Non-BPS D9-Branes in the Early Universe

We have investigated the finite temperature systems of non-BPS D-branes andD-brane-anti-D-brane pairs in the previous papers. It has been shown thatnon-BPS D9-branes and D9-anti-D9 pairs become stable near the Hagedorntemperature on the basis of boundary string field theory. This implies thatthere is a possibility that these spacetime-filling branes exist in the earlyuniverse. We study the time evolution of the universe in the presence ofnon-BPS D9-branes on the basis of boundary string field theory in this paper.We try to construct the following scenario for the early universe: The universeexpands at high temperature and the open string gas on the non-BPS D9-branesdominates the total energy of the system at first. The temperature decreases asthe universe expands. Then the non-BPS D9-branes become unstable at lowtemperature and decay through tachyon condensation. We obtain some classicalsolutions for Einstein gravity and dilaton gravity in the very simple cases.Comment: 26 pages, 9 figures, comments and references added, minor errors corrected, version to appear in JHE