The peculiar motions of galaxies and the masses of neutrinos

The universe is expanding. Because of this, we find for the most part, galaxies in space are moving away from us due to this expansion. However, because of gravity, galaxies in clusters are attracted to each other. Thus, galaxies move towards each other as well. These motions due to gravitational interactions of galaxies are called peculiar motions. On what might seem like a completely unrelated topic, neutrinos are one of the more mysterious particles in our standard model. This is at least partly because neutrinos are notorious for being difficult to detect, but also because we initially believed neutrinos were massless.

Neutrinos interact via the weak nuclear force, and only just over 20 years ago, we found out neutrinos have masses as well - via the discovery of a phenomena dubbed ‘neutrino oscillations’ where neutrinos change between different types as they travel – so they must interact gravitationally too. While experiments have confirmed that neutrino oscillations occur, they don’t tell us what the actual mass of each neutrino type is. In fact, so far, no experiment does.

The fact that neutrinos have mass, has important consequences for cosmology. The presence of massive neutrinos should have, perhaps surprisingly, influenced the distribution of matter, and thus galaxies in the universe, that we see today. In fact, this fact has been used so that galaxy surveys and even measurements of the Cosmic Microwave background (CMB) can put an upper bound on what the masses of neutrinos can be. So far, we know, thanks to the Planck satellite which has measured the CMB, they are at least half a million times lighter than the mass of even an electron.

The plots below show how the matter power spectrum (which we measure from a galaxy survey) and the CMB temperature power spectrum (something else that comes from measurements of the CMB photons in the sky) changes if the sum of masses of neutrinos changes, thus allowing us to derive constraints. It should be noted here a very wide range of neutrino masses (some values of which we know aren’t possible) have been used in these plots deliberately to exaggerate the effects of their masses.

Fortunately, we also expect that the masses of neutrinos should impact our measurements of the peculiar motions of galaxies at the present day. This is because, ultimately, we know the motions of galaxies in the universe trace the same underlying distribution of matter as the distribution of galaxies themselves, which, like the CMB also, is influenced by the presence of massive neutrinos in the early universe, and thus contains useful informaiton about their masses.

From our results we expect that galaxy peculiar motions may be able to contribute to the problem of determining the mass of each neutrino. The below plot shows forecasts for 1 and 2 sigma contours on cosmological parameters including the sum of neutrinos masses for the WALLABY redshift survey (blue), then the same when information from the WALLABY peculiar velocity survey is included (red).

by Abbé Whitford