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 Factnomenal 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 video 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 and as always thanks for watching Factnomenal