Special cases of the Multi-Measure Model -- understanding the prolonged inflation

TL;DR

This paper explores special cases of the multi-measure model in cosmology, demonstrating their ability to produce prolonged inflation and connecting them with hyperinflationary models through numerical analysis.

Contribution

It introduces two specific modifications of the multi-measure model, showing they still support key inflationary stages and revealing the role of scalar field dynamics and dark charge.

Findings

Both cases achieve sufficient e-folds of inflation.

The inflaton's behavior varies with the presence of dark charge.

A prolonged slow-roll epoch emerges from scalar field interactions.

Abstract

The multi-measure model (MMM), in which one modifies the action to include both the Riemannian measure and a non-Riemannian one, has proven to be able to produce viable Universe evolution scenarios. In this article we consider two special cases of the multi-measure model, in which we first decouple the two kinetic terms in the Lagrangian entirely and later remove the dark charge of the model. We show numerically that those special cases still possess the needed evolutionary stages of the Universe and furthermore, for them one can obtain a sufficient number of e-folds of the early inflation. In the first case, the inflaton still moves backwards on the effective potential during inflation, while in the second, it does not, meaning that this behavior comes from the dark charge. We connect the model with hyperinflationary models and investigate how the different epochs are born from the interplay between the two scalar fields. We demonstrate that there is a dynamically induced slow-roll epoch, which is prolonged by the complicated movement of the two scalars in the field space. Finally, we show that while the adiabatic speed of sound can become imaginary, the phase speed of sound remains real.