I would like to start by thanking those people who praised the series of articles that I wrote about what is impossible and what is not in the world of modern physics. Those articles were great fun to write, and I am pleased that others enjoyed them as well. This sort of excitement about scientific research is important for the future of our society.

Now on to today's commentary, regarding how the conservation of energy could be compatible with the big bang model of cosmology. A few people have asked me about this issue, as there are apparently several websites and regrettably even a handful of college instructors who seem to believe that one of these two theories must be false. In fact there are at least four main methods of resolving this apparent but non-existent conflict.

The accepted view of how our Universe began is that approximately 13.7 billion years ago it was very tiny - smaller than a single atom. (As yet experiments haven't been able to test the fraction of a second further back to the exact moment of the Big Bang). All matter in existence formed at this moment in time - time itself begins at this moment as well. At this moment the laws of physics are believed to have begun, and the Universe has been expanding and cooling ever since.

However as every high school physics student knows, energy is always conserved in an isolated system. There is no known method in which energy can be created out of nothing. It might be converted from nuclear to solar to electrical to chemical to mechanical, but the total energy always remains constant.

So how can all matter and energy suddenly appear during the Big Bang when there was nothing before? Some people claim that this disproves the Big Bang model, but they are wrong. There are in fact numerous possible resolutions of this conflict, but for now I will review only the top four.

1. Random Inflation: The first option is that our visible Universe is not the same as the entire Universe. There are several models of this (which is why I used the generic term 'random inflation') but in each there exists an eternal Universe in which random fields appear that cause a small region to suddenly inflate into a much larger area. Our visible Universe would then have formed when a large amount of energy in a small bubble suddenly inflated billions of years ago into everything we now see. Since we cannot see beyond the edge of this bubble, we think that everything formed at this time. In these models energy comes from the eternal Universe and is therefore conserved.

2. The Multiverse: Along the same lines as the previous method, it is possible that our four-dimensional Universe exists as a membrane floating through a higher dimensional multiverse. These membrane Universes could exchange energy with each other or with the multiverse, so no energy is produced or lost even though individual observers trapped on each membrane think this has happened. The Big Bang could have been simply a collision of two such brane-worlds (often referred to as the Ekpyrotic Model) and as such the Universe gained its energy from other Universes.

3. Pre-Big Bang Models: Admittedly these are not very popular right now, but they are interesting. As I wrote above, experiments have only been able to measure the effects of the Big Bang back to a fraction of a second, and so it is possible that the laws of physics actually change and there is no actual matter-from-nothing. The simplest of these is the cyclic Universe, in which the Universe collapses down to a sub-atomic scale and then bounces back and grows again. This process repeats eternally, (although there is a giant flaw in the theory as each Universe must be hotter than the previous one, and if it happened eternally we would be far too hot to generate matter). It is possible that something happens to allow a collapsing Universe to form a new one, and energy is conserved as it is transferred from one to the other.

4. Energy is Not Conserved Anyway!: This is no doubt a shock to many students of physics at all levels, but in fact energy is only conserved when the laws of physics do not change. This is part of Noether's theorem, which states that conservation laws correspond to symmetries of nature. If an experiment yields the same results when started at different times, then the experimental system conserves energy. Since the standard Big Bang model includes the start of time and the start of the laws of physics, this time shift symmetry is broken and therefore energy is not conserved at the moment of the Big Bang. It isn't that the conservation of energy has been disproven, but rather that it was never meant to be applied to systems in which the fundamental laws of physics are changed. So even if none of the first three theories are correct, the Big Bang doesn't require energy to be conserved anyway!