We review the landscape of QCD axion models. Theoretical constructions that extend the
window for the axion mass and couplings beyond conventional regions are highlighted and …

Axions comprise a broad class of particles that can play a major role in explaining the
unknown aspects of cosmology. They are also well-motivated within high energy physics …

The Early Universe has become the standard reference on forefront topics in cosmology,
particularly to the early history of the Universe. Subjects covered include primordial …

Axions are hypothetical particles that may explain the observed dark matter density and the
non-observation of a neutron electric dipole moment. An increasing number of axion …

Axions are well-motivated dark matter candidates with simple cosmological production
mechanisms. They were originally introduced to solve the strong CP problem, but also arise …

For centuries the heavens have been a natural laboratory to test the classical laws of motion,
and more recently to test Einstein's theory of gravity. Today, astrophysics has become a vast …

Very weakly interacting slim particles (WISPs), such as axion-like particles (ALPs) or hidden
photons (HPs), may be non-thermally produced via the misalignment mechanism in the …

The topic of cosmic strings provides a bridge between the physics of the very small and the
very large. They are predicted by some unified theories of particle interactions. If they exist …

This biennial review summarizes much of Particle Physics. Using data from previous
editions, plus 1900 new measurements from 700 papers, we list, evaluate, and average …

Current upper bounds on the neutron electric dipole moment constrain the physically
observable quantum chromodynamic (QCD) vacuum angle| θ¯|≲ 10− 11. Since QCD …