The emergent Big Bang scenario

TL;DR

This paper introduces a novel cosmological model where the Big Bang singularity is replaced by a smooth signature change in a purely Riemannian space, leading to an emergent universe with testable features.

Contribution

It proposes a new framework for cosmology based on signature change in Riemannian space, extending beyond the traditional singularity.

Findings

Existence of solutions with an emergent universe and primordial de Sitter phase.

A large Euclidean region with Lorentzian islands hosting expanding universes.

Potential observational signatures of the signature change scenario.

Abstract

This paper proposes a new avenue for understanding the cosmological singularity. The standard cosmological model contains a generic initial singularity usually referred to as the {\em big bang}. Herein, we present a novel idea to extend the description of our Universe beyond this limit. The proposal relies on rewriting physics in a purely Riemannian, {\em i.e.} locally Euclidean, four-dimensional space and the emergence of Lorentzian patches owing to the interaction of all matter fields to a clock field that is responsible for a signature change. If our Universe is contained within one of these patches, the initial singularity is replaced by a smooth boundary on which the signature of the physical metric flips. In this paper, we first define the model and draw the necessary conditions on its arbitrary functions for solutions to exist. Next, we prove the existence of solutions that lead to an emergent universe with a primordial (almost) de Sitter phase. To finish, we discuss the consequences of this construction for the universe on scales much larger than our observable Universe: a large ``Euclidean sea'' in which Lorentzian islands locally emerge and host an expanding universe potentially similar to ours. While speculative, this scenario has specific features that can be tested, and the present paper sets the basis for further phenomenological investigations.