Axion Dark Matter: What is it and Why Now?

Abstract

The axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the Standard Model of particle physics and the problem of charge-parity invariance of the strong nuclear force. We then discuss how the axion emerges as a dark matter candidate, and how it is produced in the early Universe. The symmetry properties of the axion dictate the form of its interactions with ordinary matter. Astrophysical considerations restrict the particle mass and interaction strengths to a limited range, which facilitates the planning of experiments to detect the axion. A companion review discusses the exciting prospect that the axion could indeed be detected in the near term in the laboratory.

Non-equilibrium scalar field dynamics starting from Fock states: Absence of thermalization in one dimensional phonons coupled to fermions

“…In particle physics it played a big role in the construction of the Higgs mechanism which leads to descriptions of massive particles [1][2][3] . In cosmology and astroparticle physics, scalar fields have been used to describe inflationary expansion of the early universe 4,5 ; they have also been projected as possible candidates for dark matter in various models [6][7][8] . They are also ubiquitous in condensed matter physics, often rearing their heads as low energy fluctuations around ordered (symmetry broken) states of matter.…”

Near-horizon microstructure and superradiant instability of black holes

“…Introduction. Ultralight bosonic particles, which are predicted in various beyond Standard Model scenarios [1][2][3][4][5][6][7], could be a promising dark matter (DM) candidate [1,2,[8][9][10][11][12][13][14][15][16][17]. Given that detecting ultralight particles is challenging in particle physics, using gravitational waves (GWs) to probe ultralight bosons has aroused much attention recently.…”

Quantum Entanglement and Axion Physics

“…The physical particle, called the axion [3,4], replaces effectively θ and the Lagrangian no longer has a CP-violating term. The axion is also one of the leading candidates for dark matter [5,6]. It appears then that the search for axions is a highly important issue for fundamental physics.…”