According to the Big Bang model, the present Universe emerged from an extremely dense and hot state around 13.7 billion years ago. This model rests on three foundations, or three types of observation:

• The Universe is expanding, and the further the galaxies are from us, the faster they are moving away.
• The composition of light elements in the Universe, such as hydrogen, helium and deuterium, can only be explained if they were formed during the cooling of the extremely dense and hot primordial Universe.
• In its youth, this very dense and hot Universe was extremely bright, but it has become much less so as it has expanded. Nevertheless we can still observe its brightness in the sky: the radiation known as the cosmic microwave background, in the microwave region of the spectrum (between infrared and radio waves).

Planck observes the relics of this first light, known as the cosmic microwave background, or relic radiation, and effectively “photographies" the Universe as it was 380,000 years after the Big Bang, well before the formation of the first stars, galaxies and galaxy clusters.

The satellite scans the heavens with a much greater ability to distinguish fine details and a much higher sensitivity to temperature than its predecessors, the American satellites COBE¹ and WMAP², which were launched in 1989 and 2001 respectively. To accomplish its mission, Planck is carrying a 1.5m-diameter telescope and two instruments: LFI³, a microwave instrument developed in Italy, and HFI⁴, a submillimetre instrument developed under French prime contractorship.

The satellite will provide a map of the sky with unprecedented precision, showing temperature and polarisation fluctuations in the cosmic microwave background radiation. The HFI instrument actually is able to detect temperature fluctuations of close to one millionth of a degree around the temperature of the cosmic microwave background radiation of 2.7K, using sensors cooled to just 0.1K above absolute zero (or -273°C).

These acutely precise data will be used to deduce fundamental information about the formation and evolution of the Universe, particularly the period of expansion which followed the Big Bang, during which all distances in the Universe were multiplied by a factor that was absolutely … astronomical. Planck should also enlighten us about how tiny specks of matter were able to give birth to galaxies and galaxy clusters under the effect of the gravity force.

Planck was launched May 14, 2009 by Ariane 5, on the same flight as the Herschel satellite, which studies the formation and evolution of stars and galaxies. Both satellites have been placed in orbit at the 2nd Lagrange Point, at a distance of around 1.5 million kilometres from the Earth in the opposite direction to the Sun. From this position it is easier to cool them and protect them from radiation from the Sun, the Earth and the Moon which could disrupt their measurements.

Planck’s production has been entrusted to Thales Alenia Space under an ESA contract. CNES contributes to the funding of the mission, via ESA, and to the realization of the HFI instrument in partnership with the national centre for scientific research (CNRS).

_{1 Cobe : COsmic Background Explorer
2 WMAP : Wilkinson Microwave Anisotropy Probe
3 LFI : Low Frequencies Instrument
4 HFI : High Frequencies Instrument}

_{Updated : August 2009}