Starting the Universe: stable violation of the null energy condition and non-standard cosmologies

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 withḢ > 0. Demanding the absence of instabilities in this model requireṡ H < ∼ 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.

Oxidised cosmic acceleration

“…Some objects in string theory (such as orientifold planes) and quantum effects (such as Casimir energies) can violate the NEC. Higher derivative terms may permit NEC violation without associated pathologies: it may be possible to construct pathology-free quantum field theories with more than two derivatives [81,82]. String and M theory have a characteristic pattern of higher-derivative terms in the low-energy effective action.…”

Inflation and String Theory

“…2 In Appendix A, we collect mathematical concepts, definitions, and results that will be helpful for following the discussion in Chapters 3-5. In Appendix B, we present the effective theory of adiabatic fluctuations during inflation [50,51]. In Appendix C, we introduce cosmological perturbation theory and derive the primordial perturbations from inflation.…”

Об Изотропном Энергетическом Условии И Космологии

“…Описание модели. В модели фантомного конденсата в плоском четырех-мерном пространстве Минковского с сигнатурой (−, +, +, +) используется лагран-жиан [13]- [16] …”

“…Члены старшего порядка S 1 и S 2 в (1) добавлены к P , чтобы обеспечить наличие слагаемого с пространственной производной. Сле-дующий квадратичный лагранжиан рассматривался в [13]- [16]:…”

“…Однако недавно были сделаны попытки построить мо-дели, к которым эти общие теоремы не применимы. Это модель фантомного кон-денсата [13]- [16] и нелокальная струнная модель [17]- [20], которые включают в себя высшие производные. В нашей работе мы анализируем эти две модели, используя метод Остроградского и представление Вейерштрасса.…”