The Axidental Universe
Abstract
Theories with several hundred axion fields have enormous numbers of distinct metastable minima. A small fraction of these local minima have vacuum energy compatible with current measurements of dark energy. The potential also contains regions suitable for inflation, and gives rise to a natural type of dark matter. Firstorder phase transitions from one minimum to the vicinity of another play the role of big bangs and produce many bubbles containing evolving FriedmannLemaitreRobertsonWalker universes. The great majority either collapse in a tiny fraction of a second, or expand exponentially forever as empty, structureless universes. However, restricting to those bubble universes that form nonlinear structure at some time in their history we find cosmologies that look remarkably similar to ours. They undergo about 60 efolds of inflation, making them flat, homogeneous and isotropic, and endowing them with a nearly scaleinvariant spectrum of primordial density perturbations with roughly the observed magnitude and tilt. They reheat after inflation to a period of radiation domination, followed by matter domination with roughly the observed abundance, followed by vacuum energy domination at roughly the observed density. None of these features require any model building or small parameters. Instead, all dimensionful parameters in the theory can be set equal to the grand unified scale 0.01 M_p, and the dimensionless parameters are order one and can be chosen randomly. The small value of dark energy ultimately comes from nonperturbative gravitational effects, giving an exponentially small vacuum energy density. Therefore, random axion landscapes can account for many of the apparently tuned features of our universe, including its current enormous size, age, and tiny energy densities compared to the scales of fundamental physics.
 Publication:

arXiv eprints
 Pub Date:
 February 2019
 DOI:
 10.48550/arXiv.1902.05952
 arXiv:
 arXiv:1902.05952
 Bibcode:
 2019arXiv190205952B
 Keywords:

 High Energy Physics  Theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 20 pages, 1 figure