Compilation of 2,884 Supernovae Adds Evidence That Dark Energy May Change Over Time

An international team led by scientists at the University of Queensland's School of Mathematics and Physics has assembled observations of 2,884 Type Ia supernovae and reports new evidence that dark energy, the mysterious force driving the accelerating expansion of the universe, may vary over time. The findings challenge a cornerstone of modern cosmology, which treats dark energy as fixed and unchanging.

The results are posted on arXiv, a repository for preprints, along with a companion paper on measurements of host galaxy masses. That means the work has not yet been peer reviewed.

Type Ia supernovae are powerful stellar explosions that astronomers use as tools for measuring distances across the universe. By comparing those distances with how the light from the explosions has been stretched, researchers can trace how fast the universe has expanded at different points in its history.

The new collection combines historical supernova measurements with findings from the Dark Energy Survey (DES) published in 2024. According to PhD candidate Ryan Camilleri, the project establishes a new international standard for supernova cosmology. "We've rebuilt 3 decades of astronomical observations into a single, consistent framework," he said.

Making decades of data comparable was a major task. The team revisited older observations using updated analytical methods, applying what scientists have since learned about how supernovae behave. The researchers also had to link observations made by different telescopes with different capabilities. In addition, they accounted for factors that can alter the light reaching Earth, including cosmic dust and the masses of the galaxies hosting the explosions. They also incorporated more subtle effects such as gravitational lensing, the bending and magnification of light around large objects as it travels from a supernova to Earth.

The team then combined the supernova data with other cosmic measurements, including relic light from the Big Bang and maps of how galaxies are distributed through space. "Instead of confirming the standard model of cosmology which assumes dark energy is fixed and unchanging, we have more evidence that dark energy may change over time," Camilleri said.

Astrophysicist Professor Tamara Davis noted that the 2024 DES supernova data first showed hints that dark energy may be time varying. The new compilation also sees a deviation from the standard model, she said, "although in a slightly different direction." That difference is a caveat: the measurements do not point to exactly the same departure from the standard model.

Still, the result echoes another line of evidence. The Dark Energy Spectroscopic Instrument (DESI) found hints of variations in dark energy in its surveys of relic sound waves from the early universe. Davis said that two completely independent measurements have now found hints of time variation in dark energy. The source describes these as hints and additional reasons to investigate, not a definitive overturning of the standard model.

The stakes extend beyond cosmic expansion. Researchers believe that learning more about dark energy's behavior might offer insight into one of the biggest unresolved problems in theoretical physics: reconciling gravity with quantum physics. "We know these 2 theories are each immensely successful in their own realms, so if we can figure out how to put them together that would be a huge step in theoretical physics," Davis said.

The next step is to grow the dataset. The researchers plan to add observations from the Dark Energy Bedrock All-Sky Supernova program (DEBASS), which is identifying hundreds of supernovae closer to Earth than those surveyed by DES. These nearby explosions will provide further measurements for testing whether dark energy truly changes over time.

The work builds on decades of effort by hundreds of astronomers and researchers worldwide. Contributing projects include the Dark Energy Survey, Foundation Supernova Survey, PanStarrs I, Sloan Digital Sky Survey, Supernova Legacy Survey, Carnegie Supernova Project and the Center for Astrophysics supernova survey.