Astrophysicist Kyu-Hyun Chae has developed an innovative new approach to measuring gravity based on the three-dimensional velocities of stars. To our knowledge, we used this novel method for the first time to wide binary stars. These binary stars are incredibly far apart at more than 2000 AU. These wide binaries serve as natural laboratories. They enable scientists to test gravity in low acceleration conditions – namely, when it’s weaker than roughly one nanometer per second squared.
Chae’s research brought these revelations to life in an exhilarating find. The effective gravity for these wide binaries is 40–50% stronger than would be expected from Newtonian gravity. This big anomaly easily falls within the theoretical bounds of Extreme Newtonian Dynamics (MOND). Physicist Mordehai (Moti) Milgrom came up with this idea roughly 40 years ago. This more recent study makes a very strong case for modified gravity theories. The sigma level of this deviation from standard gravity measures is 4.2δ, placing standard gravity beyond a remarkable 99.997% probability range.
Chae’s Innovative Methodology
Chae has come up with an alternative and more sensitive approach to measuring. This approach needs very accurate and precise values of the third velocity component, called the line-of-sight (or radial) velocity. To do this, Chae examined an estimated 300 large wide binaries chosen out of the European Space Agency’s Gaia data release 3. He noted that Gaia’s published radial velocities provide unique and powerful constraints. They lack in precision for the most detailed analyses, as they rely on sky-projected velocities.
“The existing methods to infer gravity have the limitation that only the sky-projected velocities are used. Moreover, they have some limitations in accounting for the uncertainties of various factors, including stellar masses, to derive the probability distribution of a gravity parameter.” – Kyu-Hyun Chae
Chae is quick to point out that his approach avoids these pitfalls, marking a groundbreaking breakthrough for the study of wide binaries. This new approach improves our predictive understanding of their complex, dramatic motion. Fortunately, it accomplishes this despite the long orbital periods usually restricting us to snapshot observations.
“The new method requires accurate and precise values of the third velocity component, that is, the line-of-sight (radial) velocity. In other words, only wide binaries with precisely measured radial velocities can be used.” – Kyu-Hyun Chae
Chae’s methodology requires that the binaries under study be devoid of hidden companion stars. This requirement highlights the importance of forthcoming results from dedicated ground-based follow-up studies, which will help confirm the presence or absence of any hidden third components.
Implications for Theoretical Physics
The ramifications of this discovery through research are huge both for astrophysics and theoretical physics in general. Chae is confident that the Bayesian inference from his study will provide a precise measure of gravity. It needs to be able to tell apart Newtonian dynamics and MOND with a statistical confidence of far more than 5δ.
“With new data on radial velocities, most of which have already been obtained, and results from speckle photometric observations, the Bayesian inference is expected to measure gravity sufficiently precisely.” – Kyu-Hyun Chae
Chae said he saw bright prospects for the new results in theoretical physics. He looks forward to growing developments and new opportunities in the next several years.
Pavel Kroupa, a professor of astrophysics at the University of Bonn in Germany, called Chae’s work beautiful. He described it as an extraordinary breakthrough in the study of gravity. He explained that this approach provides a new standard of precision and transparency. Beyond that, he drew to our attention a big break from Newtonian dynamics.
“This is an impressive study of gravitation using very wide binaries as probes taken to a new level of accuracy and clarity by Prof. Dr. Chae.” – Pavel Kroupa
Collaboration and Future Developments
Chae and his collaborators are now working to get higher-res, more precise radial velocities. They’re orienting these observations at remote facilities like the GEMINI North Observatory and Las Cumbres Observatory. The outcomes based on these observations are expected to greatly support their findings about gravity in low accelerations.
Milgrom gave us Kroupa’s reaction to Chae’s new findings. He added that these results corroborate a growing body of evidence supporting prior studies reporting deviations from Newtonian dynamics in low-acceleration binary stars in our galaxy.
“This new result by Prof. Chae strengthens in important ways earlier findings by him and others.” – Mordehai (Moti) Milgrom
Researchers on this topic need to work together often to improve methodologies. By exchanging information across disciplines, they are collectively laying the groundwork for innovative discoveries about the gravitational dynamics at play.
Peer Reviewer Xavier Hernandez applauded the thorough Bayesian approach taken in Chae’s new paper. In fact, he thinks this approach – taking the long view — will be the new paradigm across the discipline going forward. He cautioned against starting with two-dimensional projected velocities, pushing for a switch to three-dimensional relative velocities from the beginning for more complete analysis.
“The latest paper by Dr. Chae on wide binaries presents a fully rigorous Bayesian approach which will surely become the standard in the field.” – Xavier Hernandez