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SOMETHING in the Universe is speeding up its expansion and pushing the galaxies apart. But it wasn’t always that way, according to a group of British physicists. They’re suggesting that the accelerator may only have been pressed relatively recently, when the Universe was a few billion years old. If they’re right, there’s no reason why this mysterious repulsive force, dubbed “dark energy”, couldn’t change again or even switch off completely—meaning all bets about the future of the Universe are off.

The team, from the University of Portsmouth and Oxford University, studied a range of cosmological data sets, including observations of the brightness of distant supernovae—key measurements which first revealed the existence of dark energy in 1998—as well as surveys of the distribution of galaxies and the cosmic background radiation, the dim “afterglow” of the big bang. “If the dark energy changed, it would change the curvature of space-time,” says team member Carlo Ungarelli of the University of Portsmouth. “One way this would reveal itself is by changing the height of so-called peaks in the cosmic background radiation.”

The researchers report in a paper submitted to Monthly Notices of the Royal Astronomical Society that they have found such changes. Their results suggest that the accelerating effect of dark energy kicked in when the Universe was a few billion years old, after most galaxies had formed. “Before this time, there was only gravity decelerating the expansion,” says Ungarelli. “Afterwards, dark energy began to drive the acceleration of the Universe.”

The team decided to look for such a change simply because nobody knows anything about dark energy, so all possibilities are worth investigating. “Either it has remained constant throughout history or it has changed,” says Ungarelli. “One possibility is that it has changed radically, undergoing what physicists call a phase transition.”

One example of a phase transition is when steam condenses into liquid water. Past a critical threshold, the substance, in this case H₂O, starts behaving in a completely different way. Similarly, Ungarelli and his colleagues suggest that the Universe might contain a type of unknown matter that underwent a phase transition from a fluid with no gravitational effect into one with repulsive gravity.

But the researchers admit they have no idea how or why this change could have happened. “What is missing is a fundamental physical phenomenon which could cause such a phase transition,” says Ungarelli. “All we have done is look for evidence in the data for a change in the dark energy. If it holds up, it’s up to us theorists to come up with an explanation.”

The team knows it will need better data to convince the sceptics. “What they are saying would be tremendously important if true,” says Max Tegmark of the University of Pennsylvania. But he says that, for now, he’s sticking to the idea that dark energy has always been around.