A catalog of almost 3,000 white dwarfs that exploded as type Ia supernovas after overfeeding on companion stars indicates that dark energy, the mysterious force accelerating the expansion of the universe, is changing over time.
The discovery, in conjunction with data from the Dark Energy Survey (DES), backs results revealed from the Dark Energy Spectroscopic Instrument (DESI) published in 2024 that suggested dark energy’s influence is weakening.
That means that the team’s research doesn’t just involve the most comprehensive catalog of type Ia supernovas; it presents the clearest picture yet of the evolution of the universe and the influence of dark energy.
“We’ve rebuilt 3 decades of astronomical observations into a single, consistent framework,” team member Ryan Camilleri of the University of Queensland said in a statement.“We combined our 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.”
How do exploding white dwarfs tell us about dark energy?
White dwarfs are the smoldering stellar remnants that are left over when stars with around the mass of the sun exhaust the fuel for nuclear fusion in their cores. This ends the outward pressure that supports a star against its own gravity, meaning the star’s core collapses as the outer layers are shed. The core becomes a white dwarf, and for single stars, that is the end. But around 50% of sun-sized stars have a binary companion, and this can lead to renewed activity and a change in the star’s fate.
If the white dwarf and its companion are close enough together, the dead star begins stripping away the outer layers of its companion. As this material builds on the white dwarf, it pushes the stellar remnant’s mass over the so-called Chandrasekhar limit, allowing it to go supernova.
These cosmic explosions are called type Ia supernovas, and their light output is so uniform that these events are referred to as standard candles. They are of vital importance because measuring how their light has been redshifted as it travels means type Ia supernovas can be used to measure cosmic distances.
“Over the years we’ve learned a lot more about how supernovae behave so we’ve been able to go back and apply that improved understanding to older data,” Camilleri said. “Extensive work has been done to link observations from different telescopes with different capacities and take into account issues such as cosmic dust and galaxy mass which can affect the light coming from a supernova.
“We also incorporated more subtle effects such as gravitational lensing, which is the bending and magnification of light around large objects as it travels from a supernova to Earth.”
Combined with the 2024 results from DESI, this research could change our idea of what dark energy actually is.
“Our supernova data from DES in 2024 first showed hints that dark energy may be time-varying, and this new compilation also sees a deviation from the standard model, although in a slightly different direction,” Davis said. “Similarly, results from DESI found hints of variations in dark energy in its surveys of relic sound waves from the early universe.
“So, two completely independent measurements have found hints of time variation in dark energy, challenging the standard model that dark energy doesn’t change.”
Davis added that, as well as revealing what dark energy actually is, this research may also hold the clue to explaining how gravity and quantum physics fit together. While quantum physics (the best theory we have of how the universe operates at subatomic scales) and Einstein’s theory of general relativity (our best picture of gravity) have been highly successful in their own rights, scientists have so far been unable to unite them. In other words, there is no theory of quantum gravity.
“We know these two 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 continued.
This effort will receive a boost when data is added to the team’s already comprehensive type Ia supernova catalog. This will include observations from the Dark Energy Bedrock All-Sky Supernova program (DEBASS), which is detecting hundreds more local supernovas than even the DES survey.
The team’s results are available on the repository site arXiv.
