What is the puzzle?
Imagine an infinite forest. Wherever you looked, your gaze would eventually meet a tree trunk: the farther away the trees, the smaller they would look, but there would be more of them. An unbroken wall of trunks would be unavoidable.
The same would be true of an infinite, eternal universe. Distant stars would be dimmer, but their numbers would increase faster than their brightness decreased. Add it all up, and you get a sky glowing evenly and brightly. At night, we see exactly the opposite, and that calls for an explanation.
The sky is dark because distant stars are too faint, and dust between them absorbs their light.
Their faintness is offset by their numbers, and dust does not solve the problem: after absorbing light for centuries, it would heat up and glow itself. This objection was raised before modern cosmology existed.
The answer turned out to be about time
The universe is not infinitely old, and light does not travel instantly. That means we see only the part of it from which light has had time to reach us. There are stars farther away, but their light has not arrived yet.
The darkness of the night sky is a direct consequence of the universe having a beginning. Otherwise, the whole sky would glow.
Expansion contributes, too. Light from very distant sources stretches as it travels and shifts into wavelengths invisible to the human eye. Some radiation from the early universe really does reach us from every direction, just not as visible light, but as faint radio waves.
The result is beautiful: the sky does glow evenly in every direction, just as the paradox predicted. We simply detect this glow with instruments rather than our eyes, and it has cooled to a few degrees above absolute zero.
Why this question matters
People asked this question centuries before telescopes powerful enough to investigate it existed, and it showed that something was wrong with the picture of an eternal, unchanging universe. An observation available to anyone at night became an argument that the universe had a beginning.
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Sources
- The question is known as Olbers' paradox and has been discussed since the 17th century. The modern explanation is based on the finite age of the observable universe and the redshift of distant radiation