Spanish scientists have come up with a new hypothesis on to explain the rate of expansion of the universe: time is - literally - slowing down!

A decade ago, astronomers noticed that distant supernovae - exploding stars on the very fringes of the universe - seemed to be moving faster than those nearer to the centre, suggesting that they were accelerating as they shot through space.

Dark energy was suggested as a possible means of powering this acceleration of the expansion of the cosmos.

The problem is that no-one has any idea what dark energy is or where it comes from, and theoreticians around the world have been scrambling to find out what it is, or get rid of it.

The team's proposal, which will be published in the journal Physical Review D, does away altogether with dark energy. Instead, Prof Senovilla says, the appearance of acceleration is caused by time itself gradually slowing down, like a clock that needs winding.

"We do not say that the expansion of the universe itself is an illusion," he explains. "What we say it may be an illusion is the acceleration of this expansion - that is, the possibility that the expansion is, and has been, increasing its rate."

Instead, if time gradually slows "but we naively kept using our equations to derive the changes of the expansion with respect of 'a standard flow of time', then the simple models that we have constructed in our paper show that an "effective accelerated rate of the expansion" takes place."