what does the boundary of the universe bubbles made of?

[Secret]

1. According to string theory?
2. According to some theories which contains the concept of baby universes?

[PWrong]

1. The bubbles are probably branes. A brane is like a generalised string. Instead of a 1-dimensional string it can be any dimension. In maths it's called a manifold. You can have, say, 1-branes whose ends are stuck to a 3-brane and move around, or the 1-brane could be contained in the 3-brane, or whatever. Our universe is an 11-brane, with 3 large spacial dimensions, 6 small curled up dimensions, 1 even smaller straight dimension, and 1 time dimension. It possibly exists in a higher dimensional brane that might contain lots of universes. As for the boundary of the bubbles, I'm not certain they have a boundary, but if they do the boundaries would probably also be branes.

2. No idea. Whatever the fabric of spacetime is I guess.

[steelpillow]

String theory is not concerned with the boundary of the universe.

Other theories give all kinds of answers - usually they say there is no spatial boundary at all. Think of an ant on a ball. The ant can crawl all over the ball and never find a boundary, because there isn't one.

Usually, theories suggest boundaries in time rather than in space: the Big Bang was a boundary in time. Some theories predict a Big Crunch at the end of time, others a Big Rip as space tears itself apart into nothingness.

Stephen Hawking once proposed something called complex time, where time folds round similar to a ball so there is no boundary at all.

Penrose's latest idea, called CFC (conformal cyclic cosmology), is that the universe bounces from one bubble to the next with no boundary in time either - just an eternal sequence of bounces.

But there is a boundary to what we can see. This is sometimes called the cosmological event horizon. It is a giant ball. We do not know what lies beyond it, though it may be possible to figure that out from its effect on bits we can see.