Neutrinos seen in clustering galaxies

In the most remote times, the universe was an energetic mixture of strongly interacting particles. The first particles to break free from this dense soup were neutrinos, the lightest and least interacting particles in the standard model of particle physics. These neutrinos still surround us today, but they are very difficult to detect directly because they interact so weakly. An international team of cosmologists, including Daniel Baumann and Benjamin Wallisch of the University of Amsterdam, have now successfully measured the influence of this "cosmic neutrino background" on how galaxies have become grouped together during the evolution of the universe. The search was published in Physical Nature this week.

When a pebble falls into a pond, it creates ripples on the surface of the water that move outward in concentric circles. Similarly, regions of primordial plasma with the highest densities produced layers of matter (mainly protons and electrons) propagating outward at a speed close to the speed of light, but not all of them. did. This external thrust of matter was created by the large number of high energy photons in the primitive universe.

About 380,000 years after the Big Bang, when free electrons were captured by protons to combine into electrically neutral hydrogen atoms, the propagation of these layers of matter ceased as the photons ceased. interact with the electrons. The resulting frozen matter shells became dense regions in which an excess of galaxies would eventually form. This predicts that an increased number of pairs of galaxies should be found at a distance of about 500 million light years, corresponding to the size of the frozen shells created in the primitive universe. In 2005, this effect was observed for the first time in the galaxy distribution measured by the Sloan Digital Sky Survey (SDSS).

A neutrino effect

The presence of cosmic neutrino background affects the image described above in a subtle but relevant way. Once the neutrinos decoupled from the rest of the primordial matter, they began traveling at the speed of light, slightly faster than the rest of the matter. Neutrino shells have therefore taken precedence over shells of matter. As a result, the gravitational pull of the neutrinos has slightly deformed the material shells, creating small distortions in the seeds for galaxy formation much later. This influence of cosmic neutrinos on the large scale structure of the universe should be detectable by carefully analyzing the clustering of galaxies.

In their article, Baumann and his collaborators studied new SDSS data covering about 1.2 million galaxies over a distance of about 6 billion light years. Their statistical analysis confirms the expected signature of the cosmic neutrino bath that occupies all the space. This new measurement constitutes an interesting confirmation of the standard cosmological model that links neutrino production one second after the Big Bang to the clustering of galaxies billions of years later.

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More information:
Daniel Baumann et al. First constraint on the neutrino-induced phase shift in the spectrum of acoustic oscillations of baryon, Physical Nature (2019). DOI: 10.1038 / s41567-019-0435-6