The end of the universe and the big crunch: will time go backward?
If you are a novice to astrophysics or cosmology,
you should know that we are living in a golden age for what concerns discoveries and missions.
Every mission and tool was designed and conceived to give us a completely new understanding
of the universe. This includes new data to analyze, new worlds to explore, and corrections
to old theories to be made. There has never been a more fruitful time for science in the
history of humanity! We have never had such a huge number of discoveries
every single day: you just need to check the papers, dozens and dozens of papers are published
every day about the most intriguing and various topics, such as exoplanets search, black hole
mergers, and gravitational waves detections, or maybe… new cosmological studies giving
us a more accurate result on the value of the Hubble constant.
Every day is a new discovery, every day we get closer to ourselves because every day
we get to know more about the cosmos. All of this concerns modern astrophysics,
but today we want to turn back to let’s say a classical question because a deep understanding
of the universe is only possible if we are able to understand the more general and basic
picture. It is like observing a Monet painting: if you get too close, you can only see the
dots. If you want to appreciate the masterpiece, you have to take a step back to see it from
another perspective. That’s where cosmology kicks in.
In this video, we will talk about what will
happen to time – and to ourselves – if the universe undergoes a big crunch.
We know that everything started with the big bang, then the universe expanded (and
it is still expanding, right now, at this very moment!), and the expansion rate will
probably change and decrease over time, eventually leading to what is known as the “big crunch”.
But in the beginning, this was something unexpected.
For a long time scientists, including Albert Einstein, thought the Universe was static
and infinite. In fact, observations over the years have
shown that it is in fact expanding at an accelerating rate. This means it must have originated from
a more compact state that we call the Big Bang, implying that time does have a beginning.
And It will likely have an end. Theoretical physicists are not yet sure about how the
universe will end, but one possibility is that it will shrink and become hotter and
hotter, denser and denser as it squeezes. This end is called the big crunch. How do
we get there? And what are the consequences of it?
Here’s pretty much what we know. In 1922, Russian physicist and mathematician
Alexander Friedmann derived a famous set of equations aptly named the Friedmann equations.
These calculations showed that our universe’s destiny was determined by its density, and
it could either expand or contract, rather than remain in a steady state. With enough
matter, gravity would eventually halt the cosmos’ expansion, causing it to come crashing
back inward.
In the 1960s and 1970s, when astronomers added up all the matter in the known universe,
they calculated there was enough mass that the cosmos should ultimately collapse to an
infinitely dense state, or perhaps even a gargantuan black hole.
Some speculated that once compressed into an infinitely small point — the Big Crunch
— the universe would kickstart yet another expansion or Big Bounce.
In the 1970s and 1980s, physicist John Wheeler, who helped coin the term black hole, became
a leading proponent of the Big Crunch. To him, it was an obvious fate. A revolution
in understanding black holes was underway, and Wheeler saw each one as an “experimental
model” of the universe’s final state.
The big crunch foresees galaxies colliding
and really messy star-to-star interactions. If a big crunch kicks in, the universe will
become extremely chaotic on large timescales. This means the slow ending and cleansing of
life as we know it, pretty much everywhere – if only life existed somewhere else in
the cosmos.
To reach the big crunch, we need one main ingredient.
It concerns matter. If for any reason, instead of expanding forever, matter in the universe
reaches a point where it starts to decrease over time, this could cause gravity to become
the dominant force. Then, the gravity pull will ultimately cause
the Universe to shrink and cause stars, planets, and entire galaxies to collide with each other
and the Universe would, for all intents and purposes, collapse in on itself.
Put simply, if the expansion of the Universe slows to a crawl and the Big Bang happens
in reverse, everything will implode back into a singularity.
Initially, scientists believed that this ingredient – the decrease in matter over time – was
the only one determining the fate of our universe. But they soon discovered they were wrong.
, Just like anybody that goes against gravity,
e.g. when you throw something up, that body will eventually give in and come back down
for as long as there is no other force pushing it up.
Thus, that the gravitational forces will win in the end, once seemed like a logical prediction.
But that was until scientists discovered that the Universe was actually increasing its rate
of expansion in regions farthest from us.
To explain this phenomenon, scientists had
to assume the presence of an unknown entity, which they dubbed ‘dark energy. It is widely
believed that this entity is pushing all galaxies farther apart. With dark energy, and what
little is known about it, in the picture, there seems to be little room for the possibility
of a Big Crunch. Right now, measurements made by NASA’s Chandra
X-ray observatory indicate that the strength of dark energy in the University is constant.
Just for added information, an increasing dark energy strength would have supported
the possibility of a Big Rip, another universe ending that predicted everything (including
atoms) to be ripped apart. Even with an unchanging dark energy strength,
an ever-expanding universe is still the most likely scenario. So unless data that contradicts
these properties are collected, the Big Crunch will have to remain a less favored theory.
Still, what if new observations showed that our universe will actually end with the Big
Crunch? It’s better to be prepared for it, don’t you think? What can we expect from
it? For instance, if the universe will shrink
and end in a singularity, we can then think of the Big Bang to be the same process s the
Big Crunch, only time reversed. Let me explain it better.
The Big Bang starts, and everything expands, to the point we are today.
If we could go backward, and see the movie of the history of our universe in reverse,
we would start from today, and end up in the Big Bang singularity again.
So, it’s not that crazy to think that if the universe starts shrinking, and we go
towards the big crunch, time will go backward. Is it? To answer this question, let me introduce
you to an old friend: entropy. In physics, entropy is a measurement of
the energy in a system or process that is not available to do work.
It is most commonly associated with a state of disorder, randomness, or uncertainty, of
a thermodynamic system. One of the most important laws of physics
states that the overall entropy in the universe is always increasing. Every process, occurring
in an isolating system, every breath you take, every move you make…the entropy will be
watching you and it will increase.
As we step forward in time, a number of things always seem to happen together. Objects
move through the Universe proportional to their velocity. They change their motion due
to the effects of gravity and other forces. On large scales, the Universe expands. And
everywhere we look, the entropy of the Universe always goes up.
As the story of our cosmic evolution continues, we think all of these things will continue:
the laws of physics will still apply just as they do today, dark energy ensures that
the Universe will keep on expanding, and the laws of thermodynamics will still be obeyed.
Some scientists think that the arrow of thermodynamics and the arrow of time may be related, even
if there is still no proof for this. Moreover, others have speculated that dark
energy might evolve over time, rather than being a constant, leaving the door open to
the possibility that it might someday counteract and reverse our Universe’s expansion.
Given these two assumptions, and putting
them together, we can ask ourselves this question: if the universe were to contract towards a
big crunch, would entropy decrease; if so, would time run backward?
Let’s find out what physics has to say
about it.
One of the most important symmetries in all
of physics is known as time-reversal symmetry. Put simply, it says that the laws of physics
obey the same rules whether you run the clock forwards or backward. There are many examples
where one phenomenon if you run the clock forwards, corresponds to an equally valid
phenomenon if you run the clock backward. For example, two billiard balls colliding
would behave exactly the same if you run the clock forwards and backward, right down to
the speed and angle that the balls will go off.
This is not generally true in the quantum world, but this is a story for another video.
If we forget about this small detail, we can safely say that the time reverse is symmetrical,
but this doesn't appear to be the case for everything we experience in the real macroscopic
world.
Some phenomena clearly display an arrow of time or a preference for a particular one-way
direction. Try going to your kitchen and taking a nice
glass of red wine, then crash it against the floor: you will never see those bits of glass
rise up and spontaneously reassemble themselves.
Therefore, there clearly is a preferred direction for things: an arrow in which things
flow.
The laws of thermodynamics, on the other hand, say that basically state that there
are a finite number of ways that the particles in your system can be arranged, and the one(s)
that have the maximum number of possible configurations are the ones that all systems will tend towards
as time goes forward. Some scientists think somewhere in what
we just said lies the connection between the time arrow and the thermodynamic arrow.
In other words, scientists think there is a perceived arrow of time, and there is a
thermodynamic arrow of time, and they both always point in the forward direction. The
perceived arrow of time, however, seems largely independent of anything entropy or thermodynamics
may do.
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If the universe starts to contract, will entropy decrease?
The answer is no. To understand this you have to keep in mind
that collisions between particles, the exchange of angular momentum between planets, stars
and galaxies, and so on, are all processes that are already present today, and they increase
the entropy of the universe. If the universe was to contract, the same
processes would still be occurring, maybe at a faster pace, and therefore entropy would
increase as well. Basically, everything that makes entropy increase
today will still make entropy increase in a contracting universe.
So if the universe contracts, entropy will still go up.
Since an entropy increase is associated with a forward thermodynamic arrow and this
is ultimately associated with a forward time arrow, we can safely say time would still
be flowing as it always did.
What’s interesting about this scenario is that according to Einstein’s general
relativity theory, clocks run differently when you’re in a strong gravitational field,
for instance, close to a black hole. If the Universe were to recollapse and approach
a Big Crunch, we’d inevitably find ourselves approaching the edge of a black hole and as
we did, time would begin dilating for us.
What a nice way to die, huh? Hey! This video ends here! Thanks for watching!
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