Signs of neutrinos from the dawn of time, less than a second after the Big Bang

The first 400,000 years after the Big Bang are inaccessible to us by using light; the material that filled the entire cosmos made it opaque. However, neutrinos interact very little with ordinary matter, so they could travel right through the opaque mess. Lots of these low-mass, fast-moving particles were formed in the first second after the Big Bang, so they could provide a sensitive probe of some of the very earliest moments in the Universe.

Unfortunately, these primordial neutrinos have never been detected directly, and they may have too little energy for us to ever detect them. But a new paper published in Physical Review Letters showed an unambiguous indirect detection using measurements of the cosmic microwave background light. This article marks the first clear measurement of the cosmic neutrino background, which is a significant confirmation of one of the major predictions of the Big Bang model.

The early Universe was a very different place. In the immediate aftermath of the Big Bang, matter and energy were compressed, and particles bashed into each other at higher energies than our colliders can achieve. But as the Universe expanded, it cooled off. Much like cooling a vapor turns it into liquid and then a solid, the entire cosmos underwent several profound changes as it expanded. Particles formed the first atomic nuclei, which later went from a hot plasma to stable atoms and so on.