They say that dark matter is most inUniverse (if we talk about matter in general). And yet, in everyday life, we practically do not encounter it. We know the Sun - the most massive object in the Solar System - it consists of ordinary matter (protons, neutrons and electrons), but there are many other sources, including planets, gas, dust, plasma and the remains of stars. There is no dark matter among them - and even the Standard Model does not describe its particles. Of course, dark matter is not the only option to explain the observed gravitational phenomena in the Universe. Another option is to modify the theory of gravity, which many have tried to do. On this basis the idea of Modified Newtonian Dynamics (MOND) and other theories, which so far are popular alternatives to dark matter, have grown.
To start somewhere, we need to return to1800s and talk about the problem that existed long before the “missing mass” (or “missing light”) that dark matter and MOND are trying to solve: the problem of Uranus and Mercury. The law of gravity, put forward by Newton in the 1600s, was incredibly successful in describing everything - as far as we know - to which it was applied. From the movement of shells to rolling objects; from the weight of objects to the ticking of a pendulum clock; from the buoyancy of a boat to the orbit of the moon around the earth, Newton's gravity never failed.
Kepler’s three laws, a special case of Newton’s gravitational formula, were equally applied to all known planets:
Famous inner and outer worlds allobeyed these laws, so that no deviations revealed hundreds of years. But with the discovery of Uranus in 1781, something changed. While the last of the open planets was moving in an ellipse around the Sun, it was moving at the wrong speed when compared with the predicted gravity laws.
In the first 20 years since the discovery, she movedfaster every night and every year than the laws prescribed. Over the next 20-25 years, the planet moved in strict accordance with the laws. But then it slowed down, and the speed fell below predicted.
Was there a mistake in the law of gravity? Maybe. But it is also possible that there was a little more matter - something invisible, dark matter - that influenced Uranus, causing disturbances in its orbit. This is more like the truth. After a theoretical war between Urbain Leverrier and John Coach Adams, who worked independently and made predictions about the location of a new planet, the predictions of Leverrier were confirmed by Johann Halle and his assistant Henry d'Arre on September 23, 1846. The planet Neptune was discovered, the first object whose existence was deduced from the effects exerted by its mass: gravitational influence.
On the other hand, the inner planet Mercury -due to an increase in the accuracy of observations and in combination with secular data, she began to demonstrate an even stranger violation of the laws of gravity. If Kepler’s laws predicted that planets should move along ideal ellipses with the Sun in one of the tricks, then provided that there are no other masses that violate or influence the system. But there are no masses around, and Mercury does not move along an ideal ellipse. Its ellipse precesses over time.
Using Newton’s laws of gravity, we couldtake into account the influence of all known planets (including Neptune). Having done all this, we would find that there remains a slight discrepancy between the predicted and the observed: a precession of 43 ”per century, or 0.012 degrees per century. But this was not an accident.
What will be the explanation this time? Is this new invisible mass connected with the bowels of Mercury? Or did the real problem creep into the law of gravity? A thorough search for the answer to this question led to the new theoretical planet Vulcan, which was supposed to be closer to the Sun than everyone else. But no volcano was found. The decision came in 1915 when Einstein outlined his general theory of relativity.
Now we waste time until the 1970s - to a numberscientific observations of Vera Rubin. We observe individual galaxies - in particular, galaxies "from the edge" - and measure their velocity profiles. We look at one side of the galaxy and see that it moves towards us (at the blue shift), look at the other - it moves away from us (at the redshift), and so determine the rotation of the galaxy. What do we expect from them? Like our solar system, inner stars must rotate faster, and the farther from the center, the lower the speed should be. But that is not what we find.
Instead, each individual rotation speedthe galaxy remains constant regardless of distance. Why? Again, there are two options: either the laws of gravity require improvement, or we must assume the existence of an invisible excess mass.
The phenomenon of MOND was first noticed by Moti Milgrom in 1981to the year that he noticed that if we would change the law of gravity at very small accelerations — something like fractions of a nanometer per second squared — we could explain these rotational curves. In addition, the same modification, one single and consistent, could explain the rotation of all galaxies, from smallest to largest. MOND still does, and does well.
Dark matter, on the other hand, suggeststhat in addition to the ordinary particles of the Standard Model and ordinary matter from “protons, neutrons and electrons”, which make up almost everything we know, there is a new type of matter. To explain the rotational phenomenon, it was proposed to introduce a large halo of matter, which does not interact with light, but does not stick together, and does not interact with ordinary matter, except gravitationally. That was the idea of dark matter.
Dark matter can explain these rotationalcurves, but it doesn’t do as good as MOND. Numerical modeling for halo that produce even the simplest models of dark matter does not correspond to observations; the halo are too “downed” in the center and too “fluffy” on the outskirts. (From a technical point of view, they seem more isothermal than expected). In short, at first MOND was a clear leader.
But there, further, the whole Universe began. When you propose a new theory to replace the old - as the general theory of relativity replaced Newton's laws - your theory must satisfy three principles:
We are talking about all the successes of the previous leading theory, and they are numerous.
There is a gravitational curvature of starlightmass, strong and weak gravitational lensing. There is a Shapiro effect. There is gravitational time dilation and gravitational redshift. There is the concept of the Big Bang and the concept of an expanding universe. There are movements of galaxies inside clusters and clustering of the galaxies themselves on the largest scales.
In the case of all these examples - all - MONDsuffers a crushing defeat, either by not proposing any forecasts, or by making forecasts that come into frustrating conflict with the available data. You can rightly notice that MOND was never going to be a complete theory, but rather a description of a single phenomenon that could lead to a more complete theory. Many people are working on the MOND extension, which could explain these observations, but so far without success.
But if you continue Einstein’s law of gravity and just add a new ingredient, cold dark matter, you can explain everything, including some new unusual nuances.
You can explain the picture of clustering that is observed in the large-scale structure of the Universe, if you have five times more dark matter than usual.
And what is especially impressive, you can doa completely new prediction: when two clusters of galaxies collide, the gas in them heats up, slows down and emits X-rays, while the mass that we see with gravitational lensing follows dark matter and is replaced by X-rays. This new prediction has been experimentally confirmed and has been around for ten years, providing an indirect confirmation of the existence of dark matter.
MOND has the advantage of: she explains the rotation curves of galaxies better than dark matter. But this is not a physical theory and it does not correspond to the full set of observations that we have. Dark matter exists - at least in theory - because it gives us the same Universe, consistent, without any modifications.
But the current failure is MOND,cosmological, put it below dark matter. Let him reproduce all the successes of general relativity, explain new phenomena, make predictions that can be confirmed - and scientists will undoubtedly turn to the new faith. After all, they are good scientists.