The idea that the entropy of the universe was extremely low or even zero at the time of the Big Bang is based on our current understanding of the laws of thermodynamics and the observed properties of the universe.
Entropy is a measure of the disorder or randomness in a system. In thermodynamics, the second law of thermodynamics states that the entropy of a closed system tends to increase or remain constant over time. This means that in the absence of any external influence, the entropy of a system will naturally increase or stay the same.
At the time of the Big Bang, the universe is believed to have been in an extremely hot and dense state. According to our current understanding, all matter and energy were concentrated in a singularity, which is a point of infinite density. The laws of physics, as we know them, break down at this singularity, and our current theories cannot precisely describe the state of the universe at that exact moment.
However, as we trace the evolution of the universe back in time, based on the observations and theoretical models, we can infer that the universe was in a highly ordered and low-entropy state close to the time of the Big Bang. This inference is supported by the fact that the universe has been expanding and cooling since the Big Bang, which is consistent with the second law of thermodynamics.
The idea of a low-entropy state at the beginning of the universe is also supported by the concept of inflation. Inflation is a theory that proposes a brief period of exponential expansion in the early universe, which helps explain certain observations like the uniformity of the cosmic microwave background radiation. During inflation, the universe would have undergone a rapid expansion, smoothing out irregularities and increasing its size, which would have resulted in a decrease in entropy.
It's important to note that the concept of entropy in the context of the Big Bang is still an active area of research, and there are ongoing debates and investigations into the precise nature of the initial conditions. Our current understanding is based on theoretical models and observations, but further advancements in our understanding of quantum gravity and fundamental physics may lead to refinements or revisions in our understanding of the early universe and its entropy.