After the scientific model of the big bang has become the (almost) universally accepted theory for the beginning of the universe, such questions as the ultimate fate of our universe became possible to answer scientifically. Today, with the recent theoretical advancements and ever-more-accurate observational data, scientists are able to make quantifiable predictions about the end of the universe. And even though physicists think they have a good idea of what the universe will look like in the far future, the great debate of how everything will end is far from settled.
The End of the World
Long before the main developments of cosmology in the 20th century there have been speculations about the end of the universe in philosophy and religion. Eschatology is a term that describes the part of theology that deals with the ultimate destiny of humanity and the world. Just out of curiosity, it is interesting to see what the major religions believe to be the ultimate fate of the universe. According to the bible, “the heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything in it will be laid bare (2 Peter 3:10).” Similar beliefs are held by muslims. Buddhist version of the end of the world is a bit more interesting — the world will go through seven stages of destruction known as the “seven suns”. With each “sun” the world will dry and heat up eventually leading to the total annihilation by fire. The fun thing about this scenario is that it sounds like something that would happen at the end of the Solar System, when the Sun turns into a red giant. The interesting thing about these claims is that they all share a belief in the end of the universe in fire. Interestingly, the two main types of predictions about the ultimate fate of the universe coming from cosmologists state that the universe will either die in fiery destruction or will slowly freeze.
Big Bang Cosmology vs. the Steady State Theory
Currently there is a growing consensus among cosmologists that the universe is more or less flat and will continue to expand forever. This idea, however, is based on the knowledge of the current shape of the universe and the nature of dark matter and dark energy, which leaves a lot of room for improvement and alternative theories. Until a solid understanding of dark energy is acquired, there will be competing theories on the subject. To understand where each competing theory comes from, we need to go back to the beginning of the 20th century, when the foundation of cosmology was being established.
After the publication of Einstein’s general theory of relativity (1915) the research of how the matter distribution in the universe affects the shape and the expansion of it became possible. By solving Einstein field equations, physicists are able to make testable predictions of the behavior of the universe on the largest scales. However, the field equations are non linear and extremely hard to solve. In addition, many possible solutions exist.
The two solutions that are of particular interest to us come from Georges Lemaitre (1927) and Fred Hoyle (1948). Lamaitre’s theory has since come to be called the big bang theory, which basically states that the universe began in an extremely hot and dense state known as the singularity and has been expanding ever since. Hoyle’s competing theory suggested that the universe, even though it has been expanding, has remained statistically unchanged, since new matter was being created constantly. For almost a decade these two conflicting theories have been standing on almost equal footing, until the discovery of the cosmic microwave background, which offered undeniable evidence to the big bang theory. With the new evidence for the big bang theory and the newly developed observational techniques, scientists could now predict the future of the universe.
The Ultimate Fate of the Universe
According to cosmologists, the fate of the universe depends on the shape of it, how much dark energy it contains and on the correct equation of state. A quantity that is of crucial importance, when talking about the future of the universe, is the density parameter Ω, which is the ratio of the current total density and the critical density of matter and energy in the universe. The experimental determination of Ω is the key in finding out how the universe will end. Here are a couple of possible scenarios.
The Big Crunch & the Big Bounce
The terms big crunch and the big bounce are closely related, in that these terms are often used in describing an oscillating or a cyclic universe, which starts off with a big bang and ends with a big rip, giving rise to a new cycle. If Ω > 1, the universe is said to be closed. This means that in such a universe the initial expansion eventually slows down and turns into contraction, eventually leading into another big bang singularity. There is a catch, however, — if the universe has a significant amount of dark energy, even for Ω > 1, the expansion can continue forever. The theory of the closed universe, that starts off with a big bang and ends in a big crunch, offered an elegant picture of an infinitely pulsating universe. Such a mechanism of contraction and expansion, would, at least partially, solve the question of what happened before the big bang, which, obviously, was appreciated by many cosmologists. Unfortunately, the big crunch theory could not be reconciled with the 2nd law of thermodynamics, which is probably one of the worst things that can happen to a theory. I addition, the current observational evidence indicates that the universe is not closed. Various other cyclic models have been considered, however, the unfavorable observational evidence meant the abandonment of the big crunch theories for the most part.
Big Rip, Big Freeze and Heat Death
For Ω = 1 and Ω < 1, the universe is correspondingly flat and open. The flat universe, which has dark energy expands forever, however, the expansion rate initially slows down and then becomes higher again. In the open universe, even if there is no dark energy, the expansion continues forever as well. Adding dark energy accelerates the expansion. These two cases are of particular importance when it comes to big rip, big freeze and heat death scenarios.
Big rip refers to the idea that, given the special case of dark energy (with more negative pressure than a simple cosmological constant), the density of dark energy will grow. This will in turn cause the disintegration of all matter in the universe, starting with galaxies, into unbound elementary particles. Long story short, the universe will be torn apart.
Big freeze and heath death scenarios are quite similar in that they both consider the universe eventually approaching absolute zero temperature. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. Then the stage will be occupied by black holes, which eventually will also disappear trough Hawking radiation. Both models could theoretically happen in positively curved, negatively curved and flat universes, given certain conditions (such as a certain amount of dark energy) are satisfied.
Some alternative ideas regarding the end of the universe include the multiverse model, the end of time, the big bounce or that the universe is simply eternal in one way or another. The idea of a multiverse states that there are infinite other universes, some like ours, some radically different. So while any given universe eventually reaches a heat death, there are always other regions that haven’t, plus more universes are being constantly created. Now, when it comes to the multiverse idea, it is important to realize that it is a controversial topic with many scientists disagreeing if it is even scientific or not.
The other mentioned ideas are also rather controversial and it would take forever to described them in detail. Here’s a good list of 10 theories for the end of the universe.
To test which model offers the best explanation for the fate of the universe, scientists have to measure the contributions to the overall density by dark energy, dark matter, simple matter and radiation. In addition, the ideas can be tested by forming a better understanding of the cosmic microwave background and galaxy clustering.
So far, as mentioned before, the experimental data indicates that the universe is flat (or very close to flat) and will likely expand forever, which points towards the big freeze (and the heat death) or the big rip scenarios. However, to accurately determine the ultimate fate of the universe, more experimental data is required. Furthermore, a good understanding of dark energy and dark matter is required to form accurate predictions. So how will the universe end? Probably the best answer is that we have some good ideas, but we just can’t be certain at the given moment. To finish this off, here’s how Robert Frost sees the end of the world:
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice