In 2008, Rosenband et al. used the frequency ratio of Al and Hg in single-ion optical atomic clocks to place a very stringent constraint on the present-time temporal variation of α, namely ⁠ Δ α / α ⁠ = (−1.6 ± 2.3) × 10 per year. Note that any present day null constraint on the time variation of alpha does not necessarily rule out time variation in the past. Indeed, some theories that predict a variable fine-structure constant also predict that the value of the fine-structure constant should become practically fixed in its value once the universe enters its current dark energy -dominated epoch.