Line of research overview

Axions

Principal Investigator: Lorenzo Ubaldi

Abstract: Axions are hypothetical particles and there can be many variants of them. The best motivated one is the QCD axion, which would explain why we do not observe CP violation in the strong interactions. But there can exist other axions, not necessarily related to the solution of the strong CP problem. For example, in string theory they are ubiquitous, and can have masses and couplings to ordinary matter in a range which spans several orders of magnitude. Axions have very interesting implications in cosmology and astrophysics. They can be dark matter candidates, drive inflation, generate the baryon asymmetry of the universe, affect the propagation of light, and even provide a clue to the origin of the dark energy.

The scope of this project is to investigate several aspects of axion physics, mostly in the context of cosmology and astrophysics.

On cosmological scales, axion dark matter obtains isocurvature perturbations during inflation. Hence by studying its trace on the cosmic microwave background we can constrain axion properties as well as the physics of inflation.

Axions may also produce topological defects in the early universe such as cosmic strings and domain walls, whose evolution through the cosmic expansion poses significant challenges and there are still open questions to be addressed.

Another avenue of research is axions in the ultralight mass range; such axions possess large de Broglie wavelengths and thus affect structure formation of the universe on small scales, and may even create solitonic cores inside dark matter halos. We explore these possibilities using small-scale probes of the universe such as the Lyman-alpha forest and 21 cm radiation. We also investigate superradiance of black holes induced by axions, and analyze their implications for gravitational wave observations.

Axion dark matter is cold, and similarly to other forms of cold dark matter, like Weakly Interacting Massive Particles (WIMPs), it is predicted to form structures in good agreement with those observed in the Universe. However, due to the inherently different nature of axions compared to WIMPs, there could be differences in the structure formation process which could lead to distinct signatures that can help pinpoint the nature of the dark matter. We plan to make progress in this direction.

Status of project and perspectives: Thanks to the meeting “Structure formation with axion dark matter” in November 2019 we have made progress in sharpening some questions related to structure formation: Do axions form a Bose-Einstein condensate? If so, are there long-range correlations in such a condensate? These can have interesting implications for axion detection, and deserve more work and meetings.

Progress has been made in understanding the evolution of axion strings in the post-inflationary scenario. Gorghetto, Hardy and Villadoro have shown that numerical simulations strongly suggest a large number of axions from strings. This leads to a lower bound on the QCD axion mass in such a scenario much stronger than the naive one from misalignment.

In the pre-inflationary scenario, Kobayashi and Ubaldi have explored what happens if the scale of inflation is lower than the axion mass. While naively in this case there would be no axion relics, they have shown  that a simple kinetic mixing between inflaton and axion would be enough to produce axion cold dark matter. This scenario opens interesting questions related to mechanisms for low-scale inflation and reheating which can be investigated.