How do we know that the Big Bang happened and our universe is expanding? Edwin Hubble's research on the redshift seen in most galaxies laid the foundation for the Big Bang Theory. Redshift also explains how astronomers know the universe is expanding.




Source
Teachers' Domain, Hubble's Expanding Universe, published February 20, 2004, retrieved on February 11, 2010,
http://www.teachersdomain.org/resource/phy03.sci.phys.fund.hubble2/