Axion cosmology with long-lived domain walls (1207.3166v2)

Abstract: We investigate the cosmological constraints on axion models where the domain wall number is greater than one. In these models, multiple domain walls attached to strings are formed, and they survive for a long time. Their annihilation occurs due to the effects of explicit symmetry breaking term which might be raised by Planck-scale physics. We perform three-dimensional lattice simulations and compute the spectra of axions and gravitational waves produced by long-lived domain walls. Using the numerical results, we estimated relic density of axions and gravitational waves. We find that the existence of long-lived domain walls leads to the overproduction of cold dark matter axions, while the density of gravitational waves is too small to observe at the present time. Combining the results with other observational constraints, we find that the whole parameter region of models are excluded unless an unacceptable fine-tuning exists.

Axion Cosmology with Long-Lived Domain Walls: Implications and Constraints

The paper "Axion cosmology with long-lived domain walls" by Hiramatsu et al. offers an extensive examination of axion models where the domain wall number exceeds one. A pivotal aspect of these models is the formation of multiple domain walls attached to cosmic strings, predicted to have significant implications on the cosmological evolution of axions.

Rationale and Approach

Recognizing axions as a result of the Peccei-Quinn mechanism for addressing the strong CP problem in QCD, these models position axions as viable dark matter candidates. With domain walls possessing significant longevity, their potential overclosure of the universe constitutes a major cosmological challenge. To navigate this, the authors introduce a bias parameter, hypothesized to arise from Planck-scale physics, which facilitates domain wall collapse, enabling a more stable universe configuration.

Utilizing three-dimensional lattice simulations, the paper simulates the dynamics of axion radiation and gravitational wave production within these domain wall networks. The simulations are designed to capture the survival and disintegration patterns of these walls, allowing the authors to estimate the relic density of cold dark matter axions and the gravitational wave spectrum.

Key Findings

1. Axion Production: The simulations reveal that axions radiated from domain walls are predominantly of low momentum, contradicting earlier claims of a high-energy axion spectrum. This observation necessitates a reconsideration of earlier theoretical conclusions concerning axion contribution to cold dark matter density.
2. Gravitational Waves: Gravitational waves generated by domain wall dynamics exhibit a peak near the Hubble scale and gradually extend toward scales bounded by the domain wall width. However, the density of observable gravitational waves remains substantially low.
3. Numerical Constraints: The paper establishes stringent numerical constraints on the bias parameter $\Xi$ to prevent axion overproduction, excluding substantial parameter space if $\Xi$ is not fine-tuned—a finding that imposes significant limits on model viability unless extremely precise adjustments are invoked.

Implications

For theories predicting $N_{\mathrm{DW}>1$, this paper underscores the necessity for careful handling of domain wall dynamics and presents a thorough numerical analysis elucidating axion and gravitational wave production mechanisms. The findings stress that without precise $\delta$ parameter fine-tuning—required to mitigate unacceptable $\bar{\theta}$ contributions—most models fail to align with observed cosmological conditions without resulting in a domain wall-induced universe closure. The results question the feasibility of axion models with multiple domain walls when the Peccei-Quinn symmetry is broken post-inflation, challenging theoretical assumptions unless substantial alterations are introduced.

Theoretical and Practical Considerations

The constraints imposed by the paper offer crucial guidance for researchers exploring axionic dark matter models, highlighting areas needing refinement and presenting potential pathways for the development of models incorporating biases from Planck-scale physics. Future investigations will likely explore enhanced simulation methods across broader dynamical ranges to further elaborate these findings. Studies emphasizing alternate axion production methods and examining the signal detectability of gravitational waves may also redefine the model parameters needed for cosmic stability.

In conclusion, while the paper presents a compellingly cautious conclusion for models of axion cosmology geometry with domain walls exceeding unity, it opens avenues for further exploration and refinement in axionic theory, suggesting intricate relationships between theoretical parameters, fine-tuning requirements, and observational evidence. The cosmological implication of such axionic models continues to be an area rich for exploration and debate, with the paper providing a vital cornerstone for this ongoing research saga.