Constraining the anisotropic expansion of the Universe

We study the possibly existing anisotropy in the accelerating expansion Universe with the Union2 type Ia supernovae data and gamma-ray burst data. We construct a direction-dependent dark energy model and constrain the anisotropy direction and strength of modulation. We find that the maximum anisotropic deviation direction is $(l,b)=(126°,13°)$ [or, equivalently, $(l,b)=(306°,-13°)$], and the current anisotropy level is $g_0={0.030}_{+0.010}^{-0.030}$ ($1\sigma$ confidence level with Union2 data). Our results do not show strong evidence for the anisotropic dark energy model. We also discuss potential methods that may distinguish the peculiar velocity field from the anisotropic dark energy model.

Figure 1

Likelihoods for parameters $(g_0,g_1,l,b)$ using the full Union2 data, with the best-fitting parameters ($g_0=0.030$, $g_1=-0.031$, $l=126°$, $b=13°$).

Figure 2

68% and 95% joint posterior probability distribution of parameters $(g_0,g_1)$. The red, green, and blue contours represent the results using the GRB, SNIa, and $\mathrm{SNIa}+\mathrm{GRB}$ data, respectively.

Figure 3

Best-fitting $g(z)$ with $1\sigma$ error at different redshifts.