Note: 1. A purely non-authoritative piece. 2. This is an old draft that I am publishing after an year or so. Information presented here might have changed in the meanwhile.
Ages have passed since humans started questioning. We have always been a species with a constant endeavour to explore. We have always tried to fathom the mysteries around us. And yes, we have been quite successful in our adventures. Tremendous progress has been achieved in the fields of science and technology. Many questions have been answered. Many new have arisen. Some questions have remained unanswered, questions as old as humanity itself.
One of the first things that caught the imagination of our ancestors was the dazzling night sky. Pearls embedded in the dark depths of the universe struck wonders in the eyes of those early star-gazers. The mysteries of those pearls continue to amaze us even till today. Who put them up there? How did it all start? Rather, was there a start to it?
In a certain sense, this question is the same as “How did the Universe begin?”
Many scientists and philosophers, over the millennia, have tried answering the same question, “How did it all start?”. Today, we have a well established theory of how the Universe started and expanded to its present state. The Big-Bang Theory. Though it does not talk about the actual origins of the Universe, it explains quite satisfactorily all that happened starting from a few milliseconds after the big bang, the explosion that set into motion the Universe as we see it today.
The other question
There is another important question to be asked. We have been asking about the beginnings of time.
At the other end of the scale is the question, “Is there an end of time?”. Just like the Universe had a beginning, does it have an end? What is the ultimate fate of the Universe? What will happen in a million trillion years into the future?
The current state – An expanding Universe
To answer all these, we first need to know what is happening currently in the Universe. It is a known fact, backed up by observational evidence, that the Universe is expanding. This expansion was started by the Big Bang which took place roughly 13.75 billion years ago. The expansion continues even today, all the galaxies are moving away from each other. This has been confirmed by the red-shift of the galaxies.
The inertia of the initial big bang, which caused the expansion, will continue forever if there are no opposing forces, according to Newton’s first law. However, one of the four fundamental forces, gravity, is attractive in nature and its strength depends on the mass of the bodies. This means that the galaxies, which are moving away from each other due to the inertia of the big-bang, will also feel an attractive force that is trying to stop the expansion.
Will this gravitational force be enough to stop the expansion? It all depends on a factor called “Density Parameter”, referred to by the symbol ‘Ω‘ (omega). Ω is a measure of the density of matter in the universe. It is a ratio of the average matter density to the critical value of that density. Based on the value of Ω, < 1, = 1 or > 1, there are three different possibilities for the fate of the universe.
If Ω > 1, it means that the mass of the universe is enough to stop the expansion of the universe. At some point of time, the force of gravitation will overcome the expansion inertia and a phase will start when the universe will start collapsing on itself. This seems a perfect symmetry to the Big Bang, with some people terming it the “Big Crunch”. The big crunch causes the universe to end in a singularity, much like at the beginning of the universe. Also, a big crunch might be immediately followed by a big-bang, giving rise to a “Cyclic Model” of the universe.
A very interesting point to ponder over in a big crunch is the “arrow of time”. What will happen to the arrow of time during a big crunch? Will it reverse? Will the entropy continue to increase? These are some of the fascinating questions that can be looked at if a big crunch does occur.
If Ω < 1, then the mass of the universe is not enough to stop the expansion of the universe. Gravity will barely slow the rate of expansion. In such a scenario, the universe will continue expanding forever. It will eventually reach a stage where there is no temperature gradient, meaning no work can be performed. The entropy of the universe will be at its maximum. Such a stage is popularly termed “Heat Death” or the “Big Freeze”. It is important to understand that the temperature does not touch absolute zero. However, the temperature will be uniformly distributed with no gradient.
If Ω = 1, then the mass of the universe is just about enough to stop the expansion of the universe. Mathematically speaking, the rate of expansion asymptotically approaches zero. The ultimate fate of the universe is again the same as that of the open universe.
Dark Energy and the Accelerating Universe
The 2011 Nobel Prize for physics was given to scientists who showed that the rate of expansion of the Universe is indeed, increasing. This is counter-intuitive, given that gravity should slow down the rate of expansion of the universe. In order to explain this phenomenon, scientists often use the term “Dark Energy” and “Dark Matter”. For an accelerating universe, one needs a force which is repulsive in nature. Most scientists generally attribute this repulsive nature to the dark matter or to the cosmological constant, first introduced by Einstein. Einstein introduced the cosmological constant to account for a steady-state theory of the universe. He later discarded this after he became convinced about the big bang theory. However, a positive cosmological constant would contribute a repulsive force and might help explain the accelerating universe.
The discovery of the accelerating universe will change all of the notions about the end of the universe. Nobody has ever observed anything about the hypothetical dark matter. Until and unless we know the source of the acceleration and the nature of the force, nothing conclusive can be said about the future of our universe. For now, the dark matter gives us enough time and questions to think about before the end of time. Or, is there an end to time?
And AC said, “LET THERE BE LIGHT!”
And there was light—-
“The Last Question”