Superheavy dark matter from the string theory axiverse

We propose heavy axions as a natural superheavy dark matter candidate in string theory, with the relic density of dark matter originating in quantum fluctuations during cosmic inflation. String theory is well known for the possibility of having tens to hundreds of axionlike particles-the axiverse. Moduli stabilization generates high-scale masses for many of these, placing them naturally in the "superheavy" regime of particle physics. We consider moduli stabilization in the Kachru-Kallosh-Linde-Trivedi framework, featuring a single volume modulus and C₄ axion, and a fiducial inflation model minimally coupled to the volume modulus. We demonstrate that both the volume modulus and the axion can be abundantly produced through gravitational particle production. The former is unstable and readily decays to Standard Model particles while the latter (the axion) can be stable and survives to constitute the present day dark matter.