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 which 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 non-trivial. 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 non-trivial. 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 $5$ $e$-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.

PRD Accepted

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• Inflationary Dynamics and Swampland Criteria for Modified Gauss-Bonnet Gravity Compatible with GW170817
• Amplification of the Primordial Gravitational Waves Energy Spectrum by a Kinetic Scalar in $F(R)$ Gravity
• Importance of being nonminimally coupled: Scalar Hawking radiation from regular black holes
• Observable Gravitational Waves from Hyperkination in Palatini Gravity and Beyond
• The interaction of extended Bose-Einstein condensate dark matter with viscous $f(T,B)$ gravity
• Kinetic Axion Dark Matter in String Corrected $f(R)$ Gravity
• $R^2$ Gravity Effects on the Kinetic Axion Phase Space
• Extended Bose-Einstein condensate dark matter in $f(Q)$ gravity
• Perturbation in an interacting dark universe
• Interaction of the Cosmic Dark Fluid with Dynamic Aether: Parametric Mechanism of Axion Generation in the Early Universe
• Constraining Light Scalar Field with Torsion-Balance Gravity Experiments
• Energy conservation and axion back-reaction in a magnetic field
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