No open or flat bouncing cosmologies in Einstein gravity

Abstract: We show that, to first approximation, strings cannot propagate in bouncing open or flat Friedmann-Robertson-Walker universes. Specifically, the Virasoro constraint translates to the Ricci convergence condition in spacetime at leading order in the alpha-prime expansion. Thus one must go beyond minimally-coupled classical Einstein gravity in order to find bounce solutions that could be consistent with string theory. We map out some remaining possibilities for finding string-compatible cosmological bounces.