New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter

Though dark matter is a central a part of the usual cosmological mannequin, it is not with out its points. There proceed to be nagging mysteries in regards to the stuff, not the least of which is the truth that scientists have discovered no direct particle proof of it.

Regardless of quite a few searches, we’ve but to detect dark matter particles. So some astronomers favor an alternate, similar to modified Newtonian dynamics (MoND) or modified gravity model. And a brand new research of galactic rotation appears to assist them.

The thought of MoND was impressed by galactic rotation. Many of the seen matter in a galaxy is clustered within the center, so that you’d anticipate that stars nearer to the middle would have quicker orbital speeds than stars farther away, much like the planets of our solar system. What we observe is that stars in a galaxy all rotate at about the identical pace. The rotation curve is basically flat relatively than dropping off. The dark matter resolution is that galaxies are surrounded by a halo of invisible matter, however in 1983 Mordehai Milgrom argued that our gravitational mannequin should be incorrect.

At interstellar distances, the gravitational attraction between stars is basically Newtonian. So relatively than modifying general relativity, Milgrom proposed modifying Newton’s common regulation of gravity. He argued that relatively than the pressure of attraction as a pure inverse sq. relation, gravity has a small remnant pull no matter distance. This remnant is simply about 10 trillionths of a G, nevertheless it’s sufficient to elucidate galactic rotation curves.

In fact, simply including a small time period to Newton’s gravity signifies that you even have to change Einstein’s equations, as properly. So MoND has been generalized in varied methods, similar to AQUAL, which stands for “a quadradic Lagrangian.” Each AQUAL and the usual LCDM mannequin can clarify noticed galactic rotation curves, however there are some delicate variations.

That is the place a current research is available in. One distinction between AQUAL and LCDM is within the rotation speeds of internal orbit stars vs. outer orbit stars. For LCDM, each needs to be ruled by the distribution of matter, so the curve needs to be clean. AQUAL predicts a tiny kink within the curve as a result of dynamics of the speculation. It is too small to measure in a single galaxy, however statistically, there needs to be a small shift between the internal and outer velocity distributions.

So the creator of this paper checked out high-resolution velocity curves of 152 galaxies as noticed within the Spitzer Photometry and Correct Rotation Curves (SPARC) database. He discovered a shift in settlement with AQUAL. The info appears to assist modified gravity over commonplace dark matter cosmology.

The result’s thrilling, nevertheless it does not conclusively overturn dark matter. The AQUAL mannequin has its personal points, similar to its disagreement with noticed gravitational lensing by galaxies. However it’s a win for the underdog idea, which has some astronomers cheering “Vive le MoND!”

The analysis is printed on the arXiv preprint server.