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?

Charlie Wood | NAUTILUS

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

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.

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