The expansion rate of the universe is measured in two ways: by ancient light — the cosmic microwave background and by nearby supernovae. The results differ — 67 vs. 73 km/s per 3 million light-years. This Hubble tension, named after Edwin Hubble, long stumped cosmologists. The standard model assumes dark energy is a cosmological constant, like an engine with fixed thrust. But then the discrepancy is inexplicable. New research treats dark energy as a changing field (k-essence): it alternately boosts and slows acceleration, like a driver playing with the throttle. Calculations for a tachyonic version of such a field reduce the tension almost to zero.

If the model is correct, the universe accelerates in spurts, and gravity on cosmic scales doesn't behave as Newton predicted. As Georges Lemaître suspected, the cosmological constant might be variable.