Dark energy is a mysterious force that makes the universe expand faster and faster. The simplest explanation is the cosmological constant: its density is always the same. But recently, data from the DESI instrument hinted that it might have changed. To check this, researchers used a 'lie detector'—a Gaussian process (a flexible mathematical method that doesn't impose rigid formulas but searches for any patterns in the data).
The method relied on three types of measurements: direct measurements of the expansion rate using 37 pairs of galaxy 'clocks', the brightness of 1624 supernovae, and sound waves in the distribution of galaxies. Combined with hints from the cosmic microwave background, these data allowed researchers to reconstruct the expansion history without assuming how dark energy should behave.
Conclusion: Without a full map of the cosmic microwave background, there is no evidence yet for changing dark energy. The lie detector found no strong discrepancies with the simplest model. But that doesn't mean they don't exist — future data will clarify.
The 37 cosmic‑chronometer points are direct measurements of H(z), obtained without any model assumption — simply from the age differences of passively evolving galaxies. It is like gauging a child’s growth rate from a series of school photos taken over the years.