You may be surprised to hear this, but the universe can be modeled as a fluid. The distribution of galaxies flooded over the universe are like fluid particles, and we can find out peculiar properties about the universe from this. For example, did you know that the universe has a value for viscosity? Some researchers have used the "cosmological viscosity" in some pen and paper models and reached some eyebrow-raising conclusions about the future of the universe.
Typically, when you think of viscosity, you think of syrup pouring slowly from a bottle (very viscous) compared to water tumbling from a bottle (low viscosity). Marcelo Disconzi, the first author in the paper, published in the Physical Review D, deals with a different kind of viscosity called "bulk viscosity." This measures how much a fluid likes to expand or contract. This is appropriate when modeling a universe that is often described as expanding.
Past models that describe the universe as a fluid brushed cosmological viscosity under the rug, instead opting to say that the universe is an ideal fluid with no viscosity. This always ended up with predictions that matter within the universe could happily break the laws of physics and move faster than the speed of light. "This is disastrously wrong," said Disconzi, "since it is well-proven experimentally that nothing can travel faster than the speed of light."
So Disconzi and some other mathematicians set out to create a model of the universe with viscosity so that their results didn't break these fundamental laws of physics. Their conclusions had some startling results: a terrifying fate for the universe known as the "Big Rip."
The Big Rip is predicted to happen a long time into the future, so don't panic. This epic-sounding event is the result of the continuous, accelerating expansion of the universe.
Imagine that you have two galaxies. These galaxies naturally move away from each other because the fabric of space-time is expanding. The rate of expansion is also getting faster and faster, so the galaxies are pulled apart more and more quickly as time goes on. According to this theory, eventually the fabric of space-time will expand at faster than the speed of light. What this means for our galaxies is that the light one emits would never reach the other because the distance between them would be increasing faster than light can travel, leading to a lonely existence.
A timeline of the universe, ending in the Big Rip. Jeremy Teaford/Vanderbilt University.
If this trend were to continue, eventually the rate at which space-time expands would be more powerful than the strong nuclear force: the force that holds atoms together. If the theory is correct, then this is predicted to happen approximately 22 billion years from now; atoms will be ripped apart from one another by the unstoppable expansion of space-time and won't be able to reform. The Earth, the Sun, and even you and I would all be torn apart in the Big Rip.
“In previous models with viscosity the Big Rip was not possible,” said Scherrer. “In this new model, viscosity actually drives the universe toward this extreme end state.”
This new model for a universe with relative viscosity has only been analyzed with pen and paper. The next step is to test the model's conclusions against our observations of the universe using computers that can churn through the complex data.