  We consider the Einstein static and the de Sitter universe solutions and
examine their instabilities in a subclass of quadratic modified theories for
gravity. This modification proposed by Nash is an attempt to generalize general
relativity. Interestingly, we discover that the Einstein static universe is
unstable in the context of the modified gravity. In contrast to Einstein static
universe, the de Sitter universe remains stable under metric perturbation up to
the second order.
