Planck satellite takes off to chart the universe

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.

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/.