The adiabatic/entropy decomposition in $P({\varphi }^I,X^{IJ})$ theories with multiple sound speeds

Abstract

We consider $P({\varphi }^I,X^{IJ})$ theories of multi-field inflation and ask the question of how to define the adiabatic and entropy perturbations, widely used in calculating the curvature and isocurvature power spectra, in this general context. It is found that when the field perturbations propagate with different speeds, these adiabatic and entropy modes are not generally the fundamental (most natural to canonically quantise) degrees of freedom that propagate with a single speed. The alternative fields which do propagate with a single speed are found to be a rotation in field space of the adiabatic and entropy perturbations. We show how this affects the form of the horizon-crossing power spectrum, when there is not a single "adiabatic sound speed" sourcing the curvature perturbation. Special cases of our results are discussed, including $P(X)$ theories where the adiabatic and entropy perturbations are fundamental. We finally look at physical motivations for considering multi-speed models of inflation, particularly showing that disformal couplings can naturally lead to the kind of kinetic interactions which cause fields to have different sound speeds.

arXiv comment5 pages, 1 figure. Updated to version accepted by journal

References in corpus (17)

• Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results
• Particle Physics Models of Inflation and the Cosmological Density Perturbation
• DBI in the Sky
• Encyclopaedia Inflationaris
• A Joint Analysis of BICEP2/Keck Array and Planck Data
• Adiabatic and entropy perturbations from inflation
• Scalar perturbations during multiple-field slow-roll inflation
• Primordial perturbations and non-Gaussianities in DBI and general multi-field inflation
• An Observational Test of Two-field Inflation
• Perturbations in generalized multi-field inflation
• Adiabatic and Isocurvature Perturbations for Multifield Generalized Einstein Models
• Curvature and isocurvature perturbations from two-field inflation in a slow-roll expansion
• Inflation with multiple sound speeds: a model of multiple DBI type actions and non-Gaussianities
• Non-gaussianity from the bispectrum in general multiple field inflation
• The simplest extension of Starobinsky inflation
• Disformally coupled inflation
• Dynamics of Cosmological Perturbations in Multi-Speed Inflation

Cited by in corpus (1)

• Non-Gaussianity in multi-sound-speed disformally coupled inflation