Beyond DESI, a number of new instruments will be added in the coming years, including the 8.4-meter Vera Rubin Observatory in Chile, NASA’s Nancy Grace Roman Space Telescope, and the European Space Agency’s Euclid mission.
“Our data in cosmology have made huge leaps in the last 25 years, and they are about to make even bigger leaps,” Frieman said.
As they accumulate new observations, researchers may continue to find that dark energy appears as constant as it has been for a generation. Or, if the trend continues in the direction suggested by the DESI results, it could change everything.
New Physics
If dark energy is weakening, it cannot be a cosmological constant. Rather, it may be the same type of field that many cosmologists believe caused a moment of exponential expansion during the birth of the universe. This type of “scalar field” could fill space with an amount of energy that appears constant at first (like the cosmological constant), but eventually begins to decrease over time.
“The idea that dark energy varies is very natural,” he said. Pablo Steinhardt, a cosmologist at Princeton University. Otherwise, she continued, “it would be the only form of energy we know that is absolutely constant in space and time.”
But this variability would cause a profound paradigm shift: we would not live in a vacuum, which is defined as the lowest energy state of the universe. Instead, we would inhabit a state full of energy that slowly slides towards a true void. “We’re used to thinking we live in a vacuum,” Steinhardt said, “but no one promised you that.”
The fate of the cosmos would depend on how quickly the number formerly known as the cosmological constant decreases, and how far it could go. If it reaches zero, cosmic acceleration would stop. If it falls far enough below zero, the expansion of space would become a slow contraction, the kind of investment needed to cyclical theories of cosmologylike those developed by Steinhardt.
String theorists share a similar perspective. With their proposal that everything is reduced to the vibration of strings, they can weave universes with different numbers of dimensions and all kinds of exotic particles and forces. But they I can’t build easily a universe that permanently maintains a stable positive energy, as our universe seems to do. Instead, in string theory, energy must fall gently over billions of years or fall violently to zero or a negative value. “Essentially all string theorists believe it’s one or the other. We don’t know which one,” he said. Cumrun Vafa from Harvard University.
Observational evidence of a gradual decline in dark energy would be a boon for the soft decay scenario. “That would be surprising. It would be the most important discovery since the discovery of dark energy itself,” Vafa said.
But for now, any such speculation is based on DESI analysis only very loosely. Cosmologists will have to observe many millions more galaxies before seriously considering the possibility of a revolution.
“If this holds up, it could illuminate the path to a new and potentially deeper understanding of the universe,” Riess said. “The next few years should be very revealing.”
original story reprinted with permission of Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to improve public understanding of science by covering developments and trends in research in mathematics and the physical and biological sciences.
