“Sooner or later, everything must come to an end” is something we have all heard, but have you ever thought about how the universe will end? The universe that gave birth to everything from the biggest galaxies to the smallest microorganisms will come to an end one day, and we have some theories on how that will happen.
Since the beginning of time, the universe has been expanding, and its expansion rate has been increasing. The strange and mysterious force that is responsible for the expansion of space is called dark energy. It is still unclear what dark energy is or how it works, but we do know that it will play a significant role in determining the fate of the universe. You need to understand that the universe doesn't expand at its borders; rather, space itself expands and gets bigger, pushing things apart. Another key role will be played by gravity, which is completely opposite to dark energy. Instead of keeping things apart, gravity pulls all matter together and is the reason why we have planets, stars, and galaxies. But in the empty space between galaxies, dark energy comes into play, separating the cluster of matter held together by gravity and causing the universe to expand. The behavior of these two forces will determine the fate of our universe. [2][5]
Constant Dark energy (Heat Death): Right now, dark energy is stronger than gravity; we can prove this by the expansion of the universe. If dark energy stays the same and the universe keeps expanding at the same rate, there will be a time when everything will start moving further and further from each other. Gravity will only be able to keep together clusters of galaxies like our local group. Everything outside the local group will move away from us until it won't be visible at all, and the universe will be left with galaxy clusters with billions of light years in between them. Eventually, the stars in each cluster will run out of fuel and become black holes, neutron stars, or white dwarfs. Then, in quadrillions of years, the white dwarf and neutron stars will also run out, and the lights in the universe will be shut off forever while dark energy is still pulling everything apart. In another googol years, black holes will also run out due to Hawking radiation and no mass around to fuel them. After everything in the universe has come to an end, dark energy will keep expanding, and in the end, the universe will be an empty, dark, and lonely place with only particles moving around into nothingness. [3][4]
Increasing dark energy (Big Rip): In this scenario, Dark energy gets stronger and stronger instead of staying constant. This time, things will escalate way faster, At first, the speed of the expansion of the universe will increase, and the clusters will drift away from each other. Soon Dark energy will expand space between galaxies pulling apart individual galaxies from each other. Our own Milky Way will get separated from the local group and will drift away into the abyss of space. After a few billion years of that, the only thing you will see will be the Milky Way. It will be as if the other galaxies don't exist for you. After a few billion years, the space between star systems will start to expand, pushing individual star systems apart. Our solar system will be taken away from the Milky Way, and in this process, all the galaxies will be undone. After a few billion years of that, space between the stars and planets will start to expand, freezing all life to death. At this moment, Dark energy would have grown so strong that every single solid object would be taken apart as the space between atoms would expand to destroy stars, planets, asteroids, etc. Eventually, dark energy will take apart the atom by separating the electrons from the nucleus. But this time, it will not stop at the particles. Dark energy will become so prevalent that it will rip apart the fabric of space-time itself, and there will be no space, no time, no you, and no me. [4]
Decreasing dark energy (Big crunch): In every other scenario, gravity loses to dark energy, but if dark energy becomes weaker, gravity will take over, pulling things closer and closer to each other. First, the empty space between clusters will contract, and galaxy clusters will collide, causing one single mega-cluster to form. Soon, galaxies in the mega-cluster will start to collide and form mega-galaxies. As both a cluster and a galaxy are mostly empty space, their collisions just make a bigger version with minimal destruction. But as dark energy becomes weaker and gravity becomes more dominant, star systems, planets, asteroids, etc., all will be pulled towards each other, destroying smaller objects like planets, moons, and asteroids. In a few billion years, space would have contracted so much that the energy released by the early stars right after the Big Bang will start to collect in the now smaller universe. Space will go from cold and dark to hot and very bright. The space around stars will become hotter than them, boiling them from the outside, and they will evaporate. As the universe collapses onto itself, everything in it will merge into a hot, dense ball of plasma. Now there are two outcomes after this: either the universe will keep contracting and will become an infinitely dense point of singularity, or it will bounce back into an event called the Big Bounce, and another Big Bang will happen, giving birth to a new universe. [3][4]
Conclusion
We don't know which outcome will happen. To surely know how the universe will end, we need to know about dark energy and its properties so that we know what will happen to our beloved universe while we are not here to see it.
Sources:
Telescope, Large Synoptic Survey. “Dark Energy and the Fate of the Universe.” Rubin Observatory, 4 June 2020, www.lsst.org/science/dark-energy/universe
Dorminey, Bruce. “The Beginning to the End of the Universe: The Mystery of Dark Energy.” Astronomy Magazine, 18 May 2023, www.astronomy.com/science/the-beginning-to-the-end-of-the-universe-the-mystery-of-dark-energy
Betz, Eric. “The Beginning to the End of the Universe: The Big Crunch Vs. The Big Freeze.” Astronomy Magazine, 18 May 2023, www.astronomy.com/science/the-beginning-to-the-end-of-the-universe-the-big-crunch-vs-the-big-freeze
Kurzgesagt – In a Nutshell. “How to Destroy the Universe.” YouTube, 8 Dec. 2023, www.youtube.com/watch?v=HR2C_7G_yRQ
Lea, Robert. “What Is Dark Energy?” Space.com, 24 Nov. 2022, www.space.com/dark-energy-what-is-it