ETD

• black holes
• gravitational atoms
• long-lived
• quasi-bound states
• scalar field
• scalar wigs
• self-interaction

Ultralight scalar fields are well-motivated dark matter candidates and may form long-lived, spatially extended "clouds" around black holes, commonly referred to as gravitational atoms. The thesis investigates such configurations within a frequency-domain framework, focusing on nonrotating black-hole spacetimes and spherically symmetric scalar configurations.
For a free scalar field, established results are first reviewed and reproduced both in the test-field regime and with gravitational backreaction included. This provides a validation of the numerical method, which is then extended in the second part of the thesis. The main original contribution consists in incorporating quartic scalar self-interaction into the analysis, first in the test-field regime and subsequently together with gravitational backreaction. Within a quasi-stationary formulation, the effects of attractive and repulsive self-interactions on the scalar-cloud spatial extent, leakage rate, and compactness are systematically investigated, with particular attention to their possible phenomenological relevance. The analysis also examines the emergence of critical configurations in the mass-amplitude curves, which may be related to changes in radial stability, and assesses the temporal range of validity of the quasi-stationary description.