By Cian O’Toole.
The questions of “How did we get here?”, and “Where do we come from?”, have been asked by humans for millennia. The Greeks, Romans and Egyptians all had a shot at answering it before a modern day theory of the Big Bang became widely accepted. However, all good things must come to an end. This remains true for our very own Universe, where the manner of its demise is not so certain. Thus, the question remains. How exactly will the Universe end?
There are a number of theories about the downfall of the universe. Some end in a somewhat Biblical fashion, one in fire, the other in ice. Another theory may see us be ripped apart, atom by atom, as the Universe expands quicker and quicker. So, lets begin with the Big Crunch.
The Big Crunch:
The Big Crunch essentially sees the Universe do a U-turn on its current behavior. Currently, scientists know that the Universe is expanding, and has been expanding since its inception, approximately 13.8 billion years ago. No breaking news there. Simple knowledge of the laws of gravity would lead one to assume that after a certain time, enough matter in the Universe would accumulate together to halt the seemingly endless expansion of the Universe. This is the basis of the Big Crunch. Analogous to the phrase, “what goes up, must come down”, the expanding Universe will eventually begin to contract.
This would first be noticeable as the furthest objects in the observable Universe would start becoming more and more blue-shifted. Now what does blue shifted mean? Blue-shift, (and its opposite, red-shift), are due to a phenomenon called the Doppler Effect. You have experienced the Doppler effect many times in your life without even realizing. It is most easily observed when an ambulance approaches and then moves away from you. As the ambulance approaches, the siren seems to get louder and louder while also becoming higher in pitch. As it recedes away from you, the siren gets quieter and the pitch is lower. This is due to the fact that as the ambulance approaches, the sound waves become shorter, and as it recedes, they become longer. the same is true for light waves. The light of objects that are moving further away is stretched, leading to a longer wavelength. This is called red-shift as the longer optical wavelengths are the colour red. Conversely, the wavelengths of approaching objects are compressed, leading to a shorter wavelength. This is blue-shift as the shortest optical wavelengths are blue. Now, how does this lead to the end of the universe?
Objects that are further away in the Universe have a higher redshift, i.e. they are receding from us faster, the further they are. However, according to the theory of the Big Crunch, these objects would be the first to become more blue-shifted as they begin to stop accelerating away from us, and begin accelerating towards us. That is how the big crunch would initially be noticed by observers. From then on, everything in the Universe will begin to approach a common point, as gravity reigns supreme, the temperature always increasing. Eventually, everything will be condensed together to a single point, a singularity if you will. This singularity would contain all of the matter in the universe in an infinitely small, infinitely dense point. Does this sound familiar? Perhaps it should as this is how we currently believe the universe started out before the Big Bang. Theorists of the Big Crunch believe that this could then cause the birth of a new universe, which will then suffer its own Big Crunch, before having another Big Bang and so on and so forth in an infinite cycle. It’s poetic isn’t it?
The Big Freeze:
The Big Freeze theory can be considered to be the opposite of the Big Crunch. The Big freeze is more commonly known as the Heat Death of the Universe. Unlike the Big Crunch, where gravity ultimately brings all the matter in the Universe back to a singularity, in Heat Death, the Universe does not stop expanding. The previously used phrase, “what goes up, must come down”, does not apply here. In the case of Heat Death, it is similar to throwing a ball up, and as opposed to it coming back down (as one would expect), it flies away from you. Not only does it continue you to move away from you, it actually moves faster and faster the further away it gets. This is actually what is currently happening in our universe, the furthest objects are receding from us at a faster rate than those closer to us. It seems somewhat illogical.
The Universe can be modelled, believe it or not. The Universe can be modelled as flat, open or closed, with each resulting in a different ending for the future. The key element however to these models is the Cosmolgical Constant . As opposed to the Big Crunch, where gravity wins outright (and seems the logical next step in the Universe), in reality, the cosmological constant prevents that from happening. The most plausible cosmological constant we currently have is that of Dark Energy, a mysterious force that enables the infinite acceleration of the Universe .
Over vast timescales, this will result in far-off galaxies disappearing from our night sky, as the expansion of the universe is receding quicker than the approaching light. This will continue for eternity until eventually all the galaxies are so far from each other, they are not visible in their night skies. There will be no more stars, no more nebulae and eventually all that will be left is black-holes. They too will eventually evaporate away as a result of Hawking Radiation, on an unimaginably long timescale. Entropy will win out. Everything will begin to approach the same temperature until a maximum entropy is reached, whereby all that will be left is an infinitely large, zero Kelvin universe devoid of matter.
Entropy can be thought of as adding milk to a cup of tea. Imagine before you add the milk, the tea is the Universe before the Big Bang. Once the milk is added, the colour changes (entropy increases). The entire history and future of the Universe takes place over a few seconds until maximum entropy (disorder) is reached. A uniform, pale brown is all you can see of your tea. This represents the Universe at Heat Death. The same in all directions at maximum disorder. It is quite a bleak future in store according to this theory.
The Big Rip:
The Big Rip, is a somewhat more thrilling theory for the end of the Universe, even if it is only in comparison to the infinite bleakness of Heat Death. However, it also incorporates Dark Energy as before. Yet, in the case of a Big Rip, the Dark Energy is not a cosmological constant.
Dark Energy can be thought of as a negative pressure . This is a tough concept to get your head around, but can be thought of as analogous to pressure. Obviously. Whereas normal pressure results in a pushing force, negative pressure results in a pulling force, pulling objects apart. Dark Energy can be quantized as a ratio between itself (negative pressure) to the energy density. This parameter is called w. If w = -1, then the pressure and density are exactly opposite and the dark energy is a cosmological constant . However, the value of w may not be equal to -1. This is where things start to get interesting.
In 2003, American physicist Robert Caldwell looked into what would happen if the value of w was to be less than -1 . His findings were published in his excellently named paper, “Phantom Energy: Dark Energy with w < -1 Causes a Cosmic Doomsday”. Here, Caldwell investigates the effects of a universe where w < -1. The results were astounding. Of w was found to be infinitesimally less than -1, the Universe would be destroyed. And you could also calculate how long this destruction would take . But how exactly will this happen?
The largest objects will be the first to disperse. Galactic clusters will spread apart until eventually all that are left are individual galaxies. Then the galaxies themselves will be ripped apart, leaving isolated solar systems. This can be thought of as the matter being expanded by the space within all matter. Then stellar systems will be unbound, before planets themselves explode as they are quite literally being torn apart by the incessant Dark Energy. This continues until 10^-19 seconds before the Big Rip , atoms themselves will disassociate. The cores of blackholes will also be ripped apart before finally the fabric of space is ripped apart . Quite morbid.
So How Will It All End?
Not in fire. That much is pretty certain. It is quite likely that the Universe will die due to a Heat Death. However, the possibility of a Big Rip cannot be ruled out, as all that is needed for this to occur is the cosmological constant w to be less than 1 be *any* amount. It is all a bit doom and gloom. However, the good news is, we will not have to worry about any of these for a very long time! So sit back, relax and enjoy the Universe while you still can.
Additional information for this blog was taken solely from the following book:
 Mack, K., 2021. The End of Everything:(Astrophysically Speaking). Simon and Schuster.
With the exception of the Big Rip:
 Caldwell, R.R., Kamionkowski, M. and Weinberg, N.N., 2003. Phantom energy: dark energy with w<− 1 causes a cosmic doomsday. Physical review letters, 91(7), p.071301.