A New Source for Axion Dark Matter Production

3 December 2025

Photo: C. Eröncel, Y. Gouttenoire, R. Sato, G. Servant, and P. Simakachorn

Cartoon of the energy-density evolution of a fast-moving scalar field condensate in blue, which behaves as radiation, kination, and matter, consecutively, during the radiation era of the Universe. Kination behavior starts when the field dynamics is kinetic-energy dominated. The matter behavior happens when the field gets trapped and oscillates inside the potential barrier, leading to dark matter via kinetic misalignment or fragmentation. The red line shows the energy-density evolution of scalar-field fluctuations induced by primordial curvature perturbations. Initially produced relativistically, they eventually become non-relativistic, and can serve as dark matter. The orange line shows the Standard Model radiation. Plot is schematic and not to scale.

In a new publication in Physical Review Letters, Géraldine Servant and collaborators present a novel mechanism for scalar particle production in the early Universe that can be applied generally to scalar dark matter and specifically to rotating axion models. Rather than coming from the energy initially stored in the homogeneous background field, the energy density comes from the fluctuations of the scalar field that are induced by primordial (inflationary) curvature perturbations. Such a source term is effective if the background scalar field has a large kinetic energy.

The axion, originally introduced to solve the so-called strong CP problem in the strong nuclear force (QCD), is also a popular dark matter candidate. Usually, axion dark matter is assumed to be created through the misalignment mechanism: Early on, the axion field sits at some initial value, not necessarily at the minimum of its potential. As the universe cools, the axion ‘feels’ its potential and begins to oscillate around the minimum. These oscillations behave like a cold, pressureless fluid—the perfect behavior for dark matter. The final abundance is mostly determined by two things: 

1. how far from the minimum the axion started (the misalignment angle), and
2. the axion mass.

This new paper describes a completely different source of axion dark matter, based not on the average axion value but on fluctuations generated by tiny ripples in spacetime—curvature perturbations—left over from cosmic inflation. These fluctuations are then amplified during a period when the axion is rolling quickly around its potential and later behave like cold dark matter. In such a ‘fast-roll’ phase, the field’s evolution is controlled by the universe expansion. A curvature perturbation is simply a tiny difference in the local expansion history from one region to another. For a field whose motion closely follows this expansion, these small differences translate directly into small differences in the field’s value. In other words, each curvature perturbation naturally sources an axion fluctuation whose size is proportional to the field's velocity.

These sourced fluctuations accumulate during fast-roll and later turn into cold dark matter once the axion begins oscillating. Crucially, their contribution can exceed the usual misalignment abundance, meaning the dominant origin of axion dark matter could come from these curvature-induced fluctuations.

How exactly are the axion fluctuations generated?

During inflation, tiny quantum fluctuations are stretched to enormous scales. They freeze in as extremely small but permanent ripples in the curvature of spacetime. These ripples are only about one part in one hundred thousand, yet they shape much of the later universe. The small curvature ripples created during inflation give strong kicks to the axion field and generate significant fluctuations. In the standard axion picture, the field is almost motionless after inflation, so curvature perturbations hardly affect it. The fluctuations they generate are tiny. But in a fast-roll phase, the axion is moving quickly, so the same tiny ripples can produce large fluctuations. Later on, when the axion begins to oscillate, these fluctuations naturally behave like cold dark matter. In short, the tiny imprints of inflation become a powerful source of axion fluctuations—provided the axion experienced a fast-roll stage.

This mechanism widens the possible values of the axion mass and decay constant that yield the correct dark matter abundance. It also ties axion physics directly to inflationary physics: the observed amount of dark matter could depend partly on the curvature perturbations imprinted during inflation. This opens new paths for testing axion theories through gravitational-wave detectors and early-universe cosmology. And it is also of particular relevance for the proposed International Axion Observatory (IAXO).

In summary, axion dark matter may have been created not only through the classic misalignment mechanism, but also through a powerful new process: the amplification of inflationary curvature perturbations during a fast-roll phase. Tiny ripples in spacetime, inherited and magnified by a rapidly moving axion field, can seed an abundance of cold dark matter. This connects inflation, axion physics, and cosmic structure in a single, elegant picture.