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Astrophysics

Mysterious noise in the cosmos

Nasa and Inpe experiment captures a strong microwave signal in the Universe, the origin of which is unexplained

NASA/WMAP SCIENCE TEAMIllustration of what the first stars in the Universe were like: a possible source of noiseNASA/WMAP SCIENCE TEAM

Discovered by chance in 1965 by astronomers Arno Penzias and Robert Wilson when they were working at the Holmdel unit of Bell Labs, in the United States, background cosmic radiation is today considered the best evidence there is that 13.7 billion years ago there was a Big Bang, the primordial explosion from which the Universe originated. The evolution of research in cosmology has practically proved that this weak microwave signal, emitted some 400,000 years after the Big Bang, is a type of perennial echo of the colossal event that gave rise to all existing matter and energy – and landed the research duo the Nobel Prize in Physics in 1978. At the beginning of 2009, precisely in the International Year of Astronomy (see report on page 36), a team of scientists from the North American space agency, Nasa, and from the National Institute of Space Research (Inpe), from São José dos Campos, recorded (also accidentally) a new and loud cosmic noise. The origin of the signal, however, is a complete mystery for the group of scientists, which also includes colleagues from the Universities of Maryland and California, in Santa Barbara.

The mysterious background noise is six times more intense than the researchers expected to measure in the portion of sky that was scrutinized on July 22, 2006 by a sophisticated instrument sent up on a high altitude balloon, the Arcade (absolute radiometer for cosmology, astrophysics emission) project. The original mission of the experiment was to capture the tenuous remnants of radiation – meaning heat – generated by the first stars that were formed in the Universe in the so-called age of cosmic darkness, at least 100 million years after the Big Bang. Instead of this the flight of the Arcade over the Texan town of Palestine recorded a strong and unexpected signal coming from an unknown age in the history of the Universe. The researchers say that no radio source so far identified in space would be capable of generating such a huge noise. So, either the data are wrong, or they represent something truly new, perhaps a primordial structure or an evolution process of the young Universe so far unknown to science. “We checked all the calculations again and it was a further two years before we publicized the results because we wanted to be sure that the signal wasn’t the result of an error”, says astrophysicist, Thyrso Villela, from Inpe, one of the two Brazilians who took part in the study. One of the first concerns was to eliminate the influence of radio emissions coming from our own galaxy, the Milk Way, which could contaminate the cosmic noise that was detected. “Even if we were to place all the known galaxies side by side we’d not manage to produce an emission of radiation this intense”, explains Carlos Alexandre Wuensche, also from Inpe, the other member of Arcade.

NASA/ARCADE/ROEN KELLYDesign of the Arcade: liquid-helium cooled flightNASA/ARCADE/ROEN KELLY

Disclosure of the probable finding occurred at the beginning of January, during the annual meeting of the American Astronomy Society (AAS), at Long Beach, California. Although nothing has yet been officially published about the supposed discovery in any scientific article, although four papers have been drafted and submitted to The Astrophysical Journal, the Nasa team of Al Kogut, the main researcher heading up the project, was one of the highlights of the scientific meeting.  “The Universe played a trick on us”, says Kogut, who still does not understand the nature of the noise they found. According to the Brazilians the best recordings of the intriguing signal were picked up by the three pairs of cone-shaped antenna developed for Arcade by Inpe, and which operate at 3 and 7 giga-hertz, at low microwave frequencies.

The biggest challenge for the experiments, which are aimed at measuring the radiation produced in outer space, is to obtain a really clean recording, free from the interference that commonly contaminates this type of work. The Nasa project, with collaboration from Inpe, was conceived to minimize, as far as possible, this systematic error, say the researchers. In order to achieve this, Arcade’s instruments – seven radiometers that operate at microwave frequencies of between 3 and 90 giga-hertz – had to be cooled with 1800 liters of liquid helium at the same temperature as the background cosmic radiation, 2.725 degrees Kelvin (more or less -270°C), very close to absolute zero. In this way the heat generated by the observation instrument itself, was cancelled out, thereby avoiding one of the most common deviations in measurement. During the two and a half hours that Arcade was taking measurements of 7% of the visible sky, crossing above and below the plane of the Milky Way (at a maximum altitude of 37 km), its antennae worked plunging into this frozen environment. “Arcade was projected to measure temperature variations of 0.001 K”, comments Villela. “No radio instrument has ever had this sensitivity.”

The announcement of the discovery of a possible second type of background radiation excited both the astrophysicists who are specialists in this subject and those who study the origins of the Universe. What might give rise to a radio signal of this magnitude? The team from Arcade have avoided speculating about the source of the noise, but considers that its genesis is extra-galactic, outside the Milky Way. The hypothesis has not been discarded that the new cosmic noise comes from the first stars that appeared in the Universe, the so-called stars of population III that arose some hundreds of millions of years after the Big Bang, although there is no significant evidence pointing in this direction. The researchers believe that the signal does not originate from any specific point in space, but is likely to permeate all directions of the Universe, as happens with background cosmic radiation. However, this hypothesis has not yet been tested. At the moment the biggest efforts of the researchers seem to be concentrated on proving that their data are real and making some sense of them. “We’re going to talk to theoreticians to see if some different phenomenon may have happened in the Universe when the signal was detected”, comments Wuensche. A new Arcade flight has not yet been scheduled to check if the signal can be measured again in the same region in which the 2006 experiment was carried out, or in another part of the sky.

NASA/ARCADEColored area: the 7% of the sky observed in the studyNASA/ARCADE

Interference
Without questioning the veracity of the results generated by Arcade, some astrophysicists prefer to wait for more evidence that the unknown background signal that was picked up is real and not the result of an error in measurement or interpretation. “They seem to have been very careful in their work and the results they presented are fantastic”, is the opinion of Brazilian astrophysicist, Angélica de Oliveira-Costa, from the Massachusetts Institute of Technology (MIT), who studies background cosmic radiation. “But there’s always the possibility of an error and this question is still unanswered.” She believes that it will be necessary to wait for confirmation of the existence of the new noise by more scientists, who also should be capable of picking it up with the use of other techniques and radiation emission models. According to Angélica there is no good emission map of the galaxies in the low microwave frequencies in which the signal was detected. This is a difficulty that may lead to mistakes being made. “Personally, I believe the signal exists”, comments the astrophysicist. “The question is to know if it is as strong as was detected or as much as ten times smaller.”

When more than 40 years ago Penzias and Wilson recorded the signal that proved to be background cosmic radiation for the first time they did not know if the weak noise they had measured was real or not. They even thought that the static was a distortion caused by bird droppings that had accumulated on the radio antenna used in the experiment, or interference caused by some terrestrial source. Since the end of the 1940’s there had been theories forecasting the existence of background cosmic radiation and the work of the pair at Bell Labs ended up being the material proof of it. In the case of the new signal it is still very early to know how the story is going to end. “We’re reviewing old work in search of recordings of this noise that may have gone unnoticed”, says Villela. “There are maps of the sky from the 1980’s that recorded, without any great degree of sensitivity, emissions at even lower frequencies in mega-hertz that may have picked up this signal.” At the time the unexpected noise was probably interpreted as errors or measurement deviations. The challenge now is to discover the origin of the new background noise.

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