What? The sky gets dark at night?

I've been listening my way through astronomy lectures about black holes, but also looking forward to upcoming ones on the next disc, which included "The Paradox of the Dark Night Sky." That's a cool topic, and I thought I'd write about it. But the DVD turned out to be flawed - it came with a big crack - so I was out of luck.

But the hell with it, I'll do a blog anyway. It's about this question: "Why is the sky dark at night?"

People who hear that question for the first time usually think it's pretty silly. (Gee, do you supposed it has something to do with the sun going down?") But the issue is a lot deeper than that. Go out tonight and gaze at those umpteen stars out there. They are suns, and their combined brightness (including all those too dim for our little eyes to see) should far exceed the brightness of our Sun. Not only should the night be bright, but the day should be brighter than it is - even considering that the stars appear to dim according to the square of their distance from us.

The paradox was pondered by the likes of Kepler and Newton, but it wasn't until the mid 19th Century that Wilhelm Olbers brought other astronomers to attention by pointing out that the darkness meant that at least one of scientists' many basic assumptions about the universe (he had no idea which one) must be wrong.

Several possible answers - light blocking by interstellar dust or dark matter - are easily ruled out. The best answer, especially now that we have a rather good handle on the age of the universe, appears to be that the night is dark because the universe is relatively young. There simply hasn't been enough time for light from the countless farthest stars to reach us yet. In another 10 billion years or so, that could change. Earthlings probably won't be around to see it after our Sun goes haywire, but by then the night sky could indeed be bright.

But I'm looking even farther down the road. If the expansion of space continues to accelerate due to dark energy, eventually all the stars in the night sky (except the stars in our "local group" of galaxies that are gravitationally locked together) will be so far away as to disappear from sight. The sky will be dark indeed. The Milky Way will really stand out!

Or, at least on a somewhat shorter time scale, if the universe is vastly bigger than the part that is currently visible to us, starlight in our part of the universe may eventually get really bright.

Or, if the universe actually is a "multiverse" - with other separate universes forever beyond our sight or reach - the fate of the night sky will depend on just how big our own particular universe actually is.

All this from thinking about why it gets dark at night. Gosh, maybe it's a seriously silly question!