If the Universe is expanding and cooling today, that implies a past that was smaller, denser, and hotter. In our imaginations, we can extrapolate back to arbitrarily small sizes, high densities, and hot temperatures: all the way to a singularity, where all of the Universe's matter and energy was condensed in a single point. For many decades, these two notions of the Big Bang - of the hot dense state that describes the early Universe and the initial singularity - were inseparable.