Axion Cosmology Revisited (0910.1066v3)

Abstract: The misalignment mechanism for axion production depends on the temperature-dependent axion mass. The latter has recently been determined within the interacting instanton liquid model (IILM), and provides for the first time a well-motivated axion mass for all temperatures. We reexamine the constraints placed on the axion parameter space in the light of this new mass function. We find an accurate and updated constraint $ f_a \le 2.8(\pm2)\times 10^{11}\units{GeV}$ or $m_a \ge 21(\pm2) \units{\mu eV}$ from the misalignment mechanism in the classic axion window (thermal scenario). However, this is superseded by axion string radiation which leads to $ f_a \lesssim 3.2^{+4}_{-2} \times 10^{10} \units{GeV}$ or $m_a \gtrsim 0.20 ^{+0.2}_{-0.1} \units{meV}$. In this analysis, we take care to precisely compute the effective degrees of freedom and, to fill a gap in the literature, we present accurate fitting formulas. We solve the evolution equations exactly, and find that analytic results used to date generally underestimate the full numerical solution by a factor 2-3. In the inflationary scenario, axions induce isocurvature fluctuations and constrain the allowed inflationary scale $H_I$. Taking anharmonic effects into account, we show that these bounds are actually weaker than previously computed. Considering the fine-tuning issue of the misalignment angle in the whole of the anthropic window, we derive new bounds which open up the inflationary window near $\theta_a \to \pi$. In particular, we find that inflationary dark matter axions can have masses as high as 0.01--1$\units{meV}$, covering the whole thermal axion range, with values of $H_I$ up to $10^9$GeV. Quantum fluctuations during inflation exclude dominant dark matter axions with masses above $m_a\lesssim 1$meV.

• The paper demonstrates that the misalignment mechanism yields updated axion mass constraints (fa ≤ 2.8×10¹¹ GeV or ma ≥ 21 µeV) using a temperature-dependent mass function.
• It shows that axion string radiation supersedes misalignment, tightening bounds to fa ≲ 3.2×10¹⁰ GeV or ma ≳ 0.20 meV and refining search parameters.
• The study implies broader impacts on QCD and inflation models by revising isocurvature constraints and expanding the viable parameter space for dark matter experiments.

Axion Cosmology Revisited

The paper "Axion Cosmology Revisited" by Olivier Wantz and E.P.S. Shellard provides a thorough examination of the cosmological implications of axions, particularly focusing on their production mechanisms and the constraints on their properties arising from these mechanisms. The axion, a hypothetical particle proposed to solve the strong CP problem, has been a prime candidate for dark matter. This paper revisits the constraints placed on axion parameters with updated theories and models, notably incorporating a temperature-dependent axion mass derived from the interacting instanton liquid model (IILM).

Key Findings and Analyses

The misalignment mechanism is a prominent process for axion production, relying on the temperature-dependent axion mass. By adopting the IILM, the authors provide for the first time a well-motivated axion mass function for all temperatures. The significant updates in the axion mass function refine the classic constraints on the axion parameter space, notably offering the bounds $f_a \le 2.8(\pm2) \times 10^{11}\, \text{GeV}$ or equivalently $m_a \ge 21(\pm2) \, \mu\text{eV}$ from the misalignment mechanism alone.

A notable highlight of the paper is the comparison of misalignment-produced axions with those emitted by axion string radiation. The research argues that this radiation process supersedes the misalignment mechanism, placing the constraints at $$f_a \lesssim 3.2^{+4}_{-2} \times 10^{10} \, \text{GeV}$$ or $m_a \gtrsim 0.20 ^{+0.2}_{-0.1} \, \text{meV}$, pointing to more stringent parameters for axion searches.

Theoretical and Practical Implications

In the inflationary scenario, considerations of isocurvature fluctuations, which axions are known to induce, lead to constraints on the Hubble parameter $H_I$. The paper demonstrates that these constraints, when taking anharmonic effects into account, can be less severe than previously thought. This opens up a larger parameter space for the inflationary axion models, especially as inflationary dark matter axions can possess masses ranging from 0.01 to 1 meV and allow inflationary energy scales up to $10^9 \, \text{GeV}$.

The theoretical implications of this paper extend to the broader understanding of QCD and the strong CP problem. The IILM and its predictions regarding the axion mass serve as a powerful tool in analyzing QCD dynamics, especially during the QCD phase transition.

On the practical side, the refined constraints provide specific targets for experimental searches for axions as dark matter candidates. The identified range of axion masses, particularly those around 200 µeV, provides a focused region for experimentalists aiming to detect axions via their predicted coupling to photons.

Future Prospects and Developments

The conclusions drawn from this paper suggest several impactful directions for future research. These include further refinement of lattice QCD calculations to verify and extend beyond the results provided by the IILM and experimental efforts to detect axions, particularly in the specified mass window.

Moreover, the paper paves the way for exploring the implications of axion-like particles in various models of new physics beyond the Standard Model, potentially coupling with new particles and fields. There is also room for developing more sophisticated simulations of axion string networks to reduce uncertainties further and improve the reliability of such cosmological predictions.

Overall, "Axion Cosmology Revisited" provides a comprehensive analysis that advances our understanding of axion physics and lays a concrete groundwork for both theoretical investigations and experimental pursuit in the field of dark matter research.