Modified gravity in plane sight

Our Galaxy—the Milky Way—is a vast rotating disk containing billions of stars along with huge amounts of gas and dust. Its diameter is around 100,000 light years. The Milky Way has many satellite galaxies (e.g. the Magellanic Clouds), which orbit around it. Most of these satellites lie in a thin plane and rotate within it. The same is also true for the satellites of our neighbouring galaxy, Andromeda. Though this has been known for many years, the origin of these satellite planes has remained a mystery. Several previous studies showed that similar structures are not expected in the standard picture of galaxies obeying general relativity: the law of gravity developed by Einstein that lies at the heart of how we understand our Universe. In this picture, we expect a nearly random distribution of satellites. This contradicts observations, even when disregarding the particularly problematic velocity data.

In the Einstein-inspired picture, the fast rotation speeds of stars in galaxy outskirts imply that galaxies are held together by huge amounts of unseen mass, called dark matter. In many galaxies, this dark matter needs to vastly outweigh the visible. However, nobody has ever discovered definitive proof that dark matter actually exists. Instead, scientists have managed to prove that it can’t be made of any fundamental particles that we currently know of (e.g. it can’t just be dead stars). If it was, the gravity of these stars—which is capable of bending light—would occasionally magnify the light of background stars in the same way that moving a lens across a small light would make it appear to brighten and then fade. Although this does sometimes happen, such microlensing events are too rare, implying that the dark matter must be made of objects whose mass is at least a million times smaller than that of the Sun. Thus, the theorised dark matter has to be a new particle that has evaded numerous very sensitive attempts to detect it (e.g. LUX Collaboration 2018, PandaX-II Collaboration 2018).

An alternative theory called Modified Newtonian Dynamics (MOND) was proposed in the early 1980s by Israeli physicist Mordehai Milgrom. MOND says that gravity is much stronger than predicted by Einstein. Instead of following the usual inverse square law, gravity from any object follows a gentler inverse distance law once it gets weaker than a certain acceleration threshold typically reached in the outskirts of galaxies. As a result, instead of becoming one quarter as strong at double the distance from a galaxy, MOND says its gravity is still half as strong. Importantly, this change only applies once gravity gets weaker than a very low threshold that is one hundred billion times weaker than the gravity we experience on Earth’s surface. (Fun fact: the threshold is roughly the amount of gravity that an ant crawling on a glossy magazine would experience from just its cover). Thus, Milgrom’s proposal has very little effect here or elsewhere in the Solar System, where the inverse square law would still hold. But in the outskirts of galaxies, his proposed helping hand to Einstein’s gravity is enough to account for the fast speeds at which the stars and gas are observed to rotate. Despite its very limited freedom and its key equations being written down 35 years ago, MOND has proved amazingly good at predicting the rotation curves of a huge variety of galaxies spanning a factor of 100,000 in visible mass, all without inventing dark matter.

In a new study published by the Monthly Notices of the Royal Astronomical Society, it is claimed that MOND also provides a natural explanation for the satellite planes. The study assumes that the Milky Way and Andromeda started with plausible initial conditions (the “Hubble flow”) and had masses consistent with the stars and gas we actually detect in them. Solving the equations forwards, the galaxies end up with their observed distance and velocity—after a fashion. In MOND, their much stronger gravitational attraction implies they had a close flyby when the Universe was about half its current age.

This recent study conducted several simulations of the galaxies over the entire history of the Universe, varying parameters like just how close the galaxies got. The outer particles of both galaxies were knocked onto rather different orbits by the flyby. Amidst all this chaos, it was important to see if the outer particles of e.g. Andromeda ended up mostly rotating within a particular plane and, if so, whether this plane aligns with the real plane of Andromeda’s satellites.

Remarkably, they found some models in which the outer particles of both galaxies aligned with their observed satellite planes. The inner particles remain fairly undisturbed, so the simulations are compatible with the thin disk of the Milky Way that can be seen on a clear night. Moreover, the calculated time of the flyby is similar to when our Galaxy’s disk thickened suddenly to form its “thick disk.” Some of the particles in its simulated “satellite plane” are rotating in the opposite direction to the majority. The latest observations show that the fraction of these particles is similar to the observed fraction of “counter-rotating” satellites around our Galaxy. This phenomenon does not seem to arise around Andromeda, either in the simulations or in observations.

The study enabled researchers to observe realistic-looking satellite planes the first time anyone did a detailed investigation of the Milky Way–Andromeda flyby; this flyby is inevitable in MOND. Contrast this with the numerous efforts over more than a decade to try and explain the satellite planes in general relativity—it just doesn’t seem to work, regardless of how much dark matter you’re prepared to invent.