Neutrinos as a probe of curvature

Neutrinos, as the anisotropic stress tensor, have a damping effect on the tensor mode perturbation from inflation to the $\Lambda$ dominated era. First, we study the squared amplitude reduction for the wavelength entering the horizon during radiation and matter-dominated phases in the negatively curved de Sitter spacetime. Then, by comparing with other spatial spacetimes, $K=0$ and $K=1$, the highest difference between closed and open cases is seen in the matter-dominated era. Thus, neutrinos can be added as another candidate for determining the nature of space-time.