Signatures of Primordial Energy Injection from Axion Strings (2308.01334v1)

Abstract: Axion strings are horizon-size topological defects that may be produced in the early Universe. Ultra-light axion-like particles may form strings that persist to temperatures below that of big bang nucleosynthesis. Such strings have been considered previously as sources of gravitational waves and cosmic microwave background (CMB) polarization rotation. In this work we show, through analytic arguments and dedicated adaptive mesh refinement cosmological simulations, that axion strings deposit a sub-dominant fraction of their energy into high-energy Standard Model (SM) final states, for example, by the direct production of heavy radial modes that subsequently decay to SM particles. This high-energy SM radiation is absorbed by the primordial plasma, leading to novel signatures in precision big bang nucleosynthesis, the CMB power spectrum, and gamma-ray surveys. In particular, we show that CMB power spectrum data constrains axion strings with decay constants $f_a \lesssim 10^{12}$ GeV, up to model dependence on the ultraviolet completion, for axion masses $m_a \lesssim 10^{-29}$ eV; future CMB surveys could find striking evidence of axion strings with lower decay constants.

• The paper demonstrates that axion strings emit both axions and heavy radial modes, leading to significant primordial energy injections.
• It employs adaptive mesh refinement simulations and analytical methods to constrain axion decay constants (fₐ ≲ 10¹² GeV) and masses (mₐ ≲ 10⁻²⁹ eV).
• These findings indicate observable imprints on the CMB power spectrum and potential gamma-ray anomalies, guiding future cosmological research.

Analysis of Axion Strings and Primordial Energy Injection

The paper "Signatures of Primordial Energy Injection from Axion Strings" systematically explores the dynamics of axion strings and the resultant cosmological implications through energy injections during the evolution of the early Universe. This work elaborates on the paper of axion strings within the context of large-scale cosmology, focusing on their evolution, radiation dynamics, and constraints derived from cosmic observations.

Axion strings, which originate in scenarios where a global U(1) Peccei-Quinn symmetry is spontaneously broken, are profound cosmic entities stretching over cosmological distances. They arise when the Universe undergoes phase transitions post-reheating at temperatures above the Peccei-Quinn scale, thereby restoring the PQ symmetry. The ensuing axion-string networks evolve by emitting radiation primarily in the form of axions; however, this paper demonstrates that a notable energy fraction is relegated to heavy radial modes as well. These modes swiftly decay into Standard Model (SM) particles, subsequently interacting with and dissipating energy into the primordial plasma.

The analysis employs adaptive mesh refinement (AMR) cosmological simulations combined with analytical arguments to characterize the evolution and radiation dynamics of axion strings. Through these methods, the researchers depict how heavy radial modes inject novel energy signals that would manifest prominently during various cosmological epochs, such as big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB). This emergent energy transfer results in specific observational signatures, including potential modifications in the CMB power spectrum and anomalous gamma-ray emissions detectable by advanced astrophysical surveys.

Particularly compelling is the parameter space constraints derived for the string tension and decay constants of axions. The CMB power spectrum data notably confines the axion string networks for decay constants $f_a \lesssim 10^{12}$ GeV, contingent upon their ultraviolet completion models, and axion masses $m_a \lesssim 10^{-29}$ eV. Additionally, future CMB observational capabilities may furnish more stringent tests, enabling the identification of axion signatures through lower decay constants.

The intersection of theoretical and observational avenues in this discourse presents a structured landscape for examining the existence and characteristics of axion strings. The implications discussed are profound, offering potential insights into dark matter components, cosmic inflationary paradigms, and the standard cosmological model. The simulations elucidated provide a comprehensive framework, albeit with parameters subject to the refinement of the axion’s UV completion and the associated phenomenological model adjustments.

Envisioned future advancements in observational technology will likely enhance our ability to detect axion signatures, such as advanced CMB polarization measurements, which could probe the rotation effects proposed. Furthermore, string networks surviving beyond epochs like CMB decoupling imply potential detection via polarization modulation or discontinuities in background radiation fields.

In conclusion, this paper provides an in-depth exploration of axion strings and their cosmological implications via primordial energy injection models. The methodologies and constraints elucidated serve as cornerstones for both theoretical expansions and experimental explorations into the foundational paradigms of cosmological evolution and particle physics, unifying the domains towards a comprehensive understanding of the Universe’s fabric. Future research along these lines could significantly impact the broader field of astroparticle physics and cosmology.