause of the original tension between low and high redshift observational constraints discussed in [10,11,12,13,15], which as a solution also agree with the analysis of [19] where they proposed the existence of a local under-density to alleviate the tension in the determination of H 0 .…”

“…We have used the Union 2 set and the result of [27], a dipolar asymmetry with a maximum acceleration towards (l, b) = (309 o , 18 o ) and a minimum acceleration towards (l, b) = (129 o , −18 o ) is found. The low reshift transition, previously recognized in [10,11,12,13,14,15], appears only towards the direction of minimum acceleration, and it is absent towards the opposite direction. This result was obtained separating the data in two hemispheres constructed along the maximal asymmetry axis.…”

“…where Ω m comprises both the baryonic and non baryonic DM, and the reconstructed DE density is From this, following previous works [11,12], we can reconstruct the deceleration parameter function…”

“…Later, this phenomena was found using new SNIa data [11], [12], also using data from gas mass fraction in galaxy clusters [13], and using different parameterizations [15], showing that this effectthe low redshift transition of the reconstructed q(z) -is telling us something about our local universe, because the evidence for such transition disappears once data from BAO and CMB (high z physics) are added.…”

“…We can parametrize the equation of state (EoS) parameter w = p/ρ (where p is the pressure and ρ the energy density) as a function of redshift [10,11,12,13], or we can parametrize the DE density itself [14].…”