Omega from velocities in voids

We propose a method for deriving a dynamical lower bound on $Omega$ from diverging flows in low-density regions, based on the fact that large outflows are not expected in a low-$Omega$ universe. The velocities are assumed to be induced by gravity from small initial fluctuations, but no assumptions are made regarding their exact Gaussian nature, galaxy biasing, or $Lambda$. The derivatives of a diverging velocity field infer a nonlinear approximation to the mass density, which is an overestimate when the true value of $Omega$ is assumed. This inferred density can become ridiculously negative when the assumed $Omega$ is too low, thus bounding $Omega$. Observed radial peculiar velocities of galaxies allow the pot procedure to recover the required velocity field, Gaussian smoothed at $1200kms$. The density and the associated errors are then inferred for different values of $Omega$, searching for a void which shows negative inferred density with high confidence. A preliminary application to data in a southern void indicates that $Omegaleq 0.3$ can be ruled out at the $2.4$-sigma level. A detailed study of possible systematic errors is under way.Comment: 9 pages, compressed and uuencoded PostScrip