Astronomers once observed exploding stars (supernovae) and found the universe expanding, driven by a mysterious force called dark energy. This led to the standard cosmological model of the late 1990s, Lambda-CDM, where “Lambda” represents dark energy, assumed constant, and “Cold Dark Matter” (CDM) explains unseen mass shaping cosmic structure. Evidence for CDM includes steady star rotation speeds in galaxies, cosmic microwave background fluctuations, galaxy clustering, and light bending by gravity. Though successful, Lambda-CDM has faced ongoing scrutiny almost from inception of the theory.

Enter the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona. With 5,000 robotic fiber-optic sensors, DESI captures light from galaxies and quasars, mapping the universe’s expansion history. A new study, analyzing three years of DESI data, 15 million objects, with plans for 50 million, combines it with cosmic microwave background radiation, supernovae, and weak gravitational lensing data. Fitting all this into Lambda-CDM with a constant dark energy revealed cracks in the model. But if dark energy weakens over time, a “dynamical dark energy“, the model aligns better.

By observing objects up to 11 billion years away, DESI peers deep into cosmic history. Researchers found hints that dark energy’s strength may have peaked around 7 billion years ago, then started weakening, challenging its fixed nature in Lambda-CDM. While not certain, this could rival the 1990s discovery of accelerated expansion, potentially demanding a new model.

The universe’s fate depends on dark energy versus matter. It’s been accelerating, but a weakening dark energy might slow it down, halt it, or, if gravity overtakes sufficiently, trigger a “Big Crunch.” New data from DESI, Europe’s Euclid, NASA’s Nancy Grace Roman, and Chile’s Vera Rubin Observatory could clarify this within five years, possibly nailing dark energy’s role.

Source: “Dark Energy Seems to Be Changing, Rattling Our View of Universe” by Rey and Lawler, Phys.org, March 2025. Graphic: DESI Collaboration Photo of Galaxies.