UC Santa Cruz

This thesis investigates the observational and phenomenological consequences of large gravitationally coupled dark sectors, using black holes and high-frequency gravitational waves as the primary probes. The work spans four interconnected studies, progressing from broad model-independent constraints on dark sector size, through the physics of primordial black holes as a dark matter candidate, to a detailed reexamination of black hole evaporation dynamics, and culminating in a quantitative assessment of high-frequency gravitational-wave signals from superradiant boson clouds. The first study derives comprehensive constraints on the number of dark degrees of freedom N from multiple astrophysical and collider observables. Hawking evaporation of stellar-mass black holes observed by LIGO/Virgo provides the dominant bound: using the merger event GW190924 021846, with chirp mass 5.7 M⊙ and inferred age (1.16±0.04)×1010 yr, we constrain N ≲ 5×1058 for massless spin-0 dark sectors and N ≲ 2.5×1060 for spin-2, using full greybody-factor calculations with the BlackHawk code. Complementary limits from ultrahigh-energy cosmic-ray collisions (N ≲ 1071), LHC monojet searches (N ≲ few × 1062), and supernova cooling (N ≲ 6 × 1068) are derived via detailed cross-section calculations, confirming that black hole evaporation provides the strongest quasi-model-independent constraint on gravitationally coupled dark sectors across a wide range of dark sector particle masses. The second study examines primordial black holes (PBHs) in the asteroid-mass window (10−16–10−11 M⊙), where they can constitute the entirety of the cosmological dark matter and are inaccessible to conventional gravitational-wave detectors. We update the calculation of the gravitational-wave sight distance for narrow-band resonant microwave cavities such as ADMX, showing that it becomes chirp-mass independent at large PBH masses, saturating at dsens ≃ 0.03 AU. Constructing the most optimistic possible merger event rate— incorporating early-time clustering, local dark matter over densities, dichromatic mass functions, and fPBH = 1—we find that detectable merger events are extremely rare even under maximally favorable assumptions, motivating the search for alternative high-frequency gravitational-wave sources from PBH systems. The third study extends the theory of black hole evaporation to Kerr and Reissner–Nordström geometries in the presence of large dark sectors, using the Page factor formalism. We demonstrate that the canonical evaporation hierarchy—charge neutralized first, followed by spin loss, then mass—can be qualitatively reversed when the number of uncharged dark sector species exceeds a critical threshold Ncrit ≈ 220 (for a 1017 g black hole). Above this threshold, the mass-loss rate outpaces the chargeloss rate, causing the effective charge Q∗ = Q/M to grow during evaporation and potentially driving the black hole toward a near-extremal state. We further characterize the competing roles of Schwinger pair production, which restores rapid discharge for sufficiently light and highly charged black holes, and superradiance, which can extract angular momentum 108 times faster than Hawking radiation for optimal boson–black hole mass combinations. The fourth study treats superradiance as a gravitational-wave source rather than merely a spin-loss mechanism, presenting a unified framework for gravitational wave emission from both isolated and binary-perturbed gravitational atoms around primordial black holes. Ultralight bosons with masses µ ∼ 10−8 –10−5 eV form macroscopic coherent clouds around PBHs in the mass range 10−6 –10−3 M⊙, emitting quasi monochromatic gravitational waves in the MHz–GHz band at frequencies fann ≃ µ/π and ftr ≃ µα 2 /(4π), directly accessible to ADMX-like resonant cavities. We derive analytic frequency-domain strain templates for both the annihilation and level-transition channels, and apply the Landau–Zener formalism to binary-induced resonant transitions. Although binary-driven bursts satisfy detector ring-up criteria over a range of parameters, their characteristic strain falls orders of magnitude below current ADMX sensitivity at astrophysically plausible distances (≳ 1 kpc), with detectable events requiring source distances ≲ 1 AU. We quantify the improvements in strain sensitivity, bandwidth, and ring-up time required to render these signals observable, and identify PBH gravitational atoms as a theoretically well-motivated target for future highfrequency gravitational-wave searches. Taken together, these studies establish black holes as stellar-mass merger remnants, primordial dark matter candidates, and hosts of superradiant boson clouds—as uniquely powerful instruments for probing physics beyond the Standard Model through the gravitational window.