This guest post was written by Danny Baker, an undergraduate student at the University of Connecticut, for an assignment in the Fall 2021 Foundations of Modern Astrophysics class taught by Professor Cara Battersby. As part of the course, students were tasked with writing an Astrobite-style summary of a topic in astronomy. Danny is a computer science major minoring in both astrophysics and mathematics. He hopes to combine all three of these fields to make groundbreaking discoveries about the universe.
The sheer vastness of the universe is something that a lot of people struggle with grasping. Imagining this in combination with the immense amount of time that it has and will continue to endure can be almost overwhelming. How do you even begin to grasp these concepts? When you aim at understanding it anyway, you almost instantly encounter some pretty weighted questions: How can there possibly be never ending space? How was the universe created? And more importantly (at least for the sake of this Astrobite): how will it end? Although the end of the universe as we know it is still very uncertain, there are four theories that aim to put us closer to understanding this aforementioned inconceivable concept: a heat death, The Big Crunch, The Big Rip, and vacuum decay.
The journey to understanding the fate of the universe starts with Albert Einstein. Einstein believed that gravity would work against the commonly understood idea of a static universe that was widely accepted in his time. He thought that the gravitational force would cause things to contract. However, this was not happening. Einstein concluded that there must be some repulsive force to oppose this which he dubbed the cosmological constant. However, in 1929, Edwin Hubble discovered that a galaxy’s redshift is proportional to its distance. Applying the concepts of general relativity, this was thought of as the fabric of space and time expanding. This new information implied an expanding universe and so Einstein discarded his work with the cosmological constant – though this is not the last we will hear of it.
Now that we know the universe is expanding, we begin to gain a little insight into how the universe might end. With this expansion occurring, the galaxies are moving increasingly farther away from each other. As time continues on for billions of years, things will begin to cool down. By this, I mean stars burn out and the “ingredients” needed to form new stars disperse to the point where star formation ceases. All the lights fade and the night sky goes dark (and the day sky for that matter). There is nothing left to occur in the universe as it approaches “minimum temperature and maximum entropy”. A stable state of low, dissipated energy is reached. Of course, this implies systems will not have enough energy to produce mechanical work. This scenario is referred to as heat death.
A heat death, while bland and dreary, is not the only way an expanding universe could reach its end. Let’s fast forward our research to 1998 where the Hubble telescope discovered that the universe used to be accelerating at a slower rate. This new insight was achieved by looking at the brightnesses from distant and old supernovae. So, now the universe is expanding at a greater rate. Okay… but how? If gravity pulls in, what is causing things to be pushed away? Not only this, but how are things being pushed away faster and faster? The answer is dark energy. Dark energy is thought to make up about 70% of the universe and is theorized to be an intrinsic property of empty space. This new data actually suggests that Einstein’s cosmological constant idea was very close! Unfortunately for this discussion, there is still very little known about dark energy as it is not directly observable. The math that proves its existence implies that as space expands from the forces of dark energy, there becomes more empty space further driving the spread of galaxies. The effects of dark energy add up over these vast spans of emptiness causing the universe to expand at faster and faster rates. The theory supported by this phenomenon proposes that if the density of dark energy becomes great enough, this acceleration will continue to increase until solar systems, planets, atoms, and even quarks are ripped apart by dark energy. As the universe expands, there is more and more energy in empty space until, quite literally, the fabric of space time itself tears. This is known as The Big Rip.
Still, there is yet another prediction–one that is quite the opposite of the Big Rip. The Big Crunch will occur if there is enough collective matter in the universe so that the force of gravity can stop the expansion and pull everything back in towards a single point. Galaxies will collide and destroy planets and stars. This contraction will cause extreme density and temperatures. For clarification, collective matter refers to matter and dark matter. Dark matter is a type of matter that we know exists but we know very little about. Instead of observing it directly, we can mostly just observe the effects of its existence; we know it exists because the gravity from matter alone is not enough to hold galaxies together. Note that the only relation it has to dark energy is that we cannot really see both of them. Interestingly enough, there is one theory that as all the matter in the universe collapses into a single point, it will instantaneously explode and expand, relaunching everything back into space. What the theory is effectively suggesting is that another Big Bang could occur leading to an infinite cycle of extreme expansion and contraction called The Big Bounce. So, there is some hope that the universe might not be completely doomed.
The fourth and final prediction for the end of the universe is something called vacuum decay. The first thing you need to know about this scenario is that vacuum in this context does not mean empty space, but rather the lowest potential energy state. To start off, let’s consider the Higgs field. This is an energy field that is responsible for giving mass to our universe. This happens when a particle interacts with the field. It gains mass, but in return loses its ability to travel with the speed of light. The central idea around vacuum decay is that the Higgs field is believed to be in a stable state of potential energy but it’s not in the most stable state. As you can see in Figure 1, the Higgs field could be in the dip on the right which is a minimum, but not the overall lowest one. There, it is in a valley, but nevertheless possesses more potential energy than in the left dip. The lower the potential energy, the more stable and therefore more favorable the state is. So the Higgs field wants to be in the lower valley, but how can it get there? There either needs to be an event with such energy so that it can propel the field over the hill and into the lower valley or the field can quantum tunnel. The latter can occur by the Heisenberg Uncertainty Principle; a Higgs boson particle can spontaneously shift from one dip and fall into the other. If this happens, a little “bubble” of a more favorable energy state will be created. While there is a chance it could collapse back in on itself leaving the universe unchanged, because it is more favorable, it could also expand and grow. This could continue until it reaches the speed of light. With this shift in energy state, the rules of the universe, physics, and even chemistry would be completely rewritten. Interactions between fields and particles will be unrecognizable. As this vacuum bubble expands it will annihilate everything in its path. There will be no warning; we will simply cease to exist…
While these four described events all have quite different processes, there is one common theme: the universe will end and that is inevitable. Afterall, the second law of thermodynamics reinforces this idea by telling us that the state of the universe will flow towards entropy. Yes, these fates are all still speculated, but as we advance scientifically and uncover more knowledge about the universe we live in, these possibilities we conceive of become more and more precise. Although they bring us closer to an answer for our questions on the mind boggling nature of the universe, I believe the fact that we do not know exactly how it will end is a testament to its beauty and complexity. With its infinite vastness and age, the universe holds so many secrets that continually challenge our studies and push the frontier of science to seemingly impossible limits.
Astrobite edited by Lina Kimmig
Featured image credit: https://www.skyatnightmagazine.com/space-science/expansion-universe/