MEASURING THE TINY FLUCTUATIONS OF THE UNIVERSE FIRST LIGHT
During these 20 last years, thanks to the new instruments sensitivity, cosmology science, that is to say the study of our universe history and its composition, truly entered an observational phase. Fundamental cosmological parameters such as baryonic matter density (ordinary matter), dark matter density (matter which do not interact with light and is revealed only by its mass), the part of dark energy, or even the age of the universe, can be measured with more and more of precision and it is now possible to test the validity of the existing models.
Since its birth which happened approximately 13.7 billion years ago, the universe is expanding. In addition to the galaxy distances observation for almost 80 years, one of the pillars of this so called Big Bang theory has been the prediction and then the observation of a microwave radiation (i.e. in the field millimetre-length wavelengths): the CMB (Cosmic Microwave Background) emitted a few 380.000 years after the Big Bang. This is the epoch when the photons managed to be released for the first time from the matter formerly too dense and too heat that the least radiation escape from it. Today this fossil radiation reaches us from all the space directions after having travelled during nearly 13.7 billion years and being considerably cooled since its emission: its average temperature is today of 2.73 K (2.73 ?C above the absolute zero). Detected for the first time in 1965 by Penzias and Wilson, it is the first light of the universe, and its fossil print constitutes a wall which prevents the astronomers from directly observing what occurred before. This remarkably uniform radiation reveals some tiny fluctuations (about one out of ten thousandth), as measured respectively by the space missions COBE in 1992 and its successor WMAP since 2001. These fluctuations are tracked by the astronomers who seek there prints left at the same time by what preceded this radiation emission, and also prints produced by the obstacles met during its travel toward us.
Introduction
To observe and measure these tiny fluctuations, Dome C seems to be a particularly appropriate place. Because of the extremely weak precipitable water concentration, this is an ideal site to observe in the submillimeter-wave and millimetre-wave range in which the fossil radiation is the most intense. In addition, the combination of its localization a sub-polar latitudes (75? south) with the atmosphere stability on the Antarctic high plateau make it possible to plan uninterrupted measurements over very long periods, which are absolutely necessary to detect temperature variations of the cosmological radiation of about 1/10 000.000! Even during southern summer, the low sun trajectory above the horizon leaves the possibility for the astronomers to make measurements on the zenith without being polluted by the sunlight. And finally, because of a very cold temperature, the atmosphere?s brightness above Dome C is particularly low to not disturb measurements.
? WMAP Science team/NASA
According to the current cosmological models, the univers is expanding since the Big Bang explosion which gave birth to him 13.7 billions years ago : this is the Big Bang theory. The observation of a homogeneous microwave radiation (the Cosmic Microwave Background) coming from all the sky and emitted 380 000 years after the Big Bang confirm the theory.
? WMAP Science team/NASA
With a mean temperature of 2.73?K, the CMB contains some tiny fluctuations (from some hundredth to a few billionth of degrees) which give some precious clues on the epoch before its emission and also on the objects crossed during its travel to us. The contribution of the ?foreground? effects which disturb the CMB measurements must be removed, like our galaxy (in red on the top), our solar system and our atmosphere as well.?
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