The strongest evidence for a Universe before the Big Bang

"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."

"When cosmic inflation was first put forth and developed in the early 1980s, it separated the two definitions of the Big Bang, proposing that the early hot, dense state never achieved these singular conditions, but rather that a new, inflationary state preceded it. There really was a Universe before the hot Big Bang, and some very strong evidence from the 21st century truly proves that it's so."

"Although we're certain that we can describe the very early Universe as being hot, dense, rapidly expanding, and full of matter-and-radiation - i.e., by the hot Big Bang - the question of whether that was truly the beginning of the Universe or not is one that can be answered with evidence. The differences between a Universe that began with a hot Big Bang and a Universe that had an inflationary phase that precedes and sets up the hot Big Bang are subtle, but tremendously important."

Nearly a century ago, observations revealed an expanding Universe, implying a smaller, denser, hotter past. Extrapolating back led to a singularity concept where all matter and energy condensed to a point. In the 1970s, unresolved puzzles emerged that the singular hot-Big-Bang picture could not explain. Cosmic inflation, developed in the early 1980s, proposed a preceding inflationary state that avoided singular conditions and set up the hot, dense phase. Observational results from the 21st century provide strong support for a pre-hot-Big-Bang epoch driven by inflation. Distinguishing a true beginning from a preexisting inflationary phase requires direct evidence from cosmological observations.

Read at Big Think