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Area of Science:

  • Cosmology
  • Particle Physics
  • Gravitational Wave Astronomy

Background:

  • Axion inflation is a leading paradigm for the early universe.
  • Preheating is a crucial non-equilibrium process following inflation.
  • The effective number of relativistic degrees of freedom (N_eff) is sensitive to new relativistic species.

Purpose of the Study:

  • To investigate gravitational wave production during gauge preheating after axion inflation.
  • To determine the impact of these gravitational waves on N_eff.
  • To establish constraints on the inflaton's axial coupling to gauge fields.

Main Methods:

  • Theoretical calculations of gravitational wave production during preheating.
  • Analysis of N_eff constraints from Planck data.
  • Correlation analysis between gravitational wave efficiency and the tensor-to-scalar ratio (r).

Main Results:

  • Gravitational waves from efficient gauge preheating significantly contribute to N_eff.
  • Planck data, combined with N_eff constraints, yield the strongest limits on the inflaton's axial coupling.
  • Gauge preheating can reheat the universe irrespective of the inflationary potential.

Conclusions:

  • N_eff constraints probe or exclude efficient gauge preheating regimes for models with r≳10^{-3}.
  • Gravitational wave production efficiency correlates with the tensor-to-scalar ratio.
  • This work links early universe physics, gravitational waves, and cosmological observables.