An alternative to inflation? Critical role of neutrino anisotropic stress in CMB

We study the role of anisotropic stress in the evolution of curvature perturbations and the cosmic microwave background (CMB) power spectrum. We found that if the extra anisotropic stress preexisted before the neutrino decoupling, the cancellation of the anisotropic stress of neutrinos and extra anisotropic stress could lead to a spectrum that could reproduce the observed CMB power spectrum.