The MOND controversy heats up with wide binary systems
Wide binary systems are systems consisting of two stars orbiting at a very large distance from one another, say 2000 AU or 2000 times the distance from the Sun to the Earth. For comparison, Pluto is only 39.5 AU. 2000 AU would be somewhere in the Oort cloud in our Solar System which is technically within interstellar space (beyond the Termination Shock and Heliopause) but still within the Sun’s sphere of gravitational influence.
Wide binaries are far enough apart that we can test a controversial theory of gravity called MOdified Newtonian Dynamics (MOND).
A quick recap of how MOND works in simple, two body cases: MOND is a theory that combines Newtonian physics with another theory depending on how strong the acceleration due to gravity is. This acceleration is called a0. If accelerations are much larger than a0, Newtonian gravity should be observed. If they are much smaller, then we observe MOND.
In MOND, orbits have a more or less constant velocity with distance rather than decreasing velocity and this accounts for the high speed of objects distant from their galactic core. These speeds in dark matter models are caused by dark matter halos which are large rings of invisible dark matter that are considered a natural part of galactic formation.
MOND is controversial and seems to be in a constant state of either being debunked or proved superior to (non-interacting, non-superfluid) cold dark matter models. To be honest, neither model works that well. The conventional wisdom is that dark matter models work vastly better at scales from galaxy clusters to the universe while MOND works best with galaxies and smaller phenomenon. Given that heuristic, it makes a lot of sense to try to test MOND with the simplest possible configuration: two body systems.
Some recent papers have done just that, but they have come up with conflicting results. Wide binary systems are an ideal test case because they are far enough apart that their gravitational accelerations on one another should be within the MOND regime. Moreover, we can choose a variety of systems and try to ascertain how Newtonian physics transitions to MOND by observing how behavior changes as stars get further apart.
MOND should kick in for accelerations around
At the recent MOND40 conference, three conflicting wide binary studies were presented. Two of them, Kyu-Hyun Chae and independently Xavier Hernandez, found agreement with MOND. The deviation of wide binary systems from Newtonian physics was large enough.
A third study by Inrdanil Banik, et al. however found no MOND and ruled it out to 16 sigma. That would put the probability that MOND is correct at
One of the great things about a conference versus hashing out controversy in the journals is that real-time discussions can happen and so disagreement can become collaborative. This is what happened at MOND40.
The Banik group only looked at binaries with very wide separations, more than 2000 AU. All of these binaries would already be explained by MOND, not Newtonian physics. You would not expect to see any change in behavior.
Chae looked at binaries that are closer to each other. These would behave in a Newtonian way. He then used those binaries to calibrate his study of wider binaries. Chae therefore found that MOND starts at about 2000 AU. That is where Banik starts his analysis.
That means that Banik may have falsely calibrated his results assuming that the tighter binaries he was studying were Newtonian when they are actually MONDian!
Banik also looked at more triple systems than Chae. A triple system, because it has more mass in it, would tend to have more acceleration than a binary system and so be more Newtonian.
This all is to say that we may not be able to trust Banik’s results until they are calibrated against tighter systems and triple systems are taken into account.
Hernandez, on the other hand, filtered triple systems out entirely, so he had a smaller but cleaner sample size. Since his findings agreed with Chae’s it suggests that triple systems may be another issue with Banik.
This is pretty exciting because it means that, if Banik goes back and recalibrates, eliminates triples more, and finds MOND then we have three (mostly) independent studies confirming MOND in wide binaries, which would be a major win for the model. It would be hard to justify such behavior with dark matter, although you could, because now every star system would need its own dark matter halo.
That doesn’t answer the question about what MOND is. MOND is an empirical law but we don’t know what it is or why it happens. We don’t know if MOND is complementary with Cold Dark Matter (CDM) models or can completely replace it. For example, superfluid CDM models use both. My own Kaluza-Klein theory which is currently in peer review likewise can use both.
If MOND keeps explaining phenomena that CDM struggles to explain, even the most ardent DM proponents will have to take it into account in their models.
Thanks to Marcel S. Pawlowski for reporting on the goings on at MOND40.