A systematic study of all massive galaxy clusters in the local universe provides information on the lightest elementary particles: Scientists at the Max Planck Institute for Extraterrestrial Physics analysed an X-ray catalog to show that there is less structure in the universe today than what is expected from the cosmic microwave background observations of the very early universe. This discrepancy can be explained, if the three neutrino families have an overall mass of about half an electron-volt.
We are surrounded by them everywhere and they fly right through us, but we don’t feel them at all – the neutrinos, the strangest among the known elementary particles. They hardly interact with other matter, every second billions fly right through the Earth but only a fraction gets stuck. They are left over in large numbers from the Big Bang, about 340 million per cubic meter on average. Together with photons, the particles of light, they are the most numerous elementary particles in the universe.
Projection of the three-dimensional distribution of galaxy clusters detected in X-rays by the ROSAT satellite. The data are shown in galactic coordinates with the galactic plane in the center. The gap in the data is due to the “zone-of-avoidance”, an area around the galactic plane where the extinction by the galactic interstellar medium makes observations very difficult. Blue dots are in the northern sky, red dots in the southern sky.
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