What if the universe stops expanding?
Predicting the end of the world is a hard thing to do. But predicting the end of the universe is
even harder. Scientists know how our world will end, “naturally”. The sun will eventually run out
of its fuel and become a red giant and in the process consume our planet Earth. In the final
burst of its glory our beloved sun will expand and engulf the closest planets and then explode into a
planetary nebula leaving behind a dense white core which is known as a white dwarf. We know
that in 4.5 Billion years our galaxy will collide with our neighbouring galaxy The Andromeda Galaxy.
But the End of our universe is something very hard to predict, because we still don't know
much about how it started and the physics behind that. Seeing the current expansion of the universe
and the unknown forces behind it we can try to predict some possible fates about our universe.
And if this expansion stops, what's going to happen next? What if all the galaxies, stars,
planets – everything – stopped moving away from everything else? Would the Universe as it is just
freeze? Or would it collapse into a single point, just like it was before the Big Bang?
In this video we are going to talk about, What if the Universe stops expanding?
To predict the future of the universe astronomers need to know about its past.
Cosmologists look backwards generally, because all the evidence they can examine with telescopes
is far away and concerns things that happened long ago. They use the motion of distant stars and
galaxies to predict the possible future through some physics and some speculation. But How did
it all begin? The general idea is a theory called the Big Bang. It is the prevailing cosmological
model explaining the existence of the observable universe from the earliest known periods through
its subsequent large-scale evolution. In the early 20th century scientists thought that the expansion
of the universe might slow down. When you throw a ball up in the air, it will eventually come down
due to earth’s gravitational field. They thought the same thing should happen to our universe too.
That after a period of time the gravitational pull of the universe will cause the expansion
to slow down. But in 1920, When Hubble turned his telescope towards the sky
he found out that the expansion of the universe wasn’t slowing down. The Expansion of the Universe
was first observed by Vesto Slipher in the Early 1920s and in 1929, Edwin Hubble who had
access to some world’s largest telescopes gave the Hubble Law. According to it, every
distant galaxy is moving away from each other with a velocity proportional to its distance.
The farther away a galaxy is, the greater is the velocity with which it moves away.
Then Arno Penzias and Robert Wilson detected a background of microwave radiation known
as the Cosmic Microwave Background(CMB) radiation today, coming to Earth from all directions. It
was an afterglow of the primordial, hot and dense Fireball. Today, with the data collected from the
Cosmic Background Explorer (COBE) satellite fits perfectly with the Big Bang and that it accounted
for the light nuclear isotopes like deuterium, hydrogen, helium-3 and others. Today we have
scientific evidence that supports and proves this idea of a violent explosion followed by inflation.
We have made a very good video titled- what caused the Bang in the Big Bang? In which we explain in
more detail about the beginnings of the universe. Make sure to check the link in description.
In 1990 Astrophysicists found out that the expansion of the universe is happening
at an accelerating rate. This came as another surprise for them.
To explain this they theorized an invisible dark force that's speeding up the expansion
and called it- “Dark Energy ''. They use the word Dark because this force doesn't interact
with normal matter or any other electromagnetic forces. It must be noted that the universe’s
inflation is a kind of misnomer. Our universe is inflating not because things in it are moving.
Theories suggest that it appears to be inflating because it is filling, the space itself is
expanding. So What if the strength of this hypothesized energy vanishes over time? Well based
upon our understanding there are three possible fates of the universe. The fate of the universe
depends upon whether this expansion will continue, accelerate or reverse. The rate of this expansion
may eventually tear the Universe apart, forcing it to end in a Big Rip. Alternatively, the Universe
could 'shrink', decrease or decay, effectively reversing the Big Bang and destroying the Universe
in a Big Crunch. A third theory is described as the Big Freeze. But in this video since we are
talking about what will happen if the expansion stops we will not talk much about the other fates.
Outside our cosmic neighbourhood, every galaxy is zooming away from us; a clear
sign of expansion. . If this expansion of the universe stops, it will lead to a possible fate
called Big Crunch. When the expansion of the universe stops and everything stops
moving away from one another in the universe, things will kind of freeze for a time being.
But then the gravitational pull from Dark matter and all the other matter will eventually dominate
in our universe. The gravity would be the dominant force and act in a reverse direction,
reversing the expansion of our universe. This would ultimately cause the Universe to shrink
and cause stars, planets and entire galaxies to collide into each other and the Universe would,
for all intents and purposes, collapse in on itself. . Over time, galaxies, then
individual stars, will smash into each other more frequently, killing off any life on nearby
planets. In the final moments, as densities and temperatures soar in a contracting inferno,
all that remains will extinguish in a single point. Put simply the expansion of the universe
slows down and the Big Bang bang happens in reverse, imploding everything into a singularity.
This seems like an intriguing possibility right? The universe going back into a singularity.
This singularity might explode into another Big Bang triggering the start of another universe.
This is the cyclic model of the universe that many scientists today believe in.
But what about the laws of physics? Would the entropy of the universe decrease? Would time run
backwards? These are yet another intriguing questions we look to answer in this video.
Time reversal symmetry is one of the most important symmetries in all of physics.
Put simply, it says that the laws of physics obey the same rules whether you run the clock forwards
or backwards. Some of the examples of this phenomenon would be- A purely elastic collision,
like the collision of two billiard balls, which would behave exactly the same if you run the
time forward or backwards. Gravitational and electromagnetic interactions work the same way
forwards or backwards. The only exceptions to this symmetry are the weak nuclear interactions
and if your system is complex enough, you’d have to know its precise positions and momenta,
to a better accuracy than what is quantum mechanically possible. If we ignore it,
all other laws of physics are truly the same, no matter in which way the time runs. Put simply,
it means that if you wind up at any final state 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 the correct order,
except for the case of weak interactions of course. But this doesn’t appear to be the case
for everything we experience in our daily lives. Some phenomena clearly are in a one-way direction.
For example, if you take an egg and break it, you can never unbreak the egg. Admittedly, these are
complex systems experiencing extremely intricate sets of interactions, the second exception that
we mentioned earlier. Now if you take an expanding universe and apply time reversal symmetry to it,
you’ll get a contracting universe. The reverse of expansion is contraction, so if you reversed
the time you will have a contracting universe. But there still will be some things happening,
let's take a look at them. Gravitation is still an attractive force, and particles that fall into
(or form) a bound structure 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 and other forms of energy.
To put it bluntly, everything that makes entropy increase today will still make
entropy increase in a contracting Universe. So even if the universe contracts the entropy
will still go up. The biggest driver of entropy in our universe is the formation and existence of
supermassive blacholes. Over the past billions of years, the entropy of the universe has increased
by about 30 orders of magnitude. The supermassive blackhole at the centre of our galaxy, The milky
way, alone has more entropy than the entire Universe had just 1 second after the hot Big
Bang! So not only will time continue to run forward, but the instant that preceded the
Big Crunch would have enormously more entropy than the Universe did at the start of the hot Big Bang.
All the energy and matter, under very extreme conditions and environment will start to merge
together as all the supermassive black holes had their event horizons begin to overlap. If there
were ever a scenario where gravitational waves and quantum gravitational effects
could show up on macroscopic scales, this would be it. With all the matter and energy compressed
into such a tiny volume, our Universe would form a supermassive black hole whose event horizon was
billions of light-years across. What’s interesting about this scenario is that clocks run differently
when you’re in a strong gravitational field: where you’re at small enough distances from a
large enough mass. If the universe was to collapse again and approach a Big Crunch,
we would inevitably find ourselves approaching the edge of a black hole’s event horizon
and time would start dilating for us: stretching our final moment towards infinity!!
A sort of race would be occurring as we fell into a black hole’s central singularity,
and as all the singularities merged to lead to the ultimate demise of our Universe in a Big Crunch.
So what will happen after that? What will our universe look like, or will it even exist?
Would the universe simply wink out of existence? Would it lead to the birth of a new universe like
the cyclic model suggests? Would there be some sort of cutoff, where we’d only get so far into
the crunch scenario before the Universe rebounded, giving rise to some sort of rebirth without
reaching a singularity? These are some of the intriguing questions of theoretical physics that
scientists look to answer. But there is one thing that seems to be true for all sorts of scenarios:
the entropy of our universe still increases, and time always runs in forward direction. If
this turns out to be false, just in case, there is something very fundamental about the nature
of reality and physics that remains elusive to us, still waiting to be discovered by us.
So what do you think about these questions? Let us know your ideas and thoughts in the
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