**THURSDAY, 11 MAY 2006**

The cosmological constant represents the amount of energy in empty space and has long been an annoyance to physicists. Einstein first predicted the cosmological constant, as a means of ensuring that the universe stays the same size. Once Hubble observed that the universe is expanding, Einstein dropped the idea, describing the cosmological constant as the ‘biggest blunder’ of his life.

Now the theory has come round full circle: the cosmological constant is thought to be the most likely explanation for the acceleration of the expanding universe. But this is not the end of the cosmological constant conundrum, “When the value of the cosmological constant was calculated, it was found to be much smaller than expected,” explains Professor Turok. “The explanation as to why this constant is so small has become one of the biggest problems in physics”.

If today’s universe is part of an endless cycle of big bangs and crunches, then the current estimate of a 12-14 billion-year-old universe would be a huge underestimate. At every Big Bang, the amount of matter and energy is reset, but the cosmological constant is not. Instead, the cosmological constant gradually decreases over many cycles to the small value observed today.

The theory put forward by Turok and Steinhardt suggests that the cosmological constant decreases in steps, through a series of transitions. “The higher the value of the cosmological constant, the more rapid the transitions,” says Turok. As the constant diminishes to lower levels, it changes more slowly, remaining on the lowest positive value for a tremendously long time. “That means that today’s universe is most likely to have a small cosmological constant, just as we currently observe,” Turok says.

However, this new theory cannot explain other cosmic coincidences: for example, why is the cosmological constant’s value so similar to the density of matter in the universe today?

Turok sees this as the next challenge for the theory. “This is an initial attempt to go beyond Einstein’s theory of gravity,” he says. “It would be surprising if we solved everything first time.”

View Steinhardt and Turok's paper in Science on the cosmological constant

View their 2002 paper proposing a cyclic universe

Written by Gemma Simpson