Why the Universe Keeps the Lights Low

Imagine walking into a forest so large that whichever way you look, your line of sight eventually lands on a tree trunk. Now take that idea up into the night sky. If the universe went on for ever, unchanging, with stars scattered through all of it, then every line of sight should end on the surface of a star. The whole sky should blaze like the face of the sun. Instead we get a soft, velvety dark with a scattering of pinpricks in it.

This puzzle has a name: Olbers’ paradox, after the German astronomer Heinrich Wilhelm Olbers. Penn State’s introductory astronomy course puts it simply: if the universe were infinite and filled with stars, every line of sight would end on a star, and the night sky would be as bright as day. The course boils it down to one short question that anyone can ask: why is the night sky dark?

An obvious first guess is that dust hides the distant stars. It does not work. The Penn State course explains why: in an infinite universe full of stars, their light would heat the dust until it glowed brightly enough to light up the night sky itself. Something more fundamental is needed, and it turns out to be time.

The universe is not eternal. The European Space Agency describes it as born nearly 14 billion years ago. Light is fast but not infinitely fast, so we can only see stars whose light has had time to reach us. Beyond a certain distance, as the Penn State course puts it, there has not been enough time for the light of the most distant stars to arrive. The course credits the writer Edgar Allan Poe, in his essay Eureka, with the first published answer of this kind. However big the universe may be, one of finite age can only show us a finite number of stars, and that is not enough to fill the sky.

The expansion of space adds a second, stranger layer. ESA explains that the cosmic microwave background, the leftover radiation from the Big Bang, was released about 380,000 years after it, when the universe had cooled to around 3,000 kelvin. Since then space has stretched, and the wavelengths of that light have grown, or been redshifted, to roughly one millimetre. The radiation now fills the universe at just 2.7 kelvin. Light a millimetre long is far beyond what the eye can see.

So in a sense the night sky is not truly dark at all. In every direction, behind the gaps between the stars, the sky is filled with the stretched-out afterglow of the young universe. It is simply glowing in microwaves, which our eyes were never built to see. The universe has not switched the lights off. It has turned them down to a colour we cannot see, and left the nearest stars on as night lights.