Dark energy might influence the arrow of time.
What the heck is dark energy? Physicists have been trying to explain dark energy — the mysterious repulsive force that pushes everything in the universe apart — for years. And even though it makes up nearly 70 percent of all energy in the universe, the only reason we know it exists is due to its influence on other objects. It has never been directly detected.
But dark energy may play a hand in another fundamental quantity of physics. According to a recent paper published in the journal Physical Review E, a team of researchers have postulated that in some cases, dark energy might cause time to propagate forward.
When physicists were first peering into the depths of the cosmos, they expected to find that the universe was slowing down because of the collective gravity from all matter after the big bang. However, they discovered something rather surprising. Everything is speeding up.
As far as we know, the universe operates according to the laws of physics, and almost all the laws are time-reversible, meaning that things look exactly the same whether time runs backwards or forwards.
But why does time have an arrow pointing from the past to the present to the future?
It likely comes down to one very important tenet of physics that is not time-reversible — the second law of thermodynamics. It states that as time moves forward, the amount of disorder in the universe always increases. For this reason, it is currently accepted that the second law is the source of time’s arrow.
To test whether dark energy and the second law might be related, physicists A. E. Allahverdyan from the Yerevan Physics Institute and V. G. Gurzadyan from Yerevan State University, both located in Armenia, looked at a simple case of a planet orbiting a star with a changing mass.
They found that if dark energy either doesn’t exist or attracts space together, the planet orbits the star without us being able to tell whether it is moving forward or backward in time — the only difference is the direction of its orbit.
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. Run time one way, the planet is flung out; run it the other way, the planet moves in and gets captured by the star, according to ScienceAlert.
So dark energy can lead to an arrow of time.
The authors note that this only is the case for particular situations, and are not claiming that dark energy is the reason time only moves forward. However, they have revealed a potential link between thermodynamics and dark energy.
You might also like: The Universe Might Be Expanding Much Faster Than We Thought