Scientists can’t agree on how clumpy the universe is

A less-lumpy cosmos could hint at cracks in physicists’ understanding of the cosmos

sky survey of universe's matter clumps

Using observations taken from Paranal Observatory (foreground image), researchers charted the universe’s clumps of matter (map overlaid to scale on the sky). Yellow colors indicate denser regions of the cosmos and pink indicates lower density.

B. Giblin, K. Kuijken KiDS team (sky survey); Y. Beletsky/ESO (CC BY 4.0) (telescope panorama)

The universe is surprisingly smooth.

A new measurement reveals that the universe is less clumpy than predicted, physicists report in a series of papers posted July 30 at The discrepancy could hint at something amiss with scientists’ understanding of the cosmos.

To pin down the cosmic clumpiness, researchers studied the orientation of 21 million galaxies with the Kilo-Degree Survey at the Paranal Observatory in Chile. As light from those galaxies streams through the universe, its trajectory is bent by massive objects, a phenomenon called gravitational lensing. This lensing causes the elongated shapes of galaxies to appear slightly aligned, rather than oriented randomly.

When combined with additional data from other sky surveys, that alignment quantifies how much the matter in the universe is clumped together. The researchers found that the universe is about 10 percent more homogeneous, or smoother, than predicted based on light released just after the Big Bang, the cosmic microwave background. Previous results had hinted at the discrepancy, but the new measurement strengthens the case that the disagreement is not a fluke (SN: 7/30/19).

If the measurement is correct, the mismatch could hint at a hole in the standard model of cosmology, the theory that describes how the universe has changed over time. When combined with a similar puzzle over how fast the universe is expanding (SN: 7/15/20), physicists are beginning to suspect that the universe is putting them­­­­­ on notice.

“It’s a bit of a riddle,” says cosmologist Hendrik Hildebrandt of Ruhr-Universität Bochum in Germany, a coauthor of the studies. “Is [the universe] just telling us ‘You’re stupid and you didn’t do your measurement right,’ or … ‘Hey, I’m more complicated than you thought’?”

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