DFG-NSF: Observational Tests of Covariant Emergent Gravity
Final Report Abstract
The purpose of this project was to investigate the possibility that observations in astrophysics and cosmology have been falsely attributed to cold dark matter. Dark matter, according to the currently prevailing hypothesis, is made of clouds of elementary particles of unknown nature which neither emit nor absorb light (are “dark”) and move slowly (are “cold”). The dark matter hypothesis is known to have problems, not so much because it is in conflict with data, but because it delivers no explanation for some observed regularities in data. The competing hypothesis – modified gravity – posits that there is no additional matter in the universe, but that instead the laws of gravity are different for entire galaxies and galaxy clusters than in our immediate vicinity and the solar system. It successfully explains the observed regularities in data but struggles with some observations, notably galaxy clusters and the cosmic microwave background. In this project, we investigated the possibility that the correct hypothesis is a combination of both. During the course of our work, we came to dub these approaches “hybrid models”. The overall idea of hybrid models is straight-forward: Use cold dark matter in the regimes where it works better than modified gravity, and use modified gravity when it works better than dark matter. We found this to work very well within galaxies, where we tested several models both on the Milky Way and on a sample of galaxies. We had also already in a previous work found that it works well for strong gravitational lensing. However, in what is the main result of this project, we found that these models have a common problem which is weak gravitational lensing. The problem is that we have two types of observations about the propagation of light in galaxies. We know from the observation of a gravitational wave event with a corresponding light flash (GW170817) that light and gravitational waves travel the same in galaxies, at least to very good precision. This means if gravity is modified, it does not change much about the propagation of light inside galaxies. This is not a problem for strong gravitational lensing because these observations have a low sensitivity to dark matter anyway, but it is a problem for weak gravitational lensing which sees a strong deviation from normal gravity in a regime where hybrid models lead to a small deviation.
Publications
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The Milky Way’s rotation curve with superfluid dark matter. Monthly Notices of the Royal Astronomical Society, 498(3), 3484-3491.
Hossenfelder, S. & Mistele, T.
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Mori-Zwanzig Formalism for General Relativity: A New Approach to the Averaging Problem. Physical Review Letters, 127(23).
te Vrugt, Michael; Hossenfelder, Sabine & Wittkowski, Raphael
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Galactic mass-to-light ratios with superfluid dark matter. Astronomy & Astrophysics, 664, A40.
Mistele, T.; McGaugh, S. & Hossenfelder, S.
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Aether scalar tensor theory confronted with weak lensing data at small accelerations. Astronomy & Astrophysics, 676, A100.
Mistele, T.; McGaugh, S. & Hossenfelder, S.
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Dark matter profiles of SPARC galaxies: a challenge to fuzzy dark matter. Monthly Notices of the Royal Astronomical Society, 523(3), 3393-3405.
Khelashvili, M.; Rudakovskyi, A. & Hossenfelder, S.
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Superfluid dark matter in tension with weak gravitational lensing data. Journal of Cosmology and Astroparticle Physics, 2023(09), 004.
Mistele, T.; McGaugh, S. & Hossenfelder, S.
