INFO 04-1999a: Payload for Planck confirmed
10 March 1999
The two instruments on board ESA's spacecraft Planck were definitivelyselected on 17February by ESA's Science Programme Committee. Planck is due to belaunched in 2007. The instruments consist of two arrays of highlysensitive detectors to study what can be called the 'echo' of the BigBang, a radiation that fills the whole Universe and was emitted when theUniverse was very young. They will be designed, built and operated bymore than 40 European institutes.Astronomers have few tools to study the origin of the Universe, which is believed to have occurred about 10 000 million years ago. Certainly their telescopes do allow them to see what the cosmos was like thousands of million years ago - in astronomy looking far in space also means looking back in time - but only up to a certain point. The farthest reachable epoch is when the Universe was about 300 000 years old; not even the best telescope ever imagined would be able see beyond that time.
This is simply because before that time the Universe was 'opaque': matter and light were tightly coupled together, and it was impossible for light to travel from side to side of the Universe - because of the high temperature the matter was not in the form of neutral atoms, but in the form of charged particles that impeded light from travelling. Only when the Universe cooled down to a temperature of approximately 3000 K did the Universe become largely neutral, thus 'freeing' the light. The universe then became 'transparent', flooded with light.
That radiation has remained up to the present time and indeed fills today's Universe. Astronomers can detect it as a background radiation, and consider it an 'echo' of the Big Bang, a 'fossil' light. It is, in fact, the 'oldest' astronomical object that can be observed, the one that takes astronomers closest to the Big Bang.
In today's Universe the 'fossil' radiation has expanded and cooled, together with the entire cosmos. As a result the temperature of the 'echo' of the Big Bang has diminished from some 3000 K to a much lower temperature of 3 K (-270 °C)
Such cold objects are most easily observed at microwave frequencies, which is the reason why this radiation is called 'Cosmic Microwave Background', CMB for short. Astronomers Arno Penzias and Robert Wilson were the first to detected the radiation using a microwave horn antenna in 1964 and for this achievement they received the Nobel Prize in 1978.
The CMB has since then become one of the fundamental pillars on which the current edifice of cosmology rests. It has been observed by a variety of other experiments (notably NASA's COBE satellite), thanks to which some fundamental predictions related to the Big Bang theory have been and are being tested. Astronomers, however, still hope to extract a lot more information from the fossil radiation, using more advanced instrumentation. This will be Planck's task.
"Planck will greatly constrain cosmological models. It will determine with high accuracy fundamental characteristics of the Universe, such as its geometry, its density, and the rate at which it expands. It will examine the birth of large-scale structure in the Universe, and provide important clues as to the kind of matter that fills it", explains Planck project scientist Jan Tauber, at ESA's Science and Technology Centre (ESTEC) in The Netherlands.
Planck's payload
ESA's Science Programme Committee confirmed in February, that Planck's payload will consist of two instruments offering much greater sensitivity and frequency range than any previous spacecraft observing the CMB. ESA thus accepts the proposal made by the scientific community last year, and which originated in a call for ideas issued by ESA in 1991.
Planck's instruments will measure the temperature of the background radiation over the whole sky, and will be able to detect differences as slight as a few microkelvin (corresponding to a few parts in one million). These differences (often called 'anisotropies') are the imprints left by the matter in the radiation when both were coupled together - 'opaque' age of the Universe - and act as the 'seeds' of the large-scale structures we observe today: galaxies, galaxy clusters, etcetera. One of the instruments is called Low Frequency Instrument (LFI). It is an array of 56 tuned radio receivers that will be operated at the very low temperature of 20 K. The second is the High Frequency Instrument (HFI), an array of 56 so-called 'bolometric' detectors, which work by converting radiation to heat and are operated at the even lower temperature of around 0.5 K.
Apart from the sensitivity, Planck's major advance comes from the fact that its two instruments will cover a very broad range of frequencies. LFI will image the sky in four frequency channels between 30 and 100 GHz (gigahertz), and HFI in six frequency channels between 100 and 857 GHz.
"One of Planck's great advantages is the complementarity of its instruments", Tauber says. "The fact that they cover a wide range of frequencies makes it possible to subtract the radiation from sources other than the CMB, including our own Galaxy. These sources bury the real data within a lot of noise."
The HFI will be designed and built by a Consortium of more than 20 institutes, most of which are European, led by Jean-Loup Puget of the Institut d'Astrophysique Spatiale in Orsay (France). The LFI will be designed and built by a Consortium of more than 22 institutes, all but one European, led by Reno Mandolesi of the Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri in Bologna (Italy).
Main institutes for PLANCK
California Institute of Technology, Pasadena (USA)
Centre d'Etudes Spatiales des Rayonnements, Toulouse (F)
Centre de Recherche sur les Très Basses Températures, Grenoble (F)
Chalmers University of Technology, Göteborg (S)
Collège de France, Paris (F)
Commissariat à l'Energie Atomique, Gif-sur-Yvette (F)
Danish Space Research Institute, Copenhagen (DK)
Imperial College, London (UK)
Institut d'Astrophysique de Paris, Paris (F)
Institute of Astronomy, Cambridge (UK)
Instituto de Astrofisica de Canarias, La Laguna (E)
Instituto de Fisica de Cantabria, Santander (E)
Istituto CAISMI, Firenze (I)
Istituto IFCTR (CNR), Milano (I)
Istituto IFSI, Roma (I)
Jet Propulsion Laboratory, Pasadena (USA)
Laboratoire de l'Accélérateur Liniaére, Orsay (F)
Max-Planck-Institut für Astrophysik, Garching (D)
Millimetre Wave Laboratory, Espoo (FI)
Mullard Radio Astronomy Observatory, Cambridge (UK)
National University of Ireland, Maynooth (IR)
Nuffield Radio Astronomy Laboratories, Macclesfield (UK)
Osservatorio Astronomico di Padova, Padova (I)
Osservatorio Astronomico di Trieste, Trieste (I)
Queen Mary and Westfield College, London (UK)
Rutherford Appleton Laboratory, Chilton (UK)
SISSA, Trieste (I)
Space Science Department of ESA, Noordwijk (NL)
Theoretical Astrophysics Center, Copenhagen (DK)
University of California (Berkley), Berkley (USA)
University of California (Santa Barbara), Santa Barbara (USA)
Université de Genève, Genève (CH)
Università La Sapienza, Roma (I)
University of Oslo, Oslo (Nor)
Università Tor Vergata, Roma (I)
