A recent study reveals credible analysis of the **orbital motions** of long-period, widely spaced binary stars, commonly known as wide binaries in astronomy and astrophysics, that provides strong evidence for the breakdown of normal gravity in the low acceleration limit.

The study carried out 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 European Space Agency’s Gaia space telescope. The study was published in the 1 August 2023 issue of the Astrophysical Journal.

Chae’s study concentrated on computing the gravitational accelerations experienced by binary stars as a function of their spacing, or alternatively the orbital period, through a Monte Carlo deprojection of observed sky-projected motions to the three-dimensional space. This was a significant advancement over previous research.

Chae explains, “From the start it seemed clear to me that gravity could be most directly and efficiently tested 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, though wide binaries follow highly elongated orbits while hydrogen gas particles in a galactic disk follow nearly circular orbits.”

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

According to the study, general relativity and Newton’s universal law of gravitation both predict that an orbit between two stars will vary from one another when the accelerations are less than one millimeter per second squared.

For accelerations lower than about 0.1 nanometer per second squared, the observed acceleration is about 30 to 40% higher than the Newton-Einstein prediction. The significance is very high meeting the conventional criteria of 5 sigma for a scientific discovery.

Professor Xavier Hernandez

It is exciting that the departure from Newtonian gravity that my group has claimed for some time has now been independently confirmed, and impressive that this departure has for the first time been correctly identified as accurately corresponding to a detailed MOND model. The unprecedented accuracy of the Gaia satellite, the large and meticulously selected sample Chae uses and his detailed analysis, make his results sufficiently robust to qualify as a discovery.

In a sample of 20,000 wide binaries within a distance limit of 650 LY two independent acceleration bins respectively show deviations of over 5 sigma significance in the same direction.

The reported rise of accelerations at lower accelerations is a puzzle because the observed accelerations stronger than around 10 nanometer per second squared correspond well with the Newton-Einstein prediction from the same research.

What is intriguing is that 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 usage suggested this breakdown of the Newton-Einstein theory at accelerations weaker than about one nanometer per second squared 40 years ago.

Moreover, a MOND-type Lagrangian theory of gravity called AQUAL, proposed by Milgrom and the late physicist Jacob Bekenstein, correctly predicts the boost factor of about 1.4.

It is interesting that the correct boost factor necessitates the Milky Way galaxy’s external field effect, which is a singular prediction of MOND-type modified gravity. As a result, the vast binary data reveal both the breakdown of Newtonian dynamics and the manifestation of modified gravity’s external field effect.

On the results, Chae says, “It seems impossible that a conspiracy or unknown systematic can cause these acceleration-dependent breakdown of the standard gravity in agreement with AQUAL. I have examined all possible systematics as described in the rather long paper. The results are genuine. I foresee that the results will 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 any interested researchers.”

Wide binary dynamics cannot be influenced by dark matter, even if it existed, unlike galaxy rotation curves where the observed boosted accelerations can, in theory, be attributed to it in the Newton-Einstein standard gravity. According to the MOND framework, the weak acceleration limit is where the standard gravity just fails.

Wide binary dynamics has important ramifications for cosmology, theoretical physics, and astrophysics. Anomalies in Mercury’s orbits observed in the nineteenth century eventually led to Einstein’s general relativity.

A new theory that extends general relativity to the low acceleration MOND limit is now necessary to explain abnormalities in broad binaries. Despite all of Newton’s gravity’s triumphs, general relativity is required to explain relativistic gravitational phenomena like gravitational waves and black holes.

Likewise, despite all the successes of general relativity, a new theory is needed for MOND phenomena in the weak acceleration limit. The ultraviolet disaster of classical electrodynamics, which gave rise to quantum physics, may share some similarities with the weak-acceleration catastrophe of gravity.

Wide binary anomalies are a catastrophe for conventional gravity and cosmology, which rely on the ideas of dark matter and dark energy. Because gravity follows MOND, a large amount of dark matter in galaxies (and even in the universe) are no longer needed. 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 seems now under way. Milgrom says, “Chae’s finding is a result of a very involved analysis of cutting-edge data, which, as far as I can judge, he has performed very meticulously and carefully. 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 agrees with the most straightforward predictions of MOND, it will have enormous implications for astrophysics, cosmology, and for fundamental physics at large.”

Xavier Hernandez, professor at UNAM in Mexico who first suggested wide binary tests of gravity a decade ago, says, “It is exciting that the departure from Newtonian gravity that my group has claimed for some time has now been independently confirmed, and impressive that this departure has for the first time been correctly identified as accurately corresponding to a detailed MOND model. The unprecedented accuracy of the Gaia satellite, the large and meticulously selected sample Chae uses and his detailed analysis, make his results sufficiently robust to qualify as a discovery.”

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