HUBBLE SPACE TELESCOPE/ NASAUniversal reference: supernova 1987A (in pink), the type of star used to calculate the rate of growth of the CosmosHUBBLE SPACE TELESCOPE/ NASA
Whoever looks up to the starry sky rarely imagines that the billions of stars and galaxies, including those that cannot be seen, would be moving away from each other at faster and faster speeds. They simply appear condemned to remain where they are because of the tremendous distance that separates them. It was not easy to prove the contrary. Only at the beginning of the last century did the American astronomer Edwin Hubble prove that other galaxies had been distancing themselves from the Milky Way, where we are located. For some six years now other astronomers adjusted this vision and confirmed that the galaxies had been distancing themselves at growing velocities. This was a clear signal that the whole Universe is expanding more and more rapidly, like a cake rising with excess baking powder. It had been believed that this accelerated expansion could last for ever. But a group of Brazilian astronomers are now proposing a scenario in which the destiny of the Universe could be very different.
If the forecasts of a group from São Paulo, Rio de Janeiro and Rio Grande de Sul were to be correct, the current phase of accelerated expansion, initiated some 7 billion years ago, should only last another 6.5 billion years. “The Cosmos will continue expanding itself indefinitely, but in a decelerated mode”, says José Ademir Sales de Lima, a professor at the University of São Paulo (USP) and one of the authors of a mathematical model presented on the 25th of August in the magazine, Physical Review Letters. This study had the participation of Jailson Alcaniz and Fábio Carvalho, from the National Observatory, and Raimundo Silva Júnior, from the State University of Rio Grande do Norte.
A possible alteration in the routes of the Universe will not change in any way the day to day situation of the majority of people for one simple reason: life on Earth and of the very planet itself should disappear long before that, in 5 billion years when the Sun explodes. But the new theory tranquilizes physicists and astronomers concerned with the prospect of a Universe in eternally accelerated expansion. According to the Theory of General Relativity, formulated by Albert Einstein, in 15 billion years a frontier on the extreme edge of the Universe could spring up starting from which nothing can be observed. Although the Cosmos is infinite, light originated beyond this frontier – a type of gigantic bubble involving billions and billions of galaxies – would never reach the solar system. The brightness of a star beyond this theoretical limit of the Universe would take infinite time to reach the Earth, since the distance would always increase with growing velocity whilst the light would continue traveling at a constant 300,000 kilometers per second.
“During this stage, the Universe would go on to behave as a black hole in reverse”, says Lima. This bubble would impede the entrance of whatever matter coming from outside, whereas a black hole absorbs all of the material in its vicinity, even light.
The difficulty of predicting phenomena beyond this frontier had especially perturbed the String Theory physicists, who are looking to unite the fundamental forces of nature in order to explain all of the physical phenomena, from the behavior of atomic particles to the formation of galaxies. “This limit would impede the reconstruction of a complete history of the Cosmos”, comments Lima, an astronomer from Rio Grande do Norte State who for 15 years has dedicated himself to cosmology at the Federal University of Rio Grande do Norte before being transferred to USP in 2003.
The problem with accelerated expansion, clearly, is not in the Universe, but in the theoretical models that describes it. According to one of the most accepted possibilities, the Universe would be in accelerated expansion in response to a repulsive force associated to an unknown form of energy, known as black energy. Corresponding to 70% of the energy of the Cosmos, black energy counterbalances gravity, a force that is essentially attractive. In an era in which the acceleration of the Universe had not yet been spoken about, Einstein adapted the Relativity equations, adding in a fixed value called the cosmological constant, so that the Universe would remain static and not enter into collapse due to the action of gravity. Later the physicists commemorated the treatment of the cosmology constant as if it were black energy itself.
The fifth element
Lima, Silva, Alcaniz and Carvalho are suggesting that the origin of the force that makes the galaxies more away from each other at a faster and faster rate could well be another: a substance that the physicist called the fifth essence, in an allusion to the imponderable element that the Greek philosophers believed to be one of the essential components of the Cosmos, as well as Earth, Wind, Fire and Water. The physicists also call it the primordial scaling field. “The physical properties of the scaling field are not homogenous and vary as a function of time, different from those of the cosmology constant”, says Alcaniz, “and, as a consequence, the scaling field could decelerate the Universe”.
The Brazilian researchers arrived at this model of the evolution of the Universe by adding in a term into the equation formulated in 1988 by the physicists Philip Peebles, from Princeton University, and Bharat Ratra, from Kansas State University. Starting from this, they calculated that in 6.5 billion years the Universe must go on to expand infinitely in a decelerated manner. This will not be the first time. Since the idea of the Big Bang, the hypothetical explosion that gave origin to the Cosmos, the Universe has altered between periods of accelerated expansion and others of decelerated expansion. The work has not ended. “Now we have to prove that the model is correct and that there will in fact be a deceleration”, says Lima, who, with his team, is analyzing data from the explosion of supernova stars in search of evidence that will confirm their predictions. Parallel to this, the publication of the article in Physical Review Letters will allow other physicists to appreciate or to criticize this new proposal about the Universe’s future.
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