On the stability of de Sitter inflationary background solution in scalar-tensor cosmology

The stability of de Sitter metric in spacetimes where inflaton fields are non-minimally coupled to propagating torsion is investigated. It is shown that the de Sitter background metric is unstable against small perturbations. Axionic torsion is generated by inflaton fluctuations and the analysis of cosmological perturbations in Einstein-Cartan-Klein-Gordon (ECKG) equations is undertaken. Instabilities in the inflaton massless modes are found when the minimal coupling constant dominates. Inflaton fluctuations trigger axionic torsion vector fluctuations. The ECKG equations reduce to Einstein-de Sitter equations in the case of a constant background inflaton field.