DFG-NSF: Observational Tests of Covariant Emergent Gravity

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The here proposed project aims to test a new explanation for dark matter and dark energy. We want to investigate whether the theory fits with the observations of galactic rotation curves and strong gravitational lensing.Evidence is overwhelming that the universe contains two little-understood sources of gravity: dark matter and dark energy. They can be collectively parameterized as fluids with pressure and energy-density. This makes it easy to incorporate them into general relativity. But their microscopic properties as well as their origin are still unknown. Describing dark matter as a fluid works well, but especially for the dynamics of galaxies, it has various shortcomings. It predicts, for example, density peaks in galactic cores and too many small satellite galaxies around larger hosts, neither of which has been observed.For galaxies, a different explanation for our observation works better. This different explanation is that the law of gravity must be changed, which is known as "modified gravity". Modified gravity works well for galaxies, but not so well in other situations, notably in the early universe and for galaxy clusters.In this project we want to explore the idea that the truth may be somewhere in the middle. For this, we use a recently developed framework called "Covariant Emergent Gravity." Covariant emergent gravity modifies the normal gravitational force on galactic scales and it also gives rise to dark energy. Previous work has shown that this theory fits well on some of the existing data. One of the work-items of this project is to study in more detail whether it fits with (already existing data) of galactic rotation curves and gravitational lenses.Covariant Emergent Gravity furthermore describes a condensate at low temperature. At high temperature, however, the effects of modified gravity should fad out and go away. To understand how this happens and under exactly which circumstances, the theory must be further developed. This, too, will be done for this project.If successful, this project can deliver strong empirical evidence for the nature of dark matter, and solve a riddle that has plagued astrophysicists since the 1930s.