It all looks the same —

Is the Universe the same in all directions?

New study of the Planck data tests the Universe's isotropy.

A colorful image of the night sky in a shape similar to the Mollweide projection of the Earth.

The standard model of Cosmology makes an important assumption: that the Universe is essentially isotropic, the same in all directions. This is a pretty good assumption, and so far isotropy seems to be the case. The Cosmic Microwave Background radiation we see coming from all directions has some small blips in it, indicating minor deviations from isotropy, but nothing to write home about. So far so good.

But we don’t know for sure just how far that isotropy extends. If there were huge deviations, it would mean we live in a wholly different universe from the one we thought we did. This class of alternative universes is known as Bianchi cosmologies, and only a few of them have been tested against the data. That leaves plenty that haven’t. If our Universe turns out to be a Bianchi universe, it would rule out our standard cosmology and associated ideas, such as inflation.

In an attempt to find out what kind of universe we live in, a group of researchers constructed a more general test. If the Universe is truly anisotropic (different in different directions), it would mean it’s expanding at different rates. If so, light traveling through these differently expanding regions would be red-shifted differently. That's because the wavelength of light is stretched as it moves through expanding space, so if space were expanding at different rates, the redshift would be altered accordingly.

Any differences, were they to exist, should be detectable. The trick is disentangling that effect from the influence of all the early Universe’s chaos, which also affected the Cosmic Microwave Background.

The researchers examined Cosmic Microwave Background data from the Planck spacecraft, including polarization and temperature data. The polarization data is ideal for probing the degree of anisotropy, strengthening this analysis compared to previous studies.

The Planck probe resides at the L2 Lagrange point. It was launched as part of a package with the Herschel infrared telescope.
Enlarge / The Planck probe resides at the L2 Lagrange point. It was launched as part of a package with the Herschel infrared telescope.

Previous work had mostly focused on scenarios in which the Universe is rotating, which is one possible anisotropic universe. These studies had put tight constraints on the possibility of a rotating universe—sorry, carnival enthusiasts, we’re probably not spinning—so the researchers wanted to see if their test would reproduce these results. It did and put even tighter constraints on the spinning Universe scenario, increasing them by a full order of magnitude.

But the researchers’ more general test applies to all the other anisotropic scenarios. None of these did any better, as the researchers were able to put tight constraints on other Bianchi universes as well.

“In this work, we put the assumption that the Universe expands isotropically to its most stringent test to date,” the researchers write in their paper. “We find overwhelming evidence against deviations from isotropy.” They conclude the chances of an anisotropic universe are 121,000 to one, against.

The study confirms what we already assumed, so it isn’t the most exciting result. But it’s nonetheless important to test assumed aspects of our theories and rule out those interesting ideas that might result from our assumptions being wrong. An anisotropic universe would surely have been an interesting one, but it doesn’t seem to be the one we live in.

Physical Review Letters, 2015. DOI: doi:10.1103/PhysRevLett.117.131302 (About DOIs)

Channel Ars Technica