Bounce conditions in Kantowski-Sachs and Bianchi cosmologies in modified gravity theories

摘要

Many cosmological scenarios envisage either a bounce of the universe at early times, or a collapse locally to form a black hole which re-expands into a new expanding universe region. Energy conditions preclude this phenomena for ordinary matter in general relativistic universe, but scalar or other types of fields can violate some of these conditions, and so can, possibly, provide conditions for a bouncing universe. In this paper, we have investigated the necessary conditions for a bounce in Kantowski-Sachs (KS) and Bianchi type models in some modified theories of gravity like Hoyle-Narlikar creation field theory, Lyra geometry, general class of scalar-tensor theories, Einstein's theory with variable cosmological term.