Why is the sky black at night?

Yesterday we discussed why the sky was blue during the day. Today we'll cover Olbers' Paradox, and why the sky turns black in the wee hours.

During the day, sunlight is bright enough to drown out the light from most of the stars. In most cities, streetlights, neon lights, and those obnoxious little light-up sneakers that the kids these days wear, will obscure the stars at night. Most planetarium presentations end with the presenter encouraging kids to get away from the light pollution of the city and see the awesome display of the stars around us, reminding us of the endless stretch of the near-unimaginably ancient universe.

Well that's crap.

The universe is young, and the stars in it are younger still. If they weren't, then we wouldn't have to deal with the dilemma known as Olbers' Paradox.

Olbers' Paradox is pretty simple: if the universe is infinite, and is filled with an infinite number of stars, then when the sun goes down, all we should see is a sky full of light. There shouldn't be any black background for stars to twinkle against, or night vision, or shadowy alleyways full of criminals to give superhero movies a dramatic start. Darkness should be exotic, because every star in the universe should be shining down on us at all times.

Clearly, that isn't happening, but why? Because we live in a sad baby universe — it's only 14 billion or so years old, which is certainly a long time, but not long enough for an infinite amount of stars to illuminate us with an infinite amount of starshine. Given that it takes a lot of time to accumulate nonillians of kilograms of gas in one place, the stars are even younger. Light is the fastest thing in the universe. It barrels through the sky at around 300,000,000 meters per second, and the universe still hasn't given it enough time to trundle across space and make it so we don't have to turn on our porch lights at dusk. It's like the universe doesn't even want us to save energy. Or maybe someone bought it off. I'm looking at you, Enron.

[Via NASA and CMB.]