What is the Location of the Universe’s Center?

Published in 1915 and already widely accepted worldwide by physicists and mathematicians, this theory assumed that the universe was static, that is to say immutable and motionless. In short, Einstein thought that today's universe was the same size and the same shape as in the past.

Then, when astronomers observed distant galaxies in the night sky using powerful telescopes, they found that the universe was far from immutable. On the contrary, new observations suggested that the universe was expanding.

Scientists quickly realized that Einstein's theory did not stipulate that the universe should be static – it could completely describe an expanding universe. Using mathematical tools that those provided by Einstein theory, scientists created new models, which showed that the universe was dynamic and evolving.

I spent decades trying to understand general relativity, including in my current work as a physics teacher, since I give lessons on the subject. I know that the idea of a perpetual expansion universe may seem intimidating, and that part of the conceptual challenge that we are all confronted with here is to overcome our intuition from the way things work.

For example, if it is difficult to imagine that something as big as the universe has no center … Physics says it is reality.

Space between galaxies

First of all, let's define what we mean by “expansion”. On earth, “expansion” means that something is growing. And with regard to the universe, it is true, in a way.

Expansion can also mean that “everything moves away from us”, which is also true with regard to the universe. Just point a telescope on distant galaxies to find that they all seem to move away from us. In addition, the more distant they are, the more they seem to move quickly. And they also seem to move away from each other.

It is therefore more correct to say that everything that exists in the universe moves away from everything else, at the same time.

This idea is subtle but essential. It is easy to imagine the creation of the universe as a fireworks that explodes: it starts with a big bang, then all the galaxies of the universe fly away in all directions from a central point.

But this analogy is not correct. Not only does it wrongly imply that the expansion of the universe began from a unique point, which is not the case, but it also suggests that it is the galaxies that move, which is not entirely exact either.

It is not so much the galaxies that move away from each other: it is the space between the galaxies, the fabric of the universe itself, which grows as time passes.

In other words, it is not really the galaxies themselves that move into the universe; Rather, it is the universe itself that carries them further as it stretches.

A current analogy is to imagine that we stick points on the surface of a balloon. When you instill air in the balloon, it expands. As the points are glued to the surface of the ball, they move away from each other. Although they seem to move, the points are in fact exactly where you have placed them, and the distance between them is simply growing due to the expansion of the ball.

Imagine now that the points are galaxies and that the ball is the fabric of the universe, and you will begin to see what it is about.

Unfortunately, if this analogy is a good start, it also does not allow to understand the details.

The fourth dimension

For any analogy, it is important to understand its limits. Some faults are obvious: a ball is small enough to hold in your hand, which is not the case with the universe. Another defect is more subtle. The ball is made up of two parts: its latex surface and its interior filled with air.

These two parts of the ball are described differently in mathematical language. The surface of the ball is two -dimensional. If you walk on it, you can move forward, rear, left or right, but you cannot move up or bottom without leaving the surface.

You would think that we are named four directions here – front, rear, left and right -, but it is only movements along two basic axes: from one side to the other and from front to back. These two axes, length and width, make the surface two -dimensional.

The interior of the ball, on the other hand, is three -dimensional. You can therefore move freely in all directions: length and width, but also up or down – which constitutes a third axis, the height.

This is where confusion resides. What we consider to be the “center” of the ball is a point somewhere inside the ball, in the space filled with air which is below the surface.

But in this analogy, the universe is more like the latex surface of the ball. The interior of the balloon, filled with air, has no equivalent in our universe, and we cannot therefore use this part of the analogy – only the surface counts.

Ask “where is the center of the universe?” “, Is that a bit like asking” where is the center of the ball surface? »Just there isn't. You could travel along the ball surface in any direction, as long as you want, and you would never reach a place that you could call its center because you would never leave the surface.

In the same way, you could travel in any direction in the universe and you would never find its center, because, just like the surface of the ball, it simply does not have one.

If this can be so difficult to understand, it is partly because of the way the universe is described in mathematical language. The surface of the balloon has two dimensions, and the interior of the ball has three, but the universe exists in four dimensions. Because it is not only about how things move in space, but also how they move in time.

Our brain is designed to think about space and time separately. But in the universe, space and time are nested in a single fabric, called “space-time”. This unification modifies the functioning of the universe, in relation to what our intuition provides.

And this explanation does not even answer the question of how something can be in infinite expansion – scientists always try to understand what is at the origin of this expansion.

By questioning ourselves on the center of the universe, we therefore come up against the limits of our intuition. The answer we find (everything is expanding everywhere and at the same time) gives us an overview of the strangeness and beauty of our universe.