Starting the Universe: Stable Violation of the Null Energy Condition and Non-standard Cosmologies

TL;DR

The paper shows that violations of the null energy condition can be realized in a technically natural effective field theory without catastrophic instabilities by starting from a ghost condensate and incorporating higher-derivative corrections. Through a general EFT of adiabatic scalar fluctuations on FRW backgrounds, it derives a stability bound abla on NEC violation: "+ obreakdash{dot{H}} lesssim H^2"; when this bound holds, gradient and Jeans instabilities can be tamed, enabling exotic cosmologies such as superacceleration, a beginning from Minkowski space, cyclic expansion, and a smooth big bounce. It provides explicit constructions and a full gravitational analysis to illustrate these histories and discusses potential observational signatures and theoretical questions about UV completion and swampland considerations. Overall, NEC violation within a controlled EFT broadens the landscape of cosmological histories beyond standard inflation while remaining within tractable theoretical bounds.

Abstract

We present a consistent effective theory that violates the null energy condition (NEC) without developing any instabilities or other pathological features. The model is the ghost condensate with the global shift symmetry softly broken by a potential. We show that this system can drive a cosmological expansion with dH/dt > 0. Demanding the absence of instabilities in this model requires dH/dt <~ H^2. We then construct a general low-energy effective theory that describes scalar fluctuations about an arbitrary FRW background, and argue that the qualitative features found in our model are very general for stable systems that violate the NEC. Violating the NEC allows dramatically non-standard cosmological histories. To illustrate this, we construct an explicit model in which the expansion of our universe originates from an asymptotically flat state in the past, smoothing out the big-bang singularity within control of a low-energy effective theory. This gives an interesting alternative to standard inflation for solving the horizon problem. We also construct models in which the present acceleration has w < -1; a periodic ever-expanding universe and a model with a smooth ``bounce'' connecting a contracting and expanding phase.

Figures (3)

• Figure 1: Schematic representation of the model presented in section \ref{['example']}.
• Figure 2: Schematic representation of the periodic model presented in section \ref{['periodicexample']}.
• Figure 3: Schematic dependence of the scale factor $a(t)$ and of the Hubble rate $H(t)$ in the bouncing model of section \ref{['bounce']}, assuming that the bounce is matched to standard $\dot H <0$ phases both for $t< -T$ and for $t>T$.