Omega from Velocities in Voids

Abstract

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 $1200\kms$. 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 $\Omega\leq 0.3$ can be ruled out at the $2.4$-sigma level. A detailed study of possible systematic errors is under way.