Scientific Rationale

ESA's Planck satellite was launched in May 2009 and has surveyed the whole sky in intensity and polarization uninterruptedly between August 2009 and October 2013, over a range of frequencies spanning the radio (30 GHz) to the submillimetre (857 GHz). Planck was designed to image the anisotropies of the Cosmic Microwave Background (CMB) over the whole sky, with unprecedented sensitivity and angular resolution. Planck is now widely considered as the flagship CMB experiment.

In addition to the main cosmological goals of the mission, the Planck sky surveys are being used to study in detail the very sources of emission which "contaminate" the CMB signal, and results in a wealth of information on the properties of extragalactic sources, and on the dust and gas in our own galaxy. In this respect, Planck is already providing a completely new look on our understanding of dust in the interstellar medium, and many further insights are expected during 2014 and 2015, especially related to polarization and magnetic fields.

In March of 2013 Planck and ESA released to the community all-sky temperature maps in nine frequency bands between 30 and 857 GHz, based on its first 15.5 months of observations. At the same time, the Planck Collaboration published its first major findings in CMB cosmology and astrophysics (~30 papers covering a wide range of science can be found on arXiv and via www.cosmos.esa.int/web/planck ). The cosmological parameters determined by Planck now constitute the most precise recipe for our Universe, and its constraints on inflationary scenarios, derived from the primordial power spectrum and from the gaussianity of the initial inhomogeneities, are the most stringent. Planck has also measured the lensing distortion of the CMB by intervening matter, and produced the first all-sky image of the large-scale distribution of dark matter integrated along the line of sight. Planck has produced a catalogue of more than one thousand clusters detected via the Sunyaev-Zeldovich effect, and the deepest all-sky catalogue of radio and dusty galaxies in the submm range. Not least, Planck has produced a new spatial and spectral model of galactic dust emission which replaces the previous standard in the field.

The fact that Planck is so important for both cosmology and astrophysics is not accidental: a very detailed knowledge of the "foregrounds" is a critical element for the reliable extraction of the CMB. This is also quite important for polarized emissions, where most of the sky is contaminated by foregrounds. There are many possible ways to separate the physical components, which rely in differing measure on prior knowledge –ranging from "no assumptions" to detailed physical modelling. Within the Planck Collaboration there has been a very intense exchange of information between scientists interested in foregrounds and those specializing in the CMB, which has resulted in improvements of understanding on both sides. The FM5 meeting is a good way to foster the extension of this type of interchanges to a much wider community.

The second major release of Planck data has started to take place in January 2015; it already has delivered the majority of the data acquired by Planck, and for the first time polarized data products. The latter certainly constitute an entirely new asset to the community, to be used both for cosmology and astrophysics. This release will be completed in May 2015. As it will contain all the Planck data, this release will be the first major step towards the delivery to the community of the Legacy of Planck. The timing of the IAU's GA Assembly in August of 2015 is thus appropriate to obtain feedback from the community which may be relevant to improve the following (and very probably final) release of data by the Planck Collaboration, planned to take place in late 2015 or early 2016.

In summary, the aims of FM5 are to:
- publicize the wide variety of current and potential uses of Planck data
- foster exchanges between users of Planck data
- get feedback to optimize the last delivery of Planck data products.