Probability of the initial conditions for inflation and slow contraction
Phys. Rev. D 112, 043519 – Published 15 August, 2025
DOI: https://doi.org/10.1103/klnz-mjj2
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.
