MoND denied

Almost exactly a year ago, I wrote a post about MoND -- modified Newtonian dynamics -- a new(ish) model of gravitation that purported to explain some of odd measurements of stellar and galactic motion without the necessity of dark matter.

Here's the situation.

In the 1970s and 1980s, the brilliant astronomer Vera Rubin discovered something peculiar.  The project she was working on involved mapping the speed of revolution of stars around galactic centers.  According to Newton's Law of Gravitation and Kepler's Laws of Planetary Motion (which, after all, have the same mathematical underpinning), stars farther away from the center should be moving more slowly.  This principle works fine, for example, in our own Solar System; Neptune moves more slowly than Mercury does.

This, Rubin found, turned out not to be true on larger scales.  The velocities of stars in the farther reaches of galaxies were moving just as fast as the ones closer to the center.  Nicknamed the flat rotation curve problem, it seemed like the only possible explanation was that there was more mass in the galaxy than had been detected -- something appeared to be causing the outer stars to orbit faster than Newtonian dynamics said they should.

Rubin and others called this mysterious something dark matter. 

And you probably know the amount of this stuff is significant.  If you add up all the detectable mass/energy in the universe, only 5% of it is ordinary matter.  26.8% of it is dark matter, and 68.2% is dark energy, an unrelated type of mass/energy that is thought to be responsible for the runaway expansion of the universe, and which is even less understood than dark matter is.

Dark matter interacts with regular matter via gravity, but -- as far as we can tell -- in no other way.  It seems to be completely unaffected by the other forces that act on the ordinary stuff we see on a day-to-day basis.  There have been various experiments set up to try to detect dark matter particles, but as of the time of this writing, every single one of them has come up empty-handed.  It's bizarre to think about; a substance that makes up five times more of the mass of the universe than all the regular matter put together, and thus far, we haven't the slightest idea what it's made of.

There's also the problem that the Standard Model -- the framework that accounts for all the ordinary matter particles, and how they interact -- is one of the most rigorously-tested theories in science, and performs to a level of precision that beggars belief.  And nothing in the Standard Model appears to admit of some strange extra particle(s) that might account for dark matter.

Well, along came Mordehai Milgrom, who in 1983 tried something a little like what Einstein did with respect to the luminiferous aether -- he said, "I have a new theory that eliminates the need for dark matter entirely."  It's called modified Newtonian dynamics, MoND for short, and proposes that the problem is that Newton's Law of Gravitation doesn't work for objects experiencing really low accelerations (like the stars in the outer reaches of galaxies).  Like the Theories of Relativity, it leaves the model relatively unchanged at the velocities and accelerations we encounter on a daily basis; here on the surface of the Earth, Newton still works just fine.  But what Einstein did for systems in extreme gravitational fields or high velocities, Milgrom did for systems experiencing really low accelerations -- tweaking the mathematics to make it match the observations.

And those tweaks, in one stroke, eliminated the need for some hypothetical and undetectable form of matter.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

Well, the race was on to try to figure out which model was correct -- Newton (with dark matter) or MoND (without it).  And earlier this year, Korean astrophysicist Kyu-Hyun Chae seemed to have settled it once and for all, showing that observations of wide binaries -- pairs of stars orbiting their common centers of gravity at large distances -- matched the predictions of MoND brilliantly.

However, there were astrophysicists who immediately had objections.  Chae, they said, had not done a good job of eliminating data points that were problematic.  The difficulty is that if you're observing a binary pair from Earth, to figure out the velocities and accelerations of the stars in the pair, you have to take into account a variety of complicating factors, including:

• the speed the entire system is moving toward or away from Earth
• the eccentricity (elliptical-ness) of the orbit
• the inclination of the orbit -- how much it's tilted toward or away from us

So a group of researchers, led by astrophysicist Indranil Banik of the University of St. Andrews, has developed a technique for sifting through the data points and using the ones for which there is the best confidence in the velocity measurements (significantly, Banik's team only eliminated about twenty percent of Chae's data points).  And in their paper, which came out five days ago, they found when they do that, the agreement of the data with MoND vanishes completely.

Without the wonky data points, the measurements from wide binaries that seemed to settle the argument in favor of MoND actually agree with Newtonian dynamics...

... to a confidence of 19 σ.  To us non-scientists -- as the wonderful YouTuber Dr. Becky Smethurst explains it -- a 19 σ confidence level means there is only a one in one hundred thousand trillion trillion trillion chance that their result is a statistical fluke.  (And if you want to know more, I highly recommend watching Dr. Becky's video on the new paper, which is awesome.)

So with regards to theories of gravity, it appears that Newton is the only game in town, meaning we're stuck with dark matter.  MoND is dead in the water, so unless someone comes up with some sort of different model entirely that matches the data better than Newton does, we'll have to keep looking for this ghostly matter whose only fingerprint is its gravity.

It's an exciting time to be an astrophysicist... or just a deeply curious science nerd.

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