The legacy of Planck

In the coming weeks, the Planck Collaboration will release the third and final set of products and papers related to the Cosmic Microwave Background (CMB) studies derived from the data provided by the Planck mission of the European Space Agency (ESA). This third release, together with the two previous ones, constitutes the legacy of the Planck mission.

The Planck mission was selected in 1996 as a medium size mission aimed at obtaining an all-sky image of the CMB anisotropies in temperature and polarisation with unprecedented sensitivity and angular resolution. Planck was launched on May 14th 2009 and during four years was collecting very valuable data for the understanding of the Universe as a whole, and also critical for the understanding of the properties of the components of our own galaxy and of other galaxies and their clustering. After just over four years of a remarkable operation, the mission was turned off on October 23rd 2013, providing high quality data for which a very large number of results in the areas of cosmology and astrophysics have been derived. The results obtained so far, as well as the products in which they were based on, were made public in 2013 and later in 2015, years corresponding to the first and full data releases, respectively. Among the great variety of the results, it is worth pointing out, in particular, the best determination of the age, composition and shape of the universe, results that have made the Planck Collaboration worthy of the 2018 Royal Astronomical Society Award.

The legacy of Planck has much to do with the knowledge of the physical properties of the universe. The results published so far establish a homogeneous and isotropic cosmological model that, with only 6 parameters, is able to truly reproduce the Planck observations of the primordial and most remote radiation in the universe. In relation to its composition, it has been possible for the first time to map the all-sky distribution of dark matter and determine its abundance with sub-percent precision, and also strongly constrain the alternative models to the cosmological constant for the dark energy. In addition, the published results reinforce the existence of an inflationary period in the very early Universe in which it expanded exponentially and appeared the quantum seeds that gave rise to the galaxies and other structures that form what is known as the “cosmic web”. Another consequence derived from the Planck data is the stringent upper limit imposed on the neutrino mass that, together with the lower limit obtained from neutrino experiments, imply a narrow window of around a tenth of electron-volt (value that, on the other hand, demands an explanation within the standard model of particle physics).

The impact that Planck publications have had on the scientific community has been very remarkable. The Planck Collaboration, made up of some 200 scientists, has published 136 articles in the journal Astronomy and Astrophysics with an average of about 200 citations per article. The Planck collaboration, formed by some 200 scientists, has published 136 papers so far in the journal Astronomy and Astrophysics with an average of about 200 citations each. The most cited papers are the cosmological publications included in the Planck Core Science Program (focused on the study of the CMB), two of them being the most cited in physics in the years 2014 and 2016, respectively. In particular, this has been key to our institute having the highest impact relative to world of CSIC centres between the years 2012-2014.

In the next and final set of publications and products that will complete the Planck legacy new and more precise results will be included. These improved results are a consequence of a revision in the calibration of data and in the reduction of systematic effects that prevented the extraction of all the available information in the polarisation data. On the one hand, this will imply an even more precise determination of the cosmological parameters and of the properties of the components of our galaxy. On the other hand, the new Planck data will be essential to complement those obtained by the next cosmological experiments that will start in the next decade, such as the ESA Euclid satellite to be launched in 2021 and aimed to study the nature and properties of the dark energy and dark matter through a deep and precise mapping of galaxies; or KATRIN, which will study the mass and properties of neutrinos.

Enrique Martínez-González, head of the Observational Cosmology and Instrumentation group, participated in the proposal of the mission to the ESA scientific program in 1993 as well as in the subsequent activities of instrumental development, data analysis and derivation of the cosmological results, being Co-investigator of the Planck Low Frequency Instrument (LFI). He together with the rest of the members of the cosmology group at IFCA who participate in Planck, who hold the status of Planck Scientist and belong to the LFI Core Team, have played a relevant role in the achievement of the important legacy left by Planck. In relation to the instrumental aspect, the IFCA group coordinated the project for the development of the back-end modules of the radiometers of the LFI at 30 and 44 GHz, in close collaboration with the group at DICOM (UC), and contributed to their posterior simulation and characterization. In relation to the scientific exploitation of the data, it has significantly contributed to the two sets of publications of the Planck Core Science Program published in 2014 and 2016, respectively, leading three papers in each set: on the isotropy and statistics of the CMB, on the catalogue of point sources and on the detection of the integrated Sachs-Wolfe effect. Also in relation to both sets of publications, temperature and polarisation maps of the CMB have been produced by means of the component separation code SEVEM developed by the group, one of the four official codes of the mission. In addition, the group has led two papers on the Sunyaev-Zeldovich effect due to the hot gas in the Virgo cluster and in filaments of the cosmic web, respectively, and another one on the recently produced multi-frequency catalogue of non-thermal sources.

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