The concept of time becomes somewhat complex when discussing the origin of the universe, particularly the event known as the Big Bang. According to current scientific understanding, the Big Bang theory suggests that the universe began as a singularity—a state of infinite density and temperature—approximately 13.8 billion years ago. However, it's important to note that our understanding of what happened at or before the moment of the Big Bang is still an active area of research, and there are various theoretical models and hypotheses.
Regarding the notion of time, the prevailing scientific view is that time, as we understand it within the framework of general relativity, also began with the Big Bang. The standard Big Bang model, based on Einstein's theory of general relativity, describes the expansion of the universe from a singular point. However, the mathematical equations in this model don't extend back to the exact moment of the singularity, which is often referred to as "time zero."
In a cosmological context, time is closely intertwined with the fabric of space itself, forming what is called "spacetime." Therefore, it's difficult to speak of time in isolation from space. The current understanding is that the Big Bang marked the beginning of both space and time, and the subsequent expansion of the universe is what we perceive as the passage of time.
However, it's worth noting that our understanding of the very early universe, including what happened at the moment of the Big Bang, is limited due to the extreme conditions involved. These conditions are beyond the scope of our current scientific knowledge, and some of the fundamental questions about the nature of time and the origins of the universe still remain unanswered. Researchers continue to explore these questions through various theoretical models and experiments, such as those involving quantum gravity and string theory, in an effort to further our understanding of the early universe and the concept of time itself.