Why is the Night Sky Dark? #star #science #mystery #astronomy

hubble_probes_the_early_universeThere are billions of stars in every direction. Even more than we used to think – photons aimed straight at us from every point in the sky. As NASA puts it,

the trouble with an infinite universe is that no matter where you look in the night sky, you should see a star. Stars should overlap each other in the sky like tree trunks in the middle of a very thick forest. But, if this were the case, the sky would be blazing with light.

This is the famous Olber Paradox. Though articulated in the 1800s for an infinite steady-state universe, it still offers a puzzle today. The universe may not be infinite, but it’s very very big with very very many stars. Why do I see so much dark sky between stars?

Because the universe is expanding, the light that reaches us is subject to a phenomenon called “redshift…” the wavelengths of light [stars] emit appear to stretch out. Go far enough, and the light will redshift below the level discernible by the human eye, and eventually telescopes.

Some of this radiation shows up as background light, a faint diffuse glow of light that appears to have no source. The rest, however, disappears before it ever reaches us.

Thanks to Astronomy.com for the explanation. Try to remember this for when some little kid asks. Of course, when a kid asked “why is the night sky dark?” the best answer may be “It isn’t.” The sky, that is – isn’t – it isn’t dark. Human eyes simply don’t register the emmissions. There really is a pervasive radiation from the Big Bang. So cool.

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