Largest-Ever Test of Gravity Confirms Newton's Law Still Holds, Strengthens Case for Dark Matter

Stars in the outer parts of galaxies, and galaxies moving within galaxy clusters, travel much faster than their visible matter can explain under standard gravity — a puzzle that has led physicists to wonder whether the universe holds large amounts of invisible "dark matter," or whether the equations of gravity themselves need to be revised at very large scales. A team led by Patricio A. Gallardo, a research associate at the University of Pennsylvania, has now tested that question using the largest-scale test of gravity performed so far.

The researchers used the Atacama Cosmology Telescope (ACT), developed largely by Penn researchers, to study how gravity behaves across galaxy clusters separated by hundreds of millions of light-years. They analyzed the cosmic microwave background (CMB), ancient light released about 380,000 years after the Big Bang, looking at how its passage through hundreds of thousands of galaxy clusters is subtly altered by the clusters' motion. That effect let the team measure how strongly gravity acts on some of the universe's largest structures.

The results, published in Physical Review Letters, showed that the strength of gravity decreases with distance almost exactly as predicted by Newton's 17th-century inverse-square law and Einstein's general relativity. If a modified-gravity model such as Modified Newtonian Dynamics (MOND) explained the missing mass instead, the measurements should have shown gravity declining more gradually with distance; that is not what the team found.

Because gravity behaved as standard theory predicts, the study strengthens the case that an unseen component — dark matter — supplies the extra gravitational pull seen in galaxies and galaxy clusters, rather than the laws of gravity themselves being wrong. "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." More than 40 researchers across multiple countries contributed to the study.