Cosmological constraints on exotic injection of electromagnetic energy

Abstract: We compute cosmic microwave background (CMB) anisotropy constraints on exotic forms of energy injection in electromagnetic (e.m.) channels over a large range of timescales. These constraints are very powerful around or just after recombination, although CMB keeps some sensitivity e.g. to decaying species with lifetimes as long as $10^{25}\,$s. We review here complementary with CMB spectral distortions and primordial nucleosynthesis bounds, which dominate at earlier timescales. For the first time, we describe the effects of the e.m. energy injection on the CMB power spectra as a function of the injection epoch, using the lifetime of a decaying particle as proxy. We identify a suitable on-the-spot approximation. Our results are of interest not only for early universe relics constituting (a fraction of) the dark matter, but also for other exotic injection of e.m. radiation. For illustration, we apply our formalism to: i) Primordial black holes of mass $\in [10^{13.5},10^{16.8}]$ g, showing that the constraints are comparable to the ones obtained from gamma-ray background studies and even dominate below $\sim 10^{14}$g. ii) To a peculiar mass-mixing range in the sterile neutrino parameter space, complementary to other astrophysical and laboratory probes. iii) Finally, we provide a first estimate of the room for improvement left for forthcoming 21 cm experiments, comparing it with the reach of proposed CMB spectral distortion (PiXiE) and CMB angular power spectrum (CORE) missions. We show that the best and most realistic opportunity to look for this signal (or to improve over current constraints) in the 21 cm probe is to focus on the Cosmic Dawn epoch, $15\lesssim z\lesssim30$, where the qualitatively unambiguous signature of a spectrum in emission can be expected for models that evade all current constraints.

• The paper presents a comprehensive analysis constraining exotic electromagnetic energy injections by examining CMB anisotropies with advanced simulations.
• It employs Monte Carlo Markov chains and Boltzmann code modifications to evaluate energy deposition across cosmic epochs, setting bounds up to 10^25 s.
• The findings enhance constraints on decaying particles, primordial black holes, and sterile neutrinos, informing future 21 cm studies.

Overview of Cosmological Constraints on Exotic Injection of Electromagnetic Energy

The paper "Cosmological constraints on exotic injection of electromagnetic energy" by Vivian Poulin, Julien Lesgourgues, and Pasquale D. Serpico provides a comprehensive analysis of the constraints on energy injection into the electromagnetic (e.m.) channels in the universe from exotic sources. The authors evaluate the impact of such injections by examining cosmic microwave background (CMB) anisotropies and provide a detailed exploration of how these affect the CMB power spectra depending on the epoch of injection, using a decaying particle's lifetime as a proxy.

Summary of Numerical Results and Methodologies

The paper uses advanced Monte Carlo Markov chains and detailed Boltzmann code modifications to simulate and constrain scenarios involving exotic e.m. energy injections. One of the key contributions is the thorough examination of energy deposition over a large timescale range, particularly focusing on the epochs around and post-recombination. These epochs are considered highly sensitive to CMB observations. The study is meticulous in determining constraints based on the lifetime of particles, allowing for the bounds to span up to $10^{25}$ s. Notably, these are complementary to existing constraints from CMB spectral distortions and primordial nucleosynthesis.

Insights on Energy Injection Epochs and CMB Power Spectra

The authors present, for the first time in this context, the impact of e.m. energy injections on CMB power spectra across various epochs. They introduce an "on-the-spot" approximation that simplifies the derivation of constraints by evaluating the differences with up-to-date models. This approximation is essential for early universe relics contributing to dark matter. For decaying species, they explore constraints on primordial black holes (PBHs) within a specific mass range, where these constraints surpass those from gamma-ray background studies below $\sim10^{14}$ g. This work also assesses mass-mixing ranges in sterile neutrino parameter space.

Theoretical Implications and Future Directions

The paper suggests that with upcoming 21 cm experiments, more refined probes into non-standard energy injections can be anticipated, particularly focusing on the epoch known as the Cosmic Dawn ($15 \lesssim z \lesssim 30$). The potential for improvement in constraints is compared with proposed CMB spectral distortion missions ({\sf PiXiE}) and CMB angular spectrum observations ({\sf CORE}).

Key Outcomes and Interpretations

The implications of this research resonate well beyond short-term constraints. The methods and numerical simulations establish a paradigm for employing CMB anisotropy in testing exotic physics associated with dark matter and neutrino dynamics. Furthermore, the study aids in refining the current understanding of cosmological history via improved physical representations of exotic energy interactions.

Conclusion

This paper conducts a rigorous exploration of electromagnetically exotic energy injection constraints using CMB anisotropies. Its findings include significant improvements in the constraints compared to prior methodologies and highlight the diverse applications across varying particle mass and epoch scenarios. These results underline the growing importance of CMB studies as tools for probing new physics beyond the standard cosmological model. Future advancements in observational techniques will likely further elucidate the boundaries and possibilities of exotic physics in the universe.