We show that the Dirac-Born-Infeld conformal galileons, derived from the world-volume theory of a 3-brane moving in an anti-de Sitter bulk, admit a background, stable under quantum corrections, which violates the null energy condition. The perturbations around this background are stable and propagate subluminally. Unlike other known examples of null energy condition violation, such as ghost condensation and conformal galileons, this theory also admits a stable, Poincaré-invariant vacuum. The 2 → 2 amplitude satisfies standard analyticity conditions. The full S matrix is likely not analytic, however, since perturbations around deformations of the Poincaré invariant vacuum propagate superluminally.

The pseudo-conformal universe is an alternative to inflation in which the early universe is described by a conformal field theory on approximately flat space-time. The fields develop time-dependent expectation values, spontaneously breaking the conformal symmetries to a de Sitter subalgebra, and fields of conformal weight zero acquire a scale invariant spectrum of perturbations. In this paper, we show that the pseudo-conformal scenario can be naturally realized within theories that would ordinarily be of interest for DBI inflation, such as the world-volume theory of a probe brane in an AdS bulk space-time. In this approach, the weight zero spectator field can be associated with a geometric flat direction in the bulk, and its scale invariance is protected by a shift symmetry.arXiv:1209.5742v1 [hep-th]

We derive all single-field cosmologies with unit sound speed that generate scale invariant curvature perturbations on a dynamical attractor background. We identify three distinct phases: slow-roll inflation; a slowly contracting adiabatic ekpyrotic phase, described by a rapidly-varying equation of state; and a novel adiabatic ekpyrotic phase on a slowly expanding background. All of these yield identical power spectra. The degeneracy is broken at the 3-point level: unlike the nearly gaussian spectrum of slow-roll inflation, adiabatic ekpyrosis predicts large non-gaussianities on small scales.The observational evidence for primordial density perturbations with nearly scale invariant and gaussian statistics is compatible with the simplest inflationary scenarios. But is inflation unique? Are there dual cosmologies with indistinguishable predictions? Such questions are critical to our understanding of the very early universe.Inflation not only generates scale invariant and gaussian density perturbations, it does so on an attractor background. On super-horizon scales, the curvature perturbation on comoving hypersurfaces [1, 2], denoted by ζ, measures differences in the expansion history of distant Hubble patches [2]. In single-field inflation, ζ approaches a constant at long wavelengths. In the strict k → 0 limit, ζ → δa/a, so the perturbation simply renormalizes the scale factor of the background solution; such a perturbation can be removed by an appropriate rescaling of global coordinates. For finite k, the perturbation cannot be completely removed, but different Hubble patches experience the same cosmological evolution, up to a shift of local time coordinates and a rescaling of local spatial coordinates. See [3] for a detailed discussion.Achieving both scale invariance and dynamical attraction in alternative scenarios has proven challenging. The ζ equation of a contracting, matter-dominated universe is identical to that of inflation [4], but ζ grows outside the horizon, indicating an unstable background. The contracting phase in the original ekpyrotic scenario [5-10], with V (φ) = −V 0 e −φ/M , is an attractor [11,12], but the resulting spectrum is strongly blue [11][12][13]. A scale invariant spectrum can be obtained through entropy perturbations [14,15], as in the New Ekpyrotic scenario [14], but this requires two scalar fields.The adiabatic ekpyrotic mechanism [16][17][18][19][20] proposed recently offers a counterexample: a single-field model for which the background is a dynamical attractor and generates a scale invariant ζ. The mechanism

General relativity is a generally covariant, locally Lorentz covariant theory of two transverse, traceless graviton degrees of freedom. According to a theorem of Hojman, Kuchař, and Teitelboim, modifications of general relativity must either introduce new degrees of freedom or violate the principle of local Lorentz covariance. In this paper, we explore modifications of general relativity that retain the same graviton degrees of freedom, and therefore explicitly break Lorentz covariance. Motivated by cosmology, the modifications of interest maintain explicit spatial covariance. In spatially covariant theories of the graviton, the physical Hamiltonian density obeys an analogue of the renormalization group equation which encodes invariance under flow through the space of conformally equivalent spatial metrics. This paper is dedicated to setting up the formalism of our approach and applying it to a realistic class of theories. Forthcoming work will apply the formalism more generally.

We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.arXiv:1305.0822v2 [hep-th]