At the beginning of time, the universewas extremely small, billions upon billions of times smaller than an atom.Then suddenly, in an instant, the universe expanded trillions of times over. Matterand energy dispersed, dark matter and dark energy shaped the galaxies andstars and we come to the universe we see today. But it’s all fine and good to knowwhere we came from but what about the future.How will the universe continue to change as we move the time scale forwardmillions, billions and even trillions of years? What will be the ultimate fate ofthe universe? To understand the future of the universewe have to look back on its past.
When the Big Bang occurred, all the matter inthe universe expanded at an unprecedented rate. In the first secondalone, the universe has expanded trillions upon trillions of times overand has continued to over the next 13.8 billion years. In the previous videos, wehave taken a look at dark matter and dark energy which are known to comprise95% of the universe. These two mysterious phenomena are going to control the fateof the universe with dark matter pulling back on the expanding universe and darkenergy pushing the universe apart. A universal tug-of-war. Whoever wins willdetermine the fate of the universe. There is another term I’ll be usingthroughout this video, which is the critical density. The critical density isthe density of matter in the universe required to equal out the inward force ofgravity with the outward force of dark energy. In other words, it is the densityof the universe required to halt its expansion but only after an infiniteamount of time. This term is important because it will distinguish between aninfinitely expanding universe and one that will eventually fall back togetherdue to gravity. However in the end, we are presented with four possible futures tothe universe each quite dire in their own right.
Two where dark energy dominatesand the universe is below the critical density, one where dark energy and matterare equal in strength, the critical density and one where matter dominatesover dark energy, where the universe is above the critical density.In the early years of the universe, it is known that dark matter dominated darkenergy and was able to form the galaxy structure that we know today. But as theuniverse aged, more and more dark energy began to dominate as more space wascreated. The current standings hold dark energy comprising 68% of the universe,with dark matter and ordinary matter making up the other 32%. If we projectwhat will happen to these values in the future, we reach a pretty bleak outcome.Everything in our universe has an upper limit to its speed, the speed of light. Asdark energy accelerates expansion, the galaxies at the very edge of theobservable universe would disappear. Well not disappear. But the distantgalaxies will be pushed beyond the Milky Way at a speed greater than the speed oflight. Therefore, the light that comes from that galaxy cannot reach us and itwill simply disappear from view. This will continue to happen as time goes by. Thedistant galaxies will disappear from view and the observable universe willshrink until we can only our own galaxy. But dark energy doesn’t stopthere. It will keep expanding space until theonly objects that we can see are in our local star group.
Then only in our solarsystem. Then only Earth. Eventually dark energy will overcome the very forcesthat hold atoms together and the most basic subatomic particles will be rippedapart. Hence this theory is called the Big Rip.Interestingly enough, the estimates on how much time it would take for thisprocess to occur are quicker than you think.The amount of time it would take is startling, only 22 billion years whichmeans that our universe has already lived out a third of its life. The othertheory where dark energy dominates is known as the Big Freeze and isessentially the same thing as the Big Rip but without the ripping. Just like inthe Big Rip, the universe will continue to expand at an accelerated rate,however this prediction works off the theory of entropy or the measure ofdisorder in the universe. In other words, entropy is the measure of the number ofpossible states the universe can be in. To explain it easier, I want you toimagine a puzzle. This puzzle only has one possible solution and if you dumpall the pieces are out onto the table, it is extremely unlikely that they willform this solution. In other words, the puzzle pieces can be dumped in more waysthan they can be solved. Now pretend that the puzzle pieces are the matter andenergy in the universe. If the universe continues to expand, then there is a lotmore ways for the energy and matter to be arranged and less ways they can cometogether to form stars and galaxies.
The second law of thermodynamics states thatin an isolated system such as the universe, entropy can only increase.Because the matter and energy in the universe becomes so dilute and canexist in so many different states and places at once, the end result of thisscenario is a universe so massive that all the matter and energy are too farapart to interact with each other. All the stars, all the galaxies, will be nomore. However this theory works over a much larger timescale than the Big Riptheory. In four billion years, the Milky Way and Andromeda galaxies will mergeinto one, based on the current speeds of eachgalaxy. In 150 billion years, galaxies outsidethe local group will no longer be accessible, as they are receding from uspast the speed of light. 800 billion years in the future and the galaxiesbegin to dim as stars die and star production comes to a halt. After 100trillion years, star formation ceases completely and all that is left in theuniverse is black holes and interstellar dust. The era of stars and nebulae isgone and we now enter the black hole era, at 10 to the 40 years. But even thesestructures will not last forever. In around 10 to the 106 years, the finalsupermassive black holes will evaporate away via a process known as HawkingRadiation.
Hawking Radiation is a very strange phenomena that happens at thevery edge of the black hole’s event horizon. Two particles: a matter andantimatter particle appear at the edge of the black hole with one particlefalling into the black hole and the others shooting off into space.The one that falls into the black hole has negative energy and will decreasethe overall mass and energy of the black hole. This process is incredibly slow butwill eventually cause the evaporation of all black holes in the universe. Andfinally, we are left with nothing but a void full of individual particles, a heatdeath. The third solution to the fate of theuniverse is that the density of the universe is equal to the criticaldensity. What this means is that the universe’s acceleration will slow downuntil it stops and become static. This process will happen over an infiniteamount of time and is highly unlikely to be the actual scenario, given that itrequires an extremely specific density. The final solution to the fate of theuniverse is where gravity from matter and dark matter overcomes dark energy’soutput force.
This can only happen if the universe is above the critical densityand has enough mass to pull all the matter back together. The scenario beginswith the expansion of space slowing down as gravity begins to overcome thestretching force of dark energy. Eventually the matter and dark matterbegins to pull the galaxies back in. They accelerate towards each other faster andfaster until they begin to collide and crunch into a small point in space muchlike the singularity at the time of the Big Bang. The extension of this theoryknown as the Big Bounce suggests that the universe then undergoes another BigBang. The process continues to repeat itself over and over with an expandingthen contracting universe, followed by a big bang to restart the process. Thistheory is intriguing as it incorporates an endless universe that is constantlybeing reborn with no end and if this theory is the case and this is how theuniverse functions, then the question is, how did it start?As I said in the previous dark energy video, the current theory is that theuniverse is below the critical density and will keep expanding forever.This is because as more space is created from expansion, more dark energy iscreated and the process increases exponentially.
The current measurements of the density of the universe is about one to twoatoms per cubic meter of space. The critical density however, is estimated tobe around five to ten atoms per cubic meter of space. This would mean thateither the Big Rip or the Big Freeze is the ultimate fate of the universe andunless something changes, this is how the universe will end. And if you’ve youenjoyed the video, be sure to give it a like and subscribe to the channel formore videos like it. If you haven’t checked out the dark matter and darkenergy videos, they will help explain some of the concepts in this video and Irecommend checking them out. Until next time, goodbye.