Ask Ethan: Is dark energy no longer a cosmological constant? | by Ethan Siegel | Starts With A Bang! | Sep, 2025

The Universe isn’t just expanding; the expansion is accelerating. If different methods yield incompatible results, is dark energy evolving?

The story of the expanding Universe has been a back-and-forth one over the past 110 years: ever since General Relativity was first introduced. Initially, Einstein introduced the notion of a cosmological constant — a form of energy inherent to the fabric of space itself — to prevent a matter-filled Universe from collapsing. When we discovered that the Universe was expanding, the constant disappeared, eventually leading Einstein to declare it his biggest blunder. Then in the 1990s, a surprising collection of data indicated that the Universe’s expansion was accelerating, a discovery that revived the cosmological constant. The combination of supernova, cosmic microwave background, and large-scale structure data all appeared to demand it.

But now, more than 25 years later, an interesting set of evidence has emerged, suggesting (but not proving) that what we observe as dark energy may not be a constant, but instead is changing over time. Others, however, including me, still prefer the “cosmological constant” interpretation, and remain skeptical of the idea of evolving dark energy. How can so…