  The dynamics of a universe with an anti-gravitating contribution to the
matter content is examined. The modified Friedmann equations are derived, and
it is shown that anti-gravitating radiation is the slowest component to dilute
when the universe expands. Assuming an interaction between both kinds of matter
which becomes important at Planckian densities, it is found that the universe
undergoes a periodic cycle of contraction and expansion. Furthermore, the
possibility of energy loss in our universe through separation of both types of
matter is discussed.
