How to leave the Universe If the Universe is flat and expanding at the speed of light, that must mean that we could never leave it, because one would have to travel faster than the speed of light which is not possible. However, what about going straight up or down ? The distance is less and if the Universe is not expanding in depth but only sideways then it would be possible to exit. Could we then find another Universe once we have gone out of this one ?
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When scientists say the universe is "flat," they don't mean that it's shaped like a dinner plate.  It's no easier to travel "up" than "forward."  In fact, off-planet, those terms don't even have any meaning.

What you're referring to has to do with the shape of the universe:


  When physicists describe the universe as being flat or nearly flat, they're talking geometry: how space and time are warped according to general relativity.


Think of it like this: if you draw a triangle on a balloon, is that triangle flat?  In one sense yes, because it never rises above the surface of the balloon.  Relative to the balloon, it is one-dimensional; you wouldn't be wrong to say that the image of the triangle is "flat."  But relative to the rest of the room, it is curved, because it follows the curvature of the balloon's shape.

When most people talk about the universe, they're talking about the triangle.  The discussion you're referring to, of the "flatness" of the universe, is discussing the shape of the balloon.

As for flying "out" of the universe and potentially into another, we're like an ant walking on the surface of that balloon: no matter what direction the ant walks, it'll just keep walking around the balloon.  The surface of his universe may be curved, but there's no direction he can go that takes him off of that curve.  For all he cares, it's essentially a flat surface, because he can only move in two dimensions along it.  Similarly, the geometry of our universe may be one thing or another, flat or curved, but either way that doesn't mean we can point a ship in any direction and "leave."