A Local Hubble Bubble from Type Ia Supernovae?

We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae (SNe Ia) to test for a local void. The sample extends from 20 to 300 h^-1 Mpc, with distances, deduced from light-curve shapes, accurate to ~6%. Assuming Ω_m = 1 and Ω_Λ = 0, the most significant deviation we find from the Hubble law is an outward flow of 6.5% ± 2.2% inside a sphere of radius 70 h^-1 Mpc, as would be produced by a void of ~20% underdensity surrounded by a dense shell. This shell roughly coincides with the local great walls. Monte Carlo analyses, using Gaussian errors or bootstrap resampling, show the probability for chance occurrence of this result out of a pure Hubble flow to be ~2%. The monopole could be contaminated by higher moments of the velocity field, especially a quadrupole, which are not properly probed by the current limited sky coverage. The void would be less significant if Ω_m is low and Ω_Λ is high. It would be more significant if one outlier is removed from the sample, or if the size of the void is constrained a priori. This putative void is not in significant conflict with any of the standard cosmological scenarios. It suggests that the Hubble constant as determined within 70 h^-1 Mpc could be overestimated by ~6%, and the local value of Ω may be underestimated by ~20%. While the present evidence for a local void is marginal in this data set, the analysis shows that the accumulation of SN Ia distances will soon provide useful constraints on elusive and important aspects of regional cosmic dynamics.