universe expansion
The universe is expanding at an ever-increasing rate.NASA

What causes the accelerating expansion of our universe is one of science's greatest mysteries. Now, physicists say they might have found a way to explain this puzzling phenomenon.

Nearly two decades ago, in 1998, two separate teams of astronomers discovered that the universe is expanding at an ever-increasing rate.

This suggests that space is filled with dark energy that pushes matter away. The expansion of the universe is thought to have been accelerating ever since the universe entered its dark energy-dominated era around five billion years ago.

In 2016, Nasa and ESA scientists used the Hubble Space Telescope to show that the universe was expanding 5-9% faster than previously reported.

In the study now published in the journal Physical Review D, a team of physicists from UBC investigated the potential origins of this accelerated expansion. In it, the authors attempted to resolve the incompatibility issue between two of the most important theories about the way our universe works – quantum mechanics and Einstein's theory of general relativity.

Fluctuating space

For years, scientists have tried to find out what exactly 'dark energy' is. The most popular hypothesis is that dark energy is in fact vacuum energy – that is, the energy density of empty space.

When physicists apply the theory of quantum mechanics to vacuum energy, they find there would be an incredibly large density of vacuum energy, far more than the total energy of all the particles in the universe.

The problem comes when they want to relate this to Einstein's theory of general relativity. If their predictions with quantum mechanics are true, the theory of general relativity suggests that the energy would have a strong gravitational effect and this would potentially result in the universe exploding.

Instead of trying to alter the theories of quantum mechanics or general relativity to address this contradiction, like past research had done, the new study suggests a different way of looking at the problem.

The scientists do take the large density of vacuum energy predicted by quantum mechanics seriously, but they come up with mathematical models that describe a completely different picture of space.

In this picture, the universe is made up of constantly fluctuating space and time. It oscillates between expansion and contraction and as it swings back and forth, the two almost cancel each other.

However, a very small net effect leads the universe to expand slowly at an accelerating rate.

We cannot feel these fluctuations because the phenomenon "happens at very tiny scales, billions and billions times smaller even than an electron", lead author Qingdi Wang said. "Space-time is not as static as it appears, it's constantly moving".