Axion dark matter from topological defects (1412.0789v3)

Abstract: The cosmological scenario where the Peccei-Quinn symmetry is broken after inflation is investigated. In this scenario, topological defects such as strings and domain walls produce a large number of axions, which contribute to the cold dark matter of the universe. The previous estimations of the cold dark matter abundance are updated and refined based on the field-theoretic simulations with improved grid sizes. The possible uncertainties originated in the numerical calculations are also discussed. It is found that axions can be responsible for the cold dark matter in the mass range $m_a=(0.8-1.3)\times 10^{-4}\mathrm{eV}$ for the models with the domain wall number $N_{\rm DW}=1$, and $m_a\approx\mathcal{O}(10^{-4}-10^{-2})\mathrm{eV}$ with a mild tuning of parameters for the models with $N_{\rm DW}>1$. Such higher mass ranges can be probed in future experimental studies.

• The paper refines axion dark matter abundance estimates by simulating contributions from misalignment, global string decay, and string-wall systems.
• It employs advanced grid simulations, up to 32768², to reduce uncertainties and improve theoretical calculations from previous models.
• The study constrains axion mass and decay constants, outlining experimental prospects through initiatives like ADMX and IAXO.

Axion Dark Matter from Topological Defects: An Expert Review

The discussed paper explores the potential role of axions produced from topological defects as a substantial component of the cold dark matter in the universe. The authors embark on refining the theoretical calculations of axion abundances, focusing on the cosmological scenario where Peccei-Quinn (PQ) symmetry is disrupted temporally after cosmic inflation. They meticulously update previous estimations by employing field-theoretical simulations with advanced grid sizes, which contributes to understanding the uncertainties arising from numerical analyses.

Key Findings and Numerical Results

The authors present improved estimates of axion dark matter abundances by calculating the contributions from three distinct production mechanisms: the misalignment mechanism, decay of global strings, and decay of string-wall systems, which are haLLMarks of models where PQ symmetry breaking occurs post-inflation.

1. Misalignment Mechanism: The initial displacement of the axion field from its minimum potential results in coherent oscillations. The paper utilizes the latest models for the axion mass dependence on temperature to update this mechanism's contribution.
2. Global String Decay: Axions are continually emitted from global strings formed after PQ symmetry breaking. The authors acknowledge a critical advancement over previous controversies by assuming that the mean energy of emitted axions aligns with the cosmic horizon scale. Their refined simulations, using expansive box sizes (up to $32768^2$ grids), suggest a more accurate parameterization of axion production through string decay.
3. Decay of String-Wall Systems: The decay dynamics of string-wall systems differ based on the value of the domain wall number $N_{\rm DW}$. For $N_{\rm DW}=1$, string-wall systems are transient, contributing a modest axion population, while $N_{\rm DW}>1$ results in stable systems that risk cosmic overclosure. The paper carefully estimates the enhanced axion production and decay dynamics for these long-lived systems, leading to refined expressions and constraints.

Numerical Insights

The authors report several critical numerical improvements:

• The calculated axion abundance from global strings assumes a string length parameter $\xi = 1.0 \pm 0.5$ and a mean energy parameter $\epsilon = 4.02 \pm 0.70$.
• For short-lived string-wall systems ($N_{\rm DW}=1$), the decay contribution uses parameters $\mathcal{A} = 0.50 \pm 0.25$ and $\tilde{\epsilon}_w = 3.23 \pm 0.18$.
• Long-lived domain wall systems ($N_{\rm DW} > 1$) exhibit variations in the mean energy parameter $\tilde{\epsilon}_a$ and the exponential decay factor $C_d$, contingent on domain wall numbers ranging from 2 to 6.

Implications and Constraints

The paper imposes critical constraints on axion mass and decay constants, offering bounds like $$F_a \lesssim (4.6 \times 10^{10} - 7.2 \times 10^{10}) \, \mathrm{GeV}$$ and $$m_a \gtrsim (0.8\times10^{-4} - 1.3\times10^{-4}) \, \mathrm{eV}$$ for models with $N_{\rm DW}=1$.

For models with $N_{\rm DW}>1$, the field of viable parameter space shrinks significantly unless experimental conditions allow direct detection of higher mass axion candidates, such as through ADMX and IAXO experiments covering up to $\mathcal{O}(10^{-2}) \, \mathrm{eV}$. The prospect of axion masses in the range of $\mathcal{O}(10^{-4} - 10^{-2}) \, \mathrm{eV}$ aligns intriguingly with the capabilities of these next-generation probes, amplifying the potential for empirical validation.

Conclusion

The rigor and sophistication found in this work represent a substantive advance in estimating axion dark matter's contribution from topological defects. Through meticulous execution of large-scale simulations, the authors delineate both theoretical refinements and parameter constraints that poignantly illuminate axion physics and the broader landscape of dark matter research. The paper closes with a promising outlook towards impending experimental initiatives, bridging theoretical predictions with impending empirical inquiries in the field of particle cosmology.