It does not matter how smart you are, once you delve into the field of Physical Cosmology, you must be ready to ask questions and also be ready to end up with them unanswered. Our Universe is so large that there is just no other way around it. It is even a wonder that scientists have been able to uncover so much about the mystery of the Universe. Physical cosmology studies the dynamics of the Universe especially about its origin, structure, nature, evolution, and ultimate fate. Underlying all these concepts is the concept of time.

^{Pixabay CC0}

No matter which aspect of the Universe we want to study, we must do it with respect to time. This post is about the beginning of time (if there was one), the key things that have happened to our Universe since then, and the end of time (assuming there was a beginning).

Time

Apart from saying that time is the ticking of the clock, candid response to what time is, is that we do not know and this has been the state of knowledge on the subject for many thousands of years. It is easy to fall into the temptation of thinking that you know what time is because it is something we deal with every day. We know its properties such as its being directional and our inability to turn it back. However, a compact definition of time is surprisingly elusive. Over the years, scientists, religious leaders and philosophers, have attempted to define it. Among the best definitions, in my opinion, are:

Time is a continuum that lacks spatial dimensions (Encyclopaedia Britannica)
Time is what prevents everything from happening at once (John Wheeler)
Time is a linear continuum of instants (Adolf Grünbaum)

It was between 1905 and 1915 that Albert Einstein's theories of relativity which helped us change the way we viewed time from Newtonian absolute time to relativistic time. He realised that time was not an independent variable on its own. Instead, it is the fourth dimension of space-time continuum (in his special relativity in 1905) or the fourth dimension of a dynamically curved space-time(in his general theory of relativity). Whether we view time from the standpoint of special relativity or from the perspective of general relativity, it is pretty obvious that time is inextricably interwoven with space and both are a fundamental fabric of the Universe.

The Beginning of Time

Human beings had thought that our Universe had been there forever. If that were so, then according to Albert Einstein, time should also have been there forever. However, the evidence that scientists and cosmologists have uncovered all suggest that the Universe has a beginning which is about 15 billion years ago. This is a pretty significant discovery when you consider the timescale of the Universe compared to human life. But wait a minute. If our Universe is that old and we have been here that long, why are we not more advanced biologically and technologically? Well, it turns out that we have not been around for very long. It appears that human beings have only existed in the Universe for just a few thousand years. So if there was a beginning, does that mean there would be an end? And what was there before the beginning?

Majority of our discovery of the age of the Universe has to do with Vesto Slipher's measurements of radial velocities for galaxies, providing the empirical basis for the expansion of the Universe.^[3]

Measurement of the expanding Universe has been done, and it is apparent that galaxies are steadily moving away from each other. A plot of this separation and extrapolating to the past reveals that our Universe has not been static and that the galaxies get closer and closer as we move into the past. Without considering the effects of acceleration due to gravity, this plot would result in a straight line that reduces the space between galaxies to zero sometime around 20 billion years ago. But acceleration due to gravity should make the galaxies come towards each other due to Newton's law of gravitation. If this is so, then it means that gravity has been working against the expansion. Therefore it took the Universe less than 20 billion years because gravity bends the graph of the separation downwards bringing our estimate of the beginning of time to about 15 billion years ago. At this time, all there was in the Universe would be bound together, one on top of the other. Excellent.

Out of Nothingness?

If you are not a religious person, you would wonder what was before the beginning of time. Agreed, everything was bound together in what scientists like to call a Singularity but what was there before the singularity. Suffice it to say that if there were nothing before the singularity, there would still be something now. The fact that there is something now is proof that there was something before the singularity. That we cannot say what that something was is nothing of which to be ashamed. Humanity ought to congratulate herself on how much we know about our universe or at least how much we think we know. But if you are a Christian, hey, it is a no-brainer: In the beginning, God created the Heavens and the Earth. It does not get simpler than that.

The Big Bang

At the Singularity everything that existed was balled up together.

This means that the law of physics broke down. While you would think that something breaking down is bad thing, the breakdown of the laws of physics here means that whatever existed, whatever happened before the singularity is not only unimportant to our present understanding of the Universe, it is also completely independent of the laws of physics that existed after the Big Bang. Therefore we can regard the moment after the Big Bang as the beginning of time.^[4]

Since the Big Bang, about 13.8 billion years ago, the Universe has passed through many different phases or epochs. When you consider the state of the universe before the Big Bang, it is easy to imagine that the changes that took place a second after the Big Bang were much more than the changes that the Universe has experienced ever since.

^{Wikipedia CC0: The Big Bang and Universal Timeline}
There is no other way things could have been because one moment we have a pretty condensed universe with infinite density, and the next second we have an expanding universe with the four fundamental forces. The four forces were possibly unified into a single fundamental force and in less than the time it takes to make a sandwich, the force of gravitation separated from the other fundamental forces and our Universe is just about of a room. It is not tenable to examine all that happened during that time in one post. But the question most people are interested in is this: if time has a beginning, does it mean that it will have an end? Will our Universe one day start contracting and possibly return to another singularity, ending all life in it?

We now know much about our Universe from the formation of galaxies to the expansion of these galaxies away from each other. But the galaxies alone do not tell the whole story because the visible portion of our Universe is made possible from calculations of the gravitational forces in it. But the visible part of the Universe is just about 4% of the Universe. Another 26% is estimated to be made up of a substance called dark matter which neither emits light nor allow any form of electromagnetic energy to escape it. Apart from this, the galaxies keep speeding away from each other such that the gravitational force could not slow it down. Therefore up to another 70% of the Universe is presumed to be made up of energy which is capable of causing this increase in separation. This is called dark energy.^[1]

From our current understanding of how the Big Bang might have progressed and the things we have learned ever since, there are several predictions about the fate of the Universe, all of them bleak.

The Fate of the Universe - Possible End of Time

It was Albert Einstein's general theory of relativity which further exploration of the Universe on a large scale. The possible solutions to the equation have led to several theories of the ultimate end of the Universe. In the beginning, the Big Bang theory had opposition in the form of Steady State theory which essentially states that even though the Universe is expanding, that the steady creation of new matter makes things to remain more or less the same. The Steady State theory was put to rest in 1965 by Arno Penzias and Robert Wilson when they discovered the cosmic microwave background radiation which was a straightforward prediction of the Big Bang theory, which the Steady State theory could not explain. If the Steady State theory was upheld, then we may not have anything to worry about as the Universe would just go on forever. But, no we are sticking with the Big Bang theory.

Well, it turns out that the fate of the Universe under the Big Bang would depend on the shape of the Universe.

• Closed Universe: If the Universe was closed, the theory is that gravity may one day, overcome the acceleration of the galaxies and cause the Universe to begin to contact until another singularity known as the Big Crunch which is opposite of the Big Bang. However, new theories assume that this is not a likely fate of the Universe because there is quite enough dark energy in the Universe and its repulsive force is enough to keep the Universe expanding forever.

• Open Universe: Avoiding all the math, if the Universe is open, that is negatively curved, then the Universe could expand forever with gravity having a negligible effect on the rate of expansion. In this case, the ultimate fate of the universe is either a Universal Heat Death, the "Big Freeze" or the "Big Rip" which is a situation where the force of the dark energy overcomes the effects of the fundamental forces and the Universe tears apart.

• Flat Universe: here also, the Universe is expected to continue expanding but at a slower rate due to the effects of gravity. Eventually, the fate of a flat Universe is the same as the fate of an open Universe.

None of these theories is conclusive about the Ultimate Fate of the Universe because we always have to consider the effects of dark energy, gravity, the shape of the Universe and many other unpredictable variables. But if anybody knows what is going to happen eventually, Stephen Hawking should know, and according to him, the Universe will eventually collapse again but time will not go in reverse. Time will keep going the direction it is going, and we shall continue getting older until the end.^[5] But as my daddy used to tell me, "Relax, we won't be here when it happens!"

Thank you for reading!

[An Original Content by @churchboy]

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References

1. CERN | Early Universe
2. Physics of the Universe | Big Bang Timeline
3. Wikipedia | Vesto Slipher
4. NASA Archive | The Universe
5. Hawking | The beginning of Time

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