General Relativity prompted the introduction of a cosmological constant to balance a matter-filled Universe against collapse. The discovery of cosmic expansion removed that need, but the 1990s detection of accelerating expansion revived the cosmological constant as an explanation, supported by supernova, cosmic microwave background, and large-scale structure measurements. Recent analyses and new data raise the possibility that dark energy evolves with time rather than remaining constant, with some studies claiming multi-sigma evidence. Other researchers remain skeptical, citing potential data or analysis issues such as with DESI, and emphasize that current evidence does not yet prove dynamical dark energy.
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.
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 many different people look at the same evidence and reach different conclusions? That's what Robert Smičiklas wants to know, asking:
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