One of the most basic and accepted truths about the universe is that it’s 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 stuff—than any other. Cosmologists take this sameness for granted; it’s one of their foremost maxims, called the “cosmological principle.” But what if this dogma isn’t true at all?
A new paper published Wednesday in Nature by two physicists calls the cosmological principle into question. They argue that the universe’s structures do look significantly different depending on the direction you look. “In this survey, we find there are large-scale structures which define special 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 claim that the universe’s structures are far more complicated than existing models suggest, violating one of cosmology’s most sacred ideas.
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“I will be very interested to hear the reaction of the community,” says Katherine Freese, a cosmologist at the University of Texas at Austin who was not involved in the new paper. Freese says that the study could challenge “the basic scaffolding for the universe that we all assume in our work.”
The cosmological principle states that the universe is “homogeneous,” meaning every patch of universe holds roughly the same amount of matter as every other, and that it is “isotropic,” meaning no direction is significantly different than any other. It’s the founding mathematical premise for most models of the universe, and it underpins how cosmologists construct the equation for its shape. And it’s the basis of cosmic inflation—the theory that our universe underwent a period of extreme, rapid expansion just after the Big Bang. It’s also the simplest explanation for the universe we see.
“But in physics, there is no field in which the simplest solution applies in reality,” contends Labini.
DESI has spent the past five years measuring huge ranges of galaxies, covering much of the universe’s structure at different moments in time. So Labini and his co-author Marco Galoppo compared galaxies along different directions in this data to see if they were all the same. They found that the standard cosmological model—one based on a universe with no preferred direction—couldn’t explain the large, correlated structures DESI observed.
“This would be important if true, but requires much more careful verification,” says David Spergel, an astrophysicist and president of the Simons Foundation. Astronomers are puzzled that such a glaring inconsistency could have gone unnoticed in existing data, such as the Cosmic Microwave Background (CMB), which provides our earliest snapshot of the universe. “There would be CMB fluctuations roughly a hundred times bigger than we see if this were true,” explains Spergel.
“The claim in this paper seems to conflict with much that we know about large-scale structure in the universe,” says John Peacock, professor of Cosmology at the Institute for Astronomy at the University of Edinburgh. “And in particular, with other results established using the same DESI data.”
To bolster such a strong claim, it will need corroboration. Peacock expects the DESI collaboration to begin that work itself. “Until we can understand if/how this can be made consistent,” says Peacock, “I don't expect that many people will be persuaded by the claims in the paper.”
