Axion Cosmology (1510.07633v2)

Abstract: 1. Introduction 2. Models: the QCD axion; the strong CP problem; PQWW, KSVZ, DFSZ; anomalies, instantons and the potential; couplings; axions in string theory 3. Production and I.C.'s: SSB and non-perturbative physics; the axion field during inflation and PQ SSB; cosmological populations - decay of parent, topological defects, thermal production, vacuum realignment 4. The Cosmological Field: action; background evolution; misalignment for QCD axion and ALPs; cosmological perturbation theory - i.c.'s, early time treatment, axion sound speed and Jeans scale, transfer functions and WDM; the Schrodinger picture; simualting axions; BEC 5. CMB and LSS: Primary anisotropies; matter power; combined constraints; Isocurvature and inflation 6. Galaxy Formation; halo mass function; high-z and the EOR; density profiles; the CDM small-scale crises 7. Accelerated expansion: the c.c. problem; axion inflation (natural and monodromy) 8. Gravitational interactions with black holes and pulsars 9. Non-gravitational interactions: stellar astrophysics; LSW; vacuum birefringence; axion forces; direct detection with ADMX and CASPEr; Axion decays; dark radiation; astrophysical magnetic fields; cosmological birefringence 10. Conclusions A Theta vacua of gauge theories B EFT for cosmologists C Friedmann equations D Cosmological fluids E Bayes Theorem and priors F Degeneracies and sampling G Sheth-Tormen HMF

• The paper presents a comprehensive review of axions as dark matter candidates and their impact on cosmological models.
• It details how axions emerge from QCD and string theory frameworks, highlighting models like KSVZ and DFSZ.
• It examines experimental detection strategies, including the misalignment mechanism, ADMX haloscopes, and helioscope approaches.

Axion Cosmology: An Expert Overview

The paper "Axion Cosmology" by David J. E. Marsh explores the relevance of axions—a class of hypothetical particles—in tackling fundamental issues in cosmology and theoretical physics. Axions arise in attempts to solve the strong CP problem in quantum chromodynamics (QCD) and within various high-energy physics theories, notably string theory. This review explores the motivation for axions, their potential role as dark matter (DM), their interactions with standard model particles, cosmological implications, and prospects for detection.

Axions in Particle Physics and String Theory

Axions were initially proposed as a solution to the strong CP problem in QCD, which concerns the unexpected smallness of the neutron's electric dipole moment—a quantum mechanical inconsistency if CP violation were not somehow mitigated. The Peccei-Quinn mechanism introduces the "QCD axion," dynamically canceling CP violation through its interaction with gluons. The mass of the QCD axion is determined by its decay constant $f_a$, constrained by experimental and astrophysical observations to fall within a wide energy window, depending on the axion model (e.g., KSVZ or DFSZ).

In string theory, axions manifest naturally due to the compactification of extra dimensions, relevant in higher-dimensional forms. Numerous axion-like particles (ALPs) with a broad range of masses are predicted, collectively termed the "axiverse." These particles could span masses from as light as $10^{-33}$ eV, potentially acting as dark energy, to around $10^{-10}$ eV, appropriate for DM.

Cosmological Roles and Experimental Probes

The paper extensively reviews axions as candidates for cold dark matter (CDM). The misalignment mechanism describes how initial conditions set during or after inflation can lead to axion DM. Cosmological constraints based on structure formation, such as large-scale structure (LSS) and cosmic microwave background (CMB) observations, place stringent limits on axion properties. The axion mass must exceed $10^{-24}$ eV to not disrupt LSS and galaxy formation.

Experimentally, axions are challenging to detect due to their weak coupling to standard model particles. Axion-photon conversion in magnetic fields (the Primakoff effect) underlies many detection efforts, from helioscopes looking for solar axions to haloscopes like ADMX searching for galactic halo axions. Stellar cooling anomalies and constraints from supernova SN1987A also provide indirect astrophysical probes of axion-like particles.

Theoretical and Practical Implications

If axions compose a significant fraction of DM, their unique phenomenology, such as suppression of structure on small scales (due to a non-trivial Jeans length), could resolve some "small-scale crises" of the CDM paradigm, like the core-cusp problem and missing satellites problem. Furthermore, embedding axions into a string-theoretic framework ties cosmology to high-energy physics, offering a potential pathway to understanding dark sector phenomenology.

Future Directions and Challenges

The paper points out several promising avenues for future research and constraints on axions. Significantly, next-generation CMB and LSS surveys may enhance sensitivity to axion-like particles, even if they compose a subdominant fraction of DM. Continued development of direct detection experiments and further astrophysical surveys could probe parameter spaces that test the robustness of axion models.

Conclusion

Axions remain an enticing solution to several unresolved questions in physics and astronomy, linking the microphysical strong CP problem to macroscopic cosmological phenomena. Ongoing and future experimental efforts, alongside theoretical refinements, will be crucial in assessing the viability of axions as components of the dark universe and as messengers of new fundamental physics. The paper by Marsh serves as both a comprehensive review and a springboard for further exploration into the multi-faceted impacts of axions on modern cosmology.