Hybrid MOND dark matter models at very small accelerations

• Applicant: Dr. Tobias Mistele
• Subject Area: Astrophysics and Astronomy ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514562826

One of the biggest unsolved questions in physics is that of dark matter. That is, the question of what explains the missing mass on both cosmological and galactic scales. On cosmological scales, the simplest explanation is a pressureless fluid consisting of cold dark matter particles. This explains, for example, the anisotropies in the cosmic microwave background. On the other hand, on galactic scales, the simplest explanation is in terms of a specific modified gravity theory called MOND (Modified Newtonian Dynamics). This explains, for example, galactic scaling relations like the Radial Acceleration Relation (RAR). This motivates hybrid MOND dark matter models. These are models that combine the successes of dark matter models on cosmological scales and the successes of MOND on galactic scales by having both a pressureless fluid on cosmological scales and a MOND-like force in galaxies. Examples are superfluid dark matter (SFDM) and the recent SZ model. In these models, there must be a transition between MOND-like behavior in galaxies and dark-matter-like behavior at larger scales. Typically, this transition happens somewhere between the scale of individual galaxies and the scale of galaxy clusters. The purpose of the proposed project is to test whether the MOND-like behavior in current hybrid models extends to sufficiently large distances (corresponding to small accelerations) to explain observations. This is motivated by two recent observations that find MOND-like behavior at unprecedentedly large galactocentric distances. The first is an extension of the RAR using weak-lensing data. The second is an analysis of the relative velocities of isolated galaxy pairs. We will determine whether SFDM and the SZ model can explain these observations showing MOND-like behavior. To this end, we will first determine whether the two models can fit these observations at all. A second test pertains to a boundary condition (the chemical potential of a condensate) that is needed in these models to solve the equations of motion. The aim of both models is to explain the observed MOND-like behavior without having to carefully adjust this boundary condition separately for each galaxy. Thus, we will determine whether these models can fit the MOND-like observations without having to tune the boundary conditions in this way. This will reveal how well SFDM and the SZ model can explain observations of MOND-like behavior at very small accelerations. In addition, our investigations will give a general idea of what is and is not possible in hybrid MOND dark matter models and will thus inform future model building.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Stacy S. McGaugh, Ph.D.