“Neutrino astronomy: Ghostly messengers from the cosmos”Dr Matthew Malek, University of Sheffield,

Wolfgang Pauli first proposed the existence of the neutrino in 1930 following the discovery two decades earlier that beta-decay did not appear to conserve energy, but it was to be a quarter of a century later still that Cowan & Reines actually recorded anti-neutrinos in a nuclear reactor. As Dr Malek explained in his Zoom lecture to the Society, progress in understanding them and what they can teach us is severely hampered by the very difficulty of actually detecting them.

This is because neutrinos interact with matter only weakly – it would require one light year of lead to actually stop one! So why bother studying them? Well, they’re second only to photons in abundance in the universe and it may also be that a large fraction of dark matter consists of Big Bang neutrinos. They come from a variety of sources – including the atmosphere as a result of cosmic rays, the Sun, supernovae and deep space gamma ray bursts. From neutrino research we now have a better understanding of the Big Bang, we understand how the Sun shines, they provide a window into the process of star death and can be used to study very distant objects in the universe.

For example regarding the Sun, we now know it takes about a million years for light to leave it, we know fusion powers it and we know how hot the Sun is; studies of neutrinos verify our understanding of the “Standard Solar Model” including how different elements are produced (H, He, Be, O, C, N, etc). Concerning supernovae, we know that 99% of the energy from a core collapse is released as neutrinos and only 1% as light. Astronomers are now asking what is the role of the neutrino in astronomy and hope that these 'ghostly' particles can explain why there is matter in the Universe at all.

Dr Malek’s research has taken him from Japan (Kamioka Observatory), Argentina (Pierre Auger Observatory), Oxford and now Sheffield. He concluded his talk by describing recent work emerging from the IceCube Neutrino Observatory, a 1 km3 detector at the South Pole where the first evidence of neutrinos from other galaxies was found. In July 2018, IceCube announced that for the first time it had tracked neutrinos back to their source blazar known as TXS-0506+056, correlating the results with observations using other astronomical techniques.

As WAS President, Prof Bob Lambourne, has explained to us previously, we are now in an era of multi-messenger astronomy – neutrino research is one important component of this (as demonstrated by three Nobel Prizes awarded in the field). Whilst progress in studying neutrinos is accelerating, it remains true that almost a century has elapsed since they were first detected. But the signs are that this extremely valuable area of research has stopped being such a slow-burner!

Sandy Giles