New Low-Acceleration Gravitational Anomaly Measurements Support Altered Gravity, Researcher Says

Large binary stars, used as direct probes of the nature of gravity at low accelerations below about 1 nanometer per second squared, have been the subject of heated debate. The nature of gravity at such low accelerations is of the utmost importance because the concept of dark matter, the gravitational dynamics of astrophysical systems, and fundamental theories of physics and cosmology are all inextricably linked to it.

For example, a deviation from Newtonian expectation at such low acceleration would require a modification or extension of Einstein’s general relativity despite all its successes outside the low-acceleration regime.

Two recent independent studies by Kyu-Hyun Chae and Xavier Hernandez claim that the statistical properties of wide binary stars based on the latest Gaia database from the European Space Agency deviate from Newtonian expectations, consistent with the prediction of modified gravity theories under the new theoretical framework called modified Newtonian dynamics (MOND, sometimes called Milgromian dynamics), introduced 40 years ago by Mordehai (Moti) Milgrom.

The disappearance of Newtonian gravity at low acceleration would mean a scientific revolution with implications of immeasurable scope. Because of this importance and given other divergent claims, Chae has conducted a new analysis to answer all the questions raised so far, in particular the challenges posed by the specific gravitational anomaly at low acceleration obtained by himself.

To do this, Chae considered a full range of samples, allowing various fractions of hierarchical systems where a binary has an invisible nested inner binary, and applied various methods that cover essentially all methods published so far, including the method used by some researchers to rule against the gravitational anomaly.

The study was published in The Journal of Astrophysics September 9, 2024. “To some extent, I had an obligation to the scientific community to complete this new work. I had to clarify the issues and challenges raised by my claimed conclusion and explain the causes of the divergent claims of others,” Chae says.

Chae found that all methods using various samples yield consistent results. The magnitude of the gravitational anomaly first identified last year appears robust.

Chae explains: “The gravitational anomaly is clearly imprinted in the data. It cannot be erased. For many scientists, it is mysterious that gravity is increased by about 40% when the internal acceleration between the two orbiting stars is less than about 0.1 nanometers per second squared. However, the presence and degree of the gravitational anomaly were actually predicted by MOND or Milgromian dynamics.”

Like Newton’s universal law of gravitation and Einstein’s general relativity, MOND satisfies the universality of free fall (also called the weak equivalence principle) which was first suggested by Galileo through experiments conducted at the Leaning Tower of Pisa.

Newton and Einstein go even further and adopt (consciously or not) the principle of strong equivalence according to which the internal dynamics of a gravitational system in free fall under the effect of a quasi-constant external field is not affected by this field. On the other hand, MOND does not postulate the principle of strong equivalence and considers theoretical possibilities which are exempt from it.

So while wide binaries falling freely under the Milky Way’s external field are expected to obey Kepler’s laws according to Newton’s and Einstein’s theories, MOND predicts that the internal motion of wide binaries deviates from the Newton-Einstein prediction by an amount dictated by the strength of the Milky Way’s external field, or about 0.2 nanometers per second squared. That’s about 40 percent as measured by the new study, consistent with previous results.

Although these consistent results are striking, unlimited replications and confirmations are needed for the reported gravitational anomaly to become a true scientific fact. Moreover, the reported gravitational anomaly will need to be better characterized on an ongoing basis to provide useful constraints to theories.

The researchers are keen to obtain new data and better methodologies for this purpose. In particular, existing research results have been obtained only with the transverse velocities of stars projected onto the sky, because the line-of-sight velocity components have not yet been accurately measured. The researchers, including Chae, are considering new measurements of the line-of-sight velocities of stars.

The other study, of which Hernandez is the lead author, was published in the Monthly Notices of the Royal Astronomical Society.