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 on Unprecedented Cosmic Scales
Gallardo and his collaborators have now put gravity to an unusually large test using observations from the Atacama Cosmology Telescope (ACT), a telescope roughly three to four stories tall that was developed largely by Penn researchers led by Mark Devlin.
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 results, published in Physical Review Letters, show that the strength of gravity decreases with distance almost exactly as predicted by Newton’s equations and later incorporated into Einstein’s theory of general 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.
The finding supports one of the foundations of modern cosmology. According to Gallardo, demonstrating that established theories of gravity continue to work over enormous distances strengthens 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.
Newton originally developed the inverse square relation to describe motion within the Solar System. The principle says that gravitational strength decreases according to the square of the distance separating two objects. Scientists have now tested that same relationship using masses and distances that were “inconceivable in Newton’s day,” Gallardo says.
Why Galaxies Move Too Fast
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.”
Ancient Light Provides a Test of Gravity
To investigate which explanation better fits the evidence, the researchers analyzed ACT observations of the cosmic microwave background, or CMB. This ancient light was released about 380,000 years after the Big Bang and has been traveling through the universe ever since.
As CMB light travels through massive galaxy clusters, the movement of those clusters produces tiny changes in the light. Astronomers can detect these faint signatures and use them to study the clusters’ motions.
By examining these effects across hundreds of thousands of galaxy clusters separated by tens of millions of light years, the researchers were able to determine how strongly gravity acts on some of the universe’s largest structures.
If modified gravity models such as MOND provided the correct explanation, the measurements should have shown gravity declining more gradually with distance.
That is not what the team found. Instead, the measurements fell almost exactly in the region where Newton’s theory and Einstein’s theory agree.
Because gravity behaved as predicted, changing the law of gravity does not explain the missing mass revealed by these observations. The result therefore strengthens the case that an unseen component, dark matter, supplies the additional gravitational pull.
The Mystery of Dark Matter Remains
Although the findings add evidence for dark matter, they do not answer one of the largest remaining questions in physics: what dark matter actually consists of.
“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 measurements of the CMB, together with larger surveys of galaxies, should allow astronomers and physicists to test gravity with even greater precision.
“With so many unanswered questions, gravity remains one of the most fascinating areas of research. It’s a naturally attractive field,” Gallardo chuckles.
Patricio Gallardo is a research associate in the Department of Astronomy and Physics in the School of Arts & Sciences at the University of Pennsylvania.
The study involved more than 40 researchers representing institutional affiliations across multiple countries. Individual researchers contributing to this study were supported by a variety of fellowships and national funding agencies, including the Kavli Institute for Cosmological Physics at the University of Chicago, the Simons Society of Fellows, the U.S. National Science Foundation (AST-2206088), NASA ROSES grant 12-EUCLID12-0004, Chile’s Agencia Nacional de Investigación y Desarrollo (ANID) Basal project FB210003, the National Research Foundation of South Africa, and the Natural Sciences and Engineering Research Council of Canada (NSERC) through grants RGPIN-2023-05014 and DGECR-2023-00180. Additional support was provided by the Sutton Family Chair in Science, Christianity and Cultures at the University of Toronto Faculty of Arts and Science.
The Atacama Cosmology Telescope (ACT) project is supported primarily by the U.S. National Science Foundation through awards AST-0408698, AST-0965625, and AST-1440226 for the ACT project, as well as PHY-0355328, PHY-0855887, and PHY-1214379. Additional funding has been provided by Princeton University, the University of Pennsylvania, and a Canada Foundation for Innovation (CFI) award to the University of British Columbia. Development of ACT multichroic detectors and lenses was supported by NASA grants NNX13AE56G and NNX14AB58G, and detector research at the National Institute of Standards and Technology (NIST) was supported through the NIST Innovations in Measurement Science program.