Scalars Gliding Through an Expanding Universe

TL;DR

This paper explores how single derivative mixing in massive bosonic fields can significantly alter their dynamics, potentially enhancing particle abundances, enabling inflation, or creating frictionless scalar fields in an expanding universe.

Contribution

It introduces the effects of large derivative mixing on bosonic fields, revealing new phenomena like delayed oscillations and frictionless behavior in an expanding universe.

Findings

Enhanced axion-like particle abundance due to mixing

QCD axion abundance can be increased, affecting dark matter models

Scalar fields can experience delayed oscillations or become frictionless

Abstract

In this article we investigate the effects of single derivative mixing in massive bosonic fields. In the regime of large mixing, we show that this leads to striking changes of the field dynamics, delaying the onset of classical oscillations and decreasing, or even eliminating, the friction due to Hubble expansion. We highlight this phenomenon with a few examples. In the first example, we show how an axion like particle can have its number abundance parametrically enhanced. In the second example, we demonstrate that the QCD axion can have its number abundance enhanced allowing for misalignment driven axion dark matter all the way down to $f_a$ of order astrophysical bounds. In the third example, we show that the delayed oscillation of the scalar field can also sustain a period of inflation. In the last example, we present a situation where an oscillating scalar field is completely frictionless and does not dilute away in time.