UC Santa Cruz

We study the obstacles to constructing metastable de Sitter space in string theory. We explain that it is very difficult to find stationary points for which both the string coupling is small and compactification radii are large even allowing the possibility of arbitrarily large fluxes, and a set of small perturbations of any would-be metastable de Sitter state, classically, will evolve to uncontrollable singularities. We study the Transplanckian Censorship Conjecture and show a conflict between the TCCand conventional conjectures about the string landscape. We calculate, as a function of the primordial black holes mass and initial abundance, the combination of dark matter particle masses and number of effective dark degrees of freedom leading to the right abundance of dark matter today, whether or not evaporation stops around the Planck scale.