MSc defense by Isabella Paola Carucci – University of Copenhagen

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MSc defense by Isabella Paola Carucci

Title: "Matter around black holes: testing scalar-tensor theories of gravity" and "Particle ejection during mergers of dark matter halos"


This thesis reports on two distinct investigations: black holes surrounded by matter are studied in scalar-tensor theories of gravity and dark matter particle ejections occurring during halo mergers are analysed through various types of simulations.

In general relativity, stationary asymptotically flat electrovacuum and regular black holes are described by the Kerr-Newman family of solutions; this is still valid in scalar-tensor theories of gravity. In this thesis it is shown that within this class of theory exist two mechanisms which can render black holes unstable when matter is close-by: the coupling between the scalar field introduced by the theory and matter induces an effective mass for the scalar, changing the dynamics of the system. The square of this effective mass can either be positive or negative according to the specific theory and to the nature of the existent matter. When the effective mass squared is negative and the matter density is above a certain limit, the first mechanism is triggered: the initial general relativity configuration is not stable and the system ends up in a hairy state, with the scalar field developing a non-trivial profile. This is a black hole version of spontaneous scalarization already studied in neutron stars. The second instability is associated with superradiance and is present when the effective mass squared is positive and when the black hole spin exceeds a certain threshold; the amplitude of the unstable modes of the scalar field exponentially grows in time at the expense of rotational energy from the black hole. The second mechanism is also responsible for a resonant e↵ect in the superradiant scattering of monochromatic scalar waves, with amplification factors as large as 105 or more, prospecting detectable deviations from general relativity. The phenomena are first analysed analytically in a simplified configuration, then, using simple models for the matter profile around the black hole, a numerical check is done to confirm the emergence of an instability.

Dark matter halos are built from accretion and merging and are believed to be the framework of the present large scale structure of the universe. During merging, some of the kinetic energy has to be disposed in order to reach a new equilibrated configuration. Baryonic matter usually radiates away the energy in excess, dark matter structures relax by expelling some of their particles and let them carry energy away. A consequence of halo mergers is indeed the ejection of particles with velocities higher than the escape velocity, thus allowing the expelled particles to leave forever the resulting structure. N-body simulations and single-particle smoothfield simulations are performed in this thesis to demonstrate that this phenomenon is a mean-field e↵ect. Studying a range of minor mergers, it is confirmed that between 5 − 15% of the particles from the smaller of the two merging structures are expelled. It is argued that the rapid changes to the field potential are responsible for such ejection. These findings provide an explanation for the high-velocity component dark matter particles which are observed in cosmological numerical simulations.

Friday 6 Sept.
Time: 10:15
Place: Dark Lounge