You’re not lost in a directionless universe

A news article in appeared in Science titled “It’s official: You’re lost in a directionless universe”1 where the author Adrian Cho reported on the results of a research paper published in Physical Review Letters in September 2016. That paper is available online as a preprint.In the online Science article the conclusion of the research is stated that

“For the first time, we really exclude anisotropy,” [the lead author] Saadeh says. “Before, it was only that it hadn’t been probed.”


Top image: CMB temperature anisotropies map from Planck satellite. Bottom image: Simulated image from one of the models used where a preferred axis was introduced. Credits: (Top to bottom) ESA and the Planck Collaboration; D. Saadeh et. al., zenodo

The research involved simulations on a supercomputer where various forms of anisotropic structure and expansion of the universe were introduced in modelled universes. The authors looked for how those would affect any putative patterns that might be observed in the cosmic microwave background (CMB) radiation. The design was to see what would produce anisotropy in the CMB temperature data. See illustration to the right.

They found that none of the patterns they produced are observed in the CMB data from the Planck satellite. Ok, so that solves it! The Universe is isotropic and therefore the fundamental assumption for the big bang model—that is, matter is distributed uniformly throughout the Universe, on the largest scales–is correct and hence it validates the choice of the standard ΛCDM big bang model to describe the Universe. Well, no, not actually.

Firstly, for that to be true it would have to be assumed that the authors modelled all possible sources of anisotropy in the Universe. It would also have to be assumed that the patterns they generated in their modelled CMB temperature anisotropies were, in fact, indicative of large scale structure in the real Universe. There is no independent way to test that. All that researchers have available to them is supercomputer modelling. So how can you know what the Universe should look like with different types of anisotropic distributions of matter? There are no other universes available except this one, therefore we are always limited by this fundamental uncertainty. Continue reading