What Is The Meaning of Expanding Universe?

How our senses of space and time are diminished by the stretching of the universe.

how we calculated age of universe what is expanding universe

Once I had a nightmarish dream in which I was lost in a large desert, broad and boundless. The immensity of the scorching desert was quite frightening. So I started running in a random direction so to find the long lost habitable zone or to make human contact. But pretty soon I realized that the desert was enormously vast and more annoyingly symmetrical at all points. Which was why, I couldn't find any differences in the course of my path. All the regions around me looked exactly the same, mountains of what appeared to be deep reddish soil.

And to make things worse, in my dream, I also found that the sun remained still in the same position with no movements so I couldn't have measured time in any way. This uncertainty between space and time made me crazy, woke me up, with profuse sweating.

Thankfully our earth is not like the desert in my nightmare. While travelling to a particular destination, let's say I want to go to the theater from my home, I have the liberty to associate certain reference points in space all along my way. Not only that, I have also the idea of time, let's say the movie is going to start at 7 in the evening, for example, so I could align my schedule with respect to that timing.

How about our universe? Is it like the earth or like the desert in my nightmare? To solve this question, we need to understand the difference between the earth and the universe. The answer is straightforward, the shortest distance between any two points on earth is the same. For instance, the shortest route between New Delhi and Goa is a constant. It was the same in the past and will remain same in the future, provided no geological activities, that is.

On the contrary, when the universe is considered, which is in the big picture, the space is expanding or the distance between any two galaxies is increasing with time. But, more correctly, the galaxies themselves are not moving, whereas, it is the space between them, and also around them, which is stretching.

How do we know this fact? We know it through the observations of the great American astronomer Edwin Hubble, who was also the first to show that in the universe ours was not the only galaxy but that there were billions of other galaxies scattered all over space. In his 1922 observations, he discovered that the distant galaxies were receding from us and doing so at increasing rate.

Just to make some sense of this, consider an ideal balloon in the size of the earth, ideal in the sense that it is perfectly spherical and equally expandable at all points when air is filled in to it. Imagine two little ants A and B sitting still on the surface of the balloon. When the balloon is blown steadily, the ant A sees the ant B moving away from it and similarly the ant B sees the ant A moving away. But it is important to note that the ants themselves are not moving. It is the balloon which is expanding.

When the ant B is placed nearer to ant A it will take longer time for ant B to disappear in the horizon but if the ant B is already sitting near the horizon it will disappear almost instantaneously when the balloon is continually getting bigger. This is similar to the ships near horizon which disappear faster than those near shore.

Now keep the ant B still but allow ant A to move freely on the surface of the expanding balloon. As the balloon is huge, compared to the size of the ant, and expanding steadily, the ant moving on it will experience a similar situation as that of my nightmare. It will never be able to reach the point where it started. It will experience as if it is unable to make sense of its movements. The balloon would grow for every step that it takes.

Our Universe is similar to the expanding balloon. It appears to be the same in all the directions. Not just that, it is also expanding, which means that those galaxies which are further away are going to disappear from our sight, faster. Thus, the vast and boundless universe, like the desert, is pretty much becoming indeterminable in the sense of space.

What about time though? Could it be measured? In the case of ants, they could associate time with the expansion of the balloon provided they know that it is expanding. If the rate of stretching of the balloon is found, they could then "use maths" to know for how long their balloon has been expanding. This is pretty much what we did, by knowing the expansion rate through astronomical observations, we calculated the age of the universe or for how long it has expanded from size zero. Roughly 14 billion years.

However, there is a problem, we tend to think that time is the same throughout the universe, which it is not. As observed from our point of view, time runs slower for far away objects, which is difficult to digest at first, but is true. Because time is not absolute, as we had thought for so long, and in fact, if analyzed, time is an even more confusing phenomenon than space. Since the universe is expanding, what's our present on the Earth, in this particular surrounding, need not be the same for some thing living distantly in the Andromeda galaxy. And because the expansion is accelerating, time dilation for far away objects becomes more pronounced thanks to the theory of general relativity.

In conclusion, the universe is more like the desert and less like the earth. We have a space which is continually expanding in all the directions because of which all our reference points in the space will disappear eventually. Thus, we will lose the sense of space. Time, on the other hand, which is directed forward along the course of expansion, is more complicated. We will keep some sense of it, at least, in our local surroundings, because we are the spectators "living in the time". But further out, in the remote places, time will also lose its general meaning.