Two articles by Avishai Gal-Yam examine the question of the existence of dark matter
Darkness in the eyes - the secret of dark matter
One of the most interesting questions that have occupied astronomers in recent years is the question of "dark matter". This is the name researchers call a substance that they identify by the effect of the gravitational force it exerts on other bodies, but it cannot be seen because it does not emit light.
One of the first research topics that put the existence of dark matter on the astronomical agenda was testing the rotation speeds of stars around the centers of galaxies. Most of the stars in the universe are inside galaxies - huge clusters of billions of stars revolving around a common center. Newton's theory of universal gravitation states that the speed of rotation of stars around the center of the galaxy depends on their distance from the center of the galaxy and the mass of the galaxy. When the researchers examined the rotation speeds of stars and gas clouds far from the centers of the so-called "spiral" galaxies (due to their shape), it was found that the measured rotation speed was much greater than expected. From this, the researchers concluded that the mass of the galaxy is much higher than they estimated based on the light coming from the stars - that is, that it contains additional material that is not visible to the eye: "dark matter".
In addition to the dark matter in spiral galaxies, evidence of the existence of such matter has also been found in other types of galaxies and in galaxy clusters: groupings of thousands of galaxies revolving around a common center. In fact, according to most astronomers the vast majority of matter in the universe is dark. The obvious question is, then, what is that dark matter.
There are currently two accepted answers to this question. One answer is that dark matter is made up of different particles than the ones that make up the matter we know from everyday life (protons, neutrons and electrons). The claim is that these particles have no electric charge and that the electromagnetic force does not apply to them. From this it follows that they cannot emit any electromagnetic radiation: no light, no radio waves, no X-ray radiation. Since the observation devices that exist today absorb electromagnetic radiation, such material would be "dark" for us.
Such particles are not known from experiments, but theoretically they may indeed exist. Many research groups around the world are engaged in building experimental facilities aimed at discovering the mysterious particles. In the experiments, the scientists observe large quantities of water or ice under controlled conditions, and follow tiny flashes of light that should be emitted when a particle of the dark matter hits a molecule of water or ice. In an experiment by French scientists, called "Super Camiocanada", the scientists observe tanks with millions of liters of water. The containers are located in an underground mine, in order to prevent "normal" cosmic rays from damaging the water and causing similar sparkles. Another group of scientists is conducting a similar experiment in the ice cap above the South Pole. In this case, light-sensitive detectors are buried deep in the ice.
The second answer to the question "what is dark matter" is that it is "normal" matter, most of which is hydrogen gas, trapped inside cold and small stars (relative to the Sun) called "brown dwarfs". The brown dwarfs do not emit light, so they cannot be seen from a distance. In this sense, "normal" matter has become dark matter.
Observations in the region closest to the Sun have recently discovered some brown dwarfs, but scientists have not been able to determine how common they are in other regions of the universe and if their number is large enough to explain all the dark matter measured in galaxies. In order to answer this question, a few years ago an international group of scientists started a research project known as MACHO. Since brown dwarfs cannot be observed directly, the researchers proposed to locate them through their influence on the light coming from distant stars. The theory of general relativity teaches that heavy bodies warp space and therefore deflect the path of light rays propagating through space. This effect is similar to the effect of an optical lens - a magnifying glass, for example - which changes the path of light rays by refracting them. Because of this, the effect of heavy bodies on light rays is called "gravitational lensing". Just as a magnifying glass concentrates the light passing through it and increases its intensity, brown dwarfs that pass between us and distant stars become "gravitational lenses" and increase the intensity of the distant stars for a certain time. The astronomers began to monitor millions of stars every night, with the help of a special telescope dedicated to this purpose in Australia. Each night, the intensity of the illumination of each star was measured and compared to their intensity on the previous nights. The assumption of the researchers was that the passage of an invisible brown dwarf between us and one of the stars would increase the brightness of the star. The degree of amplification and the time of the event, they hoped, would make it possible to estimate the mass and location of that invisible "gravitational lens". These days, after about six years of intensive activity, the leaders of the MACHO project announced its end. MACHO yielded a lot of interesting astronomical information - the scientists discovered hundreds of events of gravitational dusting in our galaxy - and proved that it is possible to use this method to search for brown dwarfs. What the project failed to do is to unequivocally answer the fundamental question: what is dark matter composed of? Many researchers around the world continue to try and crack this disturbing mystery.
Prof. Mordechai Milgrom denies the existence of dark matter and proposes to update Newton's theory of gravity. Second article
Avishai Gal-Yam
In the article "Darkness in the eyes (1)" the astrophysicists' search for the "dark matter" was described. This is the name the researchers call a substance that they identify by the effect of the gravitational force it exerts on other bodies, but it is impossible to see it because it does not emit light. The existence of dark matter is a hypothesis: it originates from the unbridgeable gap between the mass of galaxies, as it is estimated based on the light coming from them, and the same mass, as it is calculated based on the rotational speeds of the stars moving around those galaxies. The mass of visible matter in galaxies is too small to explain the rotation speeds of the stars, and in order to achieve a match between the measurements and the theoretical predictions, the scientists had to assume the existence of "dark matter", which increases the mass of the galaxies but does not emit any light. According to most astrophysicists, most of the matter in the universe is dark.
There are two main hypotheses regarding the essence of dark matter: according to one of them, dark matter is composed of different particles than the ones that make up the matter we know; They have no electric charge and the electromagnetic force does not apply to them.
The second hypothesis is that the dark matter is actually "normal" matter, most of which is hydrogen gas, trapped inside stars called "brown dwarfs". The brown dwarfs do not emit light, so they cannot be seen from a distance. Many researchers have been trying to locate the brown dwarfs in recent years. They discovered hundreds of events where brown dwarfs boosted the power of stars, but were unable to answer the question of what dark matter is made of.
Prof. Mordechai Milgrom from the Weizmann Institute put forward another proposal for solving the dark matter issue in the early 80s. The reason why astrophysicists assume that dark matter exists is, as mentioned, the incompatibility between the observations and the predictions of Newton's theory of gravity. Perhaps, suggested Prof. Milgrom, the solution is not in the search for a new and invisible type of matter, but in changing the theory of gravity.
Newton's theory of gravitation describes with great precision the behavior of bodies on Earth and the orbits of most of the planets in the solar system.
On bodies that move very close to very heavy bodies, such as the planet Mercury (the planet Hema) that moves very close to the Sun, a very strong gravitational force acts. In such cases, Newton's theory of gravity is not completely accurate. In order to describe the movement of moving bodies under the influence of a strong gravitational force, Einstein's theory of general relativity must be used. In fact, the exact calculation of the orbit of the planet Mercury around the Sun was one of the first achievements of general relativity.
However, when discussing bodies moving under the influence of a weak gravitational force - for example, stars moving in distant orbits from the centers of galaxies - the predictions of general relativity are the same as those of Newton's theory. It is possible, says Prof. Milgrom, that the theory of gravity suitable for these cases is different from Newton's theory, so that the force of gravity exerted on bodies is weaker. In that case, there is no need for dark matter at all.
In the early XNUMXs, Prof. Milgrom proposed a new theory of gravity -
Modified Newtonian Dynamics (MOND). The central principle of the Torah states that bodies moving under the influence of gravity weaker than a certain threshold, behave differently than expected according to Newton's theory. Calculations according to the principles of the New Torah explain the orbits of the stars and the gas clouds in the galaxies based on visible matter only, without the need to add dark matter. Groups of scientists - among others from Israel, the USA and the Netherlands - have shown in recent years that it is possible to explain the dynamics of different types of galaxies as well as galaxy clusters - groups of hundreds of galaxies revolving around a common center in space - according to the principles of the New Torah. Since its publication, no arguments have been put forward that unequivocally prove that the MOND theory is incorrect. Nevertheless, most astrophysicists in Israel and around the world prefer to explain the universe using Newton's theory, while assuming that most of the matter in the universe is not visible to the eye.
It seems that the main weakness of the theory - MOND, a weakness that makes it unacceptable to most astrophysicists - is the fact that it is not derived from a theory equivalent to Einstein's theory of general relativity. Einstein developed the theory of general relativity according to basic physical and mathematical principles; Only after its formulation was it tested experimentally and found to be correct. MOND, on the other hand, was developed to explain a certain type of observations, and it is not derived from the basic physical and mathematical principles from which general relativity is derived. Also, if the MOND theory is correct, it is necessary to change the general theory of relativity and formulate another general theory in its place, from which MOND will be derived just as Newton's theory is derived from the general theory of relativity.
It seems that most researchers currently prefer to keep Newton's theory of gravity and search for "dark matter". Prof. Milgrom and other scientists continue to test whether MOND or other similar theories do not better describe the universe.
{Appeared in Haaretz newspaper, 25/1/2000{
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