This novel is set aboard the Bussard ramjet generation ship LEONORA CHRISTINE with a crew of 50 towards a distant star system. However, the ship collides with a nebula which increases its velocity and it can't stop. They actually survive the end of this universe and the beginning of the next Big Bang in an oscillating universe. Name this Poul Anderson novel whose title refers to the time contraction factor at the speed of light.
Tau Zero