Explaining the Dark Energy, Baryon and Dark Matter Coincidence via Domain-Dependent Random Densities

TL;DR

This paper proposes a statistical framework where the observed similarity of dark energy, baryon, and dark matter densities arises from superhorizon domains with randomly varying densities, explained through anthropic selection and specific density dependencies.

Contribution

It introduces a model linking densities to random variables and computes probability distributions, explaining the coincidence without fine-tuning, and aligns with physical scenarios like axion dark matter and quintessence.

Findings

The observed density ratios are statistically natural within the proposed model.

The model's probability distribution matches observed values for specific parameter choices.

The framework supports a physical realization involving axion dark matter and quintessence.

Abstract

The dark energy, dark matter and baryon densities in the Universe are observed to be similar, with a factor of no more than 20 between the largest and smallest densities. We show that this coincidence can be understood via superhorizon domains of randomly varying densities when the baryon density at initial collapse of galaxy-forming perturbations is determined by anthropic selection. The baryon and dark matter densities are assumed to be dependent on random variables \theta_{d} and \theta_{b} according to \rho_{dm} ~ \theta_{d}^{\alpha} and \rho_{b} ~ \theta_{b}^{\beta}, while the effectively constant dark energy density is dependent upon a random variable \phi_{Q} according to \rho_{Q} ~ \phi_{Q}^{n}. The ratio of the baryon density to the dark energy density at initial collapse, r_{Q}, and the baryon-to-dark matter ratio, r, are then determined purely statistically, with no dependence on the anthropically-preferred baryon density. We compute the probability distribution for r_{Q} and r and show that the observed values of r_{Q} and r can be naturally understood within this framework. In particular, for the case \alpha = 2, \beta = 1 and n = 4, which can be physically realized via a combination of axion dark matter, Affleck-Dine baryogenesis and frozen quintessence with a \phi_{Q}^4 potential, the range of r_{Q} and r which corresponds to the observed Universe is a quite natural, with a probability which is broadly similar to other ranges of r_{Q} and r.