Reflections in Cosmology 18: Why is the Sky Dark at Night?

Sounds simple enough. Every kid should be able to answer that shouldn’t they? But it isn’t as straight forward as many of us would imagine it to be. This question, also known as Olber’s paradox, baffled scientists for centuries. If the universe is infinte, and there are an infinite number of stars, every line of sight in the sky should terminate at a star. If this is so, why isn’t the night sky as bright as day? Why are there so many dark gaps in between stars?

Just as every line of sight in a dense forest terminates at a tree, every line of sight in the night sky should terminate at a star... or should they?

Is it because light from faraway stars are too faint for us to be able to see them? We know that a star’s brightness decreases proportionally with the square of its distance, but the number of stars in a homogenous and isotropic universe should also increase proportionally with the distance squared too, so these effects should cancel each other out and we still end up with a bright night sky.

Is it because a large number of stars are blocked from our view by dust, which absorbs starlight? But radiation from the stars would eventually heat up the dust, which in turn would reradiate the light, and we end up with the same thing.

The riddle was eventually solved by Lord Kelvin in 1901 in an article with the title ‘On ether and gravitational matter through infinite space’, although the poet Edgar Allen Poe came pretty close. The answer? I’ll leave it to Edward Harrison to summarize it:

Darkness of the night sky is due not to absorption of starlight, not to hierarchical clustering of stars, not to the finiteness of the universe, not to expansion of the universe, and not to many other proposed causes. The explanation is quite simple and can be stated in various equivalent ways. Because of the finite luminous age of stars and the finite speed of light, the number of visible stars is too few to cover the entire sky; most stars needed to cover the sky are so far away that their light has not reached us; the light-travel time from the most distant stars is greater than their luminous lifetime; the luminous lifetime of stars is shorter than the time needed to fill space with radiation to the temperature it has at the surfaces of stars; and stars do not contain enough energy to fill space with radiation of this temperature. Why is the sky dark at night? Because starlight is too feeble to fill the dark universe.

Olber's Paradox...

Out of doors at night we look up at the night sky. Between the stars we look out immense distances in space and far back in time before the formation of the galaxies and their firstborn stars. Our sight extends to the limit of the visible universe at the frontier of the big bang. In all directions we look back to the creation of the universe and see the big bang covering the sky. Twentieth century cosmology has solved the old riddle of darkness at night and shown that the night sky is covered not by stars but by the big bang mercifully veiled from view by expansion of the universe. ~ Edward Harrison, Cosmology: The Science of the Universe


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