One of the fundamental assumptions about the universe is that it’s the same everywhere and in every direction. This very democratic idea is synthesized in the cosmological principle (CP), and although that principle is at the core of astrophysics, it doesn’t make it beyond testing.

To check the validity of the CP, a team of researchers from University College London (UCL) and Imperial College has analyzed the first light in the universe for signs that the universe is expanding in a preferred direction, known as anisotropy. Fortunately for the CP, they found that this idea is disfavored by data with odds of 121,000:1.

The team has been investigating the potential anisotropy of the universe by looking at the cosmic microwave background (CMB), the original photons from the Big Bang. The researchers, led by Daniela Saadeh and Stephen Feeney, created computer simulations of the CMB in scenarios where the universe was expanding at a different rate in different directions.

"There are actually a huge variety of ways in which Einstein's theory of general relativity would allow for space to be less egalitarian: universes that spin and stretch are entirely possible," Saadeh told IFLScience.

"If the cosmos were to behave in one of these ways, many of our calculations about its history and content would have to be revisited. This is why it is essential to test the Cosmological Principle."

In a paper, published in Physical Review Letters and available online, the team compared a special property of light, called polarization, in both the mock CMBs and in the real data collected by the Planck satellite. Light is a wave created by an oscillating electromagnetic field, and when the light of a source is oscillating in a specific direction, it is said to be polarized. If the universe was indeed expanding in a specific direction, the CMB would definitely show a certain polarization.

While the simulated CMBs show these polarizations, the real CMB doesn’t, clearly indicating that the cosmological principle is indeed correct.

This has been the latest in a long series of tests on the validity of the CP. The universe is clearly not the same at small scales; stars, planets, and galaxies have (luckily) huge gaps between them in space. But the discovery of the CMB in the sixties has suggested that at large scales the universe is uniform.

Missions like the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP), and Planck, which in the last 30 years has studied the CMB in more and more detail, have continued to suggest that the CP is valid. With this further proof, we can be content that the universe doesn’t seem to care about any particular direction.