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. The first evidence for dark energy came from supernovae observations in 1998. Further evidence arrived in 2002 from a survey of 250,000 galaxies and later from observations of gravitational lensing.

Some explanations for dark energy -- such as quintessence, modified gravitational theories that include extra dimensions, or string physics -- suggest that dark energy could change with time. If dark energy became progressively weaker, the universe would eventually collapse in on itself in a "big crunch". If it became stronger, on the other hand, the universe would tear itself apart in a "big rip".

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 recent observations on supernovae that suggested that dark energy remains constant with time. 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. "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."