The motions of galaxies can put General Relativity to the test
All galaxies in our Universe (ours included!) move with respect to each other in two ways. Firstly, as has been known for over a century now, the Universe is expanding, moving distant galaxies away from each other. On top of this, galaxies move toward and around high-density regions of the Universe due to gravitational forces. For some spiral and elliptical galaxies, we can measure their velocity towards or away from us due to gravity – known as their peculiar velocity – by taking their spectra and using scaling relationships such as Tully-Fisher or Fundamental Plane.
Our model of the Universe is based on Einstein’s Theory of General Relativity (GR) which requires “dark matter” to account for some of the gravitational effects that we observe. GR provides strong predictions for how galaxies should move under the influence of gravity. In particular, it predicts the rate at which galaxies fall into high-density regions, called the growth rate. Comparing the relative positions and peculiar velocities of galaxies in the Universe allows us to measure this growth rate and gives us a way to test GR.
Australia is home to two of the largest surveys of galaxy peculiar velocities ever undertaken: the 6-Degree Field Survey velocity elliptical galaxy sample (containing 9,000 peculiar velocity measurements) and the 2MASS-Tully Fisher Survey (containing 2,000 spiral galaxies). In a recent paper, CAASTRO researcher Dr Cullan Howlett (ICRAR-UWA), together with CAASTRO members Prof Lister Staveley Smith (also ICRAR-UWA) and A/Prof Chris Blake (Swinburne University of Technology), analysed whether combining the data from these surveys will reduce the statistical error inherent to individual surveys. They showed that correlating the positions and peculiar velocities of galaxies in these surveys can provide some of the best measurements of the growth
rate – with an improvement of around 20% – and the strongest tests of GR to date.
Over the next five years, Australia will begin two new galaxy surveys, TAIPAN at the Siding Spring Observatory and WALLABY on the Australian SKA Pathfinder (ASKAP). Combined, these will measure ten times more peculiar velocities than are currently known. In their publication, the researchers forecast that this sample of galaxies will produce an unparalleled measurement of the growth rate, with an error as low as 3%. This could finally provide overwhelming evidence of our current model of gravity.
Cullan Howlett, Lister Staveley-Smith & Chris Blake in the Monthly Notices of the Royal Astronomical Society (2017): “Cosmological Forecasts for Combined and Next Generation Peculiar Velocity Surveys”