Neutrinos observed in the clustering of galaxies

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Shortly after the big bang, the universe was an energetic mixture of particles with strong mutual interaction. The first particles that managed to free themselves from this dense primordial soup were the neutrinos, the lightest and weakest interacting particles from the standard model of elementary particles. These neutrinos are still all around us today, but are very difficult to observe immediately because their interaction is so weak. An international team of cosmologists, including Daniel Baumann and Benjamin Wallisch from the University of Amsterdam, has now succeeded in measuring the influence that this 'cosmic neutrino background' has had on the way galaxy clusters formed during the evolution of the universe. The research was published in Nature Physics.

If you throw a stone into a lake, small ripples appear that move out in circles. In a similar way, high density regions in the primordial plasma of the early universe produced layers of matter (mainly protons and electrons) that moved outward with almost the speed of light. This pushing out of the matter was a consequence of the large number of high-energy photons - light particles - in the early universe.

Frozen layers

Some 380,000 years after the big bang, when most free electrons were captured by protons and thus formed electrically neutral hydrogen atoms, the spreading of matter layers stopped because photons no longer interacted with electrons. The resulting 'frozen' matter shells became areas of high matter density from which eventually a large number of galaxies would arise. From this it can be predicted that an extra large number of pairs of galaxies would have to be found at a mutual distance of about 500 million light years - a distance corresponding to the size of the frozen layers that arose in the early universe. In 2005, this effect was indeed observed for the first time in the distribution of galaxies in the universe, as measured by the Sloan Digital Sky Survey (SDSS).

The effect of neutrinos

The presence of the cosmic background of neutrinos has a subtle but relevant influence on the process described above. When the neutrinos detached from the rest of the primal matter, they began to move at the speed of light - a little faster than the rest of the matter. As a result, the gravitational attraction of the fast-moving neutrinos distorted the layers of matter a little, causing small disturbances in the seeds that would later form galaxies. This influence of the cosmic neutrinos on the structure of the universe on a large scale should be detectable by making an accurate analysis of the clustering of galaxies.

In their article, Baumann and his colleagues studied new SDSS data from about 1.2 million galaxies out to a distance of approximately 6 billion light years. Their statistical analysis confirms the expected signature of a sea of cosmic neutrinos that fills the entire universe. The new measurement provides an interesting confirmation of the standard cosmological model in which the production of neutrons, one second after the Big Bang, is linked to the clustering of galaxies billions of years later.

This research was partly made possible by a Vidi subsidy from NWO.

Source: UvA press release

Reference: First constraint on the neutrino-induced phase shift in the spectrum of baryon acoustic oscillations, Daniel Baumann, Florian Beutler, Raphael Flauger, Daniel Green, Anže Slosar, Mariana Vargas-Magaña, Benjamin Wallisch and Christophe Yèche, Nature Physics, February 2019.

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