Axion Cosmology

TL;DR

This review surveys axions as a versatile cosmological toolkit, linking QCD axions and string-axiverse ALPs to dark matter, inflation, and dark energy. It details production mechanisms, field evolution, and perturbation theory, highlighting the distinctive signatures of ULAs in CMB, LSS, and high-redshift observables, as well as non-gravitational probes (stellar bounds, haloscopes, CASPEr). The work compiles state-of-the-art constraints on axion masses and decay constants, discusses the interplay with inflationary isocurvature, and presents the observational prospects for testing the axion paradigm from small-scale structure to black hole physics and precision cosmology. Overall, axions offer a rich, testable framework with potential to address the cosmological constant problem, dark matter characterization, and inflationary dynamics, while motivating diverse experimental searches and theoretical developments.

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

Figures (38)

• Figure 1: Summary of constraints and probes of axion cosmology.
• Figure 2: A symmetry breaking potential in the complex $\varphi$ plane. The vev of the radial mode is $f_a/\sqrt{2}$ and the axion is the massless angular degree of freedom at the potential minimum.
• Figure 3: The distribution of Hodge numbers $h^{1,1}$ and $h^{1,2}$ for the known Calabi-Yau manifolds in the Kreuzer-Skarke 2000math......1106K list. Note that the frequency (=number of occurrences) color scale is logarithmic. There is a huge peak in the distribution at $h^{1,1}\approx h^{1,2}\approx 30$, which implies that a compactification picked at random from this list is most likely to contain of the order of 30 axions.
• Figure 4: Evolution of various quantities in the exact solution to the background evolution of an ALP, Eq. \ref{['eqn:exact_background']}, for a radiation-dominated universe ($p=1/2$). Dimensionful quantities have arbitrary normalization. Vertical dashed lines show the condition defining $a_{\rm osc.}$. Further discussion of this choice, and the approximate solution for the energy density, is given in the text.
• Figure 5: ULA relic density from vacuum realignment in the broken PQ scenario with high scale inflation, $H_I\approx 10^{14}\text{ GeV}$. ULAs require $\phi_i>10^{14}\text{ GeV}$ in order to contribute more than a few percent to the DM density. Even with high scale inflation, the contribution of isocurvature backreaction is less than a percent of the total DM across the entire ULA parameter space. See Fig. \ref{['fig:contours_combinedLinear']} for more details on the allowed region at lower mass.