The Universe as We Knew It
One of the most fascinating things about the Universe is how, one day, it will die.
While the spectacular destruction of all we hold dear (which, in reality, is a microscopic fragment of all that has ever existed) sounds like a grand old party, it is only human to want to know how it will end, and whether or not this end will be absolute and ultimate.
The Big Rip
This first theory postulates that at the youthful age of 35 billion years, the Universe will be ripped apart. This will happen because the gravitational forces that keep everything together will be overpowered by the force of expansion of the Universe. For a long time, scientists, researchers, and mathematicians lived under the blissful complacence that, while the Universe was definitely expanding, its rate of expansion was probably infinitesimal. Then, in 1998, came the Hubble Space Telescope which- after observing some very faraway supernovae- concluded that further back in time, the Universe was actually expanding much slower than it is today.
Soon enough, they realised that the rate of expansion of the Universe had just been accelerating. No one had seen this coming, or could figure out exactly what was happening, or explain why. Eventually, scientists grew tired of not knowing things and tried to figure out what was causing this curious expansion of the cosmos.
The answer (sort of) was dark energy.
More is unknown about dark energy than is known, which has been known to have caused countless screaming fits among scientists at the NASA headquarters.
All that is really identified about dark energy is that it affects the Universe’s expansion. To be precise, it increases it by causing an outward force that pulls everything apart. It also occupies and expands in the space that is, well, nothing. Roughly 68% of the universe is dark energy, which begs the question- what is dark energy?
While some suggest that it’s a property of space, others suggest it is the pulling caused by black holes, stars, and large planets. The most intriguing theory is that “empty space” is actually full of temporary particles that continually form and then disappear. Sadly, when physicists tried to calculate how much energy this would give empty space, the answer came out 10120 times larger than expected. That’s 1 with 120 zeros after it. It’s hard to get an answer that bad. And thus, the mystery continues.
In case the Big Rip is the way we go- Earth, and humanity with it, would slowly decay into radiation, collapse in on itself or be ripped apart as the Universe would mindlessly continue expanding. Galaxies would be destroyed, Solar Systems would break and even atoms would be ripped apart. This would leave the Universe a mess of single, disconnected particles- floating aimlessly, in search for something to pull them together, a shadow of the way they once used to be. And you thought you were lonely.
The Big Crunch
The good news is that the Universe will finally stop expanding so wildly at an ever accelerating speed. The bad news is it’ll collapse upon itself with reckless abandon.
Playing with the dynamic between gravity and the expanding forces of the Universe, this model states that eventually, gravity will win over and start pulling everything closer together. We’ll reach a point that replicates the singularity that the Universe witnessed back when it was formed. In simpler words, the Big Crunch is like a video of the Big Bang wound backwards.
Everything will be packed very tightly into the largest black hole ever, and I’ll finally be able to reconnect with my fifth grade pen pal from Austria.
Other than the gravitational force of attraction between all the galaxies and their outward momentum due to the Big Bang, the Big Crunch also depends on the density of the Universe. It is believed that if the density is larger than a certain value, known as the critical density, an eventual collapse and implosion back to singularity is quite possible.
Interestingly enough, it has even been suggested that the Universe has already lived through an iteration or two of the Big Crunch. Some scientists have theorised that the universe we see is the result of a cyclic repetition of the Big Bang, where the first cosmological event came about after the collapse of a previous universe. This is called conformal cyclic cosmology, and implies that the Universe- from its creation with a bang to the destructive crunch- is essentially a reverse looping gif.
The Big Freeze
This cold, cold, scenario is a combination of the loss of heat and energy, and the ever-present expansion of the universe.
The density of the Universe matters almost as much as the heat contained by it- if the density is found to be lower than what is known as the critical density, then a continuous expansion will ensue. If the density is equal to the critical density, then the Universe will expand forever but at a decreasing rate. Finally, if the density is found to be greater than the critical density, the Universe will eventually stop expanding and then collapse.
For the Big Freeze to occur, the density must be less than the critical density.
It will continue to get colder and colder until the temperature throughout the universe reaches absolute zero. Similarly, if the expansion of the universe continues, planets, stars and galaxies will be pulled so far apart that the stars would eventually lose access to raw material needed to keep burning. Either way, the lights would go out for good.
This would be the point when the Universe would reach a state of maximum entropy and stars would run out of viable fuel options, making galaxies that once shone so bright and fiery, hollow cemeteries of stars and planets that used to be.
Now, none of these scenarios sound particularly fun, and thus, physicists have supplied us with several other situations, presumably to keep things fresh. This rhapsodising includes theories like the Big Slurp, which alludes that the universe is eternally unstable, which would pave the way for a vacuum metastability event to occur. After all, nothing says world peace and harmony like friendly terms indicating the end of time. Not horrifying at all.