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Astrophysics

Lights from the past

Two projects register the temperature variations that show the Universe 13 billion years ago

ACE / BEASTIn February, two projects, made public almost simultaneously, made a little clearer the origin and the future of the Universe, which can now be stated to have an age very close to 13.7 billion years and a defined fate: to expand forever. On February 11, the American space agency NASA presented the data from its satellite called the Wilkinson Microwave Anisotropy Probe (WMAP), launched in June 2001 at a cost of US$ 45 million. Before that, though, a group that was working in parallel – made up of researchers for the National Institute for Space Research (Inpe), from the Federal University of Itajubá (Unifei), in the State of Minas Gerais, from three American research institutions and from two Italian universities – managed to broadcast their own conclusions, which since the 3rd are to be found at astro-ph, a space on the Internet to which scientists send unpublished results when they do not want to wait to go through the usual channels of a scientific magazine.

While the team in which the Brazilians took part commemorated their pioneering in publishing the results, albeit with a smaller work than Nasa’s, specialists in cosmology have gained two complementary databases, both built up with the same raw material: the so-called cosmic background microwave radiation, a kind of electromagnetic radiation produced in the initial moments of the Universe. The study coordinated by Nasa with another five American universities is more wide-ranging: it recorded the subtle temperature variations that correspond to the background radiation in the whole sky and in five frequencies of microwaves.

The other project, called Advanced Cosmic Explorer (ACE), was concentrated on an area equivalent to 4% of the celestial sphere, which includes part of our galaxy, the Milky Way. It has been analyzed for two years in only two frequency bands by means of the Background Emission Anisotropy Scanning Telescope (Beast) radiotelescope. Beast, though, can show a keener sight in the regions of the sky and at the frequencies that it observed, in which its image resolution is some 50% better than that of the Nasa satellite.

Each one in its own way – the satellite 1.6 million kilometers away from the Earth, and the radiotelescope at 4,000 meters in altitude, on the top of a mountain in the west of the United States -, the two apparatuses measured the variations in the temperature of the cosmic background radiation, with oscillations of millionths around the average value, which is 2.73 kelvin, a little above absolute zero, minus 273°C or 0 kelvin. This is the way they show how the Universe was 13 billion years ago, shortly after being formed. As in those days the structures of the Universe, like the planets and the galaxies, had not yet been formed, the original architecture was very different from the current one.

It is as if physicists had decided to discover how many fans had gone to a soccer match by examining, the day after the game, the remnants left on the terraces – where there were more soft drink cans, for example, there were probably more people. Now begins the reconstitution of the original scenario, redone following the distribution of the background radiation, emitted 380,000 years after the Big Bang, the explosion that is said to have originated the Universe – in those days, when the temperature was around 3,000°C, only hydrogen and helium atoms, the simplest chemical elements, could have existed.

For astrophysicists, the first conclusions that emerge from the two mappings are almost as valuable as a Christmas present that is arriving a little late. One of the points clarified, above all by the team connected with Nasa, which is at a more advanced stage of analyzing the results obtained, is the very age of the Universe, previously put at between 8 and 20 billion years old. Now, the most certain answer is something around 13.7 billion years, with a margin of error of only 1%, very small when compared with the former 20%. An analysis of the electromagnetic radiation that has traveled 13 billion years at the speed of light to reach the Earth leads to another important conclusion: the first stars began to shine only 200 million years after the Big Bang, much earlier than astrophysicists used to imagine.

A better definition of the composition of the Universe was also made possible. The major part, 73%, consists of what is called dark energy, which no one yet knows what it is; 23% is cold dark matter, equally mysterious; known matter, made of atoms that form molecules, living beings, the planets and the galaxies comes to no more than 4%, a modest share, to the point of being called an impurity.

It was also ascertained that the Universe is expanding at a speed of 71 kilometers per second per megaparsec (one parsec is the equivalent in kilometers to the number 3 followed by 13 zeroes). Put a little more concretely, two points separated by 1 million light-years (one light-year corresponds to 9.5 trillion kilometers) are moving away from each other at a speed of 21.8 kilometers a second.

The very fate of the Universe now seems certain: to expand forever, although the possibility has not been definitively discarded that it may contract in the future. The prospect of continuous expansion suggests some shocking scenarios, for which physicists have already done some calculations. In 1040 (the number 1 followed by 40 zeroes) years, matter (planets and galaxies) should disintegrate, like ice turning into gas, and in 10100 even the black holes should evaporate, resulting in a cosmic soup – not scalding, as in the Big Bang, but frozen.

Seen as a sort of celestial fossil, cosmic background radiation was discovered accidentally in 1965, by a German, naturalized American, Arno Penzias and by an American, Robert Wilson, two physicists who used to work at the Bell Telephone laboratories in the United States – in 1978, this finding earned them the Nobel Prize for Physics. The first map of the fluctuations in background radiation, associated with the original structures of the Universe, only came out in 1992, by means of the Cosmic Background Explorer (Cobe) satellite, launched three years previously by Nasa. “Cobe only recorded temperature variations in large areas of the sky, probably related to the origin of super agglomerates of galaxies”, explains Thyrso Villela Neto, a researcher at Inpe and one of the coordinators of the ACE project. ” We now have a more realistic vision of the beginning of the Universe and of how things were when it was formed.” Today, much smaller structures are known, possibly a starting point for the formation of galaxies.

Detailed vision
It is the advance in the power of definition of telescopes, known as angular resolution, that has made this detailing possible – the capacity of distinguishing between two objects that are close to each other. By opting for a smaller map, but one with as much detail as possible, the team that worked with the Beast telescope made angular resolution one of its differentials in relation to the Nasa satellite. “To improve the angular resolution of one telescope in relation to another that is observing in the same range of frequencies, the main thing to be done is to increase the diameter of its mirror or the surface that collects the radiation”, Villela explains.

With a mirror for capturing radiation that is 2.2 meters in diameter, Beast distinguishes two objects at an angular distance of 23 minutes of an arc (one minute of an arc corresponds, in geometrical terms, to 1/60 of a degree) in the frequency of 40 gigahertz (hertz is the unit of measurement of frequency, equivalent to one cycle a second). In this same range, the WMAP, with its mirror 1.6 meters in diameter, will only inform that there are two objects and not one if the two are separated by 32 minutes of an arc from each other.

“A radiotelescope works like a sophisticated radio receiver”, observes physicist Newton Figueiredo, from the Federal University of Itajubá. “But instead of looking for a broadcaster, it tunes in to a microwave band.” With the waves that are captured, the researchers build up a map of the sky in the microwave band, representing in yellow or red the regions with a higher temperature, and the colder ones appear in blue. Afterwards, they prepare a graph known as a power spectrum, which shows a continuous line, from which the curves go up and down. The final picture looks like an electrocardiogram, the test that records the electrical currents of the heart.

It was Figueiredo who designed the Beast’s optical system. By putting the focus – the point on which the waves converge – outside the main axis of the mirror, he managed to reduce the interference from the signals that reached the eight microwave receivers which are found in the focal plane of the telescope’s mirror. Set up at the beginning of 2001 on the highest point of White Mountain, on the border between the American states of California and Nevada, Beast scanned the sky for the first time from July to December 2001. Last year, there were two more weeks of observations in February, and a longer spell from August to October.

Filtering signals
There was another Brazilian innovation in the research into background radiation. Since 1998, in a parallel project, researchers from Inpe and the University of California at Berkeley, in the United States, have been observing the sky of the Southern Hemisphere, which includes the Milky Way, one of the main sources of contamination of the measurements of background radiation, using a radiotelescope which is today at Inpe, in Cachoeira Paulista, after operating in the United States, in the Canary Isles, in Antarctica and in Colombia. Camilo Tello, a researcher from Inpe, managed to separate the signals from the sky from the undesirable ones, like the radiation emitted by the Earth itself or by radio and television transmitters. The mathematical model that he made has applications in other areas, like mobile telephony, to the extent that it can improve the coverage of the antennas that transmit the signals.

Beast is a kind of prototype of the Planck satellite, which the European Space Agency (ESA) intends to launch in 2007. Its mission will be to look for even more detailed information on the Universe. “In the next decade, we will have several maps to compare”, explains Thyrso. With the new surveys, it may perhaps be possible to find out, for example, if the Universe is finite or infinite. “If it is infinite, it is going to expand for ever”, says Figueiredo. “If it is finite, one day it’s going to contract.”

The Project
Cosmic Microwave Background Radiation and the Formation of Structures in the Universe (nº 00/06770-2); Modality Thematic project; Coordinator Reuven Opher – IAG/USP; Investment R$ 2,342,292.29

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