by A survey of over 1,500 supernovae by the Dark Energy Camera has placed strong constraints on the expansion of the expanding universe.
The results suggest that the mysterious force driving this cosmic acceleration, dark energy, may change over time, varying in density, calling into question the standard model of cosmology.
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The results were delivered by the largest sample of supernovae ever taken by a single instrument as part of the Dark Energy Survey. Supernovas were integral to the discovery in the late 1990s that not only is the universe expanding, but that it is also doing so at an accelerating rate.
That was a big surprise for physicists, who expected it after the rapid inflation of the cosmos during the Big Bang. The cosmic expansion should have slowed down, but it is speeding up.
Dark energy has been suggested as a placeholder for whatever unknown aspect of the universe is causing the mysterious and troubling 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 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 are integral to solving the mysteries that prompted such investigations that was 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 them adding that they are consistent with the standard model of cosmology, known as “Lambda cold dark matter” model (ΛCDM), that has accurate features with accelerated expansion.
These place the tightest constraints on the history of expansion throughout the 13.8 billion year history of the cosmos, but they also leave breathing room for more complex models of the universe.
Exploring dark energy with standard candles
To gather this data, the 570-megapixel Dark Energy Camera built by Fermilab scanned the sky above Earth for 758 nights, observing 2 million distant galaxies. Within these, the powerful camera spotted thousands of supernovae.
From this sample, the machine learning was able to determine that 1,499 of them were a special type of stellar explosion known as a Type Ia supernova. These occur when dead stars known as white dwarfs, which have long consumed hydrogen to drive nuclear fusion and convert it into helium in their cores, exist in a binary system with another star.
The white dwarf draws material from its companion or “donor” star, and as this material accumulates on the dead star, it can push the white dwarf past the so-called Chandrasekhar limit. This is the mass limit at which a star must go supernova.
These Type Ia supernovae are so uniform that scientists refer to them as “standard candles,” and their light can be used to measure vast distances across the cosmos.
In addition, because the wavelength of light from distant objects is stretched toward the red end of the electromagnetic spectrum, a process known as “redshift,” as they move away from Earth, the uniform light output of standard candles over distances is possible use different to measure. expansion of the universe.
Comparing the redshift of closer Type Ia supernovae to the redshift of more distant, and therefore earlier, white dwarf explosions can therefore provide a clue to the strength of the expansion. this and so the density of dark matter at the corresponding periods in cosmic history.
Through the results of the new Dark Energy Survey the known number of supernovae varies by a redshift of about 0.2, which corresponds to a distance of about 2.5 billion light years away. It dwarfs the known number of standard candles at a redshift of about 0.5, which correlates to a distance of about 6 billion light-years away.
“It’s a huge increase from 25 years ago when only 52 supernovae were used to understand dark energy,” Dark Energy Survey working group member and University of Queensland Professor Tamara Davis said in a statement.
Dark energy wasn’t always so dense
With such a large sample size of Type Ia supernovae over such wide cosmic distances, the team could find a record of cosmic expansion by combining the distance of these explosions with the speed at which they are receding from Earth.
This served as an indication that dark energy density remained stable, which did not appear to be the case.
“As the universe expands, the density of matter goes down,” Dark Energy Survey director and spokesman Rich Kron said in the same statement. “But if the dark energy density is constant, that means that the total content of dark energy must be increasing as volume increases.”
This could be a challenge to the ΛCDM model of the universe, a mathematical model that describes how the universe changes with a few key features such as the density of matter, the type of matter, and the behavior of dark energy.
That is because ΛCDM assumes that the density of dark energy is constant and does not diminish as the universe expands, which the supernova survey results indicate is not true.
“There are striking hints that dark energy changes over time. We find that the simplest model of dark energy – ΛCDM – is not the best fit,” said Davis. “It’s not so far off that we’ve ruled it out, but as we try to understand what’s accelerating the expansion of the universe, this is an interesting new piece of the puzzle. It may require a more complex explanation .”
Answers to this conundrum may have to wait until the next generation of supernova surveys are launched and drawn from the Dark Energy Survey.
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“This result clearly demonstrates the value of astronomical survey projects that continue to produce excellent science well after data collection has ceased,” National Science Foundation Division of Astronomical Sciences program director Nigel Sharp said in the statement same.
“We need as many different approaches as we can get to understand what dark energy is and what is not. This is an important way to achieve that understanding.”
The results of the Dark Energy Survey have been submitted to the Astrophysical Journal.
