Sound speed and stability conditions for perturbations in f(Q), f(T), and Einstein–Cartan gravity

Determine the sound speed and stability conditions for scalar and tensor perturbations in symmetric teleparallel f(Q) gravity, teleparallel f(T) gravity, and Einstein–Cartan theory, and establish the regimes free of ghosts and gradient instabilities for cosmological backgrounds relevant to inflation.

Background

The authors note that perturbation theory in several geometric frameworks remains underdeveloped, limiting robust predictions for power spectra, non-Gaussianity, and gravitational waves.

A systematic derivation of propagation speeds and stability criteria is necessary to ensure theoretical consistency and to confront models with high-precision data.