Exotic energy injection in the Dark Ages: impact on CMB and 21cm cosmology

Dark Matter remains one of the greatest unanswered questions in modern Cosmology, known to be cold, collisionless, and weakly interacting, making it hard to detect. My research in- vestigated its impact on the thermal history of the Dark Ages, using the latest CMB data and the redshifted 21 cm line as complementary observables. The CMB angular power spectrum is sensitive to extra energy injections into the cosmological medium, while the 21 cm signal probes Dark Matter effects at lower redshifts. I focused on two key candidates: WIMPs and massive Primordial Black Holes, examining their energy injection mechanisms: WIMP annihilations and PBH accretion. My analysis considered the free electron fraction, the medium temperature, and the “boost factor,” capturing late-time enhancements of WIMP annihilation. Using MCMC methods, I computed the most updated bound on the WIMP annihilation cross-section, incorporating data from Planck, ACT, SPT, and BAO. I found the new data offers no significant improvement over Planck alone, with bounds differing by O(10%) from Planck’s official results. I also studied PBH impacts via accretion models, including Bondi-Hoyle–Lyttleton and Park-Ricotti, finding a minimal potential 21 cm signal when CMB-allowed parameters and modern accretion models are considered. While fur- ther work is needed to assess astrophysical uncertainties and signal detectability with future experiments, my findings suggest a small, constrained impact from PBHs.