Smoking Gun Evidence for Modified Gravity at Low Acceleration from Gaia Observations of Wide Binaries

A new study reports conclusive evidence for the breakdown of standard gravity in the low acceleration limit from a verifiable analysis of the orbital motions of long-period, widely separated binary stars, commonly called wide binaries in astronomy and astrophysics.

The study conducted by Kyu-Hyun Chae, professor of physics and astronomy at Sejong University in Seoul, used up to 26,500 wide binaries within 650 light years (LY) observed by the European Space Agency’s Gaia space telescope. The study was published in the August 1, 2023 issue of Astrophysical Journal.

For an important improvement over other studies, Chae’s study focused on calculating gravitational accelerations experienced by binary stars as a function of their separation or, corresponding to the orbital period, by a Monte Carlo deprojection of observed sky-projected motions into three-dimensional space.

Chae explains, “From the beginning it seemed clear to me that seriousness could be tested most directly and effectively by calculating accelerations because gravitational field itself is an acceleration. My recent research experiences with galactic rotation curves led me to this idea. Galactic disks and wide binaries share some similarity in their orbits, although wide binary disks follow very elongated orbits while hydrogen particles in a galactic disk follow nearly circular orbits.”

Unlike other studies, Chae also calibrated the occurrence rate of hidden nested inner binaries at a benchmark acceleration.

The study shows that when two stars orbit each other with accelerations lower than about one nanometer per second squared, they begin to deviate from the prediction of Newton’s law of universal gravitation and Einstein’s general theory of relativity.

For accelerations lower than about 0.1 nanometers per second squared, the observed acceleration is about 30 to 40% higher than the Newton-Einstein prediction. The significance is very high and meets the conventional criteria of 5 sigma for a scientific discovery. In a sample of 20,000 wide binaries within a distance limit of 650 LY, two independent acceleration bins show deviations of over 5 sigma significance in the same direction.

Since the observed accelerations stronger than about 10 nanometers per second squared agree well with the Newton–Einstein prediction from the same analysis, the observed increase in accelerations at lower accelerations is a mystery. What is exciting is that this breakdown of Newton-Einstein theory at accelerations weaker than about one nanometer per second squared was proposed 40 years ago by theoretical physicist Mordehai Milgrom at the Weizmann Institute in Israel in a new theoretical framework called modified Newtonian dynamics (MOND) or Milgromian dynamics in current use.

Furthermore, the boost factor of about 1.4 is correctly predicted by a MOND-type Lagrangian gravity theory called AQUAL, proposed by Milgrom and the late physicist Jacob Bekenstein. What is remarkable is that the correct boost factor requires the external field effect from the Milky Way galaxy which is a unique prediction of MOND-type modified gravity. Thus, what the wide binary data show is not only the breakdown of Newtonian dynamics but also the manifestation of the external field effect of modified gravity.

About the results, Chae says, “It seems impossible that a conspiracy or unknown systematics could cause these acceleration-dependent decays of standard gravity consistent with AQUAL. I have investigated all possible systematics described in the rather long paper. The results are genuine. I foresee the results coming to be confirmed and refined with better and larger data in the future. I have also released all my codes for the sake of transparency and to serve all interested researchers.”

Unlike galactic rotation curves where the observed enhanced accelerations can in principle be attributed to dark matter in standard Newton-Einstein gravity, broad binary dynamics cannot be affected by it even if it existed. Standard gravity simply breaks down in the weak acceleration limit in accordance with the MOND framework.

The implications of broad binary dynamics are profound in astrophysics, theoretical physics, and cosmology. Anomalies in Mercury’s orbits observed in the nineteenth century eventually led to Einstein’s general theory of relativity.

Now anomalies in wide binaries require a new theory that extends general relativity to the low-acceleration MOND limit. Despite all the successes of Newtonian gravity, general relativity is needed for relativistic gravitational phenomena such as black holes and gravitational waves. Similarly, despite all the successes of general relativity, a new theory is needed for MOND phenomena in the weak acceleration limit. The weak-acceleration gravitational catastrophe may bear some resemblance to the ultraviolet catastrophe of classical electrodynamics that led to quantum physics.

Wide binary anomalies are a disaster for standard gravity and cosmology that relies on dark matter and dark energy concepts. Since gravity follows MOND, a large amount of dark matter is no longer needed in galaxies (and indeed in the universe). This is also a big surprise to Chae who, like typical scientists, “believed in” dark matter until a few years ago.

A new revolution in physics now seems to be underway. Milgrom says: “Chae’s findings are the result of a very involved analysis of cutting-edge data, which he has, as far as I can tell, performed very carefully and meticulously. But for such a far-reaching finding—and it is indeed very far-reaching—we require confirmation by independent analyses, preferably with better future data.”

“If this anomaly is confirmed as a breakdown of Newtonian dynamics, and especially if it indeed conforms to MOND’s most simple predictions, it will have enormous implications for astrophysics, cosmology, and for fundamental physics in general.”

Xavier Hernandez, a professor at UNAM in Mexico who first proposed broad binary gravity tests a decade ago, says: “It’s exciting that the departure from Newtonian gravity that my group has argued for some time has now been independently confirmed and impressive that this departure has for the first time correctly identified as exactly corresponding to a detailed MOND model. The unparalleled accuracy of the Gaia satellite, the large and carefully selected sample Chae uses, and his detailed analysis, make his results robust enough to qualify as a discovery.”

Pavel Kroupa, professor at the University of Bonn and at the Charles University in Prague, has come to the same conclusions regarding the law of gravitation. He says, “With this test on wide binaries as well as our tests on open star clusters near the Sun, the data now strongly suggest that gravity is Milgromian rather than Newtonian. The implications for all of astrophysics are enormous.”