What is dark energy, and what does it cause in the universe? A new study published in Physical Review E has attempted to answer this very question.
Dark energy remains elusive to the world of science. What is known is that it seems to have the opposite effect to gravity: it pushes space apart. Furthermore, it apparently also exceeds gravity in the sense that it pulls apart space faster than the pull of gravity can literally keep it together. But, what is the role of this dark energy?
According to the team of physicists who authored the new paper, dark energy might be making time go forward. It was ‘discovered’ when researchers found that, in the aftermath of the big bang, everything appears to have gone faster. Initially, it was thought that after matter was pushed into all directions in the big bang event, the collective gravity of this matter would have caused it to slow down – this is a phenomenon that we can see on Earth, for instance, if you send a ball up into the air, it will eventually slow down because of the pull of the planet’s gravity. However, according to scientists, this did not happen after the big bang. So, now, we have something out there splitting space apart faster than things can come together because of gravity; this is the dark energy. This effect is not deemed to be huge though – rather, it can only be seen in distant galaxies. But, it is still there.
Now, how do we explain dark energy in terms of the laws of physics? The team wanted to test whether it might be related to the second law of thermodynamics because of the latter’s peculiarity. While the universe appears to be based on the physics law that are time-reversible (that is, their effects caused by these laws would be the same regardless of time moving forward or backward), the second law of thermodynamics begs to differ. It is not time-reversible, given that disorder increases as time moves forward, and scientists, therefore, attempt to use this second law to explain the source of time’s (unidirectional) arrow, dictating that disorder will always increase following an event.
To test the correlation between dark energy and the second law, the researchers considered a planet orbiting a star having a changing mass. Their results show that the planet will orbit the star dully without sense of direction along time if dark energy does not exist or if it does the opposite of what it is said to do, that is, pulling space together. However, if the dark energy actually pushes space apart, as scientists believe it does, the planet will move away from the star to never return again. According to the team, this provides insight into the past and future being distinct from each other: if times moves in one direction, the planet flings off from the star, and if time runs the other way, the planet will end up getting captured by its star. To conclude, dark energy apparently furthers the arrow of time.
It is to be noted that the authors say they are not saying that time only moves forward because of dark energy; rather, they have just shown a possible correlation between dark energy and thermodynamics.