Gauge field production during inflation: Theory and phenomenology

• Applicant: Dr. Oleksandr Sobol
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 565969529

Cosmic inflation, the stage of an accelerated expansion of the universe prior to the hot Big Bang, can be regarded as a natural place for generation of various particles and fields in the early universe. The proposed project addresses a range of open questions concerning inflationary gauge-field production, touching upon both the theoretical and phenomenological aspects of this scenario. On the theoretical side, we are going to revisit the problem of ultraviolet divergences in physical observables and apply the conventional quantum field-theoretical approaches allowing to regularize these divergences and renormalize physical observables in a way which respects gauge invariance and covariant energy conservation in an expanding universe. Also, we will reveal the details of a quantum-to-classical transition which is undergone by the gauge-field modes crossing the horizon during inflation. This will be achieved by introducing the criteria of “quantumness” for the gauge-field modes and studying their time evolution during inflation. The phenomenological part of this proposal touches upon three aspects. The first one is related to the regime of strong gauge-field backreaction during inflation where we will introduce, for the first time in the literature, a clear and physically motivated criterion for its occurrence and also study the possibility of backreaction in the presence of extra gauge field damping due to Schwinger pair production. The second aspect is related to various observational manifestations of the gauge fields produced during inflation. In particular, we are going to study the generation of primordial scalar perturbations induced by the presence of gauge fields during inflation, their postinflationary evolution, and, ultimately, their implications for the cosmic microwave background anisotropy spectrum. Also, we will consider the production of vector dark matter (dark photons) which is kinetically mixed with the Standard Model gauge fields. For this purpose, we will develop a multifield version of the gradient-expansion formalism. With this tool in hand, we will determine the dark matter abundance and the large-scale magnetic field in voids for a wide range of model parameters. Third, we are going to revisit the process of Schwinger pair production by strong gauge fields during inflation and combine its kinetic description with existing approaches to gauge-field production. This is extremely important because only within the kinetic approach one can properly take into account the non-locality in time of this pair creation process.

• DFG Programme: Research Grants