The legacy of Planck

  • For four years, the Planck mission collected measurements of ancestral microwave cosmic radiation
  • In 2011, 2013 and 2018, three datasets from these measurements were released, with increasing data accuracy
  • The last and final dataset was to contribute to the most accurate quantification of the age of the Universe till that moment

What was the Planck mission?

Named after the German Nobel laureate Max Planck (1858-1947), the Planck mission of the European Space Agency (ESA) was the first European space observatory whose main goal was to study the Cosmic Microwave Background (CMB), the relic radiation from the Big Bang. The CMB preserves a picture of the Universe as it was about 380 000 years after the Big Bang. As a result, it can reveal the initial conditions for the evolution of the Universe.

Artist interpretation of the Planck satellite observatory in space.
Artist interpretation of the Planck satellite observatory in space.

In the year 1996, the Planck mission was selected as a medium-size mission for obtaining an all-sky image of the temperature and polarisation fluctuations of the CMB, with an accuracy set by fundamental astrophysical limits. This would allow to chart the most accurate maps yet of the CMB. The European Space Agency (ESA) operated the Planck observatory starting in the year 2009. It was a space-based satellite-observatory that observed the “relic” cosmic microwave background (CMB). Planck became an important source of astrophysical data and helped test theories about the early Universe and the origin of the cosmic structure. After the end of the mission, Planck had contributed to the most precise measurements of key cosmological parameters, including the age of the Universe, or the density of matter and dark matter.

The mission of the Planck observatory was put to a final end on October 2013. During the Spring of 2013, the first set of data resulting of the Planck mission was released. That was followed in 2015 by a second release, together with a cautionary note about the precision of some data. By 2018, a new set, the “Legacy” data of Planck had been re-processed by the Planck consortium was to be released with enhanced temperature and polarisation determinations. These new data were to be fully suitable for performing cosmology work with, significantly improving on the two earlier datasets.

Diving into the details of the Planck project

Planck was launched on May 14th 2009 and during four years it collected data. These data would be useful for understanding better the Universe, but also the properties of the components of our own, and other galaxies, as well as their clustering. After just over four years of  remarkable operation, the mission was turned off on October 23rd 2013. It provided high quality data from which an enormous amount of results in the areas of cosmology and astrophysics were derived.

The results obtained from the two first datasets, as well as the products on which they were based on, were made public in 2013 and later in 2015. Among the results the most worth pointing out, one is the best determination of age, composition and shape of the Universe to date. Its results rendered the Planck Collaboration worthy of the 2018 Royal Astronomical Society Award.

Artist interpretation of Planck. In the background, measurements of cosmic microwave background
Artist interpretation of Planck. In the background, measurements of cosmic microwave background

The legacy of Planck, this third and last release, has much to do with the knowledge of the physical properties of the Universe. The results published so far established 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. It has been possible to strongly constrain the alternative models to the cosmological constant for the dark energy.

The results reinforce the existence of an inflationary period in the very early Universe in which it expanded exponentially, and when appeared the quantum seeds that gave rise to 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 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 for the Instituto de Física de Cantabria (IFCA) having the highest impact relative to the world of all CSIC centres between the years 2012-2014.

In the final set of publications leveraging those data will complete the Planck legacy with the new and more precise results included. The 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 certain data (that of polarisation).

The notorious Max Planck (middle) was awarded with the Nobel of physics in 1918 for his discovery of energy quanta
The notorious Max Planck (middle) was awarded with the Nobel of physics in 1918 for his discovery of energy quanta

This final “legacy” round of results will imply an even more precise determination of the cosmological parameters and of the properties of the components of our galaxy. The newest Planck data will be essential to complement those obtained by the next cosmological experiments starting during the following decade, such as the ESA Euclid satellite (to be launched in 2021) 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.

IFCA Participation

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 and the rest of the members of the cosmology group at IFCA who participate in Planck, hold the status of Planck Scientist and belonging to the LFI Core Team and have played a relevant role in the achievement of the important legacy left by Planck. Regarding 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 the Department of Communication Engineering (DICOM) of the University of Cantabria (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.

Image credits

The images “Front view of the Planck satellite” and “Planck and the cosmic microwave background” are copyright of the European Space Agency and were used according to their usage policy.

The frontpage picture of Max Planck is in the public domain and was downloaded from Wikimedia Commons.

The picture of Walther Nernst, Albert Einstein, Max Planck, Robert Millikan and Max Von Laue in 1931 is in the public domain and was downloaded from Wikipedia.