Olbers’ Paradox

Whether it’s more troubling to imagine that the Universe goes on forever in every direction, or that it has an edge, beyond which there is nothing, is hard to say.

Astrophysics doesn’t provide any guidance as to which flavour of existential crisis we should be having – while we can’t say with any level of confidence whether the Universe goes on forever or not, we can say that our observable universe has an edge, in the sense that there’s a distance beyond which, whatever may or may not exist, we absolutely cannot see it.

The existence of this cosmic horizon is part of the answer to an ancient conundrum about the darkness of the night sky, reportedly first posed by Johannes Kepler in 1610 but later attributed to fellow astronomer Heinrich Olbers in the 1800s.

Olbers’ Paradox asks: if the Universe is infinite, and if there are stars (or galaxies) throughout it, why is the sky dark? Surely, if we look in any direction in the sky, that sightline will, eventually, land on a star. Common sense therefore tells us that everywhere we look, the sky should be as bright as the Sun, constantly aglow.

The standard resolution to this paradox invokes the finite age of the cosmos and the speed of light. Even if the cosmos is endless and full of stars, one might reason, we can only see the ones that are close enough for there to have been enough time (since the beginning of the Universe) for the light to reach us from there. Anything distant enough from the Earth that the light travel time is more than the age of the Universe is invisible to us. This doesn’t entirely resolve the paradox, for reasons that involve some much stranger physics. But this light travel distance limit is responsible for our horizon – the edge of the observable universe. The most distant things we can see in the cosmos are the things whose light has been travelling to us for the age of the Universe: 13.8 billion years.

The stranger physics comes in when you ask, what are those things whose light has travelled for that long? The Big Bang theory says that the Universe 13.8 billion years ago was a hot, dense inferno, in which all of space was filled with glowing-hot plasma, rippling and churning like the surface of the Sun. Because all of space was glowing, when we look into the farthest reaches of the cosmos in any direction, that glow is in fact what we see.

So if all of space was glowing, why is the sky dark? Have we just unsolved the paradox?

The reason the Universe can be glowing all around us but still look dark comes down to the physics of light in an expanding universe. When space expands, and the distance between objects grows, the light passing between those things gets stretched out, shifting the light to lower frequencies on the electromagnetic spectrum.

For visible light, lower frequencies correspond to redder colours, so this effect is called “redshift”. You can think of it like a Doppler shift – the same kind of effect that’s responsible for a siren dropping to a lower tone when an ambulance speeds away from you, since distant objects appear to speed away from us as the Universe expands.

Science Focus