# On the dark radiation problem in the axiverse

## TL;DR

This paper investigates how axion production after inflation can contribute to dark radiation, placing constraints on axion models and inflation scenarios, and finds that many axiverse models are incompatible with current cosmological data.

## Contribution

It provides a general constraint on axion production rates from inflaton couplings, applicable across various inflation models, and demonstrates the incompatibility of a large axion number with certain inflationary reheating mechanisms.

## Key findings

- A hundred axions are forbidden if inflaton couples universally.
- In Starobinsky inflation, even a single axion conflicts with Planck data.
- Low reheating temperatures may be incompatible with light scalar fields.

## Abstract

String scenarios generically predict that we live in a so called axiverse: the Universe with about a hundred of light axion species which are decoupled from the Standard Model particles. However, the axions can couple to the inflaton which leads to their production after inflation. Then, these axions remain in the expanding Universe contributing to the dark radiation component, which is severely bounded from present cosmological data. We place a general constraint on the axion production rate and apply it to several variants of reasonable inflaton-to-axion couplings. The limit merely constrains the number of ultralight axions and the relative strength of inflaton-to-axion coupling. It is valid in both large and small field inflationary models irrespectively of the axion energy scales and masses. Thus, the limit is complementary to those associated with the Universe overclosure and axion isocurvature fluctuations. In particular, a hundred of axions is forbidden if inflaton universally couples to all the fields at reheating. In the case of gravitational sector being responsible for the reheating of the Universe (which is a natural option in all inflationary models with modified gravity), the axion production can be efficient. We find that in the Starobinsky $R^2$-inflation even a single axion (e.g. the standard QCD-axion) is in tension with the Planck data, making the model inconsistent with the axiverse. The general conclusion is that an inflation with inefficient reheating mechanism and low reheating temperature may be in tension with the presence of light scalars.