And so it is that the 21st century has brought new revolutions and new revelations on a cosmic scale. Our picture of the universe has probably changed more in the lifetime of an octogenarian today than in all of human history. Eighty-seven years ago, as far as we knew, the universe consisted of a single galaxy, our Milky Way, surrounded by an eternal, static, empty void. Now we know that there are more than 100 billion galaxies in the observable universe, which began with the Big Bang 13.7 billion years ago. In its earliest moments, everything we now see as our universe — and much more — was contained in a volume smaller than the size of a single atom.