Why is the night sky dark if the universe is full of stars?

The question is known as Olbers' paradox. The best-supported answer is that the universe has a finite age, so we can only see objects whose light has had time to reach us. A second factor is cosmic expansion, which stretches light from distant sources toward longer wavelengths. Together these keep the visible night sky dark. [1] [2]

The paradox comes from a 19th-century picture of an infinite, unchanging universe with stars spread evenly through it. Consider a thin shell of space around Earth. A shell twice as far away has four times the volume and so four times as many stars, but each star is four times fainter by the inverse square law. The two effects cancel, so every shell contributes equal light. With infinitely many shells, every line of sight would end on a star, and the whole sky would be as bright as a stellar surface. [2]

Dust and gas clouds do not rescue the argument. They would absorb starlight, heat up, and eventually glow as brightly as the stars behind them. [2]

Light travels at a finite speed, and the universe has existed for a limited time. Current estimates put its age at about 13.8 billion years. Light from stars farther away than that distance has not yet arrived, so the region we can observe is a bubble of limited size. Most lines of sight pass through that bubble without meeting a star. Older sources gave rounder figures, such as 12 or 15 billion years, but the reasoning is the same. [2] [3] [1]

Scientific American notes that expansion alone would also darken the sky, even in a hypothetical steady-state universe, because at some distance expansion carries stars away too fast for their light to reach us. In that sense the dark sky does not prove the Big Bang, but the Big Bang is a key part of understanding it. [2]

The universe is expanding, and light from receding galaxies is shifted to longer wavelengths. Light from the most distant stars is pushed beyond the colors the human eye can detect. One source describes this as a Doppler shift, and another describes light waves being stretched as they travel through expanding space. [4] [3]

The sky is not dark at every wavelength. For roughly the first 300,000 years the universe was hot, dense and opaque, glowing like a star. That early light has been stretched into microwaves and is the cosmic background radiation. To a microwave telescope, the sky glows as brightly as a star's surface. Faint distant stars and galaxies, and Earth's own atmospheric glow, add small amounts of light too. [3] [2]

As time passes, light from ever more distant stars keeps arriving. Redshift limits how much that brightens the sky to our eyes, though. Scientific American also notes that stars have finite lifetimes, and once the gas that forms them is used up the universe will dim, making the sky darker still. [4] [2]