We as humans tend to think of ourselves in a very limited capacity. Often it is very difficult to imagine how small we are in comparison to the universe we live in. On a daily basis, we rarely stop to think about matters of a cosmic scale, let alone the fate of the universe, but that hasn’t stopped scientists from making predictions.
By all accounts, the final state of the universe, be it a peaceful or catastrophic one, is something that won’t occur for billions of years (in most theories). So, without fear to hold us back, we can examine six different ways that scientists predict the universe may meet its ultimate fate.
The ability to explore these possibilities only became possible in 1916 when Albert Einstein posted his theory of general relativity. With this theory in place, there were a number of possible solutions to the equations, solutions which would yield a potential ultimate fate of the universe.
In 1922, Alexander Friedman put forth several theories of his own along with Georges Lemaitre in 1927. Some of these theories worked from the assumption that the universe was expanding from an original gravitational singularity, an event we know as “The Big Bang.”
These theories received a jolt of energy in 1931 when Edwin Hubble put forth his conclusion that the universe was expanded based on observing the Cepheid variable stars in other galaxies. From this point on both the origin of the universe and its ultimate end have been a major subject of interest for scientists.
Part of these theories were fueled by Einstein's theory of a cosmological constant. For his theories to work, the universe had to be static, so he created this constant which he described as a “hypothetical repulsive force” which was meant to balance out the warp caused by gravity to both space and time.
When Einstein heard of Hubble’s discovery, he referred to his cosmological constant as “the biggest blunder of his life.” He wasn’t far off though as we now know. A new theory of “Dark Energy” represents a modern version of this balancing force.
The universe’s shape and density are the main factors that lend credence to each of these theories. In terms of density, this term is referred to as the “Density Parameter.” In equations, it is defined with an Omega (Ω) symbol and represents the average density of matter in the universe divided by the critical value of it.
This value comes in three variations: it can be equal to, less than, or greater than 1. These values are also known as the flat, open, and closed universes in regards to their shape. Ultimately the Universe’s fate is decided upon the shape of it, the amount of dark energy within, and how the dark energy responds to the ongoing expansion of the universe.
As it stands, the current measurements favor a universe that is flat and expanding infinitely which would result in a “Big Freeze” or “Heat Death” as it is also known. These are simply theories though, there is still room for other possibilities as new discoveries are made. Let’s explore the six major theories surrounding the ultimate fate of the universe.
Knowing the factors that affects our universe, we can delve deeper now into the various ways that the density and shape of the universe affects it’s fate. Allow yourself to picture some of this cosmic finales as we explore the six major theories that predict how everything we know could end.
This first scenario is the one that is currently the most popular in terms of possibility. It gets its name from the theory that the universe will continue to expand indefinitely until it reaches a temperature of absolute zero. This ending is possible in a universe that is flat or hyperbolic in its shape.
This ending has several phases. It begins anywhere from 1 to 100 trillion years from now when the amount of gas that is required for new stars to form runs out. Without the ability to create new celestial bodies, the universe will become darker and darker as each star reaches the end of its life.
We are currently in the Stelliferous Era which encompasses all the time between 1 million and one hundred trillion years after the big bang. Towards the end of this era, stars will begin to exhaust all of the possible gases as mentioned above. Some will burn out into white dwarfs, others will go supernova, and still others will become black holes.
Simultaneously, our galaxy and the galaxy Andromeda will collide and merge with each other roughly 5 billion years from now. This collision was speculated for a time until it was confirmed using the Hubble Telescope in 2012 to track Andromeda’s movement. This is only the beginning of the collisions though.
The universe’s continued expansion will cause all of the galaxies in our local group to eventually merge into one giant galaxy that will eventually be pushed past the cosmological horizon roughly 150 billion years from now. At this point any communication or space travel will be nigh impossible.
Star formation finally ceases 100 trillion years from now as the super cluster of galaxies becomes completely cut off from the rest of the universe. This begins the “Degenerate Era.” This era spans the time it takes for orbits to decay as a result of gravitational radiation, and for planets and galaxies to fall apart as they are swallowed up by supermassive black holes or ejected into open space.
Towards the end of this era, there will be zero nucleons in the universe. This will give way to the “Black Hole Era” Where these events will be the dominating force in the universe. Even black holes decay though, according to Hawking Radiation. Once everything has dissipated, there will be nothing left in the universe but photons, neutrinos, electrons, and positrons.
Beyond this, there isn’t much else to say about what happens next. There are theories, including the potential for the laws of physics to break down. For all intents and purposes, this empty space of floating particles is how the universe ends.
This scenario isn’t as long and drawn out as the former. It is a hypothesis put forward in 2003 which suggests that the expansion of the universe will accelerate to a speed that will rip it apart. The possibility of this occurring is heavily decided by the type of dark energy that exists in the universe.
The relationship between the dark energy’s pressure and density results in a point where the universe is literally ripped apart. Essentially, after a set amount of time, the expansion causes a final singularity known as the “Big Rip” that results in all the distances between any two points of the universe become infinite in their size. The entire process would take roughly 60 million years. First, the gravity that holds galaxies together would break down and they would come apart.
Three months before the final singularity is when solar systems would come apart and in the final minutes everything would be ripped apart. The last seconds would result in the destruction of atoms themselves.
This theory claims that the expansion of the universe will eventually be reversed as a result of the combined gravitational attraction of everything within it. This pull will cause the expanding universe to collapse inward. While the expansion of the universe allowed for plenty of space, a collapse would cause a number of black holes as matter becomes more and more dense.
This would eventually lead one black hole, also known as a “Big Crunch Singularity.” The Hubble Constant plays a major role in this theory as it measures the speed at which the universe is expanding. In one scenario, we have this ending, but in another, a repulsive force like dark energy causes the expansion to outpace the gravitational pull, resulting in the “Big Freeze.”
Current measurements do not show gravity affecting the speed of the universe’s expansion, however the speed is affected somehow by dark energy, which we know very little about. It is still possible for this scenario to occur, but currently it is not possible based on what we know.
An interesting theory here that holds some semblance of hope, unlike the others. This one assumes that the universe does indeed collapse, but prior to the point of singularity, it expands outward again in a new “Big Bang” that creates a new universe. This creates a cyclical universe that is constantly expanding, collapsing, and expanding again.
While there have been arguments made that suggest that this cycle would not last indefinitely as the decay of energy would eventually cause a “Big Freeze,” new research has found ways to explain this process in a way that is scientifically possible.
Research done in loop quantum cosmology has shown theorized that another universe collapsed to form the one we know today. The collapsing did not result in a singularity, rather the quantum properties of gravity caused it to “bounce” back out. The research is still in its early stages, but continued results have begun to form a true theory that places us in one of an infinite number of universes that expand, contract, and expand again.
This is a fascinating approach that doesn’t result in any real “end” to the universe. This hypothesis states that our universe is one of an infinite number of universes that are divided into regions of space that are bordered by inflationary space.
The theory arises from the early moments of our own universe where a period of cosmic inflation caused space itself to expand extremely fast through what’s known as a false vacuum state. During this time, space was surrounded by an “inflationary field.”
The most recognized theories state that the switch from inflationary to non-inflationary happened simultaneously. A theory known as the “eternal inflation model” instead states that different parts of the universe go through a process called vacuum decay that causes them change states at different times.
This creates entire sections of space that are still surrounded by expanding borders of inflationary space. These areas would not be able to see or contact each other and therefore would be considered separate universes. Some of them may reach a “Big Freeze” but others would not. New universes would also form to replace the dead ones. The universe would never truly die.
Without requiring a degree in quantum physics, this final theory will be explained as simply as possible. This theory only works if our universe is part of a metastable or “false” vacuum. We don’t know for certain if this is the case or not, but assuming it is, this vacuum could potentially enter a lower-energy state in the pursuit of becoming more stable.
The “God Particle” or more accurately the “Higgs Boson” could trigger an event like this as well because of its existence in everything around us. Through certain triggers, an event called “Quantum Tunneling” would cause the particle or the false vacuum to enter a more stable energy state. This would create a bubble that represents a true vacuum.
This bubble would expand at the speed of light and engulf the entire universe. The resulting event would create a new universe that would be unstable, devoid of life, and would collapse almost immediately.
With all of these possibilities, we have to consider the fact that much of the true nature of things like dark energy and inflation remains a mystery to us. We don’t know anything concrete about the true purpose and possible changes that these hypothetical things could undergo. Therefore, these theories are subject to change.
Even so the universe is safe for now. The only real threat we have to be concerned about is the possible quantum tunneling of a false vacuum or of the Higgs boson, but these things are insignificant in the grand scheme or our daily lives, and not something you should be worried about.