A vast collection of exploded stars raises a dizzying question once again: could the acceleration of the Universe be changing?
To measure enormous distances, astronomers use certain stellar explosions called Type Ia supernovae. Their brightness can be calibrated, making it possible to estimate their distance. By comparing this distance with the redshift of their light, researchers trace how the Universe has expanded over time.
An international team brought together 2,884 likely Type Ia supernovae in a single dataset, named Unite. It notably combines the Pantheon+ and DES-SN5YR catalogues, the latter from the Dark Energy Survey. The work brings together around three decades of observations made with different instruments.
Timeline of the Universe, from the Big Bang to the present day.
NASA/WMAP Science Team · Public domain
The researchers reprocessed the observations using common methods. In particular, they harmonized the selection of supernovae and the corrections related to observational biases. The mass of the galaxies hosting these explosions was also recalculated uniformly for more than 98% of the sample.
Why take so many precautions? The light we receive can be altered during its journey. Dust, the properties of the host galaxy, and the gravitational effect of large masses can all slightly distort the measurements. Across thousands of objects, these small discrepancies can affect the final result.
Supernovae can then be used to test dark energy, the name given to the phenomenon associated with the acceleration of cosmic expansion. In the standard cosmological model, this component remains constant over time. Unite alone is not sufficient to establish that it evolves. The researchers therefore combined their measurements with the afterglow of the Big Bang and maps of the distribution of galaxies.
With these datasets combined, a model with variable dark energy fits some measurements better than a model in which it remains constant. These results do not yet constitute proof of non-constant dark energy; rather, they strengthen an anomaly already seen in other recent surveys, particularly those conducted by the Dark Energy Spectroscopic Instrument.
The Unite catalogue must still reach a concrete milestone. The authors plan to make the distance measurements and associated statistical tools public after their article is accepted. Other teams will then be able to reproduce the calculations, combine these supernovae with future surveys, and determine whether the discrepancy persists with more data.