The cosmos is starting to look a little weird. For a few years now, cosmologists have been concerned about a discrepancy about the speed at which the universe is expanding. They know which one it should, based on the ancient light of the early universe, but what you see is that the modern universe has picked up too much speed. It implies that scientists have overlooked some of the fundamental ingredients of the universe or some aspect of the mixture of those ingredients.
A second rift in the so-called standard model of cosmology may now be opening: at the end of July it was announced that the modern universe also appears to be not lumpy enough. Galaxies and gas and other matter have not clumped together as much as they should. A few studies already offered similar clues, but this new analysis of seven years of data represents the sharpest and most self-sufficient sign that there is an anomaly yet.
“If there were now congresses,” says Michael Hudson, a cosmologist at the University of Waterloo in Canada, who was not involved in the research, “over coffee you would only talk about those results.”
As with most measurements of the large-scale structure of the universe today, the study is fraught with difficulties. It is also possible, although unlikely, that the results are due to chance. However, some researchers wonder if the growing trend in measurements to get out of hand will herald the discovery of a new cosmic agent.
“We already have dark matter and energy,” says Hudson. “I hope we don’t need one more dark thing.”
Another series of alarm bells
The difficult thing about studying the modern universe is that for the most part it is invisible. Astronomers glimpse the big picture where galaxies meet in glowing clusters. But to a great extent they are still unable to perceive the faint threads of gas that weave these nodes into a vast network. Worse still, most believe that these galaxies and gas tracks are little more than the tinsel that decorates a massive framework of “dark matter,” the one from which above all the core of the universe is made.
The new study is the most refined application yet of a technique that reveals the unseen. For light from a distant galaxy to reach Earth, it must have passed through filaments of dark matter and quenched clouds of gas. These thick spots attract light gravitationally, so they sow its trajectory of twists. By the time the light from the remote galaxy reaches a ground-based telescope, it will be subtly distorted, perhaps squashed into an exaggerated ellipse. Astronomers will then try to map the invisible dark matter by measuring the statistical distortions of the shapes of a huge number of distant galaxies across a wide swath of the sky.
In the new research, members of the Kilo-Degree Survey, or KiDS, observed some 31 million galaxies up to 10 billion light-years away and used these observations to calculate mean distributions of the hidden gas and the dark matter of the universe. They found that the degree of agglomeration was almost ten percent less than predicted by the hegemonic cosmological model, the so-called “cold dark matter-lambda” (lambda refers to dark energy, in the form known as the “cosmological constant”). .
Statistically, the difference is such that the probability that additional data will dissipate it is about 1 in 1,400, much higher than the demanding standard of the specialty, 1 in 1,700,000, but much less than if it were like tossing a coin into the game. air. “This tension is now at a level that at least intrigues or tempts,” says Marika Asgari, a cosmologist at the University of Edinburgh and a KiDS member.
Furthermore, other independent measurements support the finding that the contemporary universe does not appear as lumpy as it should. “This is another series of alarm bells,” in Hudson’s words.
Hudson has attempted to decipher the hidden universe by observing the drift of galaxies in cosmic currents. If matter were perfectly distributed, like a fine mist, the expansion of the universe would regularly separate some objects from others with a drifting motion called the Hubble flow. But the universe is riddled with voids and studded with superclusters rich in dark matter. The gravitational pull of the superclusters brings galaxies closer together, while the voids let them fly freely. By measuring the “peculiar velocities” of supernovae (how far they deviate from their local Hubble flow), Hudson and his collaborators have created their own maps of the hidden mass of the cosmos.
Hudson came up with the first hint in 2015 that the universe has not become as crowded as previously thought. Subsequent maps of the peculiar velocity have exhibited the same disturbing regularity. Hudson and her collaborators published a study in July in which they inferred an anomalous lack of ag<2MASCULINE>ration almost as great as that found by KiDS.
In addition, at least a dozen surveys have found in the last eight years, with different techniques, a universe of today that lacks a bit, at least, of lumpiness. Each of these soundings has little significance on its own, but there are cosmologists who are increasingly suspicious that all measurements fall short of the theoretical prediction rather than being distributed indifferently around it.
“When you see the same thing in a whole bunch of different data sets,” explains Hudson, “you think it really means something.”
If it is a message from the universe, the meaning remains unclear. The standard model of cosmology agrees with observations so closely that theorists cannot add new pieces at will. The most accurate measurements of the early universe are those provided by the Planck collaboration. It published its final results in 2018. “It’s hard to find something new that doesn’t break with Planck,” says Daniel Scolnic, a cosmologist at Duke University who studies the pace of expansion.
But Planck’s straitjacket has some loose buckles; theorists have been tinkering with them for years to explain the unexpectedly rapid expansion.
Now they have to undertake two contradictory tasks. To solve the original problem, that of the expansion of the universe, they need a phenomenon that gives the universe an extra separating shove. But to solve the new anomaly they have to weaken the gravitational pull that makes the universe clump together. As the two problems come together, according to Julien Lesourges, a theoretical cosmologist at RWTH University in Aachen and a member of the Planck collaboration, “finding an explanation for both becomes a nightmare.”
For example, to set the expansion in motion, some theorists have tried adding “dark radiation” to the early universe. But they have to balance that extra radiation with the extra matter that thickens the universe. So to end up reaching the universe that we see, new interactions have to be invented between the various dark ingredients to obtain the desired agglomeration deficit.
Another possibility is that dark matter, which leads the universe to agglomerate, is transformed into dark energy, which tends to disperse it. Or perhaps Earth is in a vast void that is skewing observations. Or perhaps there is no relationship between the two anomalies. “I haven’t seen anything convincing,” says Hudson, “but if I were a theorist, I would be passionate about this now.”
One of the two tensions, or both, could still be dissipated with more data. KiDS is one of three large weak gravitational lens surveys currently being conducted, along with the international Dark Matter Survey in Chile and the Japanese Hyper Suprime-Cam survey at the Subaru telescope in Hawaii. . Each tracks a different region of the sky, with different depths. The results of the latest Dark Energy Survey campaign, which covered an area of the sky five times the size of KiDS, will be released in the coming months. “Everyone is more or less on edge,” according to Scolnic. “It’s the next big thing in cosmology.”
Duke University’s Michael Troxel, who works with the weak gravitational lenses for the Dark Energy Survey, commends the KiDS team for taking the technique to new heights of precision and for covering more sky than ever before. But it also highlights that a mountain of technical difficulties makes it difficult to delve too deeply into a specific measurement. At a distance of billions of light years, galaxies are nothing more than mere pixels; complicates the analysis of its shape. Researchers also need to know how far apart each galaxy is, and how they deal with the uncertainties that accompany those distances can either lower or exaggerate the tension.
“I wouldn’t bet a penny on what the ultimate value might be,” says Troxel.
Charlie Wood / Quanta Magazine
Article translated by Research and Science with permission from QuantaMagazine.org, an independent publication promoted by the Simons Foundation to enhance public understanding of science.
Reference: «KiDS-1000 Cosmology: Cosmic shear constraints and comparison between two point statistics», by Marika Asgari et al., In arXiv: 2007.15633 [astro-ph.CO].