Edward Macaulay, Ingunn Kathrine Wehus, Hans Kristian Eriksen
We perform a meta-study of recently published Redshift Space Distortion (RSD) measurements of the cosmological growth rate, f(z)\sigma_8(z). We analyse the latest results from the 6dFGS, BOSS, LRG, WiggleZ and VIPERS galaxy redshift surveys, and compare the measurements to expectations from Planck. In this Letter we point out that the RSD measurements are consistently less than the values expected from Planck, and the relative scatter between the RSD measurements is lower than expected. Adopting the equation-of-state of dark energy, w, as a strawman for comparison purposes, we obtain a best-fit value of w=-0.74\pm0.07 from a joint fit to all recent measurements, in tension with the \LambdaCDM value of w=-1 at greater than the three standard deviation level. However, the \chi^2 of the resulting fit is 2.70 for 8 degrees of freedom, corresponding to a probability-to-exceed (PTE) of 0.952. On fitting for \sigma_8, we find \sigma_8=.722\pm0.038, \chi^2=2.91 and PTE=0.940. Further, a simple constant in redshift for f(z)\sigma_8(z) provides an even better fit, with \chi^2=2.09 for a PTE of 0.978. On changing the LRG results to an analysis with a longer maximum correlation scale, we find w=-0.78\pm0.07, \chi^2=4.01, and PTE=0.856, a best fit constant value of 0.425\pm0.04, \chi^2=3.71, and PTE=0.883, or \sigma_8=.741\pm0.034, \chi^2=4.05 and PTE=0.852. A full resolution of this issue may require a more robust treatment of non-linear effects in RSD models, although the trend for a low \sigma_8 agrees with recent constraints on \sigma_8 and \Omega_m from Sunyaev-Zeldovich cluster counts identified in Planck.
View original:
http://arxiv.org/abs/1303.6583
No comments:
Post a Comment