I love how simple questions can reveal deep truths about how the universe works. Take “why is the night sky dark?” It’s a question a small child might ask but stumped the likes of Newton, Halley, and Kepler and wasn’t really resolved until Einstein’s theory of general relativity and the Big Bang theory rolled around. Here’s the paradox: if we live in a static infinite universe, shouldn’t the sky be unbearably bright?
Distant stars look weak, and very distant stars shine too dimly for you to see with your eyes. But when space telescopes like Hubble peer deep into the darkest spots of sky, they uncover bunches of incredibly faint galaxies. And the deeper they look, the more they find. If the universe went on forever with stars sprinkled evenly throughout — as many early stargazers assumed — the night sky would be full of so many points of light that it would never look dark.
“The fact that the stars are everywhere makes up for the fact that some of the stars are far away,” says Katie Mack, an astrophysicist at North Carolina State University. No matter which way you look, in an endless universe your line of sight would always end smack on the surface of a star, and the entire sky would always blaze with the brightness of the sun.
The answer to this paradox is that the universe is both finite & unbounded (per Einstein) and the darkness we see is the Big Bang.
The mystery of the dark sky is solved by the fact that this history has a beginning — a time before stars and galaxies. Many cosmologists think the universe started out as a very small point, and then started inflating like a balloon in an event called the Big Bang. If you look deep enough, you can see so far back in time that you get close to the Big Bang. “You just run out of stars,” Kinney says. “And you run out of stars, in the grand scheme of things, relatively quickly.”