Dark matter is a basic element of the basic cosmological model, however its secrets remain unsolved. One of the greatest puzzles is the absence of direct proof for the existence of dark matter particles, in spite of various searches. Dark matter is a main part of the basic cosmological model, its not without its issues. The dark matter option is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued that our gravitational design needs to be wrong.
The information appears to support modified gravity over standard dark matter cosmology.
Dark matter is a fundamental part of the standard cosmological model, but its mysteries stay unsolved. Among the greatest puzzles is the absence of direct evidence for the presence of dark matter particles, despite many searches. Some astronomers have actually proposed alternative theories, such as Modified Newtonian Dynamics (MoND) or customized gravity designs, to describe the observations. A current study on stellar rotation appears to lend support to these alternative theories.
Dark matter is a main part of the standard cosmological model, its not without its problems. There continue to be irritating mysteries about the stuff, not the least of which is the fact that scientists have discovered no direct particle evidence of it. In spite of many searches, we have yet to find dark matter particles. So some astronomers prefer an option, such as Modified Newtonian Dynamics (MoND) or modified gravity design. And a brand-new study of galactic rotation appears to support them.
The idea of MoND was inspired by galactic rotation. The majority of the noticeable matter in a galaxy is clustered in the center, so you d anticipate that stars closer to the center would have quicker orbital speeds than stars farther away, similar to the planets of our solar system. We observe that stars in a galaxy all turn at about the same speed. The rotation curve is essentially flat instead of dropping off. The dark matter solution is that galaxies are surrounded by a halo of undetectable matter, but in 1983 Mordehai Milgrom argued that our gravitational design needs to be incorrect.
Rotation curve of the normal spiral nebula M33 (blue and yellow points with mistake bars) and the forecasted one from the distribution of the noticeable matter (white line). The inconsistency between the two curves is accounted for by including a dark matter halo surrounding the galaxy.
At interstellar ranges, the gravitational tourist attraction in between stars is basically Newtonian. So instead of modifying general relativity, Milgrom proposed modifying Newtons Universal Law of Gravity. He argued that instead of the force of attraction being a pure inverted square relation, gravity has a small remnant pull regardless of range. This remnant is just about 10 trillionths of a gee, however its sufficient to discuss stellar rotation curves.
Naturally, simply including a small term to Newtons gravity implies that you also need to modify Einsteins equations also. So MoND has been generalized in numerous ways, such as AQUAL, which represents A Quadradic Lagrangian. Both AQUAL and the basic LCDM model can discuss observed stellar rotation curves, however there are some subtle differences.
Measured shift in between external and inner stellar motions. Credit: Kyu-Hyun Chae
This is where a recent study comes in. One difference between AQUAL and LCDM is in the rotation speeds of inner orbit stars vs outer orbit stars. For LCDM, both ought to be governed by the circulation of matter, so the curve ought to be smooth. AQUAL predicts a small kink in the curve due to the dynamics of the theory. Its too small to determine in a single galaxy, but statistically, there should be a small shift between the inner and external velocity circulations. So the author of this paper looked at high-resolution velocity curves of 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a shift in agreement with AQUAL. The data seems to support modified gravity over standard dark matter cosmology.
The result is exciting, however it does not conclusively reverse dark matter. Thye AQUAL design has its own issues, such as its dispute with observed gravitational lensing by galaxies. But it is a win for the underdog theory, which has some astronomers cheering “Vive le MoND!”
Referral: “Distinguishing Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of Galactic Rotation Curves” by Kyu-Hyun Chae, 12 December 2022, The Astrophysical Journal.DOI: 10.3847/ 1538-4357/ ac93fc.
Originally released on Universe Today.