Instabilities and particle production in S-brane geometries

We study the classical stability of a class of S-brane geometries havingcosmological horizons. By considering the perturbations of the metric in thesegeometries we establish that their horizons are unstable in the sense that anobserver trying to cross the horizon experiences an infinite flux of radiationat the instant of crossing. The backreaction of this radiation is likely toconvert the horizons into curvature singularities, similar to the instabilityof the internal Cauchy horizon of the Reissner-Nordstrom black hole. We alsocompute the particle production by the time-dependent fields in the futureregions of these geometries, and find that the spectrum of produced particlesis thermal, with temperature coinciding with the Hawking temperature computedby euclideanizing the metric in the static region. Possible implications ofthese results are discussed.Comment: 17 pages, 4 figures, JHEP3.cls. Typos corrected, section 2 extended, reference adde