What Will Happen If the Universe Stops Expanding?

 

As much as we all want to hop into a tardis and jump to different time eras unfortunately time is linear it always moves forwards one second at a time and as time moves forward so does space and everything inside it that is why the universe keeps on expanding because that's what it has been doing since the dawn of time but what would happen if the universe says no more what if the universe plants its foot down and says it won't budge anymore welcome to Science Reads and let's find out today what would happen if the universe stops expanding why does the universe expand the universe is always awake always moving along with time some might argue that it's the universe's movement that causes the change that can be explained as time moves forward whatever you may

 believe you will reach the same conclusion that space-time can be described as a chain of events that always happen congruently objects move in the universe through space and time proportional to their velocity

 their motion is affected by many forces such as gravity this is why the universe keeps expanding as time and space are linked together entropy is also responsible for the expansion of the universe everywhere you can see entropy is always going up the universe will always move from system to chaos and there is no reversing it you can unbreak a scrambled egg or put the pieces of a broken mirror back together into a new mirror the universe can disobey the laws of thermodynamics though some have argued that there is no proof that the linear path of thermodynamics is correlated with the linear path of time when Edwin Hubble discovered galaxies moving apart from each other in 1929 scientists realized that the universe must be expanding this was a perfect explanation for the paradox that has been troubling

 humankind since the time of the ancient greeks is the universe finite or infinite in the early 1800s german astronomer Heinrich Obers argued that the universe must be finite Obers once said if the universe were infinite and contain stars throughout then if you looked in any particular direction your line of sight would eventually fall on the surface of a star although the apparent size of a star in the sky becomes smaller as the distance to the star increases the brightness of the smaller surface remains constant therefore if the universe were finite the whole surface of the night sky should be as bright as a star obviously there are dark areas in the sky so the universe must be finite but when isaac newton discovered the law of gravity he realized that gravity is always attractive every object in the universe attracts every other object if the universe truly were finite the attractive forces of all the objects in the universe should have caused the entire universe to collapse on itself this clearly has not happened and so

 astronomers were presented with a paradox the idea that our universe is finite but expanding solved this problem the reason that the universe did not collapse as newton's equation said it might is that it had been ever expanding this discovery also made scientists realize that if the universe has been ever expanding then it must have been expanding from the moment of its creation it must have been smaller and smaller in the past and thus at some point in time the entire universe must have been condensed into a singularity but that doesn't answer one question

 what caused the expansion of the universe some scientists blame dark energy for it they believe that the presence of dark energy is the reason behind the galaxies repelling from each other however as human knowledge of dark energy is limited we do not know if it can evolve in the future and if it does would the universe begin to collapse back onto itself or stand still the time reversal symmetry unfortunately this isn't something we can test however there is a possible answer in the laws of physics interestingly laws of physics offer a very interesting principle called time reversal symmetry that means that the laws of physics obey the same rules whether you run the clock forwards or backward we do not need to hypothesize about it there are plenty of examples in our universe of time reversal symmetry for example elastic collisions such as two billiard balls colliding on a pool table magnetic objects attracting each other even the strong nuclear force which binds atomic nuclei together is identical forwards and backward in time and how can we forget about that amazing article of physics professor Walter Lewin

 explaining how kinetic energy works

 with a pendulum by risking his own skull however there is a known exception time reversal symmetry weak nuclear interaction the force responsible for radioactive decay if we ignore that outlier the laws of physics truly are the same regardless of whether time goes forwards or backwards according to time reversal symmetry if you wind up at any fatal state at any moment in time there's always a way to get back to your initial state if you just apply the right series of interactions in just the right order some of us do it every day and call it reverse engineering but if your system is quite complex then you'd have to know things like the precise positions and momenta of your particle to better accuracy then it is quantum mechanically possible for example if you grab an egg break it scramble it and cook it you would never be able to uncook unscramble and unbreak an egg if you push a glass off the shelf and watch it shatter against the floor you'll never see those bits of glass rise up and spontaneously reassemble themselves here the entropy wins there is no going back time will keep moving forward what we know as the thermodynamic arrow of time the laws of thermodynamics basically state that there are a finite number of ways that the particles in your system can be arranged and the ones that have the maximum number of possible configurations the ones in what we call thermodynamic equilibrium are the ones that all systems will tend towards as time goes forward is expansion slowing down it's already

 established that since the hot big bang

 the universe has been expanding but the universe isn't expanding as fast as it used to in the past also the universe has cooled down a lot that means that even though time is moving at the speed of one second per second like always the expansion of the universe is slowing down and that concludes that perhaps time and thermodynamics aren't as closely linked as many people think the universe expanded much more quickly

 in the past is expanding more slowly today and will asymptote to a finite positive value but if dark energy may evolve to have a condition that expansion of the universe may slow down to a halt then would gravity cause the universe to contract now if you take an expanding universe and apply the time reversal symmetry to it

 you'll get a contracting universe out of it but can time go back in reverse well yes and no entropy doesn't back down if the universe stood still gravity would continue to attract objects to each other particles that fall into a bound structure will still exchange energy and momentum through elastic and inelastic collisions the normal matter particles will still shed angular momentum and collapse they will still undergo atomic and molecular transitions and emit light in other forms of energy in simple english even if the universe stood still and gravity began to attract everything to contract the universe entropy would still increase that means even though a contracting universe will look like rewind footage of an expanding universe time will still move forward we can see it happening today as well because the biggest driver of entropy in our universe is the existence and formation of supermassive black holes in fact the supermassive black hole

 at the center of the milky way alone

 has more entropy than the entire universe had just one second after the hot big bang not only would time continue to run forward as far as we know but the instant that preceded the big crunch would have more entropy than the universe did at the start of the hot big bang planets will merge into stars stars will merge into black holes black holes will merge into bigger black holes until every matter and energy in the universe converge into a single small infinitesimally dense point fascinatingly time will keep on going forward but not without being affected by the gravitational pull of the big crunch we'd inevitably find ourselves approaching the edge of a black hole's event horizon and as we did time would begin dilating for us stretching our final moment out towards infinity there would be some sort of race occurring as we fell into a black hole central singularity and all the singularities would merge to lead to the ultimate demise of our universe in a big crunch and that raises the question what's next would that be the sorry end of our universe would it lead to the birth of a new universe where this big crunch would lead to another big bang or would there be a gestation period of who knows how long because time just ceases to exist that is the beauty of the universe even if it's finite and will age and die one day it begs the question what if we go to the edge of the universe and extend our hand beyond the boundary what's on the other side tell us in the comments what do you think will happen at the end of time