Every day, we are learning more about our great and wonderful Universe (like that it's actually a lot smaller than we thought, for example). But underneath all of that is one rather solid fact: that the Universe is expanding, and it's expanding everywhere, and it's expanding ever faster. The key to all of this is that the distribution and expansion are equal all over—that things are roughly homogeneous.
However, one idea seeks to shatter that homogeneity. It's known as the anisotropic hypothesis, where the Universe is expanding along a preferred axis or direction. That idea, if proven, would shatter all the models we have built of the expansion of the known universe and put our standard model of cosmology into question.
What principle would cause the cosmos to have a specific direction? Ultimately, we don't know. However, if true, this idea could provide solutions to Einstein’s field equations (specifically, it lends weight to the Bianchi models).
But before we get to the viability of the idea, let's back up a bit.
The anisotropic hypothesis was born when it was realized that galaxies and star clusters are scattered randomly. If the Universe was so homogeneous and isotropic, then why is matter not evenly distributed, why is star system formation random?
This theory was strengthened back in the 2000s, when NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft found strange 'bumps' in the universe's background radiation. In fact, astronomers have found a place where these 'cosmic bumps' line up so much that they called it the "axis of evil."
All of these contrarian finds spurred researchers from the University College London in the UK to look at the cosmic microwave background (CMB) in order to see whether the anisotropic hypothesis actually holds up.
Now, it seems that the anisotropic/isotropic debate may be coming to a close.
Put to the test
The team probed the CMB for evidence that the Universe expands at different speeds along different axes, or whether gravitational waves distorted the Universe, stretching it in one direction and compressing it in another. Cosmologists breathed a collective sigh of relief when the study of the Universe's background radiation showed (seemingly once and for all) that the cosmos is expanding everywhere uniformly, just as our models predicted.
The study used a supercomputer to search for these patterns in the utter randomness of the CMB. They determined that there is one-in-121,000 chance that the Universe is heading in a preferred direction. This is the most conclusive study of its kind to date, and it seemingly puts a lid on the question once and for all, proving that it's just one aimless ride for all of us.
There's no direction. There's No preferred place...which may be less than stellar for the Bianchi models, but it's good for our standard model of cosmology.