News in Science

Dark energy a furphy, says new paper

'Dark energy', which researchers have spent years trying to fathom, isn't necessary to explain our universe after all, according to a new solution to Einstein's theory of general relativity.

This challenges the notion that dark energy makes up 76% of our universe, as many cosmologists believe.

Research by Dr David Wiltshire, from New Zealand's University of Canterbury, accounts for recent observations of the distribution of matter in the universe, observations that hadn't been made in Einstein's time.

Wiltshire's paper, published this week in the journal Physical Review Letters, focuses on the lumpy distribution of matter in the universe as it evolved, rather than a smooth distribution that Einstein assumed.

Once this uneven distribution is taken into account, he says, we don't need dark energy to explain the accelerating expansion of the universe.

"Dark energy, this mysterious stuff in the vacuum of space which makes the universe want to accelerate, is the basis for standard cosmology today because it explains much of what we see," says Wiltshire.

Galaxies do appear to be moving away from each other and at an ever increasing rate, he says.

But he says this acceleration is apparent, and whether you see it or not depends where in the universe you are taking measurements from, a void or a galaxy.

The matter with matter

When cosmologists first solved Einstein's equations 80-90 years ago, they didn't know how matter was distributed or what the universe really looked like.

"The early solutions, which standard cosmology still uses, assumed a very simple structure where the universe is uniformly smooth and featureless, evolving the same way in all directions," says Wiltshire.

But he says through observational projects like the Sloan Digital Sky Survey and the 2 Degree Field survey, we now have a much better picture of the large-scale structure of the universe and we know that galaxies are not uniformly distributed.

"Rather, they are in clusters sprinkled thinly in filaments and 'bubble walls' surrounding huge voids hundreds of millions of light-years across," he says.

This complicates a standard view of the universe.

"Space curves around a massive object, just as a rubber sheet on a trampoline will stretch around a heavy cannon ball, and time slows down," says Wiltshire.

But he says this only represents bound systems like galaxies.

"The flat edge of the rubber sheet is the reference point for our clocks. It is only the space beyond this flat edge that is expanding. Clock rates and the curvature of space can both vary gradually as you move across an expanding void."

And, since mass slows down time, the clocks of observers in voids, where most of the empty space in the universe is, will appear to be ticking faster than the clocks of observers in galaxies.

It is this last feature, he says, that explains why dark energy is unnecessary.

"What we see as cosmic acceleration is an apparent effect, which begins when voids open up. When the universe was very young all the clocks were synchronised but because of the way voids have evolved, the clock rate is variable today," Wiltshire says.

Age is relative too

The age of the universe also depends on where you're standing, as Wiltshire discovered in calculations published in the New Journal of Physics.

The universe is 14.7 billion years old, a billion years older than the currently accepted age, from our galactic observation point.

But it is more than 18 billion years old from an average location in a void.

In a third paper, published this week in Astrophysical Journal Letters, Wiltshire and colleagues examine independent observational tests that confirm these ages.

Although the research completely changes our view of the universe, it doesn't alter standard physics.

"Einstein's revolution is still continuing," says Wiltshire. "This is a radically conservative solution to how the universe works."