What is the meaning of 'expanding universe'?

 An image taken by the NASA/ESA Hubble telescope of one of the
 galaxies in the survey to refine the measurement for how fast
 the universe expands with time. Source: NASA, ESA

What is the meaning of 'expanding universe'? Ever since the cosmologists have established that the universe is expanding, it has created lots of confusion in minds of people not belonging to the researcher's society. More often than not the explanations and reasons they get in response to their doubts are unsatisfactory. The main reason behind the confusion is our limitation to imagine and picturize the expansion of universe. Not just this, the concept of an expanding universe creates many secondary questions such as

What triggered the expansion of universe and how is it proven that the universe is expanding?

If the universe is expanding, then what is it expanding into? What is the end of it's expansion? Where is the boundary?

Is the universe expanding like a balloon?

Do galaxies also expand with the expanding universe? Why shuldn't gravity make the expanidng universe recollapse?

How do measure distance in an expanding universe?

So many questions! So much confusion! But for good reasons.

Let's first picturize the expansion of the universe. Why do we compare it with the expansion of balloon? Does the universe expands exactly like the baloon? Before it could create more confusion in your mind, let me give the answer. It desn't, not really. Still this analogy is considered.

The main prolem lies in our perspective while looking at the expansion of balloon and trying to imagine the expansion of the universe. In our daily life, when we see something expanding, say the balloon, we immidiately see what is it expanding into. We see the balloon's surface growing and expanding as the air is pumped into it and we are able to picurize it because we are a detached observer from the frame of balloon, obsering the phenomenon from 'outside'.

The universe, however, is no balloon. We are not 'outside' the universe. We are 'inside' the universe, trying to descibe it, quite similar to a fish describing the entire ocean.

Question still remains there, why is the universe's expansion not same as of the balloon? Both are expanding. After all an expansion is expansion and it is independent of our view from inside or from outside. Let's clear the confusion.

The problem with the balloon analogy is that it's a two-dimensional analogy for a three-dimensional situation. As the balloon is blown up, it's surface stretches proportionally in two dimensions, but the universe stretches (which one is a better choice, 'stretching' or 'expanding', we shall see later) proportionally in three dimensions. The third dimension of the expanding balloon, what we see as its curvature (the direction perpendicular to the surface of the balloon), is analogous to the fourth dimension in our universe (somthing we cannot observe). Asking any question about the fourth dimension makes no sense, so the balloon analogy fails.

This analogy is supposed to got started in the first place when people believed that the universe had a significant amount of 'curvature' (in the unmeasurale fourth dimension). However, recent observations show that the universe has nearly zero curvature and it is almost flat. (A flat geometry means that the light travels in straight direction. Once leaving a point, it cannot come back to where it started from). No more hint is required to understand that a flat geometry means an infinie universe and hence the concept of balloon analogy fails.

We will discuss the correct analogy in a while. First let's clear the confusion betwen 'stretching' and 'expanding'. Nearly flat geometry of universe suggests that it is infinite. An immidiate question arises, how can an infinitely large object expand? And is followed by 'what is it expanding into'?

Current theories of the universe, however, suggest us not to frame this question like how I asked just now, although a basic answer to this question would be “the universe is not expanding into anything as there is no outer space to expand into”. Instead what happens is that the space 'stretches' itself (the one line explanation of 'expansion'). The galaxies in some sense are stationary, but with time, the space in between them stretches and the universe—a collection of galaxies—seems to be expanding. This expansion is very much like stretching a rubber sheet with both hands. A better analogy proposed by Martin Gardner in his 1962 book Relativity for the Million goes as follows.

Imagine the universe as a gigantic blob of dough. A bunch of raisins representing the galaxies are  spread throughout, embedded in the dough representing space. We are sitting somewhere inside the raisin, have no idea about the size of the dough. All we know that its size is very big. As we are far away from the edges of the dough, they have no effect on us or our view inside the dough.

Now suppose the dough is somehow put inside an oven and starts to expand. The raisins start moving away from each other, but with respect to the dough they do not move. This means the particles sitting next to a particular raisin will always remain next to it. This is where the analogy lies. The gallexies don't move, but the space in between them stretches. Here the raisins do not move, but the distance between them increases.

Distance between two galaxies

Now that we have seen the space stretching between two different galaxies, new questions arises, how do we measure the distance between two galaxies? Does the distance also increase or remainss the same?

A rather unscientific, though primary suggestion would be to take a large measuring tape, go to a distant galaxy while fixing it's one end at the our present galaxy and obtain the measurement. As time passes, the universe will expand and we shall get different measurements....One Billion light-years, Two Billion light-years, Three Billion light-years and so on.

However, with the picture of raisin and dough in our mind, we realize that the galexies do not move, so the distance between them should not change. Take the measuring tape again (also keep another similar tape in your pocket, we will need it soon) and measure the distance between these two galaxies and mark the point on the tape, say it be One Billion light-years. After a significant time period, repeat this measurement, only to find the distance to be One Billion light-years. However, here is a twist, the measuring tape also stretches due to the stretching of space. To verify this, take out the reference tape from your pocket and compare the marks. You will find that the mark on tape used in measurement is not exactly at the same point (the point at One Billion light-years) at the reference tape. In fact it is equivalent to the two billion light-years at the reference tape.

Thus we see that the concept of distance gets complicated due to expansion of universe. In fact this creates fair amount of doubt on the concept of expanding universe—whether the universe is really expanding or not. Because all what we are measuring is the stretching of the space in between galaxies. However, this answers one more question asked at the beginning of this article—how do we prove that the universe is expanding? Light while travelling between two galaxies also feels the same analogy as that of measuring tape. Due to the stretching of the space, it's wavelength gets stretched which tends its color to shift towards red end in the spectrum (first noticed by Vesto Slipher while studying the spectra of light emitted from nearby galaxies). When we look at the far away galaxies, we observed red shifted light. This not only gives the evidence of an expanding universe, but also allows us to estimate the distance between two galaxies. Hubble was first to measure the distance between galaxies and to establish that the galaxies were receding from each other at a velocity proportional to their distance. The more distant the galaxy, the greater its red-shift, and therefore the higher the velocity. This is known as Hubble's law. This is the reason we observe galaxies farther from us moving at faster speed.

Where is the center of the universe?

It is not correct to try and estimate the geometric center of the universe. We can't just say that the universe has some geometric center. There are two reasons for this. First, we are sitting 'inside' the universe, not outside. We can make no sense of the actual size and shape of the universe (Imagine we are stuck inside dough and have no idea of its outer shape. All what we see is dough). Second reason is that we are far away from the edges of the universe (or that of dough), so these edges have no effect on us at all. If the edges of our universe has no impact on us, then what difference does it make to ask what point I am currently sitting at while writing this article or even to ask the geometrical center of the universe. Absolutely none. So the 'center' of the universe contains no significance and we leave this question here itself.

However, one important fact to notice is that not everything in the universe is expanding. I am not expanding, you are not expanding and the Earth is not expanding. Two galaxies which are sufficiently close to each other, drift towards each other under the attractive pull of gravity. Even our own galaxy Milky Way is not expanding. This is because on these smaller scales (yes, theses are tiny constituent of the universe), the effect of universe's stretch is dominated by the attractive gravity of the galaxies and the stars and planets they contain. It is only at the larger scale, where the effect of stretch dominates those attractive forces and we say, “the universe is expanding”. This is the reason why the reference measuring tape kept in our pocket doesn't get stretched, while the tape used for measurement does get stretched (puzzling, yes!).

There are many more questions unanswered. What is the origin of universe? How did the expansion of universe start? What keeps it expanding? We need another lecture to find answers to these questions.