Multiple reports describe recent efforts to test whether dark energy is strictly constant or could evolve over time. Dark energy is commonly modeled as a cosmological constant (Lambda) in the Lambda cold dark matter (ΛCDM) framework, which has been used for about 25 years. After late-1990s supernova observations showed the universe’s expansion is accelerating—work that later earned the 2011 Nobel Prize—ΛCDM became the leading explanation. One article notes claims that measurements can indicate dark energy may effectively “flip” its sign, implying a change in how it influences cosmic expansion. At the same time, sources emphasize that the so-called “Hubble tension” remains unresolved. The Hubble tension refers to a persistent mismatch between the Hubble expansion rate inferred from the early universe (for example, from cosmic microwave background–based methods) and the rate measured from the local universe (such as distance ladder approaches). Even where new analyses aim to improve constraints on expansion history—such as using neutron star merger events—sources report that the discrepancy between early- and late-universe expansion rates does not clearly move toward agreement.