© NASA/CXC/JPL-Caltech/Calar Alto O. Krause et alSupernovae point to expansion
In the late 1990s, observations of supernovae revealed that the universe has started expanding faster and faster over the past few billion years. Einstein's equations of general relativity provide a mechanism for this phenomenon, in the form of the cosmological constant, also known as the inherent "dark energy" of space-time. If this constant has a small positive value, then it causes space-time to expand at an ever-increasing rate. However, theoretical calculations of the constant and the observed value are out of whack by about 120 orders of magnitude.
To overcome this daunting discrepancy, physicists have resorted to other explanations for the recent cosmic acceleration. One explanation is the idea that space-time is suffused with a field called quintessence. This field is scalar, meaning that at any given point in space-time it has a value, but no direction. Einstein's equations show that in the presence of a scalar field that changes very slowly, space-time will expand at an ever-increasing rate.
Now Christophe Ringeval of the Catholic University of Louvain (UCL) in Belgium and his colleagues suggest that a quintessence field could be linked to a phase in the universe's history called inflation. During this phase, fractions of a second after the big bang, space-time expanded exponentially. Inflation is thought to have occurred because of another scalar field that existed at the time. But what if a much weaker quintessence field was also around during inflation?
According to the UCL team's models, inflation would have induced quantum fluctuations in the quintessence field. When the universe began its more sedate expansion after inflation ended, the field and its fluctuations would have been frozen into the fabric of space-time, so that the energy density of the field did not change with time.
This field would have had no impact on the early universe, which would have been dominated by matter and radiation. But eventually, as the universe grew, its expansion rate slowed down and the influence of matter and radiation diminished, the relative strength of the quintessence field increased, causing the expansion of space-time to accelerate, says Ringeval.
"The idea of mixing inflation with the dark energy problem is especially attractive," says Jérôme Martin of the Paris Institute of Astrophysics in France. But he adds that the "scenario needs additional calculations to be confirmed".
The first test of the idea could come as early as next year. The European Space Agency's Planck satellite is looking for signs of gravitational waves - fluctuations in the fabric of space-time caused by inflation. These would be imprinted in the cosmic microwave background (CMB), the radiation left over from the big bang, which the satellite will measure.
Ringeval and colleagues calculated what the strength of a quintessence field should be today, and worked backwards to estimate when inflation should have occurred. They found that it must have happened when the energy of the universe was in the teraelectronvolts (TeV) range. That would produce gravitational waves too weak to be detected by Planck, so if it does find evidence of them "our model will be destroyed", says Ringeval.
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