Why does inflation look single field to us?

TL;DR

This paper explains why cosmic inflation appears to be single-field despite many-field models, showing volume effects and quantum diffusion lead to single-field attractors in the dominant regions.

Contribution

It introduces a formalism of constrained stochastic inflation to demonstrate how volume selection effects cause inflationary dynamics to favor single-field attractors.

Findings

Volume selection effects explain the single-field appearance.

Quantum diffusion leads to attractor behavior.

Most of the universe's volume inflates via single-field dynamics.

Abstract

Most high-energy constructions that realise a phase of cosmic inflation contain many degrees of freedom. Yet, cosmological observations are all consistent with single-field embeddings. We show how volume selection effects explain this apparent paradox. Because of quantum diffusion, different regions of space inflate by different amounts. In regions that inflate most, and eventually dominate the volume of the universe, a generic mechanism is unveiled that diverts the inflationary dynamics towards single-field attractors. The formalism of constrained stochastic inflation is developed to this end.