Surveys of more than 1,500 supernovae conducted by the Dark Energy Camera have established strong constraints on the accelerating expansion of the universe.
The results show that the mysterious force driving this cosmic acceleration, dark energy, can change in density over time, challenging the standard model of cosmology.
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The results were obtained from the largest supernova sample 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 was not only expanding, but was doing so at an increasing rate.
This was a big surprise to physicists, who expected it after the initial rapid expansion of the cosmos during the Big Bang. Cosmic expansion should be slowing down, but it is accelerating.
Dark energy has been proposed as a placeholder for the unknown aspect of the universe that is causing this mysterious and disturbing cosmic acceleration, but scientists cannot say for sure what it is. This 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 universe.
The Dark Energy Survey, performed 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 are indispensable in solving the mystery that such surveys triggered 25 years ago.
The new Dark Energy Survey results 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 called “Lambda cold dark matter.” Model that posits a universe with accelerating expansion (ΛCDM).
These place the tightest constraints on the expansion history of the universe over its 13.8 billion year history, but also leave breathing 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. The powerful camera detected thousands of supernovae among them.
Machine learning was able to identify 1,499 of these samples as a special type of star explosion called Type Ia supernovae. These occur when dead stars called white dwarfs, which have long depleted hydrogen and converted to helium in their cores to enable nuclear fusion, exist in a binary system with another star.
White dwarfs pull material from their companions, or “donor” stars, and when this material accumulates on top of the dead star, it can push the white dwarf beyond what’s called the Chandrasekhar limit. This is the mass limit that a star needs to go 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 universe.
Additionally, because the wavelength of light from distant objects extends toward the red end of the electromagnetic spectrum, a process called “redshift” with distance from Earth, the uniform light output of standard candles at varying distances can be used for measurement. expansion of the universe.
Comparing the redshift of closer Type Ia supernovae with the redshift of more distant and therefore earlier white dwarf explosions may therefore give a clue to the strength of this expansion and hence to the density of dark matter at corresponding periods in cosmic history.
The new Dark Energy Survey results triple the number of known supernovae with a redshift of about 0.2, corresponding to a distance of about 2.5 billion light-years. This fivefolds the known standard number of candles with a redshift of about 0.5, corresponding to a distance of about 6 billion light-years.
“This is a truly huge increase from 25 years ago, when only 52 supernovae were used to understand the existence of dark energy,” Tamara Davis, a Dark Energy Survey working group member and University of Queensland professor, said in a statement. said.
Dark energy wasn’t always this intense
Having such a large sample size of Type Ia supernovae over such a wide cosmic distance, the team was able to trace a record of cosmic expansion when combining the distances of these explosions with their speeds away from Earth.
This served as an indicator of whether the dark energy density remained constant, but this did not appear to be the case.
“As the universe expands, the density of matter decreases,” Dark Energy Research director and spokesman Rich Kron said in the same statement. “But if the dark energy density is constant, this means that the total proportion of dark energy should increase as the volume increases.”
This could be a challenge to the ΛCDM model of the universe, which is a mathematical model that explains how the universe evolved with just a few key properties, 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; What these supernova research results suggest may not be true.
“There are promising clues that dark energy changes over time. We found that the simplest model of dark energy – ΛCDM – is not the best fit,” Davis said. he added. “It’s not as far off as we’re ruling out, but it’s an intriguing new piece of the puzzle in the quest to understand what’s accelerating the expansion of the universe. A more complex explanation may be needed.”
Answers to this riddle may have to wait for the next generation of supernova research to begin and continue from the Dark Energy Probe.
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“This result clearly demonstrates the value of astronomical research projects that continue to produce excellent science even after data collection has ended,” Nigel Sharp, program director for the National Science Foundation Division of Astronomical Sciences, said in the same statement. said.
“We need as many different approaches as possible to understand what dark energy is and what it is not. This is an important path to that understanding.”
Dark Energy Survey results have been submitted to the Astrophysical Journal.