Emergence of fluctuations from a tachyonic big bang

It has recently been speculated that the end state of a collapsing universe is a tachyonic big crunch. The time reversal of this process would be the emergence of an expanding universe from a tachyonic big bang. In this framework, we study the emergence of cosmological fluctuations. In particular, we compare the amplitude of the perturbations at the end of the tachyon phase with what would be obtained assuming the usual vacuum initial conditions. We find that cosmological fluctuations emerge in a thermal state. We comment on the relation to the trans-Planckian problem of inflationary cosmology.

Figure 1

Spacetime sketch of our tachyon condensate background. The vertical axis denotes time, the horizontal axis is space. The bottom dashed region indicates the region of strong tachyon condensation when it does not make sense to talk about space. The upper crosshatched region is the region of strong coupling due to the linear dilaton, which we assume is stabilized to yield a standard cosmology. Our analysis studies the emergence of fluctuations in the intermediate unhatched weak tachyon phase.