Beyond Infinity — Gregory Benford

Takes a scientist's imagination to the uttermost ends of time. Set more than a billion years from now, the novel begins with a young woman who yearns to escape the rigid, timeless Earth she knows. So she flees, in the company of an intelligent beast wise beyond recognition. But there are mysterious forces afoot among the planets that she never foresaw. Alien agencies have learned to span parallel universes, ones that lie only a millimeter away but are invisible to any device known to man. Soon these beings confront the travelers and a struggle beyond imagining begins.