People have been asking why space is dark despite being crammed with stars for therefore long that this query has a special name — Olbers’ paradox.
Astronomers estimate that there are about 200 billion trillion stars within the observable universe. And lots of of those stars are as brilliant as, and even brighter than, our sun. So, why isn’t space crammed with dazzling light?
I’m an astronomer who studies stars and planets — including those outside our solar system — and their motion in space. The study of distant stars and planets helps astronomers like me understand why space is so dark.
Related: Is there anything beyond the universe?
You may guess it’s because a number of the celebrities within the universe are very far-off from Earth. In fact, it’s true that the farther away a star is, the less brilliant it looks — a star 10 times farther away looks 100 times dimmer. However it seems this isn’t the entire answer.
Imagine a bubble
Pretend, for a moment, that the universe is so old that the sunshine from even the farthest stars has had time to achieve Earth. On this imaginary scenario, all of the celebrities within the universe should not moving in any respect.
Picture a big bubble with Earth at the middle. If the bubble were about 10 light-years across, it might contain about a dozen stars. In fact, at several light-years away, a lot of those stars would look pretty dim from Earth.
Should you keep enlarging the bubble to 1,000 light-years across, then to 1 million light-years, after which 1 billion light-years, the farthest stars within the bubble will look much more faint. But there would even be increasingly more stars contained in the larger and larger bubble, all of them contributing light. Regardless that the farthest stars look dimmer and dimmer, there could be rather a lot more of them, and the entire night sky should look very brilliant.
It seems I’m back where I began, but I’m actually just a little closer to the reply.
Age matters
Within the imaginary bubble illustration, I asked you to assume that the celebrities should not moving and that the universe could be very old. However the universe is just about 13 billion years old.
Regardless that that’s an amazingly very long time in human terms, it’s short in astronomical terms. It’s short enough that the sunshine from stars more distant than about 13 billion light-years hasn’t actually reached Earth yet. And so the actual bubble around Earth that accommodates all the celebrities we will see only extends out to about 13 billion light-years from Earth.
There just should not enough stars within the bubble to fill every line of sight. In fact, should you look in some directions within the sky, you possibly can see stars. Should you take a look at other bits of the sky, you possibly can’t see any stars. And that’s because, in those dark spots, the celebrities that would block your line of sight are thus far away their light hasn’t reached Earth yet. As time passes, light from these increasingly more distant stars could have time to achieve us.
The Doppler shift
You may ask whether the night sky will eventually light up completely. But that brings me back to the opposite thing I told you to assume: that each one of the celebrities should not moving. The universe is definitely expanding, with essentially the most distant galaxies moving away from Earth at nearly the speed of sunshine.
Since the galaxies are moving away so fast, the sunshine from their stars is pushed into colours the human eye can’t see. This effect known as the Doppler shift. So, even when it had enough time to achieve you, you continue to couldn’t see the sunshine from essentially the most distant stars together with your eyes. And the night sky wouldn’t be completely lit up.
Should you wait even longer, eventually the celebrities will all burn out — stars just like the sun last only about 10 billion years. Astronomers hypothesize that within the distant future — a thousand trillion years from now — the universe will go dark, inhabited by only stellar remnants like white dwarfs and black holes.
Regardless that our night sky isn’t completely crammed with stars, we live in a really special time within the universe’s life, after we’re lucky enough to enjoy a wealthy and sophisticated night sky, crammed with light and dark.