The cosmological principle holds that the universe has the same structure and basic properties wherever you go. It's been a fundamental part of physics since Copernicus, and all available evidence has supported it...until now.
The principle tells us that the universe is both homogenous and isotropic. This means that, on large enough scale, all apparent variations get smoothed out and you're left with a universe with uniform distribution of matter - meaning there are no parts of the universe with a greater concentration of galaxies than any other - and the same laws of physics everywhere.
It's a pretty big assumption, but it's proved a cornerstone of astrophysics for the last five hundred years, and no one has ever found any solid evidence against it. Now, two researchers from the Chinese Academy of Sciences have found an apparent discrepancy that suggests the universe might be expanding slightly faster in one particular direction than any other. If the result holds up, that would be a major violation of the cosmological principle.
The result stems from careful analysis of 557 Type Ia supernovae, which have been used extensively over the past decade to track the universe's accelerating expansion. As such, they've been one of the key pieces of evidence for dark energy, and now they might be revealing a second great cosmological conundrum.
Researchers Rong-Gen Cai and Zhong-Liang Tuo say that the supernovae support faster acceleration in the northern galactic hemisphere, specifically in the direction of the constellation Vulpecula. The deviation isn't huge, but it's big enough that it can't easily be explained away as a bit of random statistical noise or an artifact of where we happen to be observing the universe.
Of course, that assumes that these findings are accurate, and we're not quite there yet. Anything that might suggest a violation of cosmological principle falls under the old dictum that "extraordinary claims require extraordinary evidence", and one study isn't enough to do that. Indeed, several previous surveys of Type Ia supernovae have suggested no such discrepancy, and the cosmological principle is backed up by tons of basic features of the universe.
Whatever happens, it's unlikely that the cosmological principle is on the way out. There are too many ways in which the cosmos remains homogeneous and isotropic for it to be dismissed entirely. But this is a bit of an all-or-nothing proposition - either the universe is truly uniform, or it isn't, and finding out that it isn't would rock our understanding of astrophysics. Right now, I'd bet that these results ultimately don't stand up, if only because the cosmological principle has proven such a strong bedrock for astrophysics...but if they really are accurate, we've got a whole lot of textbooks to rewrite.