# Can we reach the end of space?

## Is the Universe Infinite?

Infinity leaves no traces, says Andrew H. Jaffe. That is why the cosmologist is looking for evidence of a finite universe. An interview

Since the beginning of science man has tried to understand what the world is like. We know more about this today than ever before, but infinity still holds many secrets. In the edition “The 7 great riddles of the universe”, GEO gets to the bottom of the most important questions - and to do this, it worked with seven scientists who research the limits of comprehension. One of them is cosmologist Andrew H. Jaffe, who studies the structure of the universe at Imperial College London.

GEO: Since ancient times people have wondered if the cosmos ends somewhere. How much closer have we got to the answer?

Andrew H. Jaffe: A big piece. First, we now know what is actually meant when we say that the universe is finite. Second, for the first time in history, our telescopes are so precise that we can search the measurement data for clues to a finite universe.

And what is meant by “finally”?

In any case, cosmologists do not believe that a wall of fire will come anywhere, for example. Because that would immediately raise the next question: What is it? In mathematics, a space can be finite without having a limit. Perhaps you still remember “Asteroids”, one of the first computer games. In this game you control a spaceship and shoot asteroids. When the ship arrives at the edge of the screen, it doesn't ricochet, but reappears on the other side. Our universe could be something like that.

How would it feel if we crossed this "edge"?

The crazy thing is, you wouldn't even notice. You can think of it this way: if you roll up a layer and join the ends together, you get the shape of a donut. Anyone who moves on the surface of this donut will at some point automatically come back close to the starting point - regardless of which direction they are going.

But we don't live in two dimensions!

That's right, that's why the picture is very simplified. You basically have to imagine a three-dimensional space that is connected to each other on all sides.

But I can't do that. How does that work?

Nobody can get an idea of ​​it, not even me. But we can set up the equations for different shapes, for example for the donut. We call these forms ↑ topologies.

That sounds like a math gimmick to me.

No, not at all. It is by no means a mind game without reference to reality. Because we can check our hypotheses. We can find out what topology our universe has.

How exactly do you do that?

We use a unique data source for this, a holdover from the early days of the universe: the cosmic one ↑ Background radiation. It originated around 380,000 years after the Big Bang, when the charged protons and electrons combined to form electrically neutral atoms. The light particles, the photons, were no longer constantly deflected. Since then, they have been flying freely through the universe. At the beginning of this decade, the Planck space telescope recorded the background radiation very precisely. It took the oldest picture of the universe, so to speak.

What are you looking for in the picture?

Even then, matter in the universe was not evenly distributed. In some places it was denser than in others, lighter cosmic clumps had formed. If the universe is finite, then we would have to see these lumps several times in our picture. Do you understand?

No, you have to explain that to me.

Think back to the donut! The photons that came from a bright lump flew in all directions. So you would have to reach us today in different ways. We should therefore be able to see the same lump in different places in the picture. I cannot see such patterns in the background radiation with the naked eye, but a computer can detect them in the data. From this he can even calculate the exact shape of the universe.

Have you found what you are looking for?

Unfortunately not yet. There seemed to be a breakthrough 14 years ago. At that time, scientists from France and the USA came across a certain shape in older data for background radiation: a twelve-sided body. With my research team, however, I have shown that this shape only matches part of the data, not all.

Will we ever know what our universe is like?

We will measure the background radiation more precisely and we will improve our analyzes. But maybe we will never solve the riddle. Because we have a fundamental problem: We can only search our data for indications of a finite universe. Infinity leaves no traces.

When man realized that the earth was a sphere, his view of the world changed radically. What is the difference for us whether the universe is finite or infinite?

It doesn't matter for space travel. Because we would never go to the frontiers; they are far too far away. But the answer helps to understand our place in the universe. If it really should be infinite and look the same in all directions, then this conversation would take place infinitely often at that very moment. This idea is crazy.

### ↑ Glossary

topology The topology divides areas and spaces into different classes depending on their linkage properties. For example, spheres, cylinders and cuboids belong to the same class: they can be transformed into one another by bending and deforming. A torus that has a hole in the middle, on the other hand, belongs to a different class.

Background radiation A weak radiation that originates from the heat of the Big Bang and has since spread throughout the universe. It is in the range of microwaves and provides a picture of the cosmos around 380,000 years after the Big Bang. Until then, protons and electrons were a thick soup in which the photons constantly collided with particles. Only when the electrons were bound to protons could the light move freely.

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