Feb 27 2019
The universe, in the early times, was an energetic blend of powerfully interacting particles. The first particles to split free from this thick soup were neutrinos, the lightest and most feebly interacting particles of the Standard Model of particle physics. These neutrinos are still present today, but are very difficult to detect straightaway as they are so feebly interacting. An international group of cosmologists, including Daniel Baumann and Benjamin Wallisch from the University of Amsterdam, have at present been successful in measuring the impact of this “cosmic neutrino background” on the way galaxies have become bunched up while the universe evolved. The study was published in Nature Physics recently.
An artist’s impression of the shell-like clustering of galaxies in the universe. The precise shape of the shells is subtly affected by neutrinos that were produced just moments after the Big Bang. (Image credit: Zosia Rostomian (LBNL), SDSS-III, BOSS.)
When a pebble is thrown into a lake, it forms ripples on the water surface which travel outwards in concentric circles. Likewise, the regions in the primordial plasma with the largest densities created shells of matter (typically electrons and protons) spreading outwards at virtually, but not quite, the speed of light. This outward push of matter was produced by the huge number of high-energy photons in the early universe.
Frozen shells
About 380,000 years after the Big Bang, when the free electrons were trapped by protons to integrate into electrically neutral hydrogen atoms, the propagation of these shells of matter halted as the photons stopped to interact with the electrons. The subsequent frozen shells of matter turned out to be the dense regions in which a surplus of galaxies would ultimately form. This predicts that a greater number of pairs of galaxies should be located at a separation of around 500 million light years, matching to the size of the frozen shells formed in the early universe. In 2005, this effect was undeniably detected in the distribution of galaxies measured by the Sloan Digital Sky Survey (SDSS) for the first time.
A neutrino effect
The existence of the cosmic neutrino background impacts in a subtle, but pertinent way. After the neutrinos decoupled from the rest of the primordial matter, they began moving at the speed of light, somewhat faster than the rest of the matter. The shells of neutrinos thus went past the shells of matter. Accordingly, the gravitational pull of the neutrinos marginally deformed the matter shells, forming small distortions in the seeds for the creation of galaxies at much later times. This impact of the cosmic neutrinos on the significant structure of the universe should be detectable by judiciously examining the clustering of galaxies.
In their paper, Baumann and collaborators examined new SDSS data of around 1.2 million galaxies, out to a distance of approximately 6 billion light years. Their statistical analysis ratifies the expected signature of the bath of cosmic neutrinos that occupies all of space. This new measurement establishes an exciting confirmation of the typical cosmological model which connects the creation of neutrinos one second after the Big Bang to the grouping of galaxies billions of years later.
This study was partly sponsored by an NWO Vidi grant.