Gravity Follows Newton’s and Einstein’s Rules, Even at Cosmic Scales
Gravity behaves as Albert Einstein expected at cosmic scales, according to a new analysis of measurements of distant galaxies and the universe’s oldest light made by the Atacama Cosmology Telescope in Chile.
The finding reaffirms a central pillar of the Standard Model of Cosmology and rules out some alternative models that would require rewriting the laws of gravity, researchers report April 15 in Physical Review Letters.
“This is another triumph for general relativity and our ‘standard model,’” says study co-author and astrophysicist David Spergel, president of the Simons Foundation. “While the standard model with dark matter and dark energy seems strange, it continues to do a remarkable job of describing the universe that we are observing.”
Gravity, as most people understand it, is the familiar force that pulls a falling apple toward the earth. But for astronomers and theoretical physicists, it is also a vexing invisible architect that guides the shape and evolution of the largest cosmic structures across the universe.
For decades, puzzling observations of unusually fast-moving galaxies have forced cosmologists like the University of Pennsylvania’s Patricio A. Gallardo to revisit the fundamentals of physics, asking questions like “Do Newton and Einstein’s laws of gravity hold true everywhere, or do we need to completely rewrite the textbooks?”
“Astrophysics has been plagued by a massive discrepancy in the cosmic ledger,” says Gallardo, a research associate in the University of Pennsylvania’s Department of Physics and Astronomy. “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.”
This mismatch forces a choice between two radical conclusions, he explains. Either the universe contains concentrations of massive invisible ‘dark matter’ that provide extra gravitational pull, or “the fundamental equations for gravity need to be modified.”
Using observations from the six-meter Atacama Cosmology Telescope, Gallardo and collaborators have tested gravity across galaxy clusters separated by hundreds of millions of light-years — the largest-scale probe of gravity to date.
Their findings, published in the new paper, show that gravity’s strength weakens with distance almost exactly as predicted by the equations developed by Isaac Newton and later modified by Albert Einstein.
“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,” says Gallardo.
The confirmation that gravity behaves as predicted by the established theory over vast extragalactic distances reinforces a fundamental pillar of modern science: the Standard Model of Cosmology, Gallardo explains. By showing that fundamental theories of gravity do not break down on the largest scales, the data effectively closes the door on a group of theories that attempt to explain cosmic motions by modifying the laws of gravity, such as Modified Newtonian Dynamics (MOND).
When Newton proposed the inverse-square law, which states that gravity weakens in proportion to the square of the distance between objects, he was primarily concerned with describing the motion of objects in the solar system. This same principle has now been tested on masses and distances that were “inconceivable in Newton’s day,” Gallardo says.
“Yet throughout these cosmic distances,” he says, “gravity seems to behave as indicated over three centuries ago.”
Understanding the Universe’s ‘Speed Limits’
The universe’s galaxies — of which there are more than 200 billion — don’t move the way gravity alone says they should.
Following Newtonian logic, gravity weakens with distance, so stars farther from a galaxy’s center should orbit more slowly. Instead, astronomers see the opposite taking place. The outermost regions move far faster than visible matter can account for. The same mismatch appears in galaxy clusters, where entire galaxies move too quickly for their mass.
“That is the central puzzle,” Gallardo explains. “Either gravity behaves differently on very large scales, or the universe contains additional matter that we cannot directly see.”
Testing Gravity Across the Cosmos
To test this, the researchers turned to ACT’s observations of the cosmic microwave background (CMB) — light released about 380,000 years after the Big Bang and traveling across the universe ever since.
As this ancient light passes through massive galaxy clusters, it is subtly altered by their motion, leaving faint imprints that astronomers can detect. By measuring these distortions and motions across hundreds of thousands of galaxies separated by tens of millions of light-years, the researchers determined how strongly gravity pulls on the largest structures in the cosmos. If modified gravity theories such as MOND were correct, the measurements would reveal a flatter gravitational falloff.
Instead, the results landed almost exactly where Newton’s and Einstein’s theories predict. Einstein’s general relativity offers a more complete picture of gravity as the bending of space and time, but under the conditions found in galaxies and clusters, it effectively simplifies to Newton’s rule — that gravity weakens at a rate proportional to the square of the distance.
Because that prediction holds, the missing-mass problem cannot be explained by changing gravity itself, strengthening the case that an unseen component — dark matter — must be providing the extra pull.
The team measured how quickly gravity fades with distance and found it follows this rule almost perfectly, with a value of 2.1, strikingly close to the expected value of 2 given the margin of error involved. Simply put, even across vast cosmic distances, gravity behaves almost exactly as the classic equations predict.
The Dark Matter Mystery
Understanding the nature of dark matter remains one of the biggest challenges in modern physics.
“This study strengthens the evidence that the universe contains a component of dark matter,” says Gallardo. “But we still do not know what that component is made of.”
Future observations of the CMB and larger galaxy surveys will allow physicists and astronomers to test gravity even more precisely.
“As the data improve, we will be able to push these tests further,” Gallardo says. “We used around 300,000 galaxies for this measurement, but the technique should work with samples of 10 million or more.”
With surveys rapidly expanding, that increase in data could allow scientists to probe gravity across the universe with unprecedented precision — searching for not only confirmation of current theories but also for the smallest hints of new physics.
“With so many unanswered questions, gravity remains one of the most fascinating areas of research. It’s a naturally attractive field,” Gallardo jokes.
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