Starfarers — Poul Anderson

When evidence of an advanced civilization is discovered by SETT astronomers, an expedition into the far reaches of the galaxy is planned and an eclectic team of scientists is chosen to make the trip. But because the origin of the alien signals is thousands of light-years away, the crew will age only a few years while millennia pass on Earth. And though they are ready to face the ramifications of such a voyage, none of the starfarers are prepared for what awaits them at the outer edge of the cosmos--or back at the planet they once called home.