Why is the universe expanding? And accelerating?

What causes the accelerating expansion of our universe?

Solve that mystery and you reconcile two successful, yet incompatible, theories that explain how our universe works: quantum mechanics and Einstein’s theory of general relativity. But it isn't that simple. It may be that space-time itself is relative and if we zoomed in – way in – on the universe, we would realize it’s made up of constantly fluctuating space and time.

In 1998, astronomers found that our universe is expanding at an ever-increasing rate, and this led to the suggestion that space is not empty, but is instead filled with what was given the blanket term "dark energy" that pushes matter away. One natural candidate for this dark energy is vacuum energy. When physicists apply the theory of quantum mechanics to vacuum energy, it predicts that there would be an incredibly large density of vacuum energy, far more than the total energy of all the particles in the universe. If this is true, Einstein’s theory of general relativity suggests that the energy would have a strong gravitational effect and most physicists think this would cause the universe to explode.

Fortunately, this doesn’t happen and the universe expands very slowly. But it is a problem that must be resolved for fundamental physics to progress.

Unlike otherss who have tried to modify the theories of quantum mechanics or general relativity to resolve the issue, a new paper takes the large density of vacuum energy predicted by quantum mechanics seriously and find that there is important information about vacuum energy that was missing in previous calculations. The calculations provide a completely different physical picture of the universe. In this new picture, the space we live in is fluctuating wildly. At each point, it oscillates between expansion and contraction. As it swings back and forth, the two almost cancel each other but a very small net effect drives the universe to expand slowly at an accelerating rate.But if space and time are fluctuating, why can’t we feel it?

“This happens at very tiny scales, billions and billions times smaller even than an electron,” says graduate student Qingdi Wang of the University of British Columbia.