Inflation in $R + R^2$ Gravity with Torsion

Abstract

We examine an inflationary model in $R + R^2$ gravity with torsion, where $R^2$ denotes five independent quadratic curvature invariants; it turns out that only two free parameters remain in this model. We show that the behavior of the scale factor $a(t)$ is determined by two scalar fields, axial torsion $\chi(t)$ and the totally anti-symmetric curvature $E(t)$, which satisfy two first-order differential equations. Considering $\dot{\chi}\approx 0$ during inflation leads to a power-law inflation: $a \sim (t+ A)^p$ where $1< p \leq 2 $, and the constant $A$ is determined by the initial values of $E$, $\chi$ and the two parameters. After the end of inflation, $\chi$ and $E$ will enter into an oscillatory phase.