Is the Universe uniform?

One of astronomy's most fundamental assumptions may be wrong. Colin Stuart investigates whether our belief that the Universe looks the same everywhere holds up to scrutiny

- Colin Stuart

Imagine you're in a woodland, with sunlight filtering through the endless green canopy overhead. There's uniqueness everywhere. Some trees grow bolt upright, but others are wizened and twisted. Here the ground is clear, but over there it's riddled with brambles and thorns.

Yet this individuality fades if you can climb a ridge and lift yourself above the canopy. Now your view is a constant sea of green for as far as the eye can see. The forest is both regular and directionless.

Most cosmologists believe that the Universe works in the same way. At small scales, there are individual quirks such as stars, planets and different types of galaxies. But zoom out far enough, to scales of hundreds of millions of lightyears, and it's like climbing that ridge: the Universe becomes uniform.

Astronomers say that, on large scales, the Universe is both homogeneous and isotropic. Homogeneous means the Universe is the same everywhere - drop into any sufficiently large patch of space and, on average, you'd see the same spread of galaxies. Isotropic means the Universe looks the same in every direction - no matter whether you look north, south, east or west, the broad brushstrokes of the cosmic picture repeat.

Together, these ideas underpin an almost sacrosanct astronomical assumption that's known as the cosmological principle, which says that we do not occupy a special place in the Universe.

"Homogeneity and isotropy are absolutely central to modern cosmology," says Blake Sherwin, a professor of cosmology at the University of Cambridge. "If we ever robustly confirmed a departure from them, it would force us to reexamine the foundations of the field."