AT THE EDGE OF EVERYTHING

Puzzling over the answers to a centuries-old cosmic paradox can teach us profound things about the Universe

- DR KATIE MACK

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.

SOLVING THE PARADOX