The dark matter puzzle

In the last decades it became clear that more than 90 percent of the mass of the universe is dark. That is, invisible and therefore not directly discoverable. How can an invisible mass be discovered and what does it consist of? And how does the discovery of the dark mass affect the understanding of the structure of the universe and its development?

Author: Haim Shmueli
From: Galileo 21, March/April 1997

In the last decades it became clear that more than 90 percent of the mass of the universe is dark. That is, invisible and therefore not directly discoverable. How can an invisible mass be discovered and what does it consist of? And how does the discovery of the dark mass affect the understanding of the structure of the universe and its development?

At the beginning of the 20th century, astrophysics took a giant leap forward in understanding the picture of the universe: it turned out that the Milky Way is but one galaxy out of many billions, and this illustrated how big the universe is compared to what was previously thought; The discovery of the redshift of the light coming from the galaxies by Edwin Hubble (Hubble), in 1929, led to the understanding that the universe is expanding and it became clear that it had a beginning, a point of creation called the "Big Bang". And so a few decades ago it seemed to astronomers and astrophysicists that the structure of the universe was deciphered almost in its entirety, and that its development was accurately described in the "big bang theory". It seemed that perfecting the means of observation and extending them to additional fields of radiation beyond the visible field, such as radio radiation or X-rays, was enough to map the universe in its entirety. However, already at that time, question marks were created around that complete and supposedly closed description. In 1933, the astrophysicist Fritz Zwicky noticed that the galaxies belonging to the Coma galaxy cluster were moving at greater speeds than expected. When he estimated the masses of the galaxies according to the intensity of the light emitted from them, it became clear to him that their gravity was too small to cause such a rapid movement. He concluded that there is an additional mass in galaxies that he cannot distinguish.

What Zwicky did was actually "weighing" galaxy clusters in two different and independent ways. One way is to estimate directly, from observations, the number of stars in the galaxy, by measuring the intensity of the light received from the galaxy and knowing the intensity of the light of the stars in it. From the knowledge of the characteristic mass of each star, the total mass of the galaxy, or of a cluster of galaxies, is calculated. Another indirect method is based on measuring the angular velocity of different regions in the galaxy, or of entire galaxies within a galaxy cluster. A well-known physical law, the virial law, makes it possible to calculate the total mass of a physical system that is in equilibrium from the knowledge of the characteristic speed of objects within it. The researchers found that in different galaxies, the average velocities of their different regions are much greater than expected based on the apparent mass of the galaxies. How can the contradiction be reconciled? The most logical explanation was to assume that the galaxies are surrounded by a halo of dark, invisible matter, and that what is visible to the eye is only a small part of the true mass of the galaxies. The same argument is also valid for galaxy clusters, and this is how Zwicky showed the apparent contradiction between the observed and calculated mass of galaxy clusters. But most astronomers assumed that the contradiction would disappear with the improvement of the measurement means or after a more accurate calculation of the motion of the galaxies.

Consider galaxies

The issue arose again only in the 21s, when the astrophysicists Roberts and Salpeter, and later Vera Rubin and her colleagues from the Carnegie Institution in Washington, showed that the problem exists not only in galaxy clusters, but also within the galaxies themselves: the stars in the outer regions of spiral galaxies move around the centers of galaxies at a speed similar to that of stars close to the center, in clear contradiction to Kepler's laws. While Rubin measured the rotation curves of galaxies in the visible range, Roberts and Salpeter's discovery was made through the use of radio telescopes, dish-like antennas designed to receive electromagnetic radiation in the long wavelength range (radio waves). The main importance of radio astronomy is its ability to discover hydrogen, which is the most common element in nature - about seventy percent of the mass of the universe. The astronomical "identity card" of hydrogen is radiation with a wavelength of XNUMX cm that the hydrogen atom emits when it goes from an excited state to its ground state. Using this radiation, it became clear to the researchers that spiral galaxies rich in hydrogen clouds inhabit a large disk that rotates around the center of the galaxy. The hydrogen disk sometimes extends a great distance beyond the edges of the galaxy observed in the visible light field, up to three and four times the visible radius. Are the hydrogen clouds - are they the mysterious dark matter that causes the stars at the edge of the galaxy to move at such high speeds? Surprisingly, the answer to that is negative. Despite the huge size of the hydrogen disc, it is so thin that despite its enormous volume, its mass is not greater than the mass in the visible light range and both together make up no more than a fifth of the total required mass.

Macho vs. Wimp

What then could that mysterious dark matter be? For many astronomers, the answer is simple and clear: this is the normal material from which the stars are made, consisting of atoms such as hydrogen, helium, carbon, and iron, this material does not participate in nuclear fusion processes, and therefore does not emit light. Such dark bodies can have almost any mass, from asteroids that are only a million billion times less than the mass of our Sun to black holes that are only millions of times the mass of the Sun. Between these two extremes there is another whole basket of possibilities: small Earth-like planets, large planets like Jupiter, brown dwarfs - stars whose mass ranges from 8 percent to XNUMX percent of the Sun's mass (and whose temperature is too low to "turn on" the thermonuclear fusion reaction which exists in suns and is the source of their energy), white dwarfs - which are actually suns that have consumed their nuclear fuel and cooled down and neutron stars.

All these possibilities are classified in one category, called by the astronomers in the abbreviation MACHO, the initials of "compact massive bodies in the halo (of the galaxy)". Against it, an alternative approach was proposed by physicists from the field of particles known by them as WIMP, which stands for "elementary massive particles, reacting with a weak interaction". According to this concept, the universe is full of particles whose identity is not yet clear, whose mass is small and whose interaction with normal matter is extremely tiny, since they react with only a weak nuclear interaction, and not with electromagnetic interaction or gravity. But they are found in such a huge amount in the universe that their overall effect is evident and in fact they constitute the bulk of the matter in the universe. The trouble is, of course, that no one is at all clear which particle it is exactly.

One popular candidate for the role of the "wimp" is the neutrino (in Italian, "the little neutral"), if only for the simple reason that it is known to exist... The existence of the neutrino was predicted by the Austrian physicist Wolfgang Pauli some 25 years before it was actually discovered by Frederick Rains (Reines) in 1956, a discovery for which he was awarded the Nobel Prize in Physics last year. The neutrino is a particularly elusive particle. It lacks an electric charge and therefore does not respond to an electromagnetic force; It does not respond either to the "color" force acting between quarks, or to the strong nuclear force acting between nucleons (an inclusive name for protons and neutrons), and thus it is able to pass through the entire earth or even the entire sun as if they were completely transparent, just like a ghost. It is still not known for sure whether the neutrino has a small mass, or is completely massless. But even if it has a mass, it is clear that at most it is a very tiny mass, a mass smaller than what can be measured with the existing technology. However, according to the accepted theory, the universe is filled with enormous amounts of neutrinos, several hundred neutrinos in every cubic centimeter! Therefore, if the neutrino has any mass, even the smallest, it means that it constitutes the missing mass component in the universe, or at any rate - a significant part of it.

The hypothetical particles of the union of forces

But this is only one possibility. The particle theorists "prepare" news for hypothetical particle controllers, which may also play the role of the dark mass of the universe. For example, in 1983 the existence of an electrically neutral particle called "Axion" (named, by the way, after... laundry bleach) was proposed to solve certain problems related to the "color" force. The axion, if it does exist, should be at least a million times lighter than the electron - the lightest particle known, but since each cubic centimeter of the universe may contain thousands or even millions of axions, this is enough for its contribution to the total mass of the universe to be decisive. Other particles "emerge" in different equations as part of physicists' attempts to unite the four fundamental forces in nature in one unified theory, a field that has become very popular in recent years. According to one theory, every particle in nature has a corresponding "super-symmetric" particle. For example, the particles Zino and Wino correspond to the particles W and Z (the particles that carry and transmit the weak force). A photon corresponds to a supersymmetric particle in the Photino sphere. Squarks are suitable for quarks. Physicists take these ideas seriously. Several groups in different parts of the world are trying to build special devices to detect these exotic particles, and others are looking for them due to the powerful collisions between particles in the large particle accelerators. There are also some who believe that they have already discovered signs of these particles, but there is still no solid basis for this.

The best way to discover those "wimps" is the same way that neutrino particles are detected, by giant detectors sunk in mines deep below the surface of the earth. Even particles that hardly react with matter (since they react, as mentioned, with only a weak interaction), rarely hit the nucleus of an atom and splash back. But since the probability of this is astonishingly small, one must use a target as large as possible, that is, a target that has a huge number of atoms and isolate the experimental system from background noise in the form of electromagnetic radiation of the gamma type or cosmic radiation of charged particles (protons, mainly), whose intensity is greater than Millions from the "vimp". For this purpose, the detectors are "hidden" deep under the surface of the ground, where the effect of cosmic radiation or any other disturbance is very small. Such detectors were indeed built in the past with a significant financial investment to measure neutrinos impact (measurements that are done routinely) and to check whether the proton decays (a measurement that has not yet produced positive results) and they are also suitable for this type of experiments.

Macho through the lens

In 1986, the astronomer Bogdan Paczynski from Princeton proposed a sophisticated way to check whether the dark matter consists of "macho", meaning a halo of normal cold matter around the galaxy. According to his calculations, if this is indeed the case, then the gravity of this dark matter should affect the light coming from the stars located in neighboring galaxies to the Milky Way. Take for example the light coming from a star in the Large Magellanic Cloud, a galaxy close to the Milky Way. Normally, its light is scattered and only a small part of it reaches us and is visible in the observation through the telescope. However, in certain situations the dark matter of the halo affects the light coming from the star, and since according to the theory of general relativity gravity also acts on light, the star's light will be deflected by the halo and will be slightly concentrated in a certain direction. The "macho" acts like a focused lens, so viewers on Earth will (rarely, admittedly) see the star brighter than usual.

In principle, Pachinsky's idea makes it possible to examine whether the dark mass originates from a "macho" or a "wimp". However, it is clear that, in a practical way, detecting tiny changes in the light intensity of a star is extremely difficult to do, and of course, you need to know what the typical cycle time of these changes is and whether there are no other mechanisms that can cause their creation. For example, one must be careful not to base observations on variable stars, cupids, whose light intensity changes due to dynamic changes within the stars themselves, or on double stars, only one of which is visible and when it is hidden by its partner, the amount of light from it decreases, as well Periodically. So far, four groups of astronomers have reported measuring results of the type Paczynski predicted, and although these results do not yet have any conclusive proof, initial estimates show that some type of "macho" may be at least part of the dark mass of the universe, as far as visible in the form of brown dwarfs.

Open or closed universe?

While the astronomers and the particle scientists "struggle" for the identification of the dark mass, the cosmologists also entered the picture. Knowing the total mass of the universe is very important for understanding its evolution. According to the big bang theory, the expansion of the universe slowed down in the past and continues to be slowed down all the time due to the force of gravity acting between all the components of matter and energy in the world. The slowdown works in the opposite direction to the expansion trend. Thus the degree to which the expansion slows down is directly dependent on the total amount of mass and energy in it.

There are three possible scenarios for the further development of the universe. One, that the total amount of mass and energy in the universe is greater than a certain critical value (easy to calculate), and therefore the expansion of the universe will gradually slow down until it stops completely and finally a re-collapse of all matter and energy will take place back to the initial state ("the big collapse", in analogy to the big bang). The second, that the total amount of mass and energy in the universe is less than the aforementioned critical value, and therefore the expansion of the universe will indeed slow down but will never stop. The universe will continue to expand forever, exhausting all the thermonuclear reactions in the hearts of the stars, until they finally reach a stage where they can no longer produce energy, and reach a state called "thermodynamic death". The first scenario describes a closed universe in terms of both space and time (although unlike the second scenario it can be assumed that it may repeat itself over and over again). The second scenario describes an open universe, meaning infinite in terms of space and time that had a definite beginning, but would have no definite end. And finally the third scenario, if the total amount of mass and energy in the universe exactly matches the critical value, the expansion of the universe will gradually slow down more and more, and tend to zero as time goes by. Although the expansion will never stop, in practice the size of the universe will converge to some finite value, without collapsing back and without expanding to infinity.

What is the true state of the universe? Which scenario is "chosen" and actually takes place? To answer this, you need to estimate the total mass of the universe, or more precisely the mass density in those regions that can be observed. And it is true that various attempts were made to give an answer to this, and in all of them it turned out that the average mass density of the measurable universe is considerably smaller, perhaps a hundred times, than the critical mass density. If the measured value was greater than the critical value, this would be a decisive answer: it would mean that the universe would have collapsed in many days. But since the measured density is less than the critical value and dark mass has not yet been observed, there is no way to know for sure if indeed the universe will continue to expand forever.

However, cosmologists feel a great discomfort from the fact that the visible mass of the universe is "only" a hundred times smaller than the critical value (and maybe even only ten times, according to the prevailing estimates of the dark mass). Some of them believe that this is no accident. After all, the mass could have been a billion times smaller than the critical mass, or a trillion times larger than it. There is no apparent reason, they argue, that the result would be so close (in astronomical terms) to the critical value. In their opinion, this is a hint that the total amount of mass and energy in the universe is equal - must be equal - to the critical size, and there is also a cosmological reason that dictates such a "critical" universe.

The inflationary universe

The main support for such a scenario comes from a relatively new cosmological model, called the "inflationary model", which was proposed in 1981 by the American physicist Alan Guth. According to his model, which has since been perfected by other physicists, shortly after the creation of the universe there was a phase of exponential (exponential) inflation, in which the dimensions of the universe swelled at an enormous rate, hence the name. The matter in the universe, the amount of which is much greater than it seemed to be until then, according to the various cosmological models, was of course not created from nothing, but "at the expense" of potential energy (as we know, according to the theory of general relativity, matter and energy are equivalent to each other). The inflationary model came to answer various difficulties that arose regarding the big bang theory, which are not directly related to the topic of the article, but in retrospect it also explains how the creation of such an enormous amount of mass was possible, which cannot be explained within the framework of the usual big bang theory.

In the meantime, the degree of validity of the inflationary model is disputed, although it seems that the measurement results of the COBE satellite - which measured the non-uniformity in the cosmic background radiation, support this model. One thing is clear: no one yet has any idea where it is possible to "locate" a mass ten to a hundred times greater than the mass of the visible universe. These big, "cosmological" questions are still waiting for a scientist of stature, who will come up with a brilliant idea or propose new means of measurement, which are not yet in our possession.

However, if the hypothesis regarding the existence of dark matter is indeed correct, it constitutes an additional reinforcement of the Copernican hypothesis, which refers to man's place in the universe. Copernicus showed humanity that the earth, meaning man, is not at the center of the universe. Edwin Hubble showed at the beginning of the twentieth century that the sun is not in the center of the Milky Way and later - that the Milky Way is but one galaxy among a hundred billion or more other galaxies similar to it. The theory of the inflationary universe teaches us that perhaps the entire visible universe is nothing more than a tiny part of the entire universe, and if dark matter indeed exists in such a large amount, it is possible that all the normal matter from which we are made and all for further reading: the stars and galaxies visible to the eye are nothing more than a tiny part of the components of the entire universe.

"The Enigma of the Disappearing Mass", Avishai Dekal, "Mada" 6-2601979, p. XNUMX)
"The Dark Matter", Zvi Peltiel, "The Matter of the Atom" 3-51989, p. XNUMX)

* This article is the thousandth article inserted on the new website, 6/2/2007