You Can't Get There From Here: Hubble Relaxation in the Local Volume

Abstract

A beginning end-point for galaxy motions within the 10-Mpc Local Volume is constructed by requiring a smooth distribution of (luminous) mass at the time of recombination, which is shown to be equivalent to a smooth Hubble flow at early times. It is found, by this purely kinematical method, that present peculiar motions are too small by a factor of at least several (and largely in the wrong direction) to have produced the observed structures within the age of the universe. Known dynamical effects are inadequate to remove the discrepancy. This result is different in origin from previously known ``cold flow'' problems. The simple dynamical picture often used within the Local Volume (for instance, in deriving masses through calculation of a zero-velocity surface) is thus called into question. The most straightforward explanation (though not the only possible) is that there exists a large quantity of baryonic matter in this region so far undetected, and unassociated with galaxies or groups.