The Irreducable Background
Forty years ago, Arno Penzias and Robert Wilson published the results of cleaning some bird crap out of a horn-shaped microwave antenna in Murray Hill, New Jersey, then listening to the universe.
NPR has a special page in comemmoration, with commentary by Ralph Schoenstein. The audio will be available by 1930 EDT -- I just heard it "live". It's worth a listen.