[hep-th] kaloper,seminar

[Maja Buric]

Cvele, moze li seminar u cetvrtak, 12 ili 11, kako mislis?
Jer Nemanja javlja da ima obaveze u petak


Geometric Precipices in String Cosmology


 Nemanja Kaloper


Abstract: We consider the effects of graviton multiplet fields on 
transitions between string gas phases. Focusing on the dilaton field, we 
show that it may obstruct transitions between different thermodynamic phases 
of the string gas, because the sign of its dimensionally reduced, $T$-
duality invariant, part is conserved when the energy density of the universe 
is positive. Thus, many interesting solutions for which this sign is 
positive end up in a future curvature singularity. Because of this, some of 
the thermodynamic phases of the usual gravitating string gases behave like 
superselection sectors. For example, a past-regular Hagedorn phase and an 
expanding FRW phase dominated by string momentum modes cannot be smoothly 
connected in the framework of string cosmology with positive sources. The 
singularity separates them like a geometric precipice in the moduli space, 
preventing the dynamics of the theory from bridging across. Sources which 
simultaneously violate the positivity of energy and NEC could modify these 
conclusions. We provide a quantitative measure of positivity of energy and 
NEC violations that would be necessary for such transitions. These effects 
must dominate the universe at the moment of transition, altering the 
standard gas pictures. At present, it is not known how to construct such 
sources from first principles in string theory