Cosmologists in the US have made the most accurate measurements ever of how
dark energy varies with time -- and found that it remains perfectly
constant. Max Tegmark at the University of Pennsylvania and Yun Wang at the
University of Oklahoma performed numerical simulations on observational data
from supernovae, the cosmic microwave background and galaxy clusters. The
results, which agree with Einstein's predictions for a non-varying
cosmological constant, lend further support to the existence of dark energy
(Phys. Rev. Lett. 92 241302).
The acceleration of the universe is driven by a force that has repulsive
rather than attractive gravitational interactions. But although this
so-called "dark energy" is thought to account for around two-thirds of the
universe, no one knows what it is made of. Possible explanations for dark
energy include a "cosmological constant" -- which remains unchanged with
time -- that was first predicted by Einstein in 1917.
But there are also more exotic explanations for dark energy -- such as
quintessence, modified gravitational theories that include extra dimensions,
or string physics -- that suggest that dark energy could change with time.
If dark energy became progressively weaker, the universe would eventually
tear apart in a "big rip". If it became stronger, on the other hand, the
universe would collapse in on itself in a "big crunch".
Tegmark and Wang used a novel model-independent approach to measuring the
dark-energy density. They analysed data from type 1a supernovae, recorded
with the Hubble Space Telescope; the cosmic microwave background (CMB) taken
with the Wilkinson Microwave Anistropy Probe (WMAP) and the Sloan Digital
Sky Survey (SDSS); and from large-scale galaxy cluster observations.
The results agree with previous data on supernovae observations that
suggested that dark energy remains constant with time and fit well with
Einstein's cosmological constant. Moreover, the physicists calculated that
if the dark energy density were to change with time, a big crunch or big rip
could not occur for at least 50 billion years for models that allow such
events. These findings could lead to these theories being widely reassessed.
"I'm struck by the fact that the dark energy seems so 'vanilla'," Tegmark
told PhysicsWeb. "Theorists have invented scores of elegant models where it
increases or decreases its density over time, yet even with this new
improved measurement, it remains perfectly consistent with Einstein's Lambda
model where its density is a mere constant."
Belle Dumé is Science Writer at PhysicsWeb
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