September 2020

A New Cosmic Tension: The Universe Might Be Too Thin

Cosmologists have concluded that the universe doesn’t appear to clump as much as it should. Could both of cosmology’s big puzzles share a single fix?

By Charlie Wood

The cosmos is starting to look a bit weird. For a few years now, cosmologists have been troubled by a discrepancy in how fast the universe is expanding. They know how fast it should be going, based on ancient light from the early universe, but apparently the modern universe has picked up too much speed—a clue that scientists might have overlooked one of the universe’s fundamental ingredients, or some aspect of how those ingredients stir together.

Now a second crack in the so-called standard model of cosmology may be forming. In late July of 2020, scientists announced that the modern universe also looks unexpectedly thin. Galaxies and gas and other matter haven’t clumped together quite as much as they should have. A few earlier studies offered similar hints, but this new analysis of seven years of data represents the cleanest stand-alone indication of the anomaly yet.

“If we were having conferences,” said Michael Hudson, a cosmologist at the University of Waterloo in Canada who is not involved in the research, “all the coffee chatter would be about these results.”

Like most measurements of the large-scale structure of the present-day universe, the study is fraught with technical difficulties. It’s also possible, though unlikely, that the results are due to chance. Nevertheless, some researchers wonder if the trend toward increasingly funky measurements may foreshadow the discovery of a new cosmic agent.

“We’ve already got dark matter and dark energy,” Hudson said. “I hope we don’t need another dark thing.”

Another Set of Alarm Bells

The hard part about studying the modern universe is that it’s mostly invisible. Astronomers catch glimpses of the big picture in the places where galaxies gather into shining clusters. But they remain largely unable to perceive the dim strands of gas that weave these nodes together into a vast cosmic web. Worse, most believe these galaxies and trails of gas to be little more than decorative tinsel on a sturdy frame of invisible “dark matter” that makes up most the universe’s bulk.

The new survey is the most refined implementation yet of a technique for revealing the unseen. As light from a distant galaxy makes its way to Earth, it passes by fibrils of dark matter and dim clouds of gas. These thick spots pull on the light gravitationally, putting kinks in its path. By the time the far-off galaxy’s light reaches a terrestrial telescope, it’s subtly distorted—perhaps squashed into an exaggerated ellipse. Astronomers then try to map the invisible dark matter by measuring the statistical distortions in the shapes of huge numbers of distant galaxies across a vast swath of sky.

nautilus how distortions matter

Samuel Velasco/Quanta Magazine; source: news.rub.de

In the July 2020 research, members of the Kilo-Degree Survey, or KiDS, observed about 31 million galaxies from up to 10 billion light-years away. They then used these observations to calculate average distributions of the universe’s hidden gas and dark matter. They found clumps that are almost 10% thinner than the forecast from the established cosmological model, known as Lambda cold dark matter, or ΛCDM.

Statistically, the difference is such that the odds of additional data washing it out are roughly 1 in 1,400—far short of the field’s stringent standard of 1 in 1.7 million, but substantial enough to turn heads. “This tension is now at the level that is at least intriguing, or tantalizing,” said Marika Asgari, a cosmologist at the University of Edinburgh and a member of KiDS.

What’s more, other independent measurements support the finding that the contemporary universe appears too thin. “This is another set of alarm bells,” said Hudson.

Hudson has attempted to decipher the hidden universe by watching how galaxies drift on cosmic currents. If matter were perfectly distributed in a fine mist, the universe’s expansion would push all objects smoothly apart in a drifting motion known as the Hubble flow. But the universe is pocked with empty voids and superclusters rich in dark matter. The gravitational draw of superclusters pulls galaxies nearer, while voids let them fly free. By measuring the “peculiar velocities” of supernovas—how much they swerve from their local Hubble flow—Hudson and his collaborators create their own maps of the cosmos’s concealed mass.

His first hint that the universe hasn’t clumped enough came in 2015, and subsequent peculiar velocity maps have featured the same troubling smoothness. Hudson and his collaborators published work in July 2020 inferring an anomalous lack of clumping nearly as strong as what KiDS found.

Moreover, from about 2012 to 2020, at least a dozen surveys using different techniques have all found a present-day universe at least slightly too thin. Each study has little significance alone, but some cosmologists are growing suspicious that all measurements fall below the theoretical prediction, rather than scattering evenly around it.

“When you start seeing the same thing in a whole bunch of different data sets,” Hudson said, “you think this is really telling you something.”

Contradictory Anomalies

If this is a message from the universe, its meaning remains unclear. The standard model of cosmology fits observations so snugly that theorists can’t just add new pieces willy-nilly. The most precise measurements of the early universe come from the Planck collaboration, which published its final results in 2018. “It’s hard to come up with something new that doesn’t break Planck,” said Daniel Scolnic, a cosmologist at Duke University who studies expansion rates.

But the Planck straitjacket has a few loose buckles, which theorists have fiddled with for years, searching for a way to explain the unexpectedly speedy expansion.

They now seek to accomplish two contradictory tasks. To solve the original problem of the expanding universe, they need a phenomenon that would give the universe an extra kick outward. But to resolve the new anomaly, they have to weaken the gravitational influence that makes the universe clump. When you put the two problems together, said Julien Lesgourgues, a theoretical cosmologist at RWTH Aachen University in Germany and a member of the Planck collaboration, “it becomes a nightmare to find an explanation for both.”

For example, to jump-start expansion, some theorists have tried adding “dark radiation” to the early universe. But they have to balance this extra radiation with additional matter, which would have thickened the universe. So to end up with the universe that we see, they have to invent additional interactions between the various dark ingredients to get the desired thinness.

Another possibility is that dark matter, which clumps the universe together, transforms into dark energy, which drives it apart. Or perhaps the Earth sits in a vast void, skewing our observations. Or the two anomalies could be unrelated. “I haven’t seen anything compelling,” Hudson said, “but if I was a theorist, I’d be very excited right now.”

One or both tensions could still melt away with more data. KiDS is one of three weak gravitational lensing surveys currently underway, along with the international Dark Energy Survey in Chile and Japan’s Hyper Suprime-Cam at the Subaru telescope in Hawaii. Each one scans different regions of the sky, to different depths. Results from the Dark Energy Survey’s latest campaign, which covered an area of sky five times larger than that of KiDS, will be released in late 2020. “Everyone’s kind of waiting on that,” Scolnic said. “That’s the next big thing in cosmology.”

Michael Troxel of Duke University, who does weak gravitational lensing work for the Dark Energy Survey, praised the KiDS team for pushing the technique to new heights of precision and for covering an unprecedented amount of sky. But he also emphasized that a mountain of technical challenges makes it hard to read too deeply into one measurement. At a distance of billions of light-years, galaxies appear as mere pixels, complicating analysis of their shapes. Researchers also need to know how far away each galaxy is, and their treatment of the uncertainties that accompany these distances can either soften or exaggerate the tension.

“I would not put money yet on where the final value will lie,” Troxel said.



Lead image: Massive objects can distort light from distant galaxies, as seen in this illustration. Credit: Sakkmesterke/Science Photo Library

Reprinted with permission from Quanta Magazine's Abstractions blog.