Mirage resolution of cosmological singularities

We study time–dependent backgrounds in the low energy regimes of string theories. In particular the emphasis is on the general study of exotic phenomena such as positive acceleration and gravitational bounces. We generalize the usual Hawking–Penrose cosmological singularity theorems to higher–dimensional spacetimes and discuss their implications for time–dependent solutions in supergravity theories. The explicit examples we consider fall in two categories. First we consider effective lower–dimensional gravitational theories obtained from compactifications of ten and eleven–dimensional supergravity. We argue and explain why non–singular solutions (e.g., with positive acceleration and possibly a bounce) can in principle be obtained. However we show that their uplift to higher dimensions is always singular as predicted by the theorems. Secondly we revisit the issue of supergravity s–branes. Our main result is to propose a generic mechanism by which the usual singularities can be resolved.