🔭 The uniform universe? A fundamental principle of cosmology disproven by observation

It's a discovery that could overturn our understanding of the cosmos at its most fundamental level: the universe might not be uniform in all directions, even on gigantic scales. Based on data from the Dark Energy Spectroscopic Instrument (DESI), a recent study challenges one of the pillars of modern cosmology.

The cosmological principle is based on the idea that, on a large scale, matter in the universe is distributed homogeneously and isotropically. It is a consequence of the Copernican principle, according to which no observer occupies a privileged position. Yet, on the scale of galaxies or clusters, the universe appears clearly anisotropic: voids and filaments organize into a 'cosmic web'. Scientists debated the scale at which this structure fades away.


To test this isotropy more generally, astronomers Francesco Sylos Labini and Marco Galoppo used a statistical method called ADPD (Angular Distribution of Pairwise Distances). It measures directional correlations without predefined parameters. Comparing DESI data with an isotropic model, they found that galaxies remain persistently clustered up to scales on the order of a gigaparsec, i.e., a thousand times larger than what previous studies indicated.

"Our results provide direct evidence that directional coherence persists on larger scales than expected," the authors write in Nature. The implications for cosmology are significant, as the cosmological principle is the basis of many models, particularly that of the Big Bang with inflation.

The study does not challenge the Copernican principle, which only excludes privileged observers, but it nuances the idea of perfect uniformity. The authors propose exploring more general solutions to Einstein's equations, or considering mechanisms such as interactions in dark matter or feedback effects from inhomogeneities.

However, the physical origin of this anisotropy is not identified, and it is possible that at even larger scales isotropy eventually prevails. Nevertheless, if these results are confirmed, certain aspects of cosmology will need to be revised.

The Copernican principle

The Copernican principle, named after the astronomer Nicolaus Copernicus, states that no observer in the universe occupies a privileged position. In other words, the laws of physics are the same everywhere, and there is no 'center' of the universe. This principle is the basis of the cosmological principle.

The Copernican principle, however, allows local heterogeneities, provided they do not confer a special status to an observer. Thus, even if the universe is anisotropic on large scales, it does not necessarily violate the Copernican principle.

In the DESI study, the authors note that their results remain compatible with the Copernican principle, although they contradict the hypothesis of isotropy on a large scale. This indicates the possibility of cosmological models where matter is not uniformly distributed, without challenging general relativity.