Scalar perturbations in two-scalar-field warm inflation

Abstract

We investigate scalar perturbations in a two-scalar-field cosmological model unifying warm inflation, dark matter, and dark energy. The inflaton is noncanonically coupled to a second scalar field that acts as dark energy at late times. Assuming negligible dissipation and perturbations of the dark-energy field, we derive the linear scalar perturbations in a gauge-ready form, including quantum and thermal noise in the inflaton sector and stochastic fluctuations of the energy transfer to radiation. We numerically solve the coupled perturbation system using a Fokker-Planck formalism and compute the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$ for dissipation coefficients scaling as $T^p$ with $p=-1,\, 1$ and $3$. For $p=-1$, we find scenarios compatible with current CMB observations in both the weak and strong dissipative regimes, while the explored scenarios with $p=1$ and $3$ are outside the observationally supported regions. Compared with standard single-field warm inflation, the two-scalar-field model yields a systematically larger spectral index for a given tensor-to-scalar ratio.