Shining Light Through the Dark: Observational Signatures of Exotic Astrophysical Dark Objects

Abstract

The precise nature of dark matter remains one of the most profound unresolved questions in fundamental physics. While traditional searches have primarily targeted microscopic, weakly interacting particle candidates, the persistent absence of direct detection motivates a broader exploration of macroscopic, structured dark configurations. This thesis investigates the theoretical frameworks and observational signatures of a class of Exotic Astrophysical Dark Objects (EADOs), focusing on astrophysical Q-balls, mini-boson stars, primordial magnetic black holes (MBHs), and millicharged dark matter plasma environments around black holes. We first establish the theoretical viability of astrophysical Q-balls by deriving the conditions required for their existence, gravitational stability, and non-gravitational self-consistency. These conditions define the allowed Lagrangian parameter space in which Q-balls can remain stable macroscopic dark objects rather than collapsing gravitationally. Building on this framework, we analyze their gravitational lensing signatures. In the photometric regime, we analyze the distinctive microlensing signatures caused by extended thin-wall and beyond-thin-wall density profiles, showing how finite-size structure modifies the caustic-crossing behavior relative to idealized point-mass lenses. Using legacy microlensing data from EROS-2, OGLE-IV, and HSC-Subaru, we constrain astrophysical Q-balls in the range $10^{-7}\text{--}10^{-2}\,M_\odot$ to constitute no more than approximately $1\%$ of the total dark matter abundance. Extending this analysis to the high-precision astrometric regime, we study the centroid shifts generated by thin-wall Q-balls and mini-boson stars. We demonstrate that astrometric microlensing provides a substantially enlarged discovery volume relative to photometric searches, since the astrometric signal decays as $\theta_{\text{\tiny S}}^{-1}$ rather than $\theta_{\text{\tiny S}}^{-4}$. We identify caustic-induced centroid jumps as a distinctive kinematic signature of extended lenses. Using Gaia DR3-based projections, we forecast optimal event yields of order $6000$ astrometric microlensing events over a 10-year observation window and derive 90\% confidence-level exclusion limits over the mass range $10^{-1}\text{--}10^{7}\,M_\odot$ and physical radii up to $10^6\,\mathrm{AU}$. In the intermediate stellar-mass regime, these bounds can constrain the fractional dark matter abundance to $f_{\text{\tiny L}}\lesssim 10^{-3}$. We subsequently probe the dark sector through electromagnetic and strong-gravity phenomena. For primordial magnetic black holes, we show that extremal magnetic charges $Q_{\text{\tiny BH}}\gtrsim 10^{22}\,\mathrm{A~m}$, corresponding to masses $M_{\text{\tiny BH}}\gtrsim 10^{-6}\,M_\odot$, can generate Faraday rotation measures within current observational sensitivity. We also identify a topological polarization discriminator: unlike standard astrophysical dipoles, which yield a positive spatial polarization measure, the unbroken axial symmetry of an MBH enforces a null measure, providing a characteristic polarization signature for primordial magnetic monopoles. We further investigate the influence of millicharged dark matter plasmas on black hole shadows. By solving the modified electromagnetic dispersion relations in these plasma environments, we show that the physical photon sphere expands while the apparent shadow radius contracts. For physically viable sub-eV millicharged particles, this refractive contraction can reach approximately $25\%$ for a uniform density profile and can approach complete shadow suppression under radial accretion. These results establish black hole shadow measurements as a potential probe of macroscopic dark plasma environments. Finally, we analyze the tidal response of thin-wall Q-balls. In the thin-wall limit, these configurations obey the volume scaling $M_{\text{\tiny Q}}\propto R_{\text{\tiny Q}}^3$ and remain highly diffuse, with compactness typically satisfying $\mathcal{C}_{\text{\tiny Q}}\lesssim 10^{-4}$. By simplifying the coupled metric-scalar perturbation equations in this limit, we derive analytical expressions for the exterior logarithmic derivative and the quadrupolar tidal Love number. We find that thin-wall Q-balls exhibit extreme tidal deformability, with $k_2\sim 10^{16}\text{--}10^{28}$ across the mass range $0.1\text{--}100\,M_\odot$, substantially exceeding the corresponding values for neutron stars and compact boson stars. Collectively, the theoretical derivations and observational forecasts presented in this thesis establish a unified, multi-wavelength, and multi-messenger strategy for identifying, distinguishing, and constraining macroscopic dark matter configurations through photometric, astrometric, electromagnetic, black-hole-shadow, and gravitational-wave observables.