Probability of the Initial Conditions for Inflation and Slow Contraction

TL;DR

This paper assesses the likelihood of initial conditions for inflation and slow contraction, finding that inflation generally has a higher maximal probability of homogeneous initial states than ekpyrosis.

Contribution

It provides a probabilistic comparison of initial conditions for inflation and ekpyrosis, challenging claims that slow contraction is more favored.

Findings

Inflation has a higher maximum probability of homogeneous initial conditions.

The likelihood depends on the parameters of the initial power spectrum.

Both scenarios can be more likely than the other for fixed parameters.

Abstract

Some recent studies based on numerical relativity simulations claim that slow contraction/ekpyrosis is strongly preferred over inflation as the smoothing mechanism that brought the universe into the homogeneous, isotropic and flat state we observe today on large scales. In this paper, we evaluate the likelihood of the initial conditions employed in the aforementioned simulations by estimating the probability that a free scalar field dominating the universe at the beginning of inflation or ekpyrosis will be sufficiently homogeneous on scales comparable to the Hubble radius at that time. We explore the space of parameters that characterize the initial power spectrum of the scalar field, finding that either can be more likely than the other for a fixed choice of parameters. On the other hand, when we extremize over these parameters, we find that the maximal probability for inflation is much higher than that of ekpyrosis.