Dark energy is the name given to the unknown component driving the observed late-time acceleration of the cosmic expansion, usually modeled in ΛCDM as a perfectly uniform cosmological constant with fixed equation of state w = -1 and no spatial structure (Peebles & Ratra 2003; Carroll 2003). Observations increasingly leave open, and in some combinations mildly prefer, the possibility that this component may vary with time or behave more dynamically than a pure constant, but ΛCDM itself offers no physical explanation for what dark energy is, why its density is so small yet nonzero, or why it becomes important precisely in the current epoch (Frieman, Turner & Huterer 2008; Kunz 2012).