A team of researchers used a flexible, model-independent method to map how the Universe has expanded at recent times. They combined the latest baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument data release 2 (DESI DR2) with a recent set of Type Ia supernova distance measurements from the Dark Energy Survey (the Dovekie compilation). When they compared this reconstructed expansion history with the prediction of the standard Planck 2018 ΛCDM model (the cosmological model fit to the early Universe), they found a localized deviation in the redshift range roughly 0.3 to 0.6, with a peak significance of about 3.5σ near redshift z ≃ 0.47.

BAO are a ripple-like pattern in the distribution of galaxies that act as a yardstick for cosmic distances. Type Ia supernovae are bright stellar explosions used as calibrated distance markers. The researchers used a spline reconstruction, which means they fit a smooth, flexible curve to the distance data without forcing a particular cosmological model. This lets the data speak more directly about the expansion history, rather than assuming the standard model from the start.

To test how robust the feature is, the team varied the reconstruction method, changed which data points were included, and changed how the early-Universe scale called the sound horizon was calibrated. They also ran mock analyses — simulated data tests — to check for method-induced biases. Across these checks the localized feature persisted, and the mock tests suggested that the reconstruction itself is unbiased. The authors conclude the anomaly is unlikely to be caused by reconstruction bias or by grossly misestimated measurement uncertainties.

If this localized deviation is real, it could have two kinds of implications. One is that some physical process affecting the Universe at relatively late times has been missed in standard models. The other is that there is a subtle mismatch between probes of the early Universe (like the Planck cosmic microwave background measurements) and late-time probes (like BAO and supernovae). Either possibility would be important for understanding cosmic expansion and dark energy.