M.Sc Student | Rozner Mor |
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Subject | Signature of BEC Dark Matter on Galactic Scales |

Department | Department of Physics |

Supervisor | Dr. Vincent Desjacques |

Full Thesis text |

In this thesis, we explore special phenomena of the ultra-light axion model, and their possible signatures in the galactic scales. We focus on two phenomena: one is the backreaction of ultra-light axions and the second is the resonance between ultra-light axions and binary pulsars.

In the backreaction research, we investigate how coherent oscillations backreact on the evolution of the condensate wave function of ultra-light axions in the non-relativistic regime appropriate to cosmic structure formation. The coherent oscillations induce higher harmonics beyond the fundamental mode considered so far when a self-interaction is present, and imprint oscillations in the gravitational potential. We emphasize that the effective self-interaction felt by the slowly-varying envelope of the wave function always differs from the bare Lagrangian interaction potential. We also point out that, in the hydrodynamical formulation of the Gross-Pitaevskii equation, oscillations in the gravitational potential result in an attractive force that counteracts the effect of the quantum pressure arising from the strong delocalization of the particles. We have pointed out that, even in the case of a simple Lagrangian bare quartic coupling λϕ^4 , higher-order harmonics backreact on the evolution of the fundamental mode. Our findings may have implications for global stability analyzes of axion condensates, in which the stability region is inferred by minimizing energy functionals of trial equilibrium solutions.

In the resonance research, we investigate how resonance between binary system and ultra-light axion background can affect the motion of the binary. These resonances lead to changes in the period of the binary, and include cumulative effects. These effects are nontrivial and might be neglected accidentally if we use integration over orbits, i.e. if we use secular treatment, rather than instantaneous analysis. We use N-body simulations in order to test the effect of the resonance and investigate the discrepancy between resonant and non-resonant orbits and the signal-to-noise ratio of the phenomenon. The signal-to-noise ratio can vary considerably depending on whether one sits near a resonance, or away from them, and stay roughly constant for the large eccentricities tested. A future research will give an analytical estimation of the signal-to-noise ratio, and will be compared to the code we wrote during this research. The effect can be probed as well far from resonance, but requires relatively high density of dark matter. By observing this phenomenon, one could do one step towards ratifying that the dark matter in the vicinity of the binary behaves as ultra-light axions. By observing the absence of the phenomenon, one could constrain the

mass range of ultra-light axions.

Our results can be generalized to any other similar type of ultra-light boson dark matter candidate. This research can yield many interesting and important results and generalizations. The backreaction research can effect on the structure formation by changing the corresponding Jeans scale. The resonance research has an important impact by introducing a dark matter background to many phenomena and processes, that signatures can be read from the interaction with it, including wakes from the passage of single star and processes that involve binaries.