Like a vast bowl of spaghetti, the universe may be stringy on length scales far larger than cosmologists have long assumed. In a study published today in Nature, two researchers argue galaxies align in enormous filaments even on scales where the cosmos should appear smooth and uniform. If correct, the bold claim would upend the cosmological principle, the conceptual cornerstone of the standard cosmological model.
“This is serious,” says Subir Sarkar, a cosmologist at the University of Oxford who was not involved in the work. “If there is a real contradiction between what you expect and what you find, that would be progress, right?” Abandoning the cosmological principle could even eliminate the need for dark energy, the mysterious space-stretching stuff thought to be accelerating the expansion of the universe, some cosmologists say. But others are skeptical of the new claim.
The cosmological principle holds that the universe is homogeneous (the same everywhere) and isotropic (the same in all directions). A quick glance at the night sky shows this cannot be true at all levels, because the void of space is dotted with dense clumps of stars. Nevertheless, cosmologists assume the universe is essentially uniform and featureless when averaged over sufficiently long length scales, much as a pixelated photo looks smooth when viewed from a distance.
That premise underlies the standard cosmological model, which is known as lambda cold dark matter (lambda-CDM). The theory specifies a recipe for the cosmos—5% ordinary matter, 27% invisible dark matter, and 68% dark energy. It traces the evolution of the vast web of galaxies to tiny density fluctuations in the hot soup of subatomic particles that leapt into existence in the Big Bang. In a sliver of a second, an exponential growth spurt called inflation stretched those fluctuations to cosmic scales and smoothed and flattened spacetime. As expansion slowed, gravity amplified the fluctuations, drawing dark matter into clumps that later attracted ordinary matter, seeding stars and galaxies.
All of this must jibe with Albert Einstein’s theory of gravity, general relativity. That’s where the cosmological principle comes in, says Glenn Starkman, a theoretical physicist at Case Western Reserve University. According to general relativity, cosmic evolution should be governed by 10 intertwined equations. Assuming the universe is uniform reduces the tangle to two. It also ensures that physics works the same way everywhere in the cosmos, Starkman says. So including the cosmological principle makes lambda-CDM mathematically tractable. However, it also assumes the underlying spacetime remains smooth and flat, and that on some length scale the universe looks uniform.
Scientists have debated where that transition to uniformity occurs since the 1980s, Sarkar says. Previous studies of the distribution of the galaxies have suggested the universe becomes homogenous at length scales of about 100 megaparsecs—roughly 300 million light-years.
However, the universe’s stringiness persists up to scales of 1 gigaparsec—3 billion light-years—argue Francesco Sylos Labini of the Enrico Fermi Research Center and Marco Galoppo of the University of Canterbury. The duo arrived at that conclusion by applying a novel statistical tool to the largest map of galaxies so far. The portrait of the cosmos was produced by the Dark Energy Spectroscopic Instrument (DESI), which mapped the 3D positions of 47 million galaxies using a 4-meter telescope in Arizona.
Previous studies typically asked the question: Given a galaxy at any one point, how likely is it to find another galaxy at a particular distance, regardless of direction? “In general, people measure the probability function of the distance,” Sylos Labini says. “Here we are measuring how it varies with both distance and angle.”
To do that, the researchers sliced the DESI map into 2D slices, examined pairs of galaxies separated by a given distance, and looked for patterns in how those pairs were oriented. In a homogenous universe, those directions should be randomly and evenly distributed. Instead, the researchers found significant correlations in the directions, suggesting the distribution of galaxies remains stringy over much larger distances than lambda-CDM simulations predict.
“I’m not surprised,” says Thomas Buchert, a cosmologist at the École Normale Supérieure of Lyon. He argues that lambda-CDM is logically inconsistent because it ignores how the web of dark matter and galaxies itself warps spacetime. In principle, he says, that feedback might explain the accelerated stretching of space without invoking some sort of dark energy.
However, David Spergel, a cosmologist at the Flatiron Institute, is skeptical of the new claim. If structures extend across billions of light-years, he says, their gravity should dramatically distort the lingering afterglow of the Big Bang, the cosmic microwave background (CMB). “The result very much appears to be in contradiction with much more sensitive measurements from the microwave background.” Sarkar counters that if Spergel and others can use observations of the CMB to rule out the new result, “then they should do it.”
The debate may ultimately boil down to whether the cosmological principle is a fundamental characteristic of the universe or merely a good approximation. Given lambda-CDM’s success in explaining myriad observations, the principle cannot be too badly in error, Starkman says. However, he argues, a variety of anomalies suggests it’s not entirely correct. “The cosmological principle can’t be horribly violated,” Starkman says. “That doesn’t mean it’s true.”
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