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University of Pennsylvania physicists have used the Atacama Cosmology Telescope to prove that gravity follows the rules of Newton and Einstein across hundreds of millions of light-years
By confirming that gravity doesn’t change at cosmic scales, the study leaves little room for alternative theories and provides the strongest evidence yet that invisible dark matter must exist.
For decades, the “cosmic ledger” hasn’t balanced. Galaxies and star clusters move much faster than the visible matter within them should allow, leading to a fierce debate in physics: Is the universe filled with invisible dark matter, or are our laws of gravity simply wrong?
The new study, led by Patricio Gallardo at the University of Pennsylvania, published in Physical Review Letters, confirmed that the gravitational theories of Isaac Newton and Albert Einstein hold true even at the largest imaginable scales.
Testing the inverse square law
At the heart of the study is Newton’s inverse square law, which dictates that gravity weakens in proportion to the square of the distance between two objects. While this works perfectly within our solar system, theories like Modified Newtonian Dynamics (MOND) suggested that gravity might “flatten out” or remain stronger over vast cosmic distances to explain why galaxies spin so fast.
To test this, the team used the Atacama Cosmology Telescope (ACT) in Chile. They specifically looked for the kinematic Sunyaev-Zel’dovich (kSZ) effect—a tiny distortion in the Cosmic Microwave Background (the universe’s oldest light) caused by the motion of galaxy clusters.
Dark matter is real
By measuring how hundreds of thousands of galaxy clusters pull on each other across deep space, the researchers found that gravity weakens exactly as Newton and Einstein predicted. This discovery effectively “closes the door” on modified gravity theories.
Key takeaways from the data:
No “leaking” gravity:
Gravity does not behave differently on large scales; the equations written in the 17th and 20th centuries remain accurate in the 21st.
Dark matter is the only answer:
Since gravity isn’t the variable that changed, the “missing” pull must come from an invisible mass—dark matter—providing the extra gravitational glue.
The standard model wins:
The findings reinforce the “Standard Model of Cosmology,” which assumes the universe is composed mostly of dark energy and dark matter.
The remaining mystery
While the study confirms that dark matter must exist to account for the speed of galaxy clusters, it does not tell us what dark matter actually is. Patricio Gallardo notes that while the “ledger” now has a confirmed source for its extra weight, identifying the particles that make up this invisible architect remains one of the greatest challenges in modern physics.
Future surveys and even more precise measurements of the ancient light from the Big Bang will continue to probe these boundaries, but for now, the classical laws of gravity appear to be a universal constant.