28 September 2021Séminaire – Margot Brouwer (Kapteyn, Groningen)

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Dark matter: ‘real stuff’ or gravity misunderstood?

Abstract
For almost a century, astronomers have been in conflict about the mysterious nature of dark matter. Is the additional mass that we observe in the Universe ‘real’, or is it the result of deviations from the laws of gravity as we know them? To shed light on this mystery the VLT Survey Telescope continuously observes millions of galaxies, in a decade-long effort called the KiloDegree Survey. Using weak gravitational lensing, we measure the distribution of gravity around more than 259,000 of these galaxies. In particular, we measure the Radial Acceleration Relation (RAR), which compares the gravitational acceleration from ordinary matter (the stars and gas in the galaxies) to the total gravitational acceleration including dark matter. Pushing the limits of RAR observations into the unknown outer regions of galaxies, we are able to perform unprecedented tests of the predictions from several dark matter models and alternative theories of gravity. The alternative gravity theories we test (both Modified Newtonian Dynamics and Erik Verlinde’s more recent theory of Emergent Gravity) do not acknowledge the existence of dark matter as a separate substance. They therefore predict a very close relation between the mass of ordinary matter and the additional ‘dark gravity’. In contrast, particle dark matter models predict that the relative ordinary-to-dark matter abundance can vary between galaxies. Luckily we have the numbers to split our galaxy sample into different types: young spiral galaxies versus older elliptical galaxies; which have very different formation histories. Measuring a different dark-to-ordinary matter abundance for both galaxy types would be very difficult for alternative gravity theories to explain…