13.8 billion years ago, our entire observable universe was the size of a peach and had a temperature of over a trillion degrees.
By Paul Sutter | SPACE.com
That’s a pretty simple, but very bold, statement to make, and it’s not a statement that’s made lightly or easily. Indeed, even a hundred years ago it would’ve sounded downright preposterous, but here we are saying it like it’s no big deal. But as with anything in science, simple statements like this are built from mountains of multiple independent lines of evidence that all point towards the same conclusion. In this case: the Big Bang, our model of the history of our universe. [The Universe: Big Bang to Now in 10 Easy Steps]
Evidence #1: The night sky is dark
Imagine for a moment that we lived in a perfectly infinite universe, both in time and space. The glittering collections of stars go on forever in every direction, and the universe simply always has been and always will be. That would mean wherever you look in the sky — just pick a random direction and stare — you are bound to find a star out there, somewhere, at some distance. That’s the inevitable result of an infinite universe. And if that same universe has been around forever, then there’s been plenty of time for light from that star, crawling through the cosmos at a relatively sluggish c, to reach your eyeballs.
Ergo, the sky should be ablaze with the combined light of a multitude of stars. Instead, it’s mostly darkness. Emptiness. Voids. Blackness. You know, space.
The German physicist Heinrich Olbers may not have been the first person to note this apparent paradox, but his name stuck: It’s known as Olbers‘ Paradox. The simple resolution? The universe must either not be infinite in size or not be infinite in time. Or maybe it’s neither.