Newton’s law of gravity has passed its largest-scale test yet, as researchers analyzing ancient cosmic light from the Atacama Cosmology Telescope confirm that gravitational strength decreases with distance almost exactly as predicted by Newtonian physics and Einstein’s general relativity.
Gravity is best known as the force that pulls objects toward Earth, but its role extends far beyond falling apples. Across the universe, gravity acts as an invisible framework that influences how galaxies, galaxy clusters, and the largest cosmic structures form and evolve. For decades, however, astronomers have faced observations that do not seem to fit neatly with what can be seen. Some stars and galaxies move much faster than their visible mass appears capable of explaining. That mystery has led cosmologists such as University of Pennsylvania researcher Patricio A. Gallardo to investigate whether the laws of gravity established by Isaac Newton and Albert Einstein remain valid everywhere in the universe.
"Astrophysics has been plagued by a massive discrepancy in the cosmic ledger," says Gallardo. "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."
According to Gallardo, this discrepancy points toward two dramatically different possibilities. The universe may contain large amounts of invisible "dark matter" whose gravity supplies the additional pull, or "the fundamental equations for gravity need to be modified."
Testing Gravity Across Hundreds of Millions of Light-Years
To investigate which explanation holds up, a research team led by University of Pennsylvania researcher Patricio A. Gallardo turned to data from the Atacama Cosmology Telescope (ACT), a three- to four-story-tall instrument developed largely by Penn researchers led by Mark Devlin. By examining galaxy clusters separated by hundreds of millions of light-years, the team executed the most extensive test of gravity yet. The team examined gravity across galaxy clusters separated by hundreds of millions of light-years, making this the largest-scale test of gravity performed so far.
The findings, published in Physical Review Letters, demonstrate that gravitational strength drops off with distance in alignment with Newtonian mechanics and Albert Einstein’s general theory of relativity.
"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.
Patricio A.
Ancient Light Reveals the Universe’s Speed Limits
The research team relied on observations of the cosmic microwave background (CMB), the faint afterglow of radiation released roughly 380,000 years after the Big Bang. As this ancient light travels across space, it passes through massive galaxy clusters. The motion of those clusters imprints tiny, detectable distortions on the light, allowing scientists to measure how fast the clusters are moving toward one another and test how strongly gravity pulls across the largest distances in the universe.
There are more than 200 billion galaxies in the universe, and their motions have long presented astronomers with a major puzzle. Under a straightforward Newtonian picture, stars orbiting farther from the center of a galaxy should travel more slowly. Observations show something very different. Stars in the outer regions of galaxies move much faster than the amount of visible matter appears able to support gravitationally. A similar problem occurs in galaxy clusters. Entire galaxies travel through these enormous structures at speeds that cannot be explained by their observed mass alone.
"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."
Patricio A.
Gallardo noted that when stars in the outer portions of galaxies or entire galaxies within clusters move faster than expected, that is the central puzzle. Because gravity continues to behave according to established physical laws on these immense scales, modifying the underlying equations of gravity fails to resolve the missing mass problem.
Strengthening the Standard Model of Cosmology
Confirming that gravitational pull operates predictably over cosmological distances reinforces the standard model of cosmology. It also sharply limits a class of alternatives, including Modified Newtonian Dynamics (MOND), that attempt to account for unusual cosmic motions by changing the laws of gravity.
While Newton originally formulated the inverse square relation to describe motion within the solar system, scientists have now tested that exact relationship using masses and distances that were "inconceivable in Newton’s day," Gallardo says.
With modified gravity models failing to account for the speed of distant galaxies, the empirical weight shifts further toward the presence of invisible dark matter as the driver behind cosmic motions.
