Possible relationship between initial conditions for inflation and past geodesic incompleteness of the inflationary spacetime

TL;DR

This paper explores how initial conditions and boundary properties of the inflationary universe relate to its past geodesic incompleteness, using a scalar field model with a non-Riemannian volume element.

Contribution

It introduces a Two-Measure Theory model showing how boundary conditions influence the initial conditions for inflation and the spacetime's geodesic structure.

Findings

Initial boundary conditions can determine the onset of inflation.

Discontinuities at the boundary affect the spacetime's geodesic extendibility.

Inflation likely occurred if initial kinetic energy dominates over gradient energy.

Abstract

According to the Borde-Guth-Vilenkin (BGV) theorem an expanding region of spacetime cannot be extended to the past beyond some boundary \mathcal{B}. Therefore, the inflationary universe must have had some kind of beginning. However, the BGW theorem says nothing about the boundary conditions on \mathcal{B}, or even about its location. Here we present a single-scalar field model of the Two-Measure Theory, where the non-Riemannian volume element $\Upsilon d^4x$ is present in the action. As a result of the model dynamics, an upper bound \varphi_0 of admissible values of the scalar field \varphi appears, which sets the position of \mathcal{B} in the form of a spacelike hypersurface \Upsilon(x)=0 with a boundary condition: \Upsilon\to 0^+ as \varphi\to\varphi_0^-. A detailed study has established that if the initial kinetic energy density \rho_{kin}^{(in)} prevails over initial gradient energy density \rho_{grad}^{(in)} then there is an interval of initial values \varphi_{in}^{(min)}\leq\varphi_{in}<\varphi_0, where \rho_{kin}^{(in)} and \rho_{grad}^{(in)} cannot exceed the potential energy density and hence the initial conditions necessary for the onset of inflation are satisfied. It is shown that under almost all possible left-handed boundary conditions on \mathcal{B}, that is where \Upsilon\to 0^-, the metric tensor in the Einstein frame has a jump discontinuity on \mathcal{B}, so the Christoffel connection coefficients are not defined on the spacelike hypersurface \Upsilon=0. Thus, if \varphi_{in}^{(min)}\leq \varphi_{in}<\varphi_0 and \rho_{kin}^{(in)}>\rho_{grad}^{(in)}, then there was an inflationary stage in the history of our Universe and the congruence of timelike geodesics cannot be extended to the past beyond the hypersurface \Upsilon=0.