Superstring Cosmology: A Review

The enhancement of superstring theory to M-theory has profound implications for cosmology. I review the most significant aspects of the M theory cosmological- picture resulting from the combined effects of (1) M-theory's (10+1)- dimensional spacetime (compared to string theory's (9+1)-dimensional spacetime), (2) M- theory's realization that strings have a width determined by the size of the additional spatial direction (transforming strings into membranes), and (3) the M theory- prediction of spatial surfaces called D-branes. The role of membranes and D-branes in the pre-Big Bang and inflationary eras are discussed. M-theory revisions of the original Brandenberger-Vafa string inflation model are briefly summarized. The Brandenberger-Vafa model offers a qualitative understanding for why only three spatial directions became large. I also discuss the Randall-Sundrum model and extensions of it, which offer a Dbrane explanation for the apparent weakness of- gravity in our 3+1 large dimensions. A fundamental question in M-theory is, "What is the string/M-theory energy scale and corresponding length scale?" I review the recent astronomical measurements of John Webb et al. suggesting this scale to be much lower than the String/ (Planck) scale of 101 8 Gev. Some of the ``physical constants'' in nature, such as the fine- structure constant, which specifies the strength of electromagnetic interaction, may have been varying significantly only a few billion years ago. This suggests a low energy M-theory scale and a related long time before some compactified directions stabilized. An implication of a low energy M-theory scale is that gravity may stop acting as a 1/r2 force at distances somewhere below a millimeter. The distance at which deviation from 1/r2 begins corresponds to the size of the eleventh dimension. Recent University of Washington experiments verify gravity as a 1/r2 force down to 0.2mm.