Olbers' Paradox: What the mystery of the night sky teaches us about our Universe | BBC Science Focus Magazine

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.

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.

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

While we may never know if the Universe as a whole is infinite or bounded, we know that the cosmic microwave background – the distant shell of fading fire that surrounds us – is the most distant light we can ever see, at the edge of our observable universe. But just like the darkness of the night sky, this edge is a matter of perspective. Someone living in a galaxy billions of light years away from us sits at the centre of their own observable universe, which may only partly overlap our own.