String theory, as a framework that attempts to describe the fundamental nature of reality, provides some speculative insights into what might have occurred before and after the Big Bang. However, it's important to note that our current understanding of string theory is still incomplete, and many aspects of its implications for cosmology are still under active research.
According to some proposals within string theory, such as the ekpyrotic or cyclic models, the universe undergoes cycles of contraction and expansion, with each cycle beginning with a "Big Bang" and ending with a "Big Crunch." In these models, the Big Bang is not considered the absolute beginning of time but rather a transition from a previous contracting phase.
The cyclic models suggest that our observable universe is a three-dimensional brane (short for "membrane") embedded in a higher-dimensional space called the bulk. During the contracting phase of the previous cycle, two branes collide in the bulk, generating the conditions for a new expansion phase and a new "Big Bang" event.
However, it's important to note that these proposals are still highly speculative, and no concrete observational evidence currently supports them. Additionally, string theory encompasses various versions and formulations, and not all of them include cyclic or ekpyrotic scenarios.
Regarding what happened before the Big Bang, string theory doesn't offer a definitive answer. The concept of "before" implies a notion of time, but the nature of time itself in the pre-Big Bang era is not well understood within the framework of string theory. Some physicists suggest that the notion of time might have emerged only after the Big Bang, making it difficult to meaningfully discuss events "before" it.
In summary, string theory provides some speculative proposals for what may have occurred before and after the Big Bang, including cyclic models and brane collisions. However, our understanding is still incomplete, and further research is needed to develop a more comprehensive and observational foundation for these ideas.