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Planck without signs of the primordial universe

Scientists of this mission, among them an IFCA group, made a few remarks that they believed were related to gravitational waves produced after the Big Bang, but they now confirm that they have another cause.

The Planck mission, in which a group of researchers from the Institute of Physics of Cantabria (IFCA, University of Cantabria-CSIC Mixed Centre), has spent years revealing data about the cosmic microwave background, or what amounts to the same, about the remains that the Big Bang and the following cosmic expansion have left. The study of this legacy of light emitted billions of years ago is the key to understanding how the universe was created and how it has evolved.


Last year, scientists managing data from the European satellite believed that they had discovered the remains of this primordial universe in full inflation, when they announced the detection of model B polarization of the cosmic microwave background. If these signals were then understood as originating from the primordial gravitational waves, it has now been demonstrated that its origin is another one: it is really a footprint left by the interstellar dust in our Galaxy, the Milky Way.


The results are the result of a combined analysis of data obtained by Planck and the BICEP2 and Keck telescopes, both located in the South Pole. In March 2014, the U.S. team who led this experiment suggested the discovery of this "first echo of the universe", which is now being dismissed.


Light, the footprint of cosmic expansion.

From 13,800 million years ago, the universe has evolved from its primordial state - hot, dense and uniform – until it became the cosmos as we know it today, full of galaxies, stars and planets. The light emitted 380,000 years after the Big Bang is the cosmic microwave background, which was observed in 2010 by Planck all over the sky with unprecedented precision.


One of the more recent challenges of astronomers is to look for the footprint of cosmic inflation experienced by the universe in its infancy. With this intention, gravitational waves play a key role, since they would have generated into this expansive phase. These disturbances leave a mark on another of the fundamental characteristics of the cosmic background, its polarization (polarized light waves vibrate preferably in a certain direction, creating complex patterns in the sky).


"The search for this unique record of the primitive universe is as difficult as it is exciting, given that this subtle signal is hidden within the polarization of the cosmic background, which in turn accounts for only a small percentage of the total light," explains Jan Tauber, a scientist on the ESA project for Planck.


On this occasion, the data seemed to point to the fact that the signals collected by scientific instruments came from primordial gravitational waves, but a more exhaustive analysis rules that out. By subtracting "the contamination" caused by interstellar dust, which also emits polarized light and is present in the whole sky, it cannot be confirmed that the signal is a trace of the primordial universe.


The scientists on that mission do not rule out discovering it in the future. "Gravitational waves could still remain in the data", points out Brenda Crill, Member of the Planck and BICEP2 of NASA teams.


Cantabrian contributions

The Planck satellite was launched by the European Space Agency (ESA) in 2009 and its scientific work lasted until 2013. The IFCA researchers involved in the mission are members of the Group of Observational Cosmology and Instrumentation, who also participate in the QUIJOTE experiment, located at the Observatorio del Teide (Teide Observatory) in Tenerife for the study of the polarization in the cosmic background at lower microwave frequencies.


This team, together with others belonging to the UC Engineering Communications Department, worked on the design, development and calibration of Low Frequency Instrument ( LFI) Planck, consisting of 22 receivers. In addition, the IFCA participates very actively in the scientific exploitation phase of the data.



"A Joint Analysis of BICEP2/Keck Array and Planck Data", BICEP2/Keck and Planck collaboration, Physical Review Letters.