Geometric Precipices in String Cosmology

TL;DR

This paper investigates how graviton multiplet fields, especially the dilaton, influence phase transitions in string cosmology, revealing that certain transitions are obstructed by conserved signs, leading to singularities and superselection sectors.

Contribution

It demonstrates that the dilaton's conserved sign prevents smooth transitions between string gas phases, highlighting geometric precipices in the moduli space of string cosmology.

Findings

Many thermodynamic phases are separated by singularities.

Certain phase transitions are obstructed by conserved dilaton sign.

Violations of energy positivity and NEC could enable transitions.

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.