Constraints on Kinetically Modified Inflation from WMAP5

TL;DR

This paper uses WMAP5 data to constrain parameters of k-inflation models, revealing that individual sound and Hubble flow parameters are largely unbounded, but certain combinations and the energy scale are constrained.

Contribution

It provides the first detailed constraints on sound and Hubble flow parameters in k-inflation models using WMAP5 data, including bounds on the energy scale and non-Gaussianity effects.

Findings

Sound and Hubble flow parameters are unbounded individually.

The energy scale of k-inflation is limited to H_inf/m_pl < 6x10^(-6).

Weak bounds on epsilon_1 for Dirac-Born-Infeld models.

Abstract

Single field inflationary models with a non-minimal kinetic term (also called k-inflationary models) can be characterised by the so-called sound flow functions, which complete the usual Hubble flow hierarchy. These parameters appear in the primordial power spectra of cosmological perturbations at leading order and, therefore, affect the resulting Cosmic Microwave Background (CMB) anisotropies. Using the fifth year Wilkinson Microwave Anisotropy Probe (WMAP5) data, we derive the marginalised posterior probability distributions for both the sound and Hubble flow parameters. In contrast to the standard situation, these parameters remain separately unbounded, and notably there is no longer any upper limit on epsilon_1, the first Hubble flow function. Only special combinations of these parameters, corresponding to the spectral index and tensor-to-scalar ratio observables, are actually constrained by the data. The energy scale of k-inflation is nevertheless limited from above to Hinf/mpl < 6x10^(-6) at two-sigma level. Moreover, for the sub-class of Dirac-Born-Infeld models, by considering the non-gaussianity bounds on the sound speed, we find a weak limit epsilon_1 < 0.08 at 95% confidence level.