DBI Realizations of the Pseudo-Conformal Universe and Galilean Genesis Scenarios

TL;DR

This paper embeds the pseudo-conformal universe within a DBI (non-linear) realization of conformal symmetry, realized on a brane in ${AdS}_5$ and, more generally, in ${AdS}_5\times S^1$; it shows that a scaling background $\bar{\phi}(t)\sim \alpha/(-t)$ spontaneously breaks $so(4,2)$ to $so(4,1)$ and yields subluminal perturbations with $c_s=1/\bar{\gamma}<1$, while weight-0 spectator fields supply the necessary scale-invariant entropy perturbations. The authors derive the quadratic actions for fluctuations using symmetry arguments, obtaining fixed mass terms for non-zero weights, an unconstrained mass matrix for weight-0 fields, and explicit mode solutions that produce red-tilted spectra for massive weights but scale-invariant spectra for weight-0 modes, with the entropy perturbations able to seed adiabatic fluctuations. They also present a DBI Galilean Genesis realization with cubic conformal DBI galileon terms that preserves subluminal propagation and analyze stability across branches $\bar{\gamma}_{\pm}$, offering a robust, UV-motivated alternative to inflation. Together, these results provide a symmetry-driven, UV-complete-inspired framework for the pseudo-conformal mechanism and its Galilean Genesis variant, with explicit predictions for the propagation speed and spectrum of perturbations in multi-field setups. The work highlights how extra bulk isometries can naturally generate scale-invariant entropy perturbations while maintaining subluminal dynamics, strengthening the case for DBI realizations as viable alternatives to inflation in the early universe.

Abstract

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.