Reported by ScienceDaily
Mathematicians are raising a major challenge to one of modern cosmology's most widely accepted explanations: the idea that dark energy is driving the accelerating expansion of the universe.
A study published in Proceedings of the Royal Society A presents mathematical findings that researchers at the University of California, Davis, say reveal a fundamental instability in the standard model of the expanding universe. Their calculations suggest that cosmic acceleration could emerge naturally from Einstein's equations, potentially eliminating the need to invoke mysterious dark energy.
The work challenges the Lambda-cold dark matter model, the leading framework scientists use to describe the universe's evolution since the Big Bang. The researchers reached their conclusions by studying the Einstein-Euler equations, which combine Einstein's theory of general relativity with equations describing how fluids behave. Together, these mathematical tools help scientists investigate everything from galaxies and black holes to the expansion of the cosmos.
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To explain the problem, Blake Temple, the study's corresponding author and a distinguished professor emeritus of mathematics at UC Davis, offered a simple comparison: a pencil balanced vertically on its tip.
"All the forces are in balance when a pencil is standing on end, so it is a 'solution of the equations,'" he said. "But it's unstable. Any breath of air and it falls away."
According to Temple, the team's mathematical analysis shows that Friedmann spacetimes, which form the foundation of conventional models of cosmic expansion, are unstable at both small and large scales in the context of the Big Bang. He described the standard solution as exceptionally unstable.
"Unstable solutions in physics and science are considered not physical," Temple said. "You'll never observe them in nature."
This instability could have far-reaching consequences. If the universe naturally evolves away from the behavior predicted by these conventional models, its accelerating expansion might not require a separate form of energy to explain it. Instead, Temple argues, the answer could already be contained within Einstein's original theory of gravity.
"The instability of all Friedmann spacetimes to accelerated expansion suggests a simpler, more natural explanation for the acceleration of the universe than dark energy," he said.
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