Look up at the night sky with your own eyes, or marvel at images of the universe online, and you’ll see the same thing: the inky, abysmal blackness of space, punctuated by bright stars, planets or spacecraft. But why is it black? Why isn’t space colorful, like the blue daytime sky on Earth?
Surprisingly, the answer has little to do with a lack of light.
“You would think that since there are billions of stars in our galaxy, billions of galaxies in the universe and other objects, such as planets, that reflect light, that when we look up at the sky at night, it would be extremely bright,” Tenley Hutchinson-Smith, a graduate student of astronomy and astrophysics at the University of California, Santa Cruz (UCSC), told Live Science in an email. “But instead, it’s actually really dark.”
Hutchinson-Smith said this contradiction, known in physics and astronomy circles as Olbers’ paradox, can be explained by the theory of space-time expansion — the idea that “because our universe is expanding faster than the speed of light … the light from distant galaxies might be stretching and turning into infrared waves, microwaves and radio waves, which are not detectable by our human eyes.” And because they are undetectable, they appear dark (black) to the naked eye.