Gravity has survived its largest-scale test yet, holding up across distances of up to 230 megaparsecs, or about 750 million light-years.
University of Pennsylvania physicists found that the force weakens with distance almost exactly as Newton’s equations predict, a relationship later incorporated into Einstein’s theory of general relativity.
Galaxies and galaxy clusters move faster than matter astronomers can see should allow.
One explanation for this discrepancy is invisible matter, known as dark matter, whose gravity could account for their motion.
The other is that “the fundamental equations for gravity need to be modified” at cosmic distances, according to Patricio Gallardo, a Penn physicist and the study’s lead author.
To test the two explanations, the researchers turned to the cosmic microwave background: faint light left over from the Big Bang that began traveling freely roughly 380,000 years afterward.
They looked for a subtle distortion known as the kinematic Sunyaev-Zeldovich effect, which occurs when that light moves through the hot gas of a moving galaxy cluster, picking up a tiny temperature shift.
Using CMB maps from the Atacama Cosmology Telescope and roughly 228,000 galaxies cataloged by the Sloan Digital Sky Survey, the researchers traced those shifts.
The distortion reveals how quickly the clusters are moving toward one another.
Because gravity drives that motion, tracking how the clusters’ speeds change as the gap between them grows reveals how gravity weakens with distance.
The results came down clearly on one side. The researchers measured how sharply gravity falls off with distance and found a value of 2.1, plus or minus 0.3.
Newton’s law predicts exactly two. The findings challenge modified Newtonian dynamics or MOND, a theory that proposes gravity behaves differently at cosmic scales and predicts a value of one.
The measurement disfavors MOND at 3.3 sigma, short of the five-sigma standard used by physicists for a definitive claim.
But the results do not settle the long-standing mystery of dark matter. The study shows that gravity behaves as expected across long distances, which makes modified gravity a less likely explanation for the missing mass.
That strengthens the case that dark matter is supplying the extra pull.
It does not, however, identify what dark matter actually is or directly detect the substance.
“We still do not know what that component is made of,” Gallardo said.
The method could become even more precise as new surveys produce larger catalogs of galaxies and better maps of the cosmic microwave background.
The researchers forecast that a catalog of roughly four million galaxies could rule out MOND at 10 sigma, up from the current 3.3.
More than another test of a centuries-old law, the study established the kinematic Sunyaev-Zeldovich effect as a tool for testing gravity at cosmological scales.
