Axion Cosmology
 

Synopsis

Axions - pseudo-Nambu-Goldstone bosons associated with the spontaneous breaking of anomalously violated global Abelian symmetries - are key players in the hunt for new physics. Originally proposed in the context of quantum chromodynamics (QCD), they may solve the strong CP problem in QCD, contribute to the relic density of dark matter, provide a form of quintessence responsible for the late-time acceleration of the Universe, or result in cosmic birefringence in the polarization of the cosmic microwave background. They are, moreover, well motivated from a theoretical perspective for several reasons:
 

1. Axions are ubiquitous in field-theoretic and string-theoretic models of physics beyond the Standard Model.

2. Axions are pseudoscalars; after the discovery of the Higgs boson (a scalar), the discovery of an elementary pseudoscalar is still pending.

3. Many scenarios of new physics are motivated by new symmetry principles (e.g., supersymmetry). Axions come with their own unique symmetry principle: an approximate shift symmetry.

4. The symmetry properties of axion ensure that they couple in a characteristic fashion to fermions and gauge bosons, which has interesting phenomenological implications.

   
The last point implies that the coupling between axions and other particles are typically suppressed by a large energy scale, the axion decay constant. This places axions at a frontier of particle physics where new physics is not necessarily related to large masses of new particles but rather to the feeble interaction with the Standard Model. In addition, the penultimate point renders axions interesting candidates for the inflaton, the (pseudo)scalar field responsible for driving the stage of cosmic inflation in the early Universe. Identifying the inflaton with an axion results in "natural inflation", where the flatness of the inflaton potential (required in order to realize slow-roll inflation) is protected by the approximate shift symmetry. On top, it is reasonable to assume that an axion inflaton field is coupled to the vector fields of an Abelian or non-Abelian gauge sector. Axion inflation thus represents a well-motivated example of a nonminimal model of inflation that goes beyond the usual toy models where the inflaton sector merely consists of one real scalar field and nothing else. In this sense, axion inflation coupled to visible or dark gauge fields may be regarded as a step towards a more realistic microscopic description of the inflaton sector, featuring more degrees of freedom and a richer phenomenology. Axion inflation can especially leave behind several primordial relics, such as primordial magnetic fields, primordial gravitational waves, and primordial black holes, all of which define important targets for future observations. Because of this, axion inflation provides an interesting playground to explore new theoretical tools and defines a valuable benchmark scenario for upcoming observations.

In our work, we are interested in various aspects of axion physics in the early Universe, including (A) relaxion models addressing the hierarchy problem of the Standard Model, (B) axion dark matter and its various production mechanisms in the early Universe, and (C) the rich phenomenology of axion inflation. In the latter case, we focus on the development of new theoretical tools that allow us to describe the highly nonlinear dynamics of axion inflation and the application of these tools to specific realizations of axion inflation, including the generation of primordial gravitational waves, primordial black holes, etc. A scenario of particular relevance in this respect is axion inflation coupled to the Standard Model hypercharge sector, which offers the possibility to produce primordial (hyper)magnetic fields during inflation. Axion inflation is in this case then responsible for primordial (hyper)magnetogenesis, which in turn can have interesting implications for the generation of the baryon asymmetry of the Universe, which is a central topic of research area (4) "Early Universe".