For the past 20 years, physicists have known that the expansion of the universe is accelerating as if some bizarre dark energy is blowing up space like a balloon. In fact, cosmologists’ well-tested standard model assumes that 69% of the content of the universe is dark energy.

On the other side, time moves forward and never backward. But, why does time have an arrow pointing from the past to the present to the future? According to some physicists, the influence of dark energy may be the reason for the forward flow of time. Perhaps the one-directional time is a permanent feature of the universe.

Whenever we hear the word dark in physics, we should get very suspicious cause it probably means we don’t know what we’re dealing with. It’s a little frightening that we don’t know what is a mysterious influence that pushes space apart faster than gravity can pull the things in it together. It has never been directly detected, yet the best idea is that is the energy of empty space itself.

Diagrammatic presentation of accelerated expansion of the universe. Image credit: NASA/WMAP

We all know that time only moves forward in our world, although nothing in the laws of physics asserts that time must move ahead. Yet, broken eggs don’t suddenly reassemble themselves, and dead things don’t suddenly come back to life. To understand why time seems to only move in one direction, many physicists believe it comes down to entropy, the level of molecular disorder in a system, which is always increasing.

Though we accepted entropy as the mechanism that imparts directionality to time, it is understanding of how time works, but not understanding of why it behaves so.

A Dark Connection

In the quest to understand the origins of Time’s Arrow, a team of Armenian researchers has postulated that in some cases, dark energy might cause time to propagate forward. To test whether dark energy and the entropy might be related, they looked at a simple case of a planet orbiting a star with a changing mass.

The researchers ran a simulation to test how the orbit of a planet would change, depending upon whether dark energy was absent, or present. The test results were intriguing. They found if dark energy is absent, a planet simply orbits its star in the accepted fashion, regardless of whether time runs forward or backward. The only difference is the direction of its orbit.

There’s something decidedly suggestive that the entropy and dark energy might be two facets of the same phenomenon. Image credit: NASA / GSFC.

On the other hand, if dark energy pushes space apart, just like it does in our universe, the planet eventually gets thrown away from the star on a path of no return, giving a distinction between the past and the future. Without dark energy, there is no time, and hence no way to tell the difference between past, present, and future. Dark energy is now seen as a “positive” constant, pushing the universe forward, rather than holding it back.

Although the research results are quite interesting, they are limited and far away from claiming that dark energy is the reason time only moves forward. However, it revealed a potential link between Time’s Arrow and repulsive mysterious force.

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