Kinetic axion F(R) gravity inflation

Abstract

In this work we investigate the quantitative effects of the misalignment kinetic axion on ${R}^{2}$ inflation. Due to the fact that the kinetic axion possesses a large kinetic energy that dominates its potential energy, during inflation its energy density redshifts as stiff matter fluid and evolves in a constant-roll way, making the second slow-roll index to be nontrivial. At the equations of motion level, the ${R}^{2}$ term dominates the evolution; thus the next possible effect of the axion could be found at the cosmological perturbations level, via the second slow-roll index which is nontrivial. As we show, the latter elegantly cancels from the observational indices; however, the kinetic axion extends the duration of the inflationary era to an extent that it may cause a 15% decrease in the tensor-to-scalar ratio of the vacuum ${R}^{2}$ model. This occurs because as the ${R}^{2}$ model approaches its unstable quasi--de Sitter attractor in the phase space of $F(R)$ gravity due to the $⟨{R}^{2}⟩$ fluctuations, the kinetic axion dominates over the ${R}^{2}$ inflation and in effect the background equation of state is described by a stiff era, or equivalently a kination era, different from the ordinary radiation domination era. This in turn affects the duration of the inflationary era, increasing the $e$-foldings number up to $5e$-foldings in some cases, depending on the reheating temperature, which in turn has a significant quantitative effect on the observational indices of inflation and especially on the tensor-to-scalar ratio.