New light on dark energy

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 constant 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."