edwin hubble had changed our understanding of the cosmos forever. mountain: the whole universe was clearly expanding. and so this was the discovery of the expansion of the universe. you know, which einstein said, "well, that's crazy, right?" the discovery of the expansion of the universe of course was a very strong piece of evidence for a beginning. if everything is now expanding, you can run this backwards and see that everything should have started from a certain point or singularity, what we today call a big bang. mountain: that was pretty radical and mind-blowing stuff back in the '20s and '30s, and some people today even find it mind-blowing. narrator: hubble's discovery opened up the modern era of astronomy and raised huge questions, like "how old is the universe?" but astronomers couldn't see clearly and deeply enough into the cosmos, so the mystery would endure for decades. livio: determining the age of the universe was definitely one of the key goals of the launch of the hubble space telescope. astronomers used to have fistfights whether th