Our understanding of the universe is underpinned by the cosmological principle: the assumption that, on the grandest scales, it looks more or less the same in all directions. What if that’s wrong?
IMAGINE you are marooned in a vast, featureless expanse. Everywhere you look, no matter how far you travel, it all looks the same. It sounds like a disturbing dream. Believe it or not, though, this is the universe you live in. If you zoom out far enough, past nearby stars, through the Milky Way to clusters of galaxies and the filament-like structures that connect them, and then you keep going, eventually, everything starts to look smooth and uniform wherever you glance.
Or does it? This idea that, on the grandest scales, the cosmos looks largely the same regardless of position or direction is called the cosmological principle, and it underpins our best theory of how the universe evolved. For cosmologists, it is gospel. But some heretics are now calling the principle into question, pointing to fresh evidence that even at its largest scales, the cosmos is not only lumpy, but fundamentally off-kilter.
If they are right, it would upend cosmology. We would have to start our description of the universe’s evolution from scratch – and possibly even admit there can be no single model capable of describing it right up to today.
If they are right. Most cosmologists are a long way from convinced. Similar challenges have been seen off before, they shrug. And yet there is a reckoning on the horizon because, one way or another, the cosmological principle – for so long considered untouchable – is finally going to run the empirical gauntlet.
The notion that the universe is the same everywhere in all directions grew out of another essential tenet of cosmology: Nicolaus Copernicus’s argument, made in the …