How dark matter has become a popular hypothesis
Why do we consider this or that theory or hypothesis convincing, and its alternatives unlikely? The answer to this question has been worrying the philosophers of science for many decades: the disputes of Karl Popper and Thomas Kuhn are already more than 40 years old. A relatively recent example in which to discuss it is the development of ideas about dark matter. Signs of its existence were discovered in the 1930s, but it was not until the 1970s that this hypothesis was started to be considered as the main hypothesis. Why?
The problem of dark matter today is one of the central issues for both astrophysics and physics in general. It is known that, apparently, about 80% of the mass of galaxies consists of a substance unknown to science that does not manifest itself except for its gravity. Surprisingly, although the first signs of the existence of this hidden mass from our eyes were discovered as far back as the 1930s, truly scientists took up this hypothesis only in the 1970s. Why did this happen? One of the possible answers is offered by the authors of the article How Dark Matter Came to Matter , recently posted on arxiv.org. I will cite a brief retelling of her theses.
In 1933, the now famous article by Fritz Zwicky came out in which he, after studying the observed speeds of galaxies in the Veronica Hair Cluster, discovered that their spread is too large - the cluster should have scattered, since its apparent total mass is clearly not enough to hold fast galaxies. Two more astronomers, Eric Holmberg and Sinclair Smith, who studied other galactic systems, arrived at a similar conclusion.
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As one of the explanations, Zwicky introduced the hypothesis of dark matter - an invisible substance that creates additional gravity and does not allow clusters to scatter. However, a serious discussion of these observations began only after two decades.
In the late 1950s, more accurate measurements of galactic clusters appeared, which confirmed the observations of the 1930s and made them more thoroughly. Then, in 1958, Victor Ambartsumian put forward an alternative hypothesis: what we see is not an equilibrium state. The galaxies really fly away, we were just lucky to watch them at that moment when they are still relatively close to each other.
This hypothesis, however, had a big problem. According to estimates, the expansion of galaxies should occur in tens or hundreds of millions of years, which is hundreds of times less than the age of the Universe. To our time there should not be a single galactic cluster. But we see that in fact there are many clusters.
However, the idea that the visible substance represents only one-fifth of the total mass looked no less radical, so many alternative explanations arose. They tried to explain the hidden mass with hypothetical huge areas of ionized hydrogen, a large number of dwarf galaxies, the need to modify the laws of gravity, high density of gravitational radiation, the existence of relict black holes, massive neutrinos, and, of course, just observation errors.
At the same time, a problem arose in parallel in a completely different field of astronomy, which could also be solved by the dark matter hypothesis. In the 1950s – 1960s, radio astronomy actively developed, making it possible, on the basis of the famous 21 cm line, to “see” previously invisible neutral hydrogen. With its help, we measured the speed of gas rotation in galaxies and constructed their rotation curves — dependencies of the speed of rotation of stars and gas around the center of the galaxy on the distance to this center.
And in the early 1970s, it was found that these curves are too “flat”. It was expected that, in accordance with Kepler's laws, the apparent mass of the velocity away from the main cluster would fall, but they remained almost constant. There was a problem, where does “additional” gravity come from, providing higher speed.
The situation, however, was ambiguous. Although in some works it was directly stated that the observed rotation curves can be explained by some additional mass not available for direct observation, this viewpoint did not prevail. Part of the problem was that there were many models of galaxy rotation, and by varying the parameters in them, it was possible to obtain relatively good agreement with the observed curves.
Well, even though the dark matter hypothesis could have solved both of the existing problems at once, it was only one of many possible hypotheses, at that time not very convincing, and besides, not without its flaws. And, in general, few people considered these two problems together.
A significant change in the perception of the dark matter hypothesis was due to the development of a completely different area - cosmology.
Back at the beginning of the 20th century, most scientists believed that our galaxy was the whole Universe. But by the middle of the century it became clear: this is far from the case. All the same radio astronomy led to the discovery of a large number of galaxies, and later in 1963 and objects of a completely new nature - quasars, distant super-bright objects that existed at the dawn of the Universe. It became clear that the Universe is not static, but somehow evolves. So cosmology arose.
The main task of cosmology was to find out what scenario the Universe is developing. According to the most popular model, bearing the name of Friedman, there are three variants of the Universe: flat, open and closed. Which of them corresponds to our Universe depends on the parameters in the equations. These parameters should, naturally, be determined from observations, and the main one is the density of matter in the Universe.
The most attractive - mostly from some abstract aesthetic senses - to many cosmologists seemed to be a model of a closed Universe. For this, the density of the substance in it should be about 10 −29 g / cm³. In fact, the estimate of the apparent mass was a hundred times less - 10 −31 g / cm³. This discrepancy could not be ignored.
And it was here that cosmologists came to the rescue of the well-known problems with the rotation curves and the velocity spread of galaxies in clusters. In 1974, Jim Peebles, Jerry Ostrajker and Amos Yahil put everything together and showed that the dark matter hypothesis solves all three problems. The elegance of this conclusion, however, blurred the fact that even taking into account the hidden mass of galaxies, the density of the Universe was still five times lower than that required for the “closed” scenario. However, this was no longer a hundred times, and astrophysicists learned to put up with a coefficient less than 10 long before that - yet measurements in this field of science were always not too accurate.
In addition, another group of scientists came at the same time - the Estonian group from the Tartu Observatory consisting of Jaan Einasto, Ants Kaasik and Enn Saar. Their article was published in Nature magazine at the same time as the article by the Americans, although they sent it a few weeks earlier.
Be that as it may, it was precisely cosmologists and their young and rapidly developing field of science that ultimately became the “glue” that combined the scattered and not very convincing evidence into a single and rather well-proportioned theory, based on only one hypothesis - the hypothesis dark matter.
The problem of dark matter today is one of the central issues for both astrophysics and physics in general. It is known that, apparently, about 80% of the mass of galaxies consists of a substance unknown to science that does not manifest itself except for its gravity. Surprisingly, although the first signs of the existence of this hidden mass from our eyes were discovered as far back as the 1930s, truly scientists took up this hypothesis only in the 1970s. Why did this happen? One of the possible answers is offered by the authors of the article How Dark Matter Came to Matter , recently posted on arxiv.org. I will cite a brief retelling of her theses.
The problem of the velocity variation of galaxies in clusters
In 1933, the now famous article by Fritz Zwicky came out in which he, after studying the observed speeds of galaxies in the Veronica Hair Cluster, discovered that their spread is too large - the cluster should have scattered, since its apparent total mass is clearly not enough to hold fast galaxies. Two more astronomers, Eric Holmberg and Sinclair Smith, who studied other galactic systems, arrived at a similar conclusion.
')
As one of the explanations, Zwicky introduced the hypothesis of dark matter - an invisible substance that creates additional gravity and does not allow clusters to scatter. However, a serious discussion of these observations began only after two decades.
In the late 1950s, more accurate measurements of galactic clusters appeared, which confirmed the observations of the 1930s and made them more thoroughly. Then, in 1958, Victor Ambartsumian put forward an alternative hypothesis: what we see is not an equilibrium state. The galaxies really fly away, we were just lucky to watch them at that moment when they are still relatively close to each other.
This hypothesis, however, had a big problem. According to estimates, the expansion of galaxies should occur in tens or hundreds of millions of years, which is hundreds of times less than the age of the Universe. To our time there should not be a single galactic cluster. But we see that in fact there are many clusters.
However, the idea that the visible substance represents only one-fifth of the total mass looked no less radical, so many alternative explanations arose. They tried to explain the hidden mass with hypothetical huge areas of ionized hydrogen, a large number of dwarf galaxies, the need to modify the laws of gravity, high density of gravitational radiation, the existence of relict black holes, massive neutrinos, and, of course, just observation errors.
The problem of rotation curves of galaxies
At the same time, a problem arose in parallel in a completely different field of astronomy, which could also be solved by the dark matter hypothesis. In the 1950s – 1960s, radio astronomy actively developed, making it possible, on the basis of the famous 21 cm line, to “see” previously invisible neutral hydrogen. With its help, we measured the speed of gas rotation in galaxies and constructed their rotation curves — dependencies of the speed of rotation of stars and gas around the center of the galaxy on the distance to this center.
And in the early 1970s, it was found that these curves are too “flat”. It was expected that, in accordance with Kepler's laws, the apparent mass of the velocity away from the main cluster would fall, but they remained almost constant. There was a problem, where does “additional” gravity come from, providing higher speed.
The situation, however, was ambiguous. Although in some works it was directly stated that the observed rotation curves can be explained by some additional mass not available for direct observation, this viewpoint did not prevail. Part of the problem was that there were many models of galaxy rotation, and by varying the parameters in them, it was possible to obtain relatively good agreement with the observed curves.
Well, even though the dark matter hypothesis could have solved both of the existing problems at once, it was only one of many possible hypotheses, at that time not very convincing, and besides, not without its flaws. And, in general, few people considered these two problems together.
Cosmology problems
A significant change in the perception of the dark matter hypothesis was due to the development of a completely different area - cosmology.
Back at the beginning of the 20th century, most scientists believed that our galaxy was the whole Universe. But by the middle of the century it became clear: this is far from the case. All the same radio astronomy led to the discovery of a large number of galaxies, and later in 1963 and objects of a completely new nature - quasars, distant super-bright objects that existed at the dawn of the Universe. It became clear that the Universe is not static, but somehow evolves. So cosmology arose.
The main task of cosmology was to find out what scenario the Universe is developing. According to the most popular model, bearing the name of Friedman, there are three variants of the Universe: flat, open and closed. Which of them corresponds to our Universe depends on the parameters in the equations. These parameters should, naturally, be determined from observations, and the main one is the density of matter in the Universe.
The most attractive - mostly from some abstract aesthetic senses - to many cosmologists seemed to be a model of a closed Universe. For this, the density of the substance in it should be about 10 −29 g / cm³. In fact, the estimate of the apparent mass was a hundred times less - 10 −31 g / cm³. This discrepancy could not be ignored.
Combining three problems into one and solution
And it was here that cosmologists came to the rescue of the well-known problems with the rotation curves and the velocity spread of galaxies in clusters. In 1974, Jim Peebles, Jerry Ostrajker and Amos Yahil put everything together and showed that the dark matter hypothesis solves all three problems. The elegance of this conclusion, however, blurred the fact that even taking into account the hidden mass of galaxies, the density of the Universe was still five times lower than that required for the “closed” scenario. However, this was no longer a hundred times, and astrophysicists learned to put up with a coefficient less than 10 long before that - yet measurements in this field of science were always not too accurate.
In addition, another group of scientists came at the same time - the Estonian group from the Tartu Observatory consisting of Jaan Einasto, Ants Kaasik and Enn Saar. Their article was published in Nature magazine at the same time as the article by the Americans, although they sent it a few weeks earlier.
Be that as it may, it was precisely cosmologists and their young and rapidly developing field of science that ultimately became the “glue” that combined the scattered and not very convincing evidence into a single and rather well-proportioned theory, based on only one hypothesis - the hypothesis dark matter.
Source: https://habr.com/ru/post/403921/
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