Elucidation of 'Cosmic Coincidence'

In the standard cosmological model the dark energy (DE) and nonrelativistic (NR) matter densities are observationally determined to be comparable at the present time, in spite of their greatly different evolution histories. This 'cosmic coincidence' enigma - also referred to as the 'why now? problem' - relies, by its very definition, on the implicit prior expectation for our 'typicality' in the cosmic (expanding) spacetime volume. Otherwise, this conundrum does not exist in the first place. It is shown here that this apparent coincidence could be explained as a non-anthropic observational selection effect: for us to be typical observers in the comoving (static) spacetime volume, the cosmic energy budget must contain a non-vanishing DE component. In addition, it is shown that irrespective of the cosmological initial conditions and assuming no 'new physics', the Universe is most likely to be observed at a time when the conformal Hubble radius, H-1, attains a maximum. The latter takes place at the epoch when rho(DE) and rho(m), the energy densities of DE and NR matter, respectively, are comparable. Specifically, our presumed 'typicality' along the conformal timeline, coupled to a few other plausible assumptions, implies that R equivalent to rho(DE)/rho(m) is 'sampled' from a Beta Prime probability distribution function. A priori 68% (95%) confidence range for the ratio is 0.20 < R < 3.46 (0.033 < R < 17.20), with an expectation value of R = 3.5. These are in agreement with the observationally inferred value, R-obs = 2.23.

Automated summary

In the standard cosmological model, the similarity between the densities of dark energy and nonrelativistic matter at the present time poses a cosmic coincidence problem. This study suggests that this coincidence can be explained as an observational selection effect and proposes a probability distribution function to describe it.