Stars would litter everything. Dumb stars everywhere wrecking the view. It's stars all the way down, people. So, shouldn't the entire sky be as bright as a star, since there's a star in every possible minute direction you could ever look in? If you've ever asked yourself this question, you probably won't be surprised to know you're not the first. Also, at this point you can tell people you were wondering about it and they'll never know you just watched it here and then you can sound wicked smart and impress all those dudes.
This question was famously asked by the German astronomer Heinrich Wilhelm Olbers who described it in We now call this Olbers' Paradox after him. Here let me give you a little coaching, you'll start your conversation at the party with "So, the other day, I was contemplating Olbers' Paradox… Oh what's that?
You don't know what it is… oh that's so sweet! The paradox goes like this: if the Universe is infinite, static and has existed forever, then everywhere you look should eventually hit a star.
Our experiences tell us this isn't the case. So by proposing this paradox, Olbers knew the Universe couldn't be infinite, static and timeless.
It could be a couple of these, but not all three. In the s, debonair man about town, Edwin Hubble discovered that the Universe isn't static. In fact, galaxies are speeding away from us in all directions like we have the cooties. This led to the theory of the Big Bang, that the Universe was once gathered into a single point in time and space, and then, expanded rapidly.
Our Universe has proven to not be static or timeless. Here's the short version. We don't see stars in every direction because many of the stars haven't been around long enough for their light to get to us. Which I hope tickles your brain in the way it does mine. Not only do we have this incomprehensibly massive size of our Universe, but the scale of time we're talking about when we do these thought experiments is absolutely boggling.
Well, not exactly. Shortly after the Big Bang, the entire Universe was hot and dense, like the core of a star. A few hundred thousand years after the Big Bang, when the first light was able to leap out into space, everything, in every direction was as bright as the surface of a star. So, in all directions, we should still be seeing the brightness of a star.. As the Universe expanded, the wavelengths of that initial visible light were stretched out and out and dragged to the wide end of the electromagnetic spectrum until they became microwaves.
This is Cosmic Microwave Background Radiation, and you guessed it, we can detect it in every direction we can look in. So Olbers' instinct was right. If you look in every direction, you're seeing a spot as bright as a star, it's just that the expansion of the Universe stretched out the wavelengths so that the light is invisible to our eyes.
But if you could see the Universe with microwave detecting eyes, you'd see this: brightness in every direction. More from Astronomy and Astrophysics. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page.
In other words, Olbers' Paradox is resolved with the assumption that the universe has a finite age something which is supported by the Big Bang Theory , that the speed of light is finite, and thus the observable universe has a horizon beyond which we can't see the stars. Fifty years later, Lord Kelvin used math to prove that in a finite universe, or one in which stars were born and died, the night sky should be dark.
There are other contributing factors to the darkness out there. Cosmic expansion over billions of years means that the energy from the radiation which was emitted following the Big Bang has been red-shifted , or reduced to the low temperature of microwaves. That puts it beyond the visible spectrum. And other radiation in space—infrared and ultra-violet light, radio waves, and X-rays—are all invisible to the human eye. If we could see them, space would seem a little less dark.
Universe Today has another explanation: "Space is black to our perception because there are few molecules of matter that can reflect or scatter light like our atmosphere on Earth.
Since light goes in a straight line it seems to be absorbed by the void and vacuum of space. Related: How long is a galactic year? 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.
Another reason interstellar and interplanetary space appear dark is that space is a nearly perfect vacuum. Recall that Earth's sky is blue because molecules that make up the atmosphere, including nitrogen and oxygen , scatter a lot of visible light's component blue and violet wavelengths from the sun in all directions, including toward our eyes. However, in the absence of matter, light travels in a straight line from its source to the receiver.
And with no light reaching the eyes, they see black.
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