The visible universe could be a four-dimensional membrane, or brane for short, moving through unseen dimensions. Other branes, or parallel universes may also exist. floating through a fifth dimension that we cannot perceive.
Some scientists propose that our would-be universe existed as a brane into which another brane collided in a uniform fashion, generating the heat energy and expansion accounted for in the big bang, but these theories now exist mainly as mathematical models that require much additional investigation.
Types of Brane
In string theory, branes are fundamental objects that exist in a specific number of spatial dimensions. These are the higher dimentional objects in string theory with dimension from zero (points) to nine. In terms of branes, what we call a membrane we would be a two brane. The “p” in p-brane stands for the number of dimensions that brane has. For example, a string is a 0-brane, a membrane is a 2-brane, and we could live on a 3-brane.
These are actually dynamical objects which have fluctuation can move around for example they intract with gravity. D-branes provide a simple description of various nonperturbative objects required by string duality, and give new insight into the quantum mechanics of black holes and the nature of spacetime at the shortest distances.
The Brane Multiverse
“An intensely hot, thoroughly dense environment in which particles stream this way and that sounds much like the conditions just after the big bang. Perhaps, then, when two branes collide they wipe out whatever structures may have coalesced during either of their histories, from galaxies to planets to people, while setting the stage for a cosmic rebirth. Indeed, a three-brane filled with a blistering plasma of particles and radiation responds just as an ordinary three-dimensional spatial expanse would: it expands. And as it does, the environment cools, allowing particles to clump, ultimately yielding the next generation of stars and galaxies. Some have suggested that an apt name for this reprocessing of universes would be the big splat…
… As the branes fall back together, each builds up speed, they collide, and through the ensuing firestorm the conditions on each brane are reset once again, initiating a new era of cosmological evolution. The essence of this cosmology thus involves worlds that repeatedly cycle through time, generating a new variety of parallel universes called the Cyclic Multiverse. If we are living on a brane in the Cyclic Multiverse, the other member universes (in addition to the partner brane with which we periodically collide) are in our past and future. Steinhardt and his co-workers estimated the time scale for a full cycle of the colliding cosmic tango— birth, evolution, and death— and came up with about a trillion years. In this scenario, the universe as we know it would merely be the latest in a temporal series, some of which may have contained intelligent life and the culture they created, but are now long ago extinguished. In due course, all of our contributions and those of any other life-forms our universe supports would be similarly erased.
… Part of the appeal of a cyclical cosmology is its apparent ability to avoid the knotty issue of how the universe began. If the universe goes through cycle after cycle, and if the cycles have always happened (and perhaps always will), then the problem of an ultimate beginning is sidestepped. Each cycle has its own beginning, but the theory provides a concrete physical cause: the termination of the previous cycle. And if you ask about the beginning of the entire cycle of universes, the answer is simply that there was no such beginning, because the cycles have been repeating for eternity.
~ Brian Greene. “The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos”.