# The ALP miracle: unified inflaton and dark matter

## TL;DR

This paper proposes a unified model where a single axion-like particle explains both inflation and dark matter, predicting specific ALP properties that are testable by upcoming experiments and compatible with cosmological observations.

## Contribution

It introduces a minimal axion hilltop inflation model where the ALP accounts for both inflation and dark matter, with specific mass and coupling predictions within experimental reach.

## Key findings

- ALP mass around 0.01 eV and photon coupling ~10^{-11} GeV^{-1}
- Predicted hot dark matter contribution with ΔN_eff ≈ 0.03
- Parameter space accessible to next-generation axion searches like IAXO

## Abstract

We propose a scenario where both inflation and dark matter are described by a single axion-like particle (ALP) in a unified manner. In a class of the minimal axion hilltop inflation, the effective masses at the maximum and mimimum of the potential have equal magnitude but opposite sign, so that the ALP inflaton is light both during inflation and in the true vacuum. After inflation, most of the ALPs decay and evaporate into plasma through a coupling to photons, and the remaining ones become dark matter. We find that the observed CMB and matter power spectrum as well as the dark matter abundance point to an ALP of mass $m_\phi = {\cal O}(0.01)$ eV and the axion-photon coupling $g_{\phi \gamma \gamma} ={\cal O}(10^{-11})$GeV$^{-1}$: the ALP miracle. The suggested parameter region is within the reach of the next generation axion helioscope, IAXO. Furthermore, thermalized ALPs contribute to hot dark matter and its abundance is given in terms of the effective number of extra neutrino species, $\Delta N_{\rm eff} \simeq 0.03$, which can be tested by the future CMB experiments. We also discuss a case with multiple ALPs, where the coupling to photons can be enhanced in the early Universe by an order of magnitude or more, which enlarges the parameter space for the ALP miracle. The heavy ALP plays a role of the waterfall field in hybrid inflation, and reheats the Universe, and it can be searched for in various experiments such as SHiP.