Newton’s 300-year-old law just passed its biggest test yet

Gravity, best known for its pull on objects toward Earth, plays a far-reaching role in shaping the universe. Astronomers have long observed that stars and galaxies move faster than visible matter alone can explain, leading researchers like University of Pennsylvania's Patricio A. Gallardo to question if Isaac Newton's and Albert Einstein's laws of gravity remain valid across the cosmos.

The discrepancy observed is so significant that it suggests either the existence of vast amounts of invisible dark matter whose gravity provides extra pull, or the need to modify the fundamental equations for gravity. To test these possibilities, Gallardo and his team used observations from the Atacama Cosmology Telescope (ACT) to study gravity across galaxy clusters separated by hundreds of millions of light-years.

Their findings, published in Physical Review Letters, reveal that gravity's strength decreases with distance almost exactly as Newton's inverse square law and Einstein's theory of general relativity predict. This supports the foundation of modern cosmology and rules out modified gravity theories like MOND, which attempt to account for unusual cosmic motions by altering gravity's laws.

The results, obtained by analyzing cosmic microwave background light traveling through massive galaxy clusters, show gravity behaves as expected over enormous distances. This supports the existence of an unseen component, dark matter, providing the additional gravitational pull. However, the study does not identify what dark matter is composed of, leaving one of physics' biggest questions unanswered.