D-branes and cosmology

D-branes, topological defects in string theory on which string endpoints ca n live, may give new insight into the understanding of the cosmological evo lution of the Universe at early epochs. We analyze the dynamics of D-branes in curved backgrounds and discuss the parameter space of M-theory as a fun ction of the coupling constant and of the curvature of the Universe. We sho w that D-branes may be efficiently produced by gravitational effects. Furth ermore, in curved space-times the transverse fluctuations of the D-branes d evelop a tachyonic mode and when the fluctuations grow larger than the hori zon the branes become tensionless and break up. This signals a transition t o a new regime. We discuss the implications of our findings for the singula rity problem present in string cosmology, suggesting the existence of a lim iting value for the curvature which is in agreement with the value suggeste d by the cosmological version of the holography principle. We also comment on possible implications for the so-called brane world scenario, where the Standard Model gauge and matter fields live inside some branes while gravit ons live in the bulk, (C) 1999 Elsevier Science B.V. All rights reserved.