The passing of time has puzzled humans for millennia, and while our understanding of the universe has become deeper and deeper, the true nature of time still escapes us. The latest attempt to bring some clarity into the matter takes a bold stance: Dark energy helps to push time forward.
This is the hypothesis put forward by Armen Allahverdyan and Vahe Gurzadyan from the Yerevan Physics Institute in Armenia. In a paper published by Physical Review E, they suggest that once dark energy, which is responsible for the accelerated expansion of the universe, becomes dominant, it helps time to move forward.
Their theoretical setup is simple. They investigate what would happen to a relatively small mass, like a planet, moving through the gravitational field of a larger mass, like a star, in a scenario where time didn’t have to go forward. This little system starts at a certain energy and obeys the laws of Newtonian physics, although with a twist so that dark energy can be introduced. They found that if the contribution was positive (like it is in our universe), then the system suddenly had a preferred direction of time.
While this piece of research is certainly interesting, it is not conclusive evidence of a causal link between time and dark energy, which the researchers themselves admit. The system was timeless before dark energy took over, but we know that's not the case for our universe.
Dark energy remains as mysterious as when it was discovered almost 20 years ago. Although there are many possibilities regarding its nature, the simplest solution treats dark energy as a cosmological constant related to the energy of empty space.
Energy fluctuations happen in a vacuum (see, for example, the Casimir effect), so the cosmological constant idea is not outlandish, but when we approach the dark energy problem from the point of view of quantum mechanics, things don’t match up. This vacuum energy should be a much bigger number than what is pushing the galaxy apart.
So we can see the effect of dark energy, but not know exactly what is causing it, and in that respect, it is very similar to time. We believe time is related to the entropy of the universe. Entropy, the measure of the disorder of a system, always increases and it does so in the same direction of time.
The research does highlight something, though. Dark energy is intrinsically connected with spacetime on the cosmic scales, so we have to research how it might affect things even here in a lab.