# The Axidental Universe

## TL;DR

The paper proposes that a landscape of axion fields naturally produces a multiverse with universes resembling ours, explaining dark energy, inflation, and structure formation without fine-tuning.

## Contribution

It demonstrates that axion landscapes can generate cosmologies similar to ours with minimal assumptions and no small parameters, linking fundamental physics to observed universe features.

## Key findings

- Universes with properties matching our universe emerge naturally from axion landscapes.
- Inflation, dark energy, and matter content are explained without fine-tuning or special model building.
- The smallness of dark energy arises from non-perturbative gravitational effects.

## Abstract

Theories with several hundred axion fields have enormous numbers of distinct meta-stable 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. First-order phase transitions from one minimum to the vicinity of another play the role of big bangs and produce many bubbles containing evolving Friedmann-Lemaitre-Robertson-Walker 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 non-linear 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 scale-invariant 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 non-perturbative 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.