by Imagine a future where the universe, too soon, tears itself apart. Eventually, space-time is torn apart, making the universe uninhabitable.
Imagine a future where the universe, too soon, tears itself apart. First come the clusters, with their galaxies drawn away from each other. Then, the galaxies dissolution. Then the star systems and the planets. And now atoms themselves. Finally, space–time it is torn asunder, rendering the universe uninhabitable.
This is a potential future known as the Big Rip. It sounds terrifying and almost impossible to imagine, but the truly horrifying part is that some evidence seems to point directly to that fate.
Related: Was Einstein wrong? The case against space-time theory
Phantom energy
A quarter of a century ago, astronomers discovered dark energywhich is the name given to the apparent acceleration expansion of the universe. This dark energy is deeply mysterious; we do not currently understand what causes it, where it came from or what it is going to do. But that hasn’t stopped theorists from speculating.
The simplest possible form of dark energy is the so-called cosmological constant. In this simple picture, dark energy is a substance that permeates all space and time. Dark energy is everywhere, including in the room you are in right now. This dark energy is perfectly stable. It is exactly the same throughout space and time. This substance accelerates the expansion of the universe, but otherwise, it never changes.
Another possibility is that the substance behind dark energy can double back on itself, causing it to increase over time. This situation is called dark phantom energy (or just phantom energy). In this case, the acceleration would increase with time.
Read more: 25 years after its discovery, dark energy is still very elusive
Ghosts in the machine
Ironically, increasing the acceleration would make the observable universe much smaller. That’s because the velocity between any two points would continue to grow, even beyond the speed of light. In this case, galaxies flew apart so quickly that they would never see each other again. This would reduce the detectable limit of what we could see over time in an uncontrolled way.
If two points were ripped apart faster than light, they would no longer interact through any force of physics. While constant dark energy would leave objects that are already intact, such as galaxy clusters, phantom energy could tear them apart. In a finite amount of time, billions of years ago, clusters would break apart, followed by ever smaller objects. Even atomic and nuclear bonds would not withstand the attack.
Eventually, space itself would dissolve in an event known as the Big Rip. Any two points, no matter how close, would be ripped infinitely far apart. The structure of space-time, the causal foundations that make our universe work, would no longer behave. The universe would just break down.
However, fortunately, most physicists do not believe that this scenario could actually happen. For one, it is not clear how this tearing process interacts with other laws of physics. For example, quark it can’t be torn apart — when you try to do that, you need so much energy that a new quark emerges from the vacuum. So tearing a quark apart can lead to other interesting interactions.
Also, phantom energy does not behave according to normal physics. To do this work, the phantom substance must have negative kinetic energy. But negative kinetic energy does not normally occur in the universe — an example of this would be a ball rolling naturally up a hill — so this would be a major exception to our established understanding of physics.
Evidence for the Big Rip
For decades, astronomers have been trying to measure the strength of dark energy. They do this through a number called the equation of state parameter, which measures, for dark energy matter, the ratio of its pressure to its energy density. A cosmological constant corresponds to a parameter equal to -1, and the case of phantom energy corresponds to anything with this parameter less than -1.
So far, all astronomical measurements are compatible with a cosmological constant, an equation of state parameter -1. But strangely, all those same measurements, year after year, prefer a value slightly less than -1. All of those measurements have uncertainties, including the “boring” cosmological constant case, but it’s interesting that the exact data favors phantom energies.
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Some physicists believe that further evidence from us will pull the magic case firmly into the safety of the cosmological constant. Others, however, are taking this as a sign that the Universe may be telling us something interesting. Perhaps some combination of physics is allowed by current laws that give the appearance of phantom energy. (For example, if you kick a ball hard enough, it may temporarily go uphill.) Or maybe there are some brand new physics that seem impossible now but will make sense with a new understanding.
But even if we have the phantom energy scenario, it doesn’t look like the universe is going to tear itself apart anytime soon. With the known measurement constraints, the Big Rip would not play out for hundreds of billions of years. So, in the meantime, we can enjoy the nice, calm, steady cosmos.
