The Opposite of Dark Energy: Limits on $w = 2/3$ Ultralight Energy in the Early Universe

Abstract

Might stable energy species "lighter" than radiation, with $w > 1/3$, exist? A dimensional expansion of the cosmological Friedmann Equation of energy has a clear place for them. Such energies would affect the universe much differently than dark energies, and so are here dubbed "ultralight." As the universe expands, ultralight dilutes even faster than light. Although any specie of energy can be mimicked by a properly evolving scalar field, ultralight energy species are hypothesized here to be stable and not related to dynamics of a scalar field. Ultralight is not considered a candidate to make a significant contribution to the energy budget of the universe today, although ultralight might have affected the universe in the distant past. In particular, the $w=2/3$ ultralight energy specie appears to have relatively mundane physical attributes. A discussion of properties and falsifiable attributes of ultralight is given. The duration of primordial nucleosynthesis is extrapolated to limit the present density of $w=2/3$ ultralight to below one part in 100 billion of the critical density.