XMM-Newton/ESA/CosmosCosmic watercolor: each circle represents a cluster of galaxies. The closest ones are shown in blue and those farthest away, in redXMM-Newton/ESA/Cosmos
The European Space Agency (ESA) released in late January an image showing the concentration of galaxies at different distances in a small region of the Universe. Each colored point of the image corresponds to a group of hundreds to thousands of galaxies – each of them formed by hundreds of billions of stars and a high volume of very hot gas. These are what astronomers call clusters of galaxies, the largest structures ever identified in the cosmos in terms of their size and mass, that are in equilibrium. Calculating the number of celestial bodies that might exist there, it is hard to imagine that they only help to make up a meager 4.6% of everything within the Universe. The rest, which is actually almost everything, cannot be seen. The other 95.6%, according to the vast majority of physicists and astronomers, consists of two types of elements that were only discovered in the last 80 years: dark matter and dark energy, about which we know almost nothing, except that they need to exist for the Universe to be the way we imagine it is. They are the target of a series of international experiments where there are some Brazilians involved, among other nationalities. Some of these experiments are already under way, whereas others are due to begin in forthcoming years. This type of matter and of energy neither absorbs nor emits light and is therefore invisible to the human eye.
No equipment in use to date has been able to detect them directly. However, physicists foresee the existence of both in their models of the evolution of the cosmos, while astronomers infer their presence from the signatures they leave in the structure of the Universe, identifiable in images such as this one from ESA, which resulted from its Cosmic Evolution Survey (Cosmos). This project uses the largest telescopes on earth and in space to comb a region of the sky that is the size of eight full moons.
It was only in the last century that man’s understanding of the Universe became so complicated. In the 1920’s, the American astronomer Edwin Hubble realized that the cosmos consisted of large clusters of stars (the galaxies) that were pulling away from each other. The finding that the Universe was expanding led physicists and astronomers to review their ideas, because until then it was generally believed that the Universe was static and finite.
Studying the galaxies, the Bulgarian astronomer Fritz Zwicky, considered by many to have been an ill-humored man, noted in 1933 that they would need some 10 times more mass than they had to form clusters just through the force of gravitation, a notion put forth by Isaac Newton to explain the attraction between bodies with high mass over very large distances, such as planets and stars. This invisible mass was dubbed dark matter. Dark energy, however, was only proposed some 70 years later, when the groups of Adam Riess and Saul Perlmutter, who were investigating supernovas (stars that explode and consequently shine millions of times more strongly), were pulling away from us increasingly fast. The Universe was not only expanding, but doing so at a high speed. Something unknown, a sort of force contrary to gravitation (later named dark energy) was causing the cosmos to grow at increasingly greater speeds, as if it were a rubber sheet being pulled at the corners.
HST/CXC/NasaCollision of titans: the clash of two clusters of galaxies separated common matter (pink) from dark matter (blue)HST/CXC/Nasa
Few scientists now doubt the existence of dark matter and of dark energy, also known as the dark component of the Universe. The main challenge (many researchers see this as one of the most important issues to be solved) is to determine the nature of both, i.e., what they are made of.
Concerning this point, physicists and astronomers do not know anything for sure. At most, they make good guesses. And, like the other inhabitants of the planet, they will have to remain in the dark until an avalanche of data about more galaxy clusters and other structures in the Universe that are older than these observed at present start feeding their computers.
“Our ignorance has never been quantified with such precision”, comments the astronomer Laerte Sodré Júnior, from the University of São Paulo (USP), in reference to the most widely accepted calculations about the amount of dark matter and dark energy in the cosmos: 22.6% and 72.8% respectively. For almost 30 years, Sodré has been studying galaxy clusters, which bring together about 10% of the existing galaxies. These clusters can be seen as cosmic metropolises: like metropolises on Earth, they are few but huge and they are heavily populated.
Based on information about galaxy clusters and other very ancient and distant celestial bodies, the theoretical physicists Élcio Abdalla, Luis Raul Abramo and Sandro Micheletti, from USP’s Physics Institute, in collaboration with the Chinese physicist Bin Wang, recently decided to check whether or not the data about these astronomic observations confirmed an idea put forth years ago by another Brazilian, the physicist Orfeu Bertolami, a researcher from Lisbon’s Technical Superior Institute (Instituto Superior Técnico – IST) in Portugal. Almost a decade ago, soon after the early evidence of dark energy was found, Bertolami proposed that, if dark matter and dark energy interact, as suggested by the Italian astronomer Luca Amendola, then this mutual influence should leave some traces in very large structures of the cosmos, such as the galaxy clusters.
Particles
For those who are not used to the idea, it may seem strange that something that is more or less unknown can in some way affect another thing about which nothing at all is known. However, physicists do not think like this. Whatever the nature of dark matter and dark energy, they expect the behavior of both on the scale of infinitely small things (the world of atomic particles) to influence the world of infinitely large things. Therefore, understanding the interaction between them and between one of them and visible matter might help us to understand how and over how long the Universe was formed and became the way it is, enabling, among other things, the existence of life. “If we believe minimally in the standard model of particle physics, which explains the composition of baryonic matter [i.e., common matter, composed of protons, neutrons and electrons, which is the matter that forms the stars, the plants and everything else that we know] and how the particles that form it interact among themselves, then there is no reason to doubt that there may also be some interaction between dark matter and dark energy”, states Abdalla.
At first, Abdalla, Micheletti and Bin Wang, from Fudan University, in Shanghai, developed a rudimentary model in which they described dark matter and dark energy with properties similar to those of liquids and gases such as water and air. Physicists call them fluids, i.e., matter consisting of layers that move continuously in relation to each other and that, in the process of this dislocation, can become reciprocally deformed. The model constructed consists of a series of mathematical equations that attempt to describe what happened in the past and to predict what will happen in the future. It resorted to information obtained over the course of years, through the observation of quasars, which are the very old nuclei of very brilliant galaxies, of supernovas, i.e., the stars that exploded and started emitting light millions of times stronger than normal, and cosmic background microwave radiation, a form of electromagnetic energy produced in the first instants after the Big Bang, the initial explosion that generated the Universe and time itself, some 13.7 billion years ago.
Even without determining how dark matter and dark energy interact, as the researchers merely assumed that there was some interaction, they found that upon solving these equations and those formulated by Einstein in his general relativity theory, a Universe similar to what we know today was obtained, expanding at high speed, with everything in it becoming separated faster and faster, according to an article in June 2009 in Physical Review D. This model indicates that as a result of interaction, dark energy releases radiation and turns into dark matter, a consequence of the famous equation E=m.c2, according to which, under certain circumstances, matter can turn into energy and vice-versa.
Interaction
But all this was not sufficient. Together with physicist Luis Raul Abramo, Abdalla improved the model. This time, he looked for a signature of the interaction between dark energy and dark matter in information about 33 galaxy clusters, 25 of which were studied years ago in detail by Laerte Sodré and Eduardo Cypriano, both of them researchers at USP’s Astronomy, Geophysics and Atmospheric Sciences Institute (IAG). In collaboration with Sodré, Abdalla, Abramo and Wang used three known methods to estimate the quantity of matter (mass) of the galaxy clusters. If there were no interaction, the results would have to be the same or close, at least. On the other hand, if dark matter turned into dark energy or vice-versa, one of these values, sensitive to the conversion, would differ from the others. In the work published in 2009 in Physics Letters B, they stated that there is a real though small possibility that the interaction does indeed occur, with dark energy turning into dark matter.
High-Z Supernova Search Team/HST/NasaAt the end of the Universe: observation of supernovas; stellar explosions such as this one (brilliant spot beneath the galaxy) revealed the accelerated expansion of the cosmosHigh-Z Supernova Search Team/HST/Nasa
The group itself looks upon these results cautiously, because there are various uncertainties in the measuring of the mass of galaxy clusters. Some of these techniques depend on the clusters being in equilibrium and not interacting with other galaxies or clusters. This, however, is unlikely, because the mass of the clusters is very high and attracts everything that lies nearby. “The uncertainty surrounding the measurement of the mass of each cluster is high”, comments Abramo. “This model will have to be tested for quite a few years. We analyzed 33 galaxy clusters, but, to be sure, we would have to assess hundreds or thousands of them”, states Sodré, who is currently negotiating with Spanish physicists and astronomers a Brazilian participation in the project Javalambre Physics of the Accelerating Universe Survey (J-PAS), that aims to understand the properties of dark energy and the evolution of galaxies better, by measuring the distance that they are at more precisely.
Structures of the Universe
Abdalla, who undertook to verify the signals of this interaction a few years ago, knows that many people disagree with his proposal. “Once a rude referee [scientific reviewer] said that this work was speculation squared”, recalls the USP physicist. “However, if we are right and this interaction is clearly defined, it should be verifiable in particle physics experiments.”
About five years ago the Brazilian theoretical physicists Gabriela Camargo Campos and Rogério Rosenfeld, both from IFT-Unesp (the Theoretical Physics Institute of Paulista State University) created a model of the interaction between dark matter and dark energy that also treated them as fluids. In the study, conducted jointly with Luca Amendola, from the Astronomical Laboratory of Rome, who came up with the notion of the interaction between these elements, the Brazilian duo took into account both the information on supernovas and on cosmic background radiation. Once all the calculations had been made, they concluded that this conversion should not occur, according to an article published in Physical Review D in 2007.
However, with the information currently available about the structures of the Universe, it is hard to figure out who is right. “There is little information and it is fragmented”, comments Cypriano, an astronomer at IAG-USP. “We need homogeneous data and in great quantity”. This is why more than a dozen major international projects have gone through the planning stage at least.
The study of the structure and evolution of galaxies has led a substantial number of physicists and astronomers to consider the existence of dark matter a certainty. They also believe that its composition will be discovered shortly, perhaps within one decade. “If it is comprised of cold particles with very high mass, several particle physics models forecast that it could be produced within the Large Hadron Collider [LHC]”, states Abramo. Set up on the French-Swiss border, LHC started running experimentally in late 2009 and is designed to smash atomic particles traveling at a speed close to that of light against each other. This should break them down into their smallest components. As for an answer on the nature of dark energy, it is likely take a lot longer because it depends on broad surveys of the galaxies and stars in different parts of the sky.
MPE/V.SpringelIn 12 billion years: under the influence of dark matter, gas agglomerates to form stars and galaxies (brilliant areas) that start to pull away from each other, driven by dark energyMPE/V.Springel
One of these surveys, due to start in the second half of 2011, is the Dark Energy Survey (DES). Some 30 Brazilians are to take part in this, including researchers, students and technicians from institutions in the states of Rio de Janeiro, São Paulo and Rio Grande do Sul. This project plans to use a digital supercamera, able to take pictures with an extremely high resolution (500 megapixels, 40 times greater than regular cameras can produce), coupled with the Blanco telescope of the Interamerican Observatory of Cerro Tololo, in Chile, to collect, over the course of four years, information about approximately 400 million galaxies. “We want to study the distribution of the mass of the galaxy clusters at different distances”, tells us Luiz Alberto Nicolaci da Costa, an astronomer from the National Observatory (ON) in Rio de Janeiro and coordinator of the Brazilian participation in DES. Depending on the total mass of the Universe and the existence or not of interaction between dark matter and dark energy, there could be a larger or smaller number of these clusters at certain distances.
Repulsion
Even before the experiment begins, Nicolaci knows that it will not provide a definitive answer about the nature of dark energy, i.e., repulsion, a sort of anti-gravity, which causes objects to draw away from each other at increasing speeds in the Universe. “At the beginning of this decade an international group of researchers convened and tried to outline the most suitable experiments, to be carried out in four stages, to try to find out what dark energy is”, explains the National Observatory’s astronomer. The simplest ones have now been completed and DES is stage three. “With DES, we hope to limit the dark energy candidates”, Nicolaci tells us.
One of the most likely candidates is so-called vacuum energy. Vacuums, contrary to how one imagines, are not empty, being very rich in fleeting particles that appear and disappear before they can be detected and that might be providing the antigraviational force that drives celestial bodies apart. Where particle physics is concerned, vacuum force corresponds to the cosmological constant, a term coined by Albert Einstein, which he added to his general relativity equations so that this theory could depict a static Universe. “However, this would be an inelegant solution, because the density of the vacuum energy would have to be 10120 [the number followed by 120 zeros] greater than astronomers observe”, comments Élcio Abdalla.
Nasa/ESA/STSclEinstein’s cross: galaxy at the center of the image causes space to curve and transforms a single quasar into four, proving the effect foreseen by the German physicistNasa/ESA/STScl
Another possibility is that dark energy is a kind of unknown fluid, which astronomers have named the quintessence, in an allusion to the four elements that were believed, in the past, to form the Universe: air, water, fire and earth. Alternatively, perhaps dark energy does not exist and the effects that have been ascribed to it result from the Universe not being homogeneous the way we have imagined. In this case, the Milky Way would be in a region that contains little matter. The astrophysicist Filipe Abdalla, a researcher from University College London who is also Élcio Abdalla’s son, is working on two more advanced experiments that are part of the fourth stage of the search for dark energy. These will only go into operation in a few years: the Euclid satellite and the Square Kilometer Array microwave telescope, to be built in South Africa or Australia. “If it is an error in Einstein’s equations, which explain the gravitational attraction in galaxies well”, commented Filipe during a visit to São Paulo in August 2009, “these experiments will enable us to find out.”
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