A survey of more than 1,500 supernovae conducted by the Dark Energy Camera has placed strong constraints on the accelerating expansion of the universe.
The results suggest that the mysterious force driving this cosmic acceleration, dark energy, can change over time and vary in density, challenging the standard model of cosmology.
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The results come from 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 doing so at an increasingly rapid rate.
That was a huge surprise to physicists, who had expected it after the initial rapid inflation of the cosmos during the Big Bang. The cosmic expansion should have slowed down, but it is accelerating.
Dark energy has been proposed as a placeholder for whatever unknown aspect of the universe is causing this mysterious and disturbing cosmic acceleration, but scientists can’t say for sure what it is. That problem is compounded by the fact that dark energy is now thought to account for 65% to 70% of the total energy and matter in the cosmos.
The Dark Energy Survey, 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 observations of supernovae remain an integral part of solving the mystery that such research sparked 25 years ago.
The new Dark Energy Survey results were presented at the 243rd meeting of the American Astronomical Society on January 8, 2024, with the team behind it adding that they are consistent with the standard model of cosmology, the so-called ‘cold dark matter of Lambda’. model (ΛCDM), which shows a universe with an accelerating expansion.
These place the strictest constraints on the history of expansion over the 13.8 billion year history of the cosmos, but also leave breathing room for more complex models of the universe.
Dark energy research 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. Inside, the powerful camera has spotted thousands of supernovae.
From this sample, machine learning was able to determine that 1,499 was a special type of stellar explosion, called a Type Ia supernova. These form when dead stars called white dwarfs, which have long since consumed hydrogen to power nuclear fusion and converted to helium in their cores, exist in a binary system with another star.
The white dwarfs drag material from their companion or ‘donor’ star, and as this material accumulates on the dead star, it can push the white dwarf beyond the so-called Chandrasekhar limit. This is the mass limit a star needs to become a supernova.
These Type Ia supernovae are so uniform that scientists call them “standard candles,” and their light can be used to measure vast distances in the cosmos.
In addition, because the wavelength of light from distant objects extends toward the red end of the electromagnetic spectrum, a process called “redshift,” the uniform light output of standard candles at different distances can be used to determine the wavelength of light from distant objects to measure. the expansion of the universe.
Comparing the redshift of closer Type Ia supernovae with the redshift of more distant, and thus earlier, white dwarf explosions may therefore provide a hint as to the strength of this expansion and thus to the density of dark matter in the corresponding periods in cosmic history.
The new results from the Dark Energy Survey triple the known number of supernovae at a redshift of about 0.2, which corresponds to a distance of about 2.5 billion light-years away. It fivefolds the known number of standard candles with a redshift of about 0.5, which corresponds to a distance of about 6 billion light years.
“It’s really a huge increase in scale from 25 years ago, when only 52 supernovae were used to derive dark energy,” Tamara Davis, a member of the Dark Energy Survey working group and professor at the University of Queensland, said in a statement. declaration.
Dark energy wasn’t always so dense
With such a larger sample size of Type Ia supernovae over such large cosmic distances, the team could piece together a record of cosmic expansion by combining the distances of these explosions with the speed at which they are moving away from Earth.
This acted as an indication of whether the dark energy density had remained stable, which did not appear to be the case.
“As the universe expands, its matter density decreases,” Rich Kron, director and spokesperson for Dark Energy Survey, said in the same statement. “But if the dark energy density is constant, this means that the total proportion of dark energy must increase as the volume increases.”
This could pose a challenge to the ΛCDM model of the universe, a mathematical model that describes how the universe evolves using just a few key features such as the density of matter, the type of matter and the behavior of dark energy.
That’s because ΛCDM assumes that the density of dark energy is constant and does not dilute as the universe expands, something these supernova research results suggest may not be true.
“There is tantalizing evidence that dark energy changes over time. We find that the simplest model of dark energy – ΛCDM – is not the best fit,” Davis added. “It’s not so far off that we’ve ruled it out, but in the quest to understand what’s accelerating the universe’s expansion, this is an intriguing new piece of the puzzle. A more complex explanation may be needed.”
The answers to this riddle may have to wait until the next generation of supernova surveys take off and build on the Dark Energy Survey.
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“This result clearly demonstrates the value of astronomical research projects that continue to produce excellent science long after data collection has ended,” Nigel Sharp, program director of the National Science Foundation Astronomical Sciences Division, said in the same statement.
“We need as many different approaches as possible to understand what dark energy is and what isn’t. This is an important route to that understanding.”
The results of the Dark Energy Survey have been submitted to the Astrophysical Journal.