ESA’s satellite mission to map the cosmic microwave background.
Planck part cartographier l’Univers
Planck est le premier satellite européen consacré à
l’étude du rayonnement micro-onde cosmique. Il sera envoyé dans l’espace le
16 avril prochain à bord d’une fusée Ariane 5 à partir de
la base de lancement de l’ESA à Kourou, en Guyane française. Le satellite
transporte deux instruments : l’instrument haute fréquence et l’instrument
basse fréquence. Ensemble, ils vont balayer l’Univers à la recherche d’une
large gamme de fréquences, avec une sensibilité 10 fois
supérieure à celle des sondes précédentes. L’objectif est de
cartographier le rayonnement cosmique de fond et d’analyser de façon très
précise les anisotropies de température. Ces mesures devraient permettre
aux chercheurs de décoder énormément d’informations sur les
propriétés de l’Univers.
ESA’s Planck spacecraft is the first European satellite dedicated to the study of the
cosmic microwave background (CMB) radiation. Due to be launched on 29 April aboard
an Ariane 5 rocket from ESA’s launch site in Kourou, French Guyana, Planck’s primary
goal is to determine the cosmological parameters of the universe and to survey
astronomical sources. Scientists are hopeful that it should also answer many other
fundamental and astrophysical questions.
The satellite will orbit at the second Lagrangian point (L2) of the Earth–Sun
system at 1.5 million km from the Earth (figure 1). From this position, Planck
will explore the unknowns of the cosmic background radiation – the relic radiation
that brings with it many secrets of the history and evolution of the universe. For
380,000 years following the Big Bang, all of the dramatic events that steered the
evolution of the universe, its geometry and properties were imprinted and memorized in
The CMB today permeates the universe and has an average temperature of 2.725 K,
though observations have revealed slightly colder and hotter spots known as anisotropies.
Highly accurate studies of where these anisotropies are and what produced them may allow
researchers to decode a wealth of information about the properties of the universe.
Planck’s task – 13.7 billion years after the Big Bang – is not an easy
one, however, because the radiation signal is feeble and is embedded in all of the other
galactic and extragalactic signals, each emitting at different frequencies.
Following in WMAP’s footsteps
The first two scientific missions to map the CMB and its anisotropies were NASA’s
Cosmic Background Explorer (launched in 1989) and the Wilkinson Microwave Anisotropy Probe
(WMAP, launched in 2001). The data from these two satellites confirmed that the universe
is flat, that its expansion is accelerating and that only 4% consists of known forms of
matter. Nevertheless, given the lower accuracy of previous experiments, many questions
remain concerning the nature of dark energy (73% of the universe) and dark matter (23%),
as well as the processes that marked the infancy of the universe.
Planck comes eight years after WMAP and is designed to improve significantly on those
results. The satellite is equipped with both the Low Frequency Instrument (LFI) and the
High Frequency Instrument (HFI). “Together, the two instruments will scan the universe in
nine frequency channels, with a sensitivity that is 10 times better than that of
WMAP,” says Reno Mandolesi of the Italian Institute of Space Astrophysics and Cosmic
Physics in Bologna (IASF-BO/INAF). He is also the principal investigator of the
consortium that built the LFI. “However, the main improvement of Planck, with respect to
previous missions, is in the suppression and control of systematic effects. The HFI and
LFI employ two different detection techniques and this drastically reduces the systematic
effects. Both instruments operate at cryogenic temperatures, at which the intrinsic noise
coming from the devices is reduced to a minimum,” he adds.
The systematic effects can also be controlled by an appropriate choice of orbit- and
sky-scanning strategy. “WMAP was the first satellite to orbit round L2 and Planck will
fly in a similar orbit. From L2 the noise from the Earth is drastically reduced,”
confirms Mandolesi. Also, from this position the satellite’s telescope can always be
protected from illumination from the Earth, the Sun and the Moon, thanks to the optimal
design and observational strategy.
The LFI is an array of 22 radiometers, each one made of an antenna to capture the
signal and cryogenically cooled (20 K) electronics – a combination of
ultralow-noise amplifiers and high-electron-mobility transistors – for read-out.
“Low-noise temperature fluctuations in the amplifiers are a crucial factor in the
measurement,” says Mandolesi. “The LFI radiometers meet the requirements for both noise
and bandwidth, with low power consumption at all frequencies – and they establish
world-record low-noise performances in the 30–70 GHz range. This is
particularly important considering that the main noise sources come from our own galaxy
and have their minimum around the 70 GHz frequency,” he explains.
The HFI is an array of 48 bolometric detectors that is placed at the focal plane
of the Planck telescope. These will measure the energy of the incident CMB radiation in
six frequency channels between 100–857 GHz, with sensitivity in the lower
frequencies close to the fundamental limit set by the photon statistics of the
background. The HFI was designed and built by a consortium of scientists led by Jean-Loup
Puget of the Institut d’Astrophysique Spatiale in Orsay. The detectors operate at the
cryogenic temperature of 0.1 K, obtained using a cryochain of sorption, mechanical
and dilution coolers.
Signals from the CMB are polarized in two types of mode, known as E-modes and B-modes.
The E-modes have already been measured (Kovac et al. 2002; Page
et al. 2007). All of the LFI channels and four of the HFI channels can
measure the intensity of the CMB radiation as well as its linear polarization. “By
combining the signals measured by the LFI and HFI, Planck might be able to discover the
B polarization mode, which is linked to the existence of the primordial
gravitational waves” says Mandolesi.
“In some cosmological models it could even be possible to find signatures that might
correspond to scenarios with extra dimensions of the universe. Also, the mass and quantum
fluctuations that occurred at 10–35s after the Big Bang, and might have
affected the cosmic inflation, can be explored by studying the polarization modes of the
CMB with high accuracy. Furthermore, Planck’s excellent sensitivity might allow the
discovery of interesting physics hidden behind the non-Gaussian distribution of the
temperature anisotropies predicted by many cosmological models,” he explains.
Planck will start collecting physics data after a three-month period of commissioning
in orbit. Six months later the scientific teams will start the analysis, aimed at the
early release of a catalogue of compact sources, the first to be made at so many
frequencies. It is expected to become public about 15 months after the launch. A
core team of about 100 scientists supporting the Data Processing Centre in Trieste
will carry out the processing and analysis of LFI data. The HFI data will be processed by
a distributed system involving several institutes in France and the UK. The satellite
will accomplish two complete surveys of the sky over 14 months and the hope is that
this will be extended to four surveys.
• For more about Planck, see www.esa.int/SPECIALS/Planck/.