What’s the matter with dark matter?

Comparison of observed and expected
rotation curves of galaxy M33
(Wikipedia)

Nine years ago, I published here a post entitled Dark Matter or New Theory, where I pointed out that the motion of stars in galaxies does not match theoretical predictions. The problem can be solved in two ways:

1.      Assuming that galaxies have much more mass than we can see. The mass we cannot see was called dark matter, where the word dark means that we don't know what it is.

2.      Assuming that Newton's laws should be corrected in the outer part of galaxies, where the acceleration of stars is very small. Various corrections to Newton's equations have been proposed that would satisfactorily solve the problem, which are called MOND (MOdified Newtonian Dynamics) theories.

When the standard cosmological model was developed, the first assumption was adopted, which led to the calculation that the amount of dark matter in the universe must be about five times greater than normal matter, the matter we can see. The problem is that such dark matter should be made up of different particles than those predicted by the Standard Model of particle physics, which applies to the microscopic world. Consequently, for the moment, our two standard models are incompatible.

To play the role of dark matter, several new types of elementary particles have been proposed:

·         WIMP (weakly interacting massive particles), with very large masses, greater than 1 Giga-electron-Volt (1 GeV).

·         Axions, with very small masses, less than 1 electron-Volt (1 eV), which should be extremely abundant.

Another type of object has also been proposed:

·         MACHO (MAssive Compact Halo Objects), which could be black holes, neutron stars, or brown dwarf stars that are not part of any known planetary system and would be very difficult to locate. In that case, dark matter would be baryonic matter, indistinguishable from the normal matter we are used to, but imperceptible.

The latest news is that an experiment (Lux-ZEPLIN) that began in 2021, with the aim of searching for traces of WIMPs in an old gold mine in South Dakota, has ended with a negative result. Not a single particle has been detected. This implies that WIMPs with a mass greater than 9 GeV cannot exist. Therefore, if they exist, their mass must be between 1 and 9 GeV. A new experiment (XLZD) will fill this gap, and if its result is also negative, WIMPs would be eliminated.

The other option is MOND theories, which modify Newton's equations when the acceleration is very small, while leaving them unchanged when the acceleration reaches higher values, as is the case, for example, in the solar system, where Newton's equations are well-proven, except in the vicinity of the sun, where relativistic corrections must be applied. This figure, taken from Wikipedia, shows how the acceleration predicted by a MOND theory deviates from Newtonian acceleration for very low accelerations.

MOND theories have been able to fit perfectly to the conditions existing on the outskirts of galaxies, where the low acceleration conditions that led to the proposal of the existence of dark matter take place. However, these theories have some outstanding problems, which is the reason why many cosmologists refuse to accept them. These problems are the following:

·         So far, it is not clear that they have made any surprisingly accurate predictions that validate them. It is true that the standard cosmological model hasn’t either, although it has been very well fitted to previously known data.

·         They have trouble adjusting to the acoustic waves of the cosmic background radiation, while the standard cosmological model has been perfectly adjusted to them. In contrast, what is known about the oldest galaxies tends to be closer to the predictions of MOND theories than to those of the standard cosmological model.

·         Although there have been several attempts to design a relativistic MOND theory, none have been convincing so far.

In a previous post, I criticized the fact that dark matter and dark energy had been mentioned among the greatest scientific discoveries of the 20th century in a list published in Science News. A quarter of the way into the 21st century, both remain entities about which we know nothing.

More information in this article of PhysicsWorld.

The same post in Spanish

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Manuel Alfonseca