(Intro)

For hundreds of thousands of years, people have been looking up at the stars shining against the blackness of space, but have you ever stopped to wonder why the night sky is so dark?  If we live in an infinite universe, you'd think there'd be starlight everywhere, because there would be stars all around us.  Sure, some would be farther away, but every point in the sky would have a star in that direction, and if there's a star for every point in the sky, then they should combine to light up the whole sky, and you don't have to be an astronomer to realize that that isn't happening, so why isn't the whole sky lighted up?

The answer has to do with the universe's expansion and the fact that stars eventually die.  The dilemma is known as Olber's Paradox after 19th century astronomer Heinrich Olbers, but the basic question dates as far back as the late 1500s.  Over the centuries, people have had a lot of different ideas.  One of the most popular was that the universe contains enormous clouds of dust that block the light from all but  the visible stars, but that explanation doesn't really work.  Starlight carries energy and billions of years of absorbing that energy would heat up the dust clouds until they were shining as brightly as the stars themselves.

To actually resolve the paradox, you need to knock out some of its assumptions.  Once we realized that our universe was expanding, it was only a matter of time before someone got the answer.   That's because Olber's paradox assumes that the observable universe is infinite, but if you wind back time on an expanding universe, there comes a point where everything was in the same place.

That was the Big Bang, and it happened around 13.8 billion years ago, and remember, light takes time to travel.  So we can only see as far out into space as light can travel in 13.8 billion years, and that's called the observable universe, and we don't really know what's beyond the observable universe, so we don't know for sure whether the full universe is infinite, but the part we can see is definitely a finite size and that's what matters here.

The observable universe is mind-bogglingly huge and stars are tiny in comparison.  So even across the whole observable universe there aren't nearly enough stars to fill up the whole sky.  So okay, but what if we waited a really long time?  After like, a trillion years, the observable universe would be so big that you'd think there would be enough stars to fill up the whole sky of whatever world you happened to be living on.  Well, it is true that the observable universe would be so big that it would effectively be infinite, but the sky would probably still be dark at night because of a problem with another one of the paradox's assumptions.  

Olbers and others assumed that the universe was constant and never really changed, but today we know that isn't true.  Stars and galaxies have life spans.  They only stick around for so long.  By the time the light from those super distant galaxies reached you, the nearby stars would have long since died out, which means you'd still be left with a situation where lots of the sky is dark.  

So Olbers paradox is easy to resolve and get rid of some of its assumptions, but the idea that the sky should shine in every direction isn't all wrong.  From studying the Big Bang, we know that the entire universe used to be really, really hot.  All that heat generated a tremendous amount of light, which, billions of years later, we can still detect in every direction we look.  It's just not in the visible part of the spectrum.  We call it the cosmic microwave background, and it's incredibly useful for astronomers.  It's helping us learn more about things like the structure of the early universe and how matter is distributed through space.

So it'll be dark out tonight because there aren't stars covering every point in the sky, but the universe is still bright in some ways, even if you can't see it.

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