Violation Of Null Energy Condition Research Articles

Propagation of adiabatic and entropy perturbations in abouncing universe

By studying some bouncing universe models dominated by aspecific class of hydrodynamical fluids, we show that the primordialcosmological perturbations may propagate smoothly through a generalrelativistic bounce. We also find that the purely adiabatic modes,although almost always fruitfully investigated in all other contextsin cosmology, are meaningless in the bounce or null energy condition(NEC) violation cases since the entropy modes can never be neglectedin these situations: the adiabatic modes exhibit a fake divergencethat is compensated in the total Bardeen gravitational potential byinclusion of the entropy perturbations.

Model of graceful exit in string cosmology

We construct, for the first time, a model of graceful exit transition from a dilaton-driven inflationary phase to a decelerated Friedman-Robertson-Walker era. Exploiting a demonstration that classical corrections can stabilize a high curvature string phase while the evolution is still in the weakly coupled regime, we show that if additional terms of the type that may result from quantum corrections to the string effective action exist, and induce violation of the null energy condition, then evolution towards a decelerated Friedman-Robertson-Walker phase is possible. We also observe that stabilizing the dilaton at a fixed value, either by capture in a potential minimum or by radiation production, may require that these quantum corrections are turned off, perhaps by non-perturbative effects or higher order contributions which overturn the null energy condition violation.