by A study of more than 1,500 supernovae by the Dark Energy Chamber has placed strong limitations on the accelerated expansion of the universe.
The results suggest that the mysterious force driving this cosmic acceleration, dark energy, can change over time, varying in density, calling into question the standard model of cosmology.
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The results were returned by the largest sample of supernovae ever collected by a single instrument as part of the Dark Energy Survey. Supernovae were integral to the discovery in the late 1990s that the universe is not only expanding, but also doing so at an accelerating rate.
This came as a big surprise to physicists, who expected it after the initial rapid inflation of the cosmos during the Big Bang. Cosmic expansion should have slowed down, but it is accelerating.
Dark energy has been suggested as a placeholder for whatever unknown aspect of the universe is causing that mysterious and worrying cosmic acceleration, but scientists can’t say for sure what it is. That problem is compounded by the fact that dark energy is now believed to account for between 65% and 70% of the total energy and matter in the cosmos.
The Dark Energy Study conducted by the Dark Energy Camera mounted on the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in northern Chile shows that supernova observations remain critical to solving the mystery that such investigations were unleashed 25 years ago.
The new results from the Dark Energy Survey were presented at the 243rd meeting of the American Astronomical Society on January 8, 2024, and the team behind them added that they are consistent with the standard model of cosmology, the so-called “Lambda cold dark matter.” . model (ΛCDM), which presents a universe with accelerated expansion.
These impose the strictest constraints on the history of expansion over the 13.8 billion-year history of the cosmos, but they also leave room for more complex models of the universe.
Investigating dark energy with standard candles
To collect this data, the 570-megapixel dark energy camera built by Fermilab observed the sky above Earth for 758 nights, observing 2 million distant galaxies. Within them, the powerful camera detected thousands of supernovae.
Of this sample, machine learning was able to determine that 1,499 were a special type of stellar explosion called a Type Ia supernova. These occur when dead stars called white dwarfs, which have long depleted hydrogen to drive nuclear fusion into helium in their cores, exist in a binary system with another star.
White dwarfs drag material from their companion or “donor” star, and as this matter accumulates on the dead star, it can push the white dwarf beyond the so-called Chandrasekhar limit. This is the mass limit that a star needs to become a supernova.
These Type Ia supernovae are so uniform that scientists refer to them as “standard candles” and their light can be used to measure great distances across the cosmos.
Additionally, because the wavelength of light from distant objects extends toward the red end of the electromagnetic spectrum, a process called “redshift,” as they move away from Earth, the uniform light output of Standard candles at different distances can be used to measure the expansion of the universe.
Comparing the redshift of closer Type Ia supernovae with the redshift of more distant and therefore earlier white dwarf explosions can therefore give insight into the strength of this expansion and therefore therefore, of the density of dark matter in the corresponding periods of cosmic history.
The results of the new Dark Energy Survey triple the known number of supernovae with a redshift of around 0.2, corresponding to a distance of about 2.5 billion light years. It is five times the known number of standard candles at a redshift of about 0.5, which correlates to a distance of about 6 billion light years.
“This is a truly massive scale-up from 25 years ago, when only 52 supernovae were used to infer dark energy,” Tamara Davis, a member of the Dark Energy Survey working group and a professor at the University of Queensland, said in a statement.
Dark energy wasn’t always so dense
With such a large sample size of Type Ia supernovae over such vast cosmic distances, the team could plot a record of cosmic expansion by combining the distances of these explosions with the speed at which they move away from Earth.
This acted as an indication of whether the dark energy density had remained stable, which appeared not to be the case.
“As the universe expands, the density of matter decreases,” Dark Energy Survey director and spokesperson Rich Kron said in the same statement. “But if the dark energy density is constant, that means that the total proportion of dark energy must increase as the volume increases.”
This could be a challenge for the ΛCDM model of the universe, a mathematical model that describes how the universe evolves with just a few key characteristics such as the density of matter, the type of matter, and the behavior of dark energy.
This is because ΛCDM assumes that the density of dark energy is constant and does not dilute as the universe expands, something that the results of these supernova studies suggest may not be true.
“There are tantalizing signs that dark energy changes over time. We found that the simplest model of dark energy, the ΛCDM, is not the best fit,” Davis added. “It’s not so far away that we could have ruled it out, but in the quest to understand what is accelerating the expansion of the universe, this is an intriguing new piece of the puzzle. A more complex explanation may be necessary.”
Answers to this puzzle may have to wait until the next generation of supernova studies begins and is picked up from the Dark Energy Survey.
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“This result clearly shows the value of astronomical survey projects that continue to produce excellent science long after data collection has ended,” said the program director of the Division of Astronomical Sciences at the National Foundation for Astronomy in the same statement. Science, Nigel Sharp.
“We need as many diverse approaches as we can to understand what dark energy is and what it is not. This is an important route toward that understanding.”
The results of the Dark Energy Survey have been submitted to the Astrophysical Journal.
