Is the universe infinite?

Nowadays we know more about the universe than ever before. For example the fact that the universe’s expansion is acceleration. This is strange because as we’ve learned Einstein’s laws say that every object attracts every other object in the universe, so all this gravitational force should slow down the expansion. In spite of everything we do know, there are also subject we are not yet certain about, for example what is the shape of the universe? There are three mayor theories about this question, which don’t agree of the fact whether the universe is infinite or if there is a border at some point.

The first theory states the universe is positively curved. In this case the universe can be seen as a soccer ball. If we look at this  universe in 2D then we can state that this universe has no edges, you can go up or down and left or right without ever running into any sort of wall stopping you. In this case space would be finite because there is no way of escaping the surface of the ball. Think about it, it’s possible to go up, down, left or right but it would not make any sense to go through the surface to go inside the ball. This means that going in a straight line until we’ve gone ‘’around’’ the universe we would end up on the exact same spot making the universe finite. The last surprising fact about this universe is that when two parallel lines are drawn, they will always intersect at some point. For example, the longitude lines on Earth intersect at the north and the south Poles (just like the lines in the picture)

Contradicting the first theory, the universe could also be negatively curved. This second theory is basically the exact opposite of the first theory: the universe can be seen as a hollow saddle instead of a sphere. The fact that the universe is saddle shaped means that it is infinite. It is possible to move forward, backward, left or right but the object can keep going for as long as it wants because there will be no point at which it returns to the place it originally came from. One last thing we can conclude from this is the fact that two parallel lines will separate more and more, infinitely so that the distance between the two will become increasingly big.

Luckily I’ve left the most simple theory for the end. This theory states that the universe is flat and non-curved, it can be seen as a flat sheet of paper, but infinite. The sheet of paper would expand infinitely in all dimensions. Again, it is infinite because when moving in a straight line in any direction there will be no point at which you return to the starting point. To compare this universe with the others, two parallel lines will not separate nor will the intersect, they will stay parallel infinitely, meaning the distance between them will never change.

Up to this moment scientists are not sure about the shape and size of the universe, currently they think the most likely theory is…   the last theory in which the universe would be flat and infinite. However, they haven’t found enough evidence to confirm this theory is correct, and I don't think they ever will. If you don’t agree with scientists and you think that the universe is positively or negatively curved (or you just like it better), let me know your thoughts in the comments.

NOTE: The triangles in each of the images are to show the effect of the curves on lines once more: on a flat surface a triangle’s corners added up always equals 180°, on a positively curved surface a triangle’s corners will add up to more than 180°, and finally on a negatively curved surface a triangles corners, when added up will equal less than 180°