In early 2010, a string theorist at the University of Amsterdam named Erik Verlinde first published his alternative theory of gravity. In this theory, gravity isn’t a fundamental interaction or field as described by Einstein, but an emergent phenomenon that is a result of physical systems’ tending toward disorder as described by the second law of thermodynamics.
Verlinde’s theory polarized physicists, whose mixed responses were concisely described by Harvard physicist Andrew Strominger to the New York Times: “Some people have said it can’t be right, others that it’s right and we already knew it—that it’s right and profound, right and trivial. What you have to say is…it’s just a very interesting collection of ideas that touch on things we most profoundly do not understand about our universe.”
Interesting, yes, but in physics intrigue alone doesn’t cut it. The real question remained to be answered: could Verlinde’s theory be experimentally validated?
Recently a team of Dutch astronomers led by Margo Brouwer at Leiden Observatory sought to shed light on this question by being the first to test Verlinde’s theory by using it to predict the gravity distribution for some 33,000 galaxies. When these predictions were compared against observed gravitational distribution, Brouwer found that the gravity distribution predicted by Verlinde’s theory matched well with actual observations.
One of the big questions in physics is why the observed force of gravity around a galaxy is so much stronger than Einstein’s general theory of relativity would predict, even at distances that are up to a hundred times the radius of the galaxy. So far, this strange distribution of gravity has been explained by dark matter and energy, which supposedly account for 95 percent of the mass and energy of the universe.
Yet as Verlinde points out in a paper published on arXiv in November, “the fact that 95% of our Universe consists of mysterious forms of energy or matter gives sufficient motivation to reconsider this basic starting point.” Instead, Verlinde thinks it is possible to describe this distribution of gravity without relying on substances like dark matter and dark energy if gravity is considered an emergent phenomenon arising from entropic principles as outlined in his 2010 theory.
“The real reason why most physicists believe in the existence of particle dark matter is not the observations, but because there was no theoretical evidence nor a conceptual argument for the breakdown of these laws at the scales where the new phenomena are being observed,” Verlinde wrote in his latest paper. “Once there is a conceptual reason for a new phase of gravitational force, which is governed by different laws, and this is combined with a confirmation of its quantitative behavior, the weight of evidence tips in the other direction.”
In short, Verlinde had his theory of emergent gravity—all that was needed to tip the weight of evidence away from dark matter and energy dominating the universe was quantifiable confirmation of his theory. Enter Brouwer and her colleagues at Leiden Observatory, who became the first team to put Verlinde’s theory to the test.
To do this, Brouwer and her colleagues measured the surface mass density of 33,613 galaxies using gravitational lensing—a way to measure gravity distribution by observing how light is bent by a galaxy—and then compared these measurements to the surface mass density that would be predicted by Verlinde’s theory. What they found was that “the prediction from emergent gravity…is in good agreement with the observed galaxy-galaxy lensing profiles.”
In other words, Verlinde’s theory seems to be able to account for the strange distribution of gravity around galaxies that doesn’t make sense based on general relativity theory—and it does it only based on observable matter, rather than requiring the existence of mysterious dark particles.
While these quantitative results are encouraging for Verlinde’s theory, they are only a first step towards its validation. According to Brouwer, there are still a number of questions that Verlinde’s theory of emergent gravity can’t explain, so “further advancements on both the theoretical framework and observational tests of emergent gravity are needed before it can be considered a fully developed and solidly tested theory.”