Plate Nº 87 · recorded October 13, 2026
Space & AstronomyReported finding
Newton's gravity law passes largest test yet, dark matter gains ground
Newton's 300-year-old law of gravity held up across galaxy clusters hundreds of millions of light-years apart in the largest-scale test yet, strengthening the case for dark matter.
By Nathan Brooks3 min read566 words
In brief
- Study published August 24, 2026 in Physical Review Letters by a University of Pennsylvania-led team
- Largest-scale test of gravity ever performed, across galaxy clusters separated by hundreds of millions of light-years
- Led by Patricio A. Gallardo, with more than 40 collaborators across multiple countries
- Data came from the Atacama Cosmology Telescope (ACT), a roughly three- to four-story-tall instrument developed largely at Penn
- Universe holds more than 200 billion galaxies, most of which rotate faster than visible matter alone can explain
By Nathan Brooks · August 24, 2026
Newton's 300-year-old law of gravity held up across galaxy clusters separated by hundreds of millions of light-years, a University of Pennsylvania-led team reported on August 24 in Physical Review Letters.
The result — the largest-scale test of gravity ever performed — weakens a class of alternatives to dark matter and reinforces the case for unseen mass as the explanation for why galaxies spin faster than they should.
"Astrophysics has been plagued by a massive discrepancy in the cosmic ledger," said Patricio A. Gallardo, a research associate in Penn's Department of Astronomy and Physics. "When we look at how stars orbit within galaxies or how galaxies move within galaxy clusters, some appear to be traveling way too fast for the amount of visible matter they contain."
What did the researchers test?
The team used data from the Atacama Cosmology Telescope (ACT), a roughly three- to four-story-tall instrument developed largely by Mark Devlin at Penn. They examined the gravitational pull across hundreds of thousands of galaxy clusters spread across the cosmos.
Newton's inverse-square rule — gravitational strength drops with the square of the distance between two objects — was first written in the 17th century and later folded into Einstein's general relativity. Gallardo and his colleagues checked whether that relationship still holds at scales Newton never imagined.
"It is remarkable that the law of the inverse of the squares — proposed by Newton in the 17th century and then incorporated by Einstein's theory of general relativity — is still holding its ground in the 21st century," Gallardo said.
How do you test gravity across the universe?
The team measured ancient light known as the cosmic microwave background, or CMB, which escaped about 380,000 years after the Big Bang and has been streaming through space ever since.
As that light passes through massive galaxy clusters, the clusters' motion imprints tiny changes on it — a faint signature astronomers call the kinematic Sunyaev-Zel'dovich effect. By examining those signatures across hundreds of thousands of clusters, the researchers tracked how strongly gravity acts on the universe's largest structures.
If modified-gravity theories such as Modified Newtonian Dynamics, or MOND, were correct, gravity should fade more slowly with distance. Instead, the data fell almost exactly where Newton's and Einstein's predictions intersect.
What does this mean for dark matter?
Two competing ideas have long tried to explain the missing mass. Either the cosmos is full of invisible dark matter that supplies the extra pull, or the equations of gravity need rewriting. The new measurements favor the first explanation.
"This study strengthens the evidence that the universe contains a component of dark matter," Gallardo said. "But we still do not know what that component is made of."
For context, the universe holds more than 200 billion galaxies, and most rotate in ways visible matter alone cannot explain.
What's next?
Future CMB measurements and larger galaxy surveys should let astronomers test gravity with even greater precision. The ACT collaboration alone involves more than 40 researchers across multiple countries.
"With so many unanswered questions, gravity remains one of the most fascinating areas of research," Gallardo said. "It's a naturally attractive field."
Funding came from the U.S. National Science Foundation, NASA, Chile's ANID, the National Research Foundation of South Africa, and NSERC in Canada, among others.
via penntoday.upenn.edu (Original)
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