“When will the big crunch happen?”
No one has a definite answer to the above question. In fact, there are multiple theories explaining how the universe will end, and the Big Crunch is only one of them. The other two being The Big Freeze and The Big Rip.
The Big Crunch explained
The easiest way to understand Big Crunch is using the analogy of an inflatable balloon. When blowing air into a balloon, you sometimes lose control of the open end and all the air contained within squeezes out, thus leaving the balloon in its original state.
Most scientists believe that the universe was created as a result of The Big Bang, and that it was initially a single primordial atom which then expanded to become what it is today. In fact, there are some studies that show it is continuously expanding – even today.
Having consensus on how the universe came into being, scientists have now come up with theories about how it will come to an end.
The Big Crunch, being one of those theories, suggests that there may come a time that the universe starts shrinking instead of expanding. With gravity becoming the dominant force, galaxies, planets and stars would start moving closer to each other instead of drifting apart like they are doing right now. Eventually, they would collide with each other, and the universe would collapse. Thus, Big Crunch will basically be The Big Bang, happening in reverse.
The force that is currently opposing the gravity and supporting the Big Bang is called the dark energy. In view of the current strength of dark energy, the answer to when will the big crunch happen may be never. Reason being, three cosmologists have discovered that instead of the rate of expansion slowing down as an effect of gravity (like they expected), it is actually increasing. They won a Nobel Prize for this discovery in 2011.
However, the supporters of The Big Crunch theory argue that there ought to be a threshold for density, crossing which may strengthen the gravitational force to the extent that it is able to stop the expansion and pull everything in the opposite direction. They present this argument based on the fact that the strength of gravity is believed to be directly proportional to density of the universe.
Alternatives to the Big Crunch
The Big freeze
With the strength of dark energy increasing, there seems to be a low probability for the big crunch to happen. In fact, if the universe continues to expand, galaxies will keep moving further apart from each other, and that too at an increasing rate.
This may result in the space becoming an evacuated place to the extent that astronomers are not able to see anything beyond The Milky Way. This will result in what is known as the “Heat Death”. The Heat Death is predicted to occur when heat in the entire system will be evenly distributed, thus significantly reducing the amount of usable energy.
This is worrisome because at least some temperature difference is required for anything to function in routine life, be the machines that warm up when started, or the human body that survives on food providing energy to it. Similarly, the temperature difference between the Sun and rest of the universe is essential for it to survive.
When this difference will no longer be present, the universe will end up becoming extremely cold and dark, thus halting everything. This theory is referred to as The Big Freeze.
The ideas of Big Freeze and Big Crunch are extremely opposite to each other, but suggest one thing in common, i.e., the universe will ultimately come to an end – one way or the other.
The Big Rip
If the belief about the increasing rate of dark energy turns out to be true, scientists suggest another probability instead of the Big Crunch.
When will the Big Crunch happen vs when will the Big Rip happen?
If the strength of dark energy remains constant and the universe continues to expand, but at a uniform rate, the probability of Big Crunch is higher.
If dark energy continues to become more forceful and the universe starts expanding at an increasing rate, there are greater chances of the Big Rip.
The Big Rip suggests that at an increasing expansion rate, even the clusters within a galaxy may not be able to stay together due to the reducing effect of gravity. As a result, the galaxies would “rip apart”, thus causing the universe to explode.
As to when will the big crunch, or any of its suggested alternatives happen, cosmologists do not have a definite answer. Some say it would occur approximately 6 billion years later whereas others think it could happen any time between 2.8 and 22 billion years from now.
Even though the above-mentioned three theories suggest different ways in which the universe may come to an end but they are a powerful reminder of the fact that the universe is mortal.