If you’ve ever stepped outside on a clear night here on the Western Slope, especially away from the lights of the town, you’ve probably noticed something so obvious we rarely question it; the sky is dark. Not just shadowed – but truly, deeply dark.
It seems simple, but for astronomers, this idea leads to one of the most interesting questions in science, If the universe is full of stars, why isn’t the night sky bright everywhere you look?
This puzzle is known as Olbers’ Paradox, which is named after a German astronomer who wondered why an infinite universe that is filled with stars doesn’t glow like the inside of a light bulb. If the stars were all scattered in even directions, and if the universe has always existed, then every line of sight from someone should land on a star. The whole sky should be blazing with brightness.
But it isn’t. And that darkness tells us something profound about the universe.
The first part of the answer is distance. There are stars so far away that their light just hasn’t reached us yet. Light travels very fast, around 186,000 miles per second, but the universe is so vast that even at that speed, some starlight is still on its way.
Secondly, the universe has a beginning. It hasn’t existed forever. Because we only know the universe to be about 13.8 billion years old, we can only see light that has had time to travel that far distance. Anything that is beyond that distance isn’t visible to us, but not because it isn’t shining, but because the light hasn’t arrived yet.
Then there is expansion, the universe is consistently stretching, and as it does, the light from other distant galaxies is being stretched too; the light is shifted into wavelengths that our eyes aren’t able to see. What could have maybe been visible starlight becomes microwave radiation or very faint infrared.
Therefore, the darkness of the night sky isn’t empty at all, it’s more of a clue. It tells us the universe has a history, a beginning, and it is all still unfolding. When you look up from a quiet spot, and see the deep darkness between the stars, you are seeing evidence of the universe's expansion and its age.
Music written and produced by Kenny Mihelich (ma-HELL-itch). Western Slope Skies is produced by the Colorado Mesa University Astronomy Club, the Western Slope Dark Sky Coalition, and KVNF Community Radio. This feature was written and voiced by Dalton Poulignot.