The Big Bang, that giant explosion of more than 13 billion years ago, provides the accepted description of our Universe's beginning. We can trace with exquisite precision what happened during the expansion and cooling that followed that cataclysm, but the presence of neutrinos in that earliest phase continues to elude direct experimental confirmation.
Neutrinos, once in thermal equilibrium, were supposedly freed from their bonds to other particles about two seconds after the Big Bang. Since then they should have been roaming undisturbed through intergalactic space, some 200 of them in every cubic centimeter of our Universe, altogether a billion of them for every single atom. Their presence is noted indirectly in the Universe's expansion. However, though they are presumably by far the most numerous type of material particle in existence, not a single one of those primordial neutrinos has ever been detected. It is not for want of trying, but the necessary experiments are almost unimaginably difficult. And yet those neutrinos must be there. If they are not, our whole picture of the early Universe will have to be totally reconfigured.
Wolfgang Pauli's original 1930 proposal of the neutrino's existence was so daring he didn't publish it. Enrico Fermi's brilliant 1934 theory of how neutrinos are produced in nuclear events was rejected for publication byNature magazine as being too speculative. In the 1950s neutrinos were detected in nuclear reactors and soon afterwards in particle accelerators. Starting in the 1960s, an experimental tour de force revealed their existence in the solar core. Finally, in1987 a ten second burst of neutrinos was observed radiating outward from a supernova collapse that had occurred almost 200,000 years ago. When they reached the Earth and were observed, one prominent physicist quipped that extra-solar neutrino astronomy "had gone in ten seconds from science fiction to science fact". These are some of the milestones of 20th century neutrino physics.
In the 21st century we eagerly await another one, the observation of neutrinos produced in the first seconds after the Big Bang. We have been able to identify them, infer their presence, but will we be able to actually see these minute and elusive particles? They must be everywhere around us, even though we still cannot prove it.