Ω from velocities in voids

We propose a method for deriving a dynamical lower bound on Ω from diverging flows in low-density regions, based on the fact that large outflows are not expected in a low-Ω universe. The velocities are assumed to be induced by gravity from small initial fluctuations, but no assumptions are made regarding galaxy biasing, the exact statistical nature of the fluctuations or Λ. The derivatives of a diverging velocity field infer a nonlinear approximation to the mass density, which is an overestimate when the true value of Ω is assumed. This inferred density can become ridiculously negative when the assumed Ω is too low, thus bounding Ω. Observed radial peculiar velocities of galaxies allow the POTENT procedure to recover the required velocity field, Gaussian smoothed at 1200 km s^-1. The density and the associated errors are then inferred for different values of Ω, searching for a void which shows negative inferred density with high confidence. A preliminary application to data in a southern void indicates that Ω < 0.3 can be ruled out at the 2.4 σ level. A detailed study of possible systematic errors is under way.