Brian Greene | What Was There Before The Big Bang?

The Big Bang explains the evolution of the universe from a starting density and temperature that is currently well beyond humanity's capability to replicate. Thus the most extreme conditions and earliest times of the universe are speculative and any explanation for what caused the big bang should be taken with a grain of salt. Nevertheless that shouldn't stop us to ask questions like what was there before the big bang.

Brian Greene mentions the possibility that time itself may have originated with the birth of the cosmos about 13.8 billion years ago.

To understand how the Universe came to be, scientists combine mathematical models with observations and develop workable theories which explain the evolution of the cosmos. The Big Bang theory, which is built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time. 

However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, which is the earliest period of time in the history of the universe, and correcting this will require the development of a correct treatment of quantum gravity.

Certain quantum gravity treatments imply that time itself could be an emergent property. Which leads some physicists to conclude that time did not exist before the Big Bang. While others are open to the possibility of time preceding the big bang.

One of the common misconceptions about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.