The prevailing cosmological model, supported by various lines of evidence, including the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements, is the Big Bang theory. According to this theory, the universe originated from a hot and dense state approximately 13.8 billion years ago.
However, it is important to note that the Big Bang theory describes the evolution of the observable universe from its earliest moments up to the present. It does not provide a definitive explanation for what might have occurred before the Big Bang or outside the observable universe. The concept of the "Big Bang" refers specifically to the rapid expansion and subsequent evolution of the universe that we can observe.
The nature of what may have existed prior to the Big Bang is still a topic of active scientific investigation and speculation. Some cosmological models propose the existence of a multiverse or a cyclic universe, but these ideas are currently speculative and lack direct observational evidence.
As for the idea that the Big Bang could be a local phenomenon within a larger and older universe, it is not ruled out by known physics. In fact, various cosmological models and theories, such as inflationary cosmology and eternal inflation, suggest the possibility of multiple universes or a larger-scale structure beyond our observable universe.
Regarding your second question, the application of physical laws outside their known domain of applicability is a possibility that scientists consider when exploring such speculative scenarios. However, without empirical evidence or a well-developed theoretical framework, it is difficult to make definitive conclusions about the nature of the pre-Big Bang universe or any potential physical laws that may apply in such a context.
In summary, while known physics does not rule out the possibility of the Big Bang being a local phenomenon within a larger and older universe, the nature of what might have existed before the Big Bang is still an open question in cosmology, and further research is needed to provide more insights into these intriguing possibilities.