I was recently participating in an online Q&A session for theoretical physicists, and I was intrigued that people still believe that there is a flaw in the Big Bang model, as it needs to conserve energy and doesn't. Even worse was the realization that many of the people with this false belief are trained in physics and really should know better!

The simple fact is, the Big Bang might or might not conserve energy. And even the best theories we have and the best experimental data from both particle physics and astrophysics cannot answer this simple question. However we also know that the Big Bang is not expected to conserve energy, and there is nothing wrong with that.

First let me explain why energy is conserved at all.

All of physics, both classical and quantum, is derived from a mathematical object called the action. In classical physics, the laws of nature will always act to minimize the action. All equations of classical physics can be derived from this one requirement. Quantum physics is similar, except instead of the laws of nature always minimizing the action we find a very small probability that the action is slightly higher than the minimum. That is all there is to physics - the action is either a minimum or there is a probability distribution around the minimum.

This then leads to one of the most beautiful theories in modern physics, Noether's Theorem. According to Noether's theorem, each and every symmetry of the action will generate a conservation law that is observed in nature. If the action is the same at each point in space, then momentum is conserved. And if the action is the same when the coordinates are rotated, then angular momentum is conserved. And if the action does not change over time, which means that the the laws of nature are constant, then energy will be conserved. (It is important here to understand that this means the laws of physics are constant, but individual particles or objects can move and change over time so long as the laws governing their motion and transformation do not change.)

And so energy is conserved if and only if the laws of physics do not change over time. This leads us to consider two versions of the Big Bang.

The first possibility is that the Big Bang was the start of our Universe, but that the laws of physics existed before. Maybe the Universe expands and contracts repeatedly, or maybe our Universe is a bubble in a bigger Universe, or maybe the Universe exists as a an object in a much larger Multiverse that contains many Universes. In each of those cases, the laws of physics exist outside of the Universe and existed before the Big Bang. And in each of those cases, energy is conserved because the Big Bang draws its energy from something older and larger. So energy is conserved.

The second possibility, which is perhaps more interesting when discussing energy conservation, is that the laws of physics began when the Universe did. Time did not exist before the Big Bang, and neither did the laws of nature. By definition, if something did not exist before the Big Bang and then came into existence, it is not constant. The laws of physics started and evolved in the earliest moments of the Universe. And that means that according to Noether's Theorem, energy is not going to be conserved. There is no problem with conservation of energy because energy is not expected to be conserved at the moment of the Big Bang.

And so there it is in a nutshell. The laws of physics either exist outside of our Universe, and energy is conserved at the Big Bang, or the laws of physics exist only inside our Universe and energy is not expected to be conserved at the Big Bang anyway. Either way, there is no problem with energy conservation at the moment of the Big Bang.