Résumé

Planck : une année dans les étoiles

En mai 2009, le satellite Planck, de l'Agence spatiale européenne, a été lancé dans l'espace avec pour mission d'explorer le rayonnement cosmique diffus. Les premiers résultats donnent déjà de nouvelles perspectives sur l'astrophysique. Le 11 janvier, la collaboration a publié son premier catalogue de sources astrophysiques. Ce catalogue, le plus complet jamais produit, inclut des sources très diverses : des objets proches de la galaxie, différents types de radiogalaxies et de galaxies poussiéreuses, ou encore des amas distants de galaxies. Planck étudie les propriétés des amas de galaxies au moyen de la signature qu'elles laissent dans le fonds commun cosmologique.

The cosmic microwave background (CMB) is one of the most powerful resources that cosmologists have to investigate the evolution of the universe since its earliest moments. Like a "fabric" that permeates the cosmos, it holds information about the temperature distribution, keeping a permanent memory of all of the events that the universe has gone through. In particular, its anisotropies – deviations from the isotropic distribution that characterizes the universe – contain the signatures of the primordial perturbations that gave birth to the large-scale structure of the universe observed today.

Reading among these ripples in the CMB is by no means easy because they appear as tenuous fluctuations (1 part in 100,000) in a cold background at 3 K. In May 2009, ESA's Planck spacecraft was launched into space to prise out the secrets hidden there (CERN Courier July/August 2009 p8). The result of about 20 years of work by the international Planck collaboration, it is a third-generation satellite that follows on from the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). Since mid-July 2009, Planck has been orbiting at the second Lagrangian point (L2) of the Earth–Sun system, 1.5 million kilometres from Earth. It carries on board a Low Frequency Instrument (LFI), consisting of an array of 22 radiometers, and a High Frequency Instrument (HFI), which has 48 bolometric detectors (CERN Courier April 2009 p26). Since its launch, Planck has performed extremely well. The two instruments have so far scanned the whole sky almost three times in nine different frequency channels, with a sensitivity that is up to 10 times better and an angular resolution up to 3 times better than that of its most recent predecessor, WMAP (figure 1).

On 11 January, the Planck collaboration released its first catalogue of compact astrophysical sources. This is the first full-sky source catalogue to cover the frequency range 30–857 GHz at nine different frequencies. It includes a variety of different types of sources, from nearby objects in the galaxy to various classes of radio galaxies, and dusty galaxies to distant clusters of galaxies.

Because Planck is optimized to measure the CMB, the catalogue turns out to be an extremely powerful tool for identifying the cold objects that populate the interstellar medium (ISM) and measuring their temperature accurately. In this task Planck is allied with the Herschel space observatory, which was launched by ESA on board the same spacecraft. Herschel, designed to study cold objects, is not a survey telescope; rather, its purpose is to look closely at one part of the sky at a time. Planck and Herschel are thus good companions, whereby Planck provides the whole-sky survey and points Herschel to interesting locations that it can focus on.

Among the sources detected by Planck are "protostellar objects", that is, clusters of matter that could give rise to a star. The complex processes at the origin of stars are among the hottest topics for astronomers, who carefully investigate the properties of the ISM to identify the trigger factors for star formation. Researchers at many Earth-based observatories will be able to use data from Planck to improve our understanding of these processes.

After only a few months of observation, Planck is also shedding light on another component of the ISM: namely, spinning dust grains. These are tiny aggregates of matter that appear to be slightly bigger than molecules such as CO2. They spin and radiate with a particular spectrum. Planck has for the first time been able to reconstruct this spectrum at high frequencies and so confirm that the spinning dust grains really do exist. This opens up a completely new field of study for astronomers, who will now have to understand the exact nature and behaviour of this intriguing component of the ISM.

Moving away from the interior of the Galaxy, one of the major contributions of the first part of Planck's scientific programme is the identification of clusters of galaxies and the study of their properties through the signature that they leave in the CMB when its photons travel through the hot gas of the cluster. This is the Sunyaev-Zel'dovich effect, in which photons in the CMB increase in energy through inverse Compton scattering off hot electrons in the galaxy clusters. As a consequence, along the cluster direction, the CMB temperature increases at high frequency (>217 GHz) and decreases at low frequency (<217 GHz) with a well defined frequency spectrum, observable by Planck thanks to its wide frequency coverage.

Matter in the universe is grouped in enormous clusters surrounded by vast, empty spaces. These clusters can contain hundreds of galaxies and large amounts of dark matter. Dark matter consists of particles observed so far only through their gravitational effect; their exact nature remains unknown. Observing clusters of galaxies is crucial to understanding why matter of any kind aggregates in this fashion. The Sunyaev-Zel'dovich effect can be used to estimate the total mass of the cluster, which, when combined with X-ray observations, can in turn provide evaluations of the proportion of dark matter. The list of sources of this type that Planck has identified through the Sunyaev-Zel'dovich effect is 2–3 times larger than those published so far by the best observatories on Earth.

Planck has also been able to extend the spectrum of conventional radio sources. Previously this was known up to about 100 GHz but Planck has now pushed this to 857 GHz, giving new insight into the behaviour of these sources and the physical processes involved.

This first set of results is just the beginning of the Planck adventure. There will be more accurate catalogues and further findings in astrophysics, followed in early 2013 by Planck's crucial contributions to cosmology. While the theoretical models used at present in cosmology seem to fit the current observations well, they require important components whose nature is not yet known – dark matter and dark energy. A major aim of the Planck mission is to cast light on both of these enigmatic components.

Dark energy is yet another contribution to the energy density of the universe, being different from dark and ordinary matter. It is presumed to provide the current acceleration to the expansion of the universe and its existence is inferred from observations of Type Ia supernovae, of the CMB and of the baryon acoustic oscillations that are determined by surveying galaxies at different cosmic epochs. The equation-of-state of dark energy characterizes the late and future evolution of the universe. Planck will be able to measure the parameters ρ (energy density) and w (ratio of pressure to ρ) of the equation-of-state with an accuracy that is expected to be an order of magnitude greater than for the previous data from WMAP. Moreover, studies of CMB anisotropies will allow the Planck collaboration to distinguish between various theoretical models that do not consider new ingredients in the energy-budget of the universe (such as dark energy and dark matter) but, rather, change the Einstein equations (as for example in "modified gravity" models).

As far as gravity is concerned, Planck's contribution will depend on which theoretical model best describes the evolution of the universe. Among the many models that try to explain the initial conditions of the Big Bang, two have gained particular prominence: one is the inflationary model, in which the early universe underwent a period of exponential expansion; the other is the "bouncing model", where the universe is described as something that was contracting and then "bounced" at the time when quantum gravity was important and began to re-expand. Inflationary models generally generate gravitational waves that can in principle be detected by Planck, depending on their amplitude, the value of which is a feature of the specific inflationary model. By contrast, the bouncing models do not predict gravitational waves. Planck will also constrain the expected deviation from the Gaussian distribution of the primordial fluctuations that are imprinted in the CMB. This feature characterizes more the bouncing models than the inflationary ones. While Planck will not have the final say in this field, it will indeed have the opportunity to rule out several models.

In addition to questions directly related to cosmology and astrophysics, Planck will also address a number of problems that are linked to particle physics and the Standard Model. It will increase by at least a factor three the accuracy of limits on the mass and number of neutrino species that WMAP currently sets at 0.56 eV and 4.3+/–0.8, respectively. Planck may also provide limits on the mass of the Higgs boson in certain theoretical models for a non-minimally coupled Higgs-inflation field with gravity.

According to current theories, the conditions of the universe today were set at the time of inflation, about 10–35 s after the Big Bang. The LHC below ground and Planck in deep space are allies in probing these first moments of the universe's evolution. While the physicists at CERN are seeking to reproduce the conditions of the early universe, with Planck we observe the first light that came out of this "soup" of matter and radiation. Particle physicists, as well as astrophysicists and cosmologists, must work towards a concordant description of this early epoch in which data from the different sources fit together to give a consistent picture of the universe that we all inhabit.