The fundamental principles of the universe questioned by two physicists

One of the most fundamental and accepted truths about the universe is that it is pretty much the same everywhere you look. In other words, there is no “up” or “down” in the cosmos. No direction has more structure, more galaxies – more things – than another. Cosmologists take this similarity for granted; this is one of their main maxims, called the “cosmological principle”. But what if this dogma isn’t true at all?

A new article published Wednesday in Nature by two physicists calls into question the cosmological principle. They argue that the structures of the universe look very different depending on which direction you look. “In this study, we discovered large-scale structures that define particular directions,” says Francesco Sylos Labini, a physicist at the Enrico Fermi Research Center in Rome, Italy and one of the study’s authors.

Using data from the Dark Energy Spectroscopic Instrument (DESI), Labini and his co-author argue that the structures of the universe are far more complicated than existing models suggest, violating one of the most sacred ideas in cosmology.

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“I’ll be very interested to hear the community’s reaction,” says Katherine Freese, a cosmologist at the University of Texas at Austin who was not involved in the new paper. Freese says the study could challenge “the basic scaffolding of the universe that we all assume in our work.”

The cosmological principle states that the universe is “homogeneous”, meaning that each part of the universe contains approximately the same amount of matter as the others, and that it is “isotropic”, meaning that no direction is significantly different from the others. It is the founding mathematical premise of most models of the universe and underlies the way cosmologists construct the equation for its form. And this is the basis of cosmic inflation – the theory that our universe experienced a period of extreme and rapid expansion just after the Big Bang. It is also the simplest explanation of the universe we see.

“But in physics there is no area in which the simplest solution actually applies,” says Labini.

DESI has spent the last five years measuring vast swaths of galaxies, covering much of the structure of the universe at different times. So Labini and his co-author Marco Galoppo compared galaxies in different directions in this data to see if they were all the same. They discovered that the standard cosmological model, based on a universe without a preferred direction, could not explain the large correlated structures observed by DESI.

“If true, it would be important, but it requires much more careful verification,” says David Spergel, an astrophysicist and president of the Simons Foundation. Astronomers are perplexed that such a glaring inconsistency could have gone unnoticed in existing data, such as the cosmic microwave background (CMB), which provides our first snapshot of the universe. “If this were true, the CMB fluctuations would be about a hundred times larger than what we observe,” says Spergel.

“The claims in this paper appear to conflict with what we know about the large-scale structure of the universe,” says John Peacock, professor of cosmology at the Institute of Astronomy at the University of Edinburgh. “And in particular with other results established from the same DESI data.”

To support such a strong statement, it will need corroboration. Peacock expects the DESI collaboration to begin this work itself. “Until we can understand if/how this can be made consistent,” Peacock says, “I don’t expect many people to be convinced by the paper’s claims.”