How realistic is can be checked by observations. Ok, you have the Cyclic model, in which there are two branes colliding cyclically. One of the assumptions of the Cyclic model is that the density perturbations that will produce the large scale structure observed nowadays are produced before the collision between the branes, that is before what we can call the Big Bang. If you prefere the Standard Inflationary scenario, the density perturbations are produced after the Big Bang. Is possible to detect if the density perturbations were created after or before the Big Bang by searching for tensor fluctuations, non-gaussianity and non-adiabaticityand how realistic this theory really is...
String theory is a theoretical framework in physics that attempts to reconcile the two major theories of modern physics - general relativity and quantum mechanics. It proposes that the fundamental building blocks of the universe are not particles, but tiny vibrating strings.
String theory has the potential to explain some of the biggest mysteries of the universe, such as the nature of gravity and the origin of the universe. It also suggests the existence of extra dimensions beyond the three we can observe.
Membranes, also known as branes, are objects that can vibrate in string theory. They can have different numbers of dimensions, such as 0-dimensional point particles, 1-dimensional strings, and 2-dimensional membranes.
String theory proposes that the Big Bang was not a singularity, but rather a violent collision between two branes in a higher-dimensional space. This collision caused the expansion of our universe and the creation of matter and energy.
Currently, there is no direct evidence for string theory. However, it has been shown to mathematically unify general relativity and quantum mechanics, and it also predicts the existence of particles that have not yet been observed, such as gravitons and supersymmetric particles.