Constraining the kinetically dominated Universe

TL;DR

This paper uses Planck 2015 data to analyze a universe that was kinetically dominated early on, comparing inflationary models and finding a slight preference for a large-scale cutoff in the primordial power spectrum.

Contribution

It introduces a detailed analysis of kinetically dominated initial conditions in inflation and compares their Bayesian evidence to standard models using Planck data.

Findings

Kinetically dominated initial conditions perform similarly to slow-roll models in Bayesian evidence.

Data slightly favors a large-scale cutoff in the primordial power spectrum.

Kinetic initial conditions do not significantly reduce model plausibility despite extra parameters.

Abstract

We present cosmological constraints from Planck 2015 data for a universe that is kinetically dominated at very early times. We perform a Markov chain Monte Carlo analysis to estimate parameters and use nested sampling to determine the evidence for a model comparison of the single-field quadratic and Starobinsky inflationary models with the standard $\Lambda$CDM cosmology. In particular we investigate how different amounts of inflation before and after horizon exit affect the primordial power spectrum and subsequently the power spectrum of the cosmic microwave background. We find that the model using kinetically dominated initial conditions for inflation performs similarly well in terms of Bayesian evidence as a model directly starting out in the slow-roll phase, despite having an additional parameter. The data show a slight preference for a cutoff at large scales in the primordial and temperature power spectra.