Cosmic Background To explore

The satellite COBE ( Cosmic Background To explore ) was launched to proceed to the study of the cosmological diffuse Fond. The cosmological diffuse bottom is the electromagnetic Rayonnement resulting from the dense and hot phase of the paramount Univers. It was emitted approximately: 380000 years after the Big Bang. The standard Modèle of cosmology predicted that this radiation was to present a electromagnetic Specter of black body, because it was to be emitted very early in the history of the universe, at the time where the interactions between light and matter were to be sufficiently important to reach a thermal balance perfect. Moreover, this radiation was not to be perfectly uniform, but administrative duty of observation. If the universe has structures today (Galaxies, Galaxy cluster), the germs of these structures were to exist at the time of the emission of the cosmological diffuse bottom, in the form of negligible surdensities, which were to seem hotter. In 1989, date of the launching of COBE, neither the exact form of black body of the cosmological diffuse bottom, nor the presence of these fluctuations of temperature had been detected. Mission COBE filled these two objectives (see below), whose importance was recognized in 2006 by the attribution of the Nobel Prize of physics to John C. Mather and George Fitzgerald Smoot, persons in charge of the two instruments which allowed measurements (FIRAS and DMR respectively).

Characteristics

Satellite COBE was launched the November 18th 1989 (after a delay due to the explosion of the Space shuttle Challenger), and placed on a Orbite to 900 km of altitude. At this altitude the satellite is subject to only one weak influence of the Earth and avoids the elements which could disturb its measurements at a higher altitude. It is in rotation on itself and thus carries out a turn per minute. The purpose of this technique of observation is to make it possible the instruments of the satellite to observe the whole of the areas of the sky in a short time. The long life of life of the satellite allows the repeated observation a great number of times of each area of the sky, and this in order to minimize the problems involved in nonthe constancy of the performances of the various detectors.

Instruments

In order to conclude its observations, it embarked three principal instruments:

DMR ( Differential Microwave Radiometer ), whose function was to detect differences in temperature between various areas of the sky,
FIRAS ( Far InfraRed Absolute Spectrometer ) whose function was to very precisely measure the spectrum of the cosmological diffuse bottom by comparing this one with an embarked artificial black body on board the satellite,
DIRBE ( Diffuse InfraRed Background Experiment ), whose function was the observation of the sky in the field Infrarouge, corresponding to wavelengths slightly higher than that of the cosmological diffuse bottom.

DMR and FIRAS had the role of testing into force the Paradigme at the time (and confirmed since), namely the fact that the cosmological bottom diffuse had been emitted very early in the history of the universe, but that at the time this one showed already the germs of what was going to become several hundreds of million years later the first galaxies. DIRBE had the role of detecting the radiation emitted by these first galaxies of which very little were known at the time.

DMR

This apparatus consists of a whole of three Radiomètre S differentials carrying out of measurements on distinct wavelengths, 3,7 mm, 5,7 mm, 9,6 mm, which corresponds to Fréquence S of 81,52 and 31 GHz respectively. A radiometer is an instrument which measures the relative intensities of the radiation for a given wavelength. The reason of the use of measures to three different wavelengths is due to the fact that, to be certain that the differences in temperature observed come indeed from the emission of the cosmological diffuse bottom and not of radiations of foreground (Lumière zodiacale, radiation Synchrotron or Fre-free of the Milky Way, Effet Sunyaev-Zel' dovich, etc), it is necessary that the differences in luminous intensity in each one these wavelengths satisfy certain relations. For example, a hot black body will emit more than one cold black body whatever the wavelength observed. If DMR observes an area more luminous than another to 3,7 mm wavelength, but less luminous to 5,7 mm, it is certain that a significant part of the radiation observed is not due to a variation of intensity of the cosmological diffuse bottom. The use of a great number wavelengths thus allows a certain redundancy in order to as well as possible eliminate all Artifact related to the presence from radiation of foreground.

FIRAS

This apparatus primarily consists of a Spectrophotomètre: it compares using a interferometer of Michelson the spectrum of the cosmological radiation received since an area of the sky of 7° with that of a black Corps of reference embarked on board satellite. The radiation is then divided into two beams which one makes interfere. Figures of interferences thus formed, one deduces finally nature from the spectrum of the radiation by analyzes Furrier.
Just like the precedent, this instrument acquires its precision from its differential analysis.

DIRBE

This apparatus is a Photomètre with Infrarouge which measures the intensity of the radiations emitted by the first generations of celestial objects. It operates in the remote infra-red range.

Results

Since 1990, and only a few minutes after the startup of instrument FIRAS, the observations of COBE allowed to confirm the thermal nature of the cosmological radiation according to the law of the black body of Planck with a temperature of 2,728 K. Because of structure of instrument FIRAS, the precision of the spectrum of black body was better than that of its absolute temperature (0,001% against 0,1%). This result constituted an extremely solid proof of the validity of the model of Big Bang. The results of experiment DMR were made wait more than one year, fact, there still, structure of the instrument which enabled him to give the full measurement of its capacities only after one very great number of redundant observations of various areas of the sky. The first official results were announced on April 23rd, 1992, and were accommodated like an major event in the field of the Astrophysique. The detailed analysis of the fluctuations of temperature of the cosmological bottom diffuse, improved considerably since, allowed the precise measurement of the majority of the cosmological parameters and emergence of what (2006) is called today the standard Modèle of cosmology.

Distinction and price

Two members of the team of satellite COBE received the Nobel Prize of physics in 2006 for their contribution to the results obtained with this satellite: George Smoot and John C. Mather. This last, as well as the whole of the team of the satellite, also were rewarded by the Prix Peter Gruber for cosmology the same year.

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