What we can learn from the spectral index of the tensor mode

TL;DR

This paper explores how the spectral index of the tensor mode can reveal early inflation dynamics and the influence of other fields, emphasizing the importance of precise measurements to distinguish different inflation scenarios.

Contribution

It demonstrates that the tensor spectral index is a valuable probe of inflationary dynamics and non-inflaton field effects, highlighting the significance of measuring spectral runnings.

Findings

Tensor spectral index reflects early inflation dynamics.

Other fields can alter the tensor spectrum's scale dependence.

Precise measurements of spectral runnings can discriminate inflation models.

Abstract

If the beginning of inflation is defined at the moment when the vacuum energy of the inflaton starts to dominate, the energy density of the other fields at that moment is (by definition) comparable to the inflaton. Although the fraction will be small at the horizon exit due to the inflationary expansion, they can alter the scale dependence of the spectrum. At the same time, velocity of the inflaton field may not coincide with the slow-roll (attractor) velocity. Those dynamics could be ubiquitous but can easily alter the scale dependence of the spectrum. Since the scale dependence is currently used to constrain or even exclude inflation models, it is very important to measure its shift, which is due to the dynamics that does not appear in the original inflation model. Considering typical examples, we show that the spectral index of the tensor mode is a useful measure of such effect. Precise measurement of the higher runnings of the scalar mode will be helpful in discriminating the source.