Why is the night sky dark? The answer isn't as simple as "because the sun isn't shining." Think of the stars in the universe like trees in a forest. A forest has too many trees to see to the other side—all you see is trees, with no empty space. The universe has many more stars than a forest has trees, so why do we see areas of empty space in the night sky instead of nothing but the blazing light of stars? This is known as Olbers' Paradox, named after the 19th century astronomer Heinrich Olbers who formalized the conundrum that had plagued astronomers of history.
That was the astronomy of the 19th-century. Scientists have since made a large number of discoveries that explain this phenomenon. The most notable: Olbers' Paradox assumes that the universe is infinitely old and infinitely large, but we now know that the universe had a beginning and is always expanding. This provides two explanations for our dark night sky. For one, the universe was formed about 14 billion years ago, so we can only see about 14 billion light years in any direction. The light from stars further away hasn't had time to reach us yet. The other explanation is that galaxies are constantly moving away from us. When objects move away from an observer, the Doppler effect causes their light to shift toward the red end of the spectrum—move away fast enough, and that light goes into the infrared, which is invisible to the human eye. Explore our expanding universe with the videos below.
Why Is The Night Sky Dark?
Scientists of history wondered, but today we have an answer.
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The History of Olbers' Paradox
Hear why scientists used to be confounded by the dark sky.
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Key Facts In This Video
If we're living in an infinite static universe with an infinite number of stars in a uniform distribution, the night sky should be as bright as the surface of the sun. 00:08
The birth of the universe occurred 13.8 billion years ago, so we can only see about 13.8 billion light years of our universe -- light from stars beyond that wouldn't have had time to reach us. 00:44
Our universe is expanding. As stars move further away from us, they "redshift," eventually disappearing from the visible portion of the electromagnetic spectrum. 01:21
Why Is The Universe Expanding?
One reason we see a dark night sky is because the universe is expanding. But why is it expanding in the first place?