Gravity is so weird that it essentially forces cosmologists into subscribing to one of two equally radical conclusions: dark matter exists, or fundamental gravity rules need massive revisions. To settle this debate, scientists embarked on the largest investigation of gravity to date, finding that old physics wisdom held strong for the most puzzling observations.
In a study published this week in Physical Review Letters, an international team of astronomers tested cosmic observations against Albert Einstein’s general relativity, which is built upon Isaac Newton’s basic laws of gravity. Now, general relativity—which defines gravity as distortions in spacetime—is among the most successful ideas in the history of science. What’s “new” about the study is that it definitively demonstrates that even the largest, most distant structures in the universe follow similar gravity rules as those we experience daily.
This puts a damper on alternative theories that attempted to modify fundamental rules of gravity to explain observational inconsistencies in cosmology. That is, the basic rules are correct—there’s just something else we’re not seeing. Many scientists say this “something” is dark matter.
“With so many unanswered questions, gravity remains one of the most fascinating areas of research,” Patricio Gallardo, one of the study’s authors and an astrophysicist at the University of Pennsylvania, said in a statement. “It’s a naturally attractive field.”
Something beyond
In the 1970s, the famous astronomer Vera Rubin found that spinning galaxies seemed to defy the laws of gravity. Incidentally, Newtonian logic states that stars farther away from a galaxy’s center should orbit slower than stars at the center, where starlight is concentrated. But Rubin’s observations revealed the complete opposite was true and that stars at the fringe were moving just as fast.
According to Gallardo, similar discrepancies appear with galaxy clusters, some of which travel “way too fast for the amount of visible matter they contain,” he said. “That is the central puzzle. Either gravity behaves differently on very large scales, or the universe contains additional matter that we cannot directly see.”
Enter dark matter
Dark matter—invisible stuff presumably making up 85% of the universe’s mass—was brought in (partly) to address this issue. Although researchers haven’t found direct evidence for dark matter, there is strong consensus that its presence solves many unexplained phenomena in the cosmos.
Of course, because we’ve yet to actually discover dark matter, not all scientists believe in dark matter, with many proposing alternative explanations. However, many recent studies, such as this extensive map of dark matter, consistently arrive at conclusions in favor of dark matter’s existence.
Newton, Einstein, gravity
That includes the newest study, which gathered data on galaxy clusters observed by the Atacama Cosmology Telescope (ACT) in Chile and the Sloan Digital Sky Survey in New Mexico. The researchers tracked a particular light source emitted roughly 380,000 years after the Big Bang. Every time this ancient light source flies past massive galaxy clusters, its motion slightly warps from gravitational effects.
In the analysis, the team measured these tiny changes across hundreds of thousands of galaxy clusters separated by tens of millions of light-years. The calculations were a near-perfect match for what Newton and Einstein’s theories predicted for gravity’s influence on this light source.
“Our analysis here is quite general—it does not rely on a specific cosmology,” the researchers noted in the paper. However, modified laws of gravity weren’t as successful as the conventional equations, they added. Therefore, if the laws of gravity are fine, the discrepancy must lie in something we’re not seeing—something like dark matter.
