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astrophysics

The dark side of the Universe

Physicists select the particles that are candidates for making up the matter that can still not be detected

S. COLOMBI/ IAPFrontal view: the deviation of the light makes the galaxies appear elongated when observed in the direction of the longest axis of the cobblestone S. COLOMBI/ IAP

Simpzillas and wimpzillas. Nothing to do with Godzilla, the prehistoric Japanese monster in the shape of a dragon that has been terrorizing the population of Tokyo for almost 50 years, at least in the cinema. Simpzillas and wimpzillas are two families of special subatomic particles that in the last few years have been spurring the imagination amongst physicists. Not without reason: they could explain the current structure of the Universe. They have not yet been found, but, should they really exist, they ought to solve an old problem, posed in 1933 by Bulgarian astrophysicist Fritz Zwicky.

Specialized in the study of sets of galaxies, Zwicky stated that ordinary matter alone (made up of protons, neutrons and electrons) was not enough to explain how galaxies could remain united just by gravity, the only force capable of acting in such vast spaces. This sort of cohesion would only be possible if there existed one hundred times more matter than it was possible to observe. Zwicky called what could not be seen the dark matter, as it neither absorbed nor emitted light.

Hypothetical particles, the simpzillas and the wimpzillas may be precisely the so much sought after components of dark matter, responsible for 85% of the mass of the Universe – six times the quantity of ordinary matter concentrated in planets, stars and clouds of gas. Although the existence of these exotic particles has not been detected, headway has now been made in the selection of hypotheses that may explain what dark matter is made of. In a study published on June 6 in the Physical Review Letters, one of the most important scientific magazines of the area, Brazilian physicist Ivone Freire da Mota e Albuquerque, currently at the University of California, in Berkeley, in the United States, practically rules out the possibility of the very existence of simpzillas, at least the way one used to imagine, since up until now they have not been recorded by today’s neutrino telescopes and particle detectors.

This means that the wimpzillas come out of this strengthened, even though it is not yet known how they can be recognized. The prospect enhances the construction of new observatories, like the IceCube, which the Americans should be inaugurating in two years time in Antarctica, and the Pierre Auger, a multinational project in the final stage of construction in Argentina, with the participation of research groups from São Paulo, Rio de Janeiro and Bahia. “A real possibility exists of our detecting the particles that form dark matter”, claims physicist Brian Fick, from the University of Utah, United States, who for ten years has been accompanying the advance of the Auger.

Quite apart from proofs, there are indications that dark matter really does exist. Concentrated in a sort of sphere around the galaxies, it could act as a sort of glue that keeps the Universe united. Two years ago, French physicists, using information collected by a telescope in Hawaii, drew up a three-dimensional map of a stretch of the Universe that shows the deviation in the trajectory of light caused by the dark matter, presented as a net that holds the galaxies up. More difficult is still solving the problem posed 70 years ago: what is dark matter made of?

As in an election campaign, there are several candidate particles. Very popular amongst physicists, the simpzillas and the wimpzillas differ only in the way how they interact with ordinary matter. The simpzillas interact strongly with ordinary matter, as its very name indicates, an abbreviation for Strongly Interacting Massive Particle. With the wimpzillas, which stands for Weakly Interacting Very-Massive Particle, the opposite happens. The zilla bit indicates only that they have a very high mass, up to 100 billion times greater than the mass of a proton, which is 1 giga electron volt or 1 GeV originally used as a unit of energy, the electron volt also indicates the mass of particles, obeying the equivalence between mass and energy put forward by Einstein in the famous formula (E=mc2), which established that energy (E) corresponds to the mass of the particle (m) multiplied by the speed of light (c) raised to the power two.

Neutralinos in vogue
Invisible to telescopes for not emitting light, the two zillas could only be identified in a direct manner when, crossing through the Earth, they collide with particle detectors installed in laboratories hundreds of meters below the surface. Or, in an indirect fashion, by the recognition of another kind of high-energy particles, the neutrinos, emitted when the simpzillas collide with each other and annihilate themselves in the center of the Sun, the same happening with the wimpzillas. As for the time being it is only possible to observe the candidates for dark matter when they interact with ordinary matter, in theory at least, it should be easier to detect simpzillas than wimpzillas.

In the article in the Physical Review Letters, written in partnership with Laura Baudis, from Stanford University, in the United States, Ivone analyzed the characteristics of the two main observatories of component particles of dark matter now functioning: Edelweiss (Expérience pour Detecter les Wimps en Site Souterrain, or Experiment for Detecting Wimps at an Underground Site), equipped with detectors of the very purest germanium installed at a depth of 1,600 meters in the French Alps, and the CDMS (Cryogenic Dark Matter Search), with detectors of germanium and silicon, set up at a depth of 12 meters underneath the campus of Stanford University.

Designed to detect a third kind of particle that is a candidate for making up dark matter the neutralino, similar to the wimpzilla in that it interacts little with ordinary matter, but with a mass millions of times less, these two laboratories could also easily observe simpzillas in their supposedly most natural form, with a mass of 1,000 GeV. After comparing the recent data captured by the equipment with the theoretical forecasts for simpzillas, Laura and Ivone concluded that simpzillas should not exist. Not, at least, with the predicted mass. “Should they exist with this mass, they would already have been detected”, Ivone explains. But this line of thought does not rule out completely the chances of simpzillas existing. It may be that they have a mass at least a thousand times greater and would only be captured by two other observatories still under construction, the Pierre Auger, in Argentina, and the IceCube, in Antarctica, which would have better conditions for filtering high-energy particles.

Even so, the simpzillas chances seem slight. This comes out to strengthen the models that predict a dark matter made up of particles that interact little with ordinary matter, such as wimpzillas and their likes with less mass, the neutralinos, also called wimps (weakly interacting massive particles). It is the neutralinos that nowadays respond better to the requirements imposed by computer simulations and by the model most accepted to day for explaining the origins of the cosmos, the Big Bang, the gigantic explosion that is said to have occurred 13.7 billion years ago and originated the Universe. In theory, neutralinos are sufficiently stable particles to the point of having existed ever since the Big Bang. For moving at speeds lower than the speed of light, they may be capable of interacting amongst each other and agglomerating, generating sufficient gravitational force to unite ordinary matter into galaxies.

Well rated by the physicists, the neutralino hypothesis explains well the structure of the agglomerates with thousands of galaxies, but not very well the isolated galaxies. It was to be expected that the concentration of mass both ordinary matter and dark matter would increase progressively from the edge towards the center in spiral galaxies like the Milky Way, until it reaches its maximum value at the nucleus, where the density of matter is provenly greater. Observations show that, from a given point, the concentration of dark matter in these galaxies becomes constant, in an indication that the theoretical model for neutralinos may perhaps need some adjustments. The proposed alternatives there are at least another four do not resolve this impasse either.

It was precisely because theory still did not manage to agree with reality that Fritz. Zwicky launched the idea of dark matter. From then until now, there has been more consensus on the composition of the cosmos. Now there is even talk of several kinds of dark matter, which, according to the preferences of the physicists, may be cold, made up of zillas and neutralinos, which would travel relatively slowly; hot, with particles so quick that they would never join up into galaxies; repelling; and another kind, which interacts strongly with its own, or even of a category that annihilates itself emitting radiation. Take your choice.

“Without dark matter, the Universe would have remained too uniform to permit the formation of galaxies, stars and planets”, is the comment made by physicists Jeremiah Ostriker and Paul Steinhardt, both from Princeton University, United States, in an article in the Science magazine of June 20. The majority of physicists are betting like this: if the Big Bang model really is correct and the Universe is expanding, about 27% of the cosmos is made up of dark matter and 70% of an also unknown form of energy, dark energy. Ordinary matter, which forms all that we know, would add up to the remaining 3%.

Correcting Newton
But there are physicists a limited group, it is true who doubt the existence of dark matter. As an alternative, they put forward a way out that sound like heresy when it as presented, 20 years ago, by Israeli physicist Mordehai Milgrom, today with the Weizmann Institute, in Israel. According to him, there was nothing wrong with the mass of rotating galaxies. The error lay where few would dare to point to: in the formula for the force of attraction between bodies, the Universal Law of Gravitation, deduced in 1665 by the English physicist Isaac Newton.

The speed of the clouds of gas of a galaxy, which spin around an imaginary axis, diminishes in accordance with the distance from the center, according to the law of gravity. But Milgrom found that this speed became constant after a certain distance and there is a detail that is inconsistent with Newton’s Law of Gravity. “I analyzed the properties of the galaxies and looked for those that showed great differences in relation to what is to be seen in the Solar System, where Newton’s Law of Gravity recognizedly works well”, Milgrom commented to Pesquisa FAPESP. “I noticed that in galactic systems acceleration is much lower than what is seen in the Solar System or on Earth. The gravitational attraction at the extremities of the Milky Way is 100 billion times less than that of a free-falling body on Earth.” The conclusion that he arrived at is that Newton?s law worked very well in regions of space with very high acceleration, but not in regions where acceleration is small, like the agglomerations of galaxies.

The alternative was to alter Newton’s law, rebaptized as Modified Newtonian Dynamics, or Mond, which up until now has explained in a satisfactory way the observations of spiral galaxies made by satellites like Chandra. But it is also not perfect and balks at the moment for analyzing the agglomerations containing thousands of galaxies. “The results of applying Mond to the agglomerates indicate that it attenuates the problem of dark matter, but it does not resolve it”, comments physicist Reuven Opher, from the University of São Paulo (USP), who uses this approach in studying agglomerations of galaxies. “The difference between using Mond and Newtonian gravitation is that the quantity of dark matter that one infers to exist is less in the first case.” While Milgrom tries to perfect his model, the physicists who believe in dark matter are planning tests to reveal finally the nature of this enigmatic portion of the Universe. In the article in Science, Ostriker and Steinhardt stimulate the quest for varied alternatives: sometimes, important clues appear where one least expects.

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