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New galaxy rotation measurements point to modified gravity as an explanation for dark matter


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Although dark matter is an important component of the standard cosmological model, it is not without flaws.

There are still unanswered questions about the substance, not the least of which is the fact that scientists have found no direct particle evidence of it.

Despite extensive searches, we have yet to find dark matter particles. As a result, some astronomers prefer a different approach, such as modified Newtonian dynamics (MoND) or a modified gravity model. A new study of galactic rotation appears to back them up.

Galactic rotation inspired the concept of MoND. Because the majority of visible matter in a galaxy is concentrated in the center, you’d expect stars closer to the center to have faster orbital speeds than stars farther away, similar to our solar system’s planets.

We can see that stars in a galaxy all rotate at roughly the same speed. Instead of dropping off, the rotation curve is essentially flat.

The dark matter solution proposes that galaxies are surrounded by an invisible halo of matter, but Mordehai Milgrom argued in 1983 that our gravitational model must be incorrect.

New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter
The rotation curve of the typical spiral galaxy M 33 (yellow and blue points with error bars) and the predicted one based on visible matter distribution (white line). The difference between the two curves is explained by including a dark matter halo around the galaxy. Source: Wikipedia

The gravitational attraction between stars at interstellar distances is essentially Newtonian. Milgrom proposed modifying Newton’s universal law of gravity rather than general relativity.

He contended that, rather than being a pure inverse square relation, gravity has a small remnant pull regardless of distance.

This remnant is only a few ten trillionths of a G in size, but it’s large enough to explain galactic rotation curves.

Of course, simply adding a small term to Newton’s gravity requires modifying Einstein’s equations as well.

As a result, MoND has been generalized in a variety of ways, including AQUAL, which stands for “a quadradic Lagrangian.”

There are some subtle differences between AQUAL and the standard LCDM model in explaining observed galactic rotation curves.

A recent study comes into play here. One distinction between AQUAL and LCDM is the rotation speeds of inner and outer orbit stars.

Both should be governed by matter distribution in LCDM, so the curve should be smooth. Because of the dynamics of the theory, AQUAL predicts a tiny kink in the curve.

It is too small to be measured within a single galaxy. It’s too small to detect in a single galaxy, but statistically, a small shift between the inner and outer velocity distributions should be observed.

New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter
The difference between inner and outer stellar motions is measured. Credit: Kyu-Hyun Chae

As a result, the author of this paper examined high-resolution velocity curves from 152 galaxies in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database.

He discovered a shift in AQUAL’s agreement. The evidence appears to favor modified gravity over standard dark matter cosmology.

The outcome is exciting, but it does not definitively disprove dark matter. The AQUAL model has its own problems, such as its disagreement with observed galaxies’ gravitational lensing.

However, it is a victory for the underdog theory, with some astronomers exclaiming, “Vive le MoND!”

The study is available on the arXiv preprint server.

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