phinds said:
I had had the same impression that you did, which was that the Big Bang Theory started immediately after the singularity but Peter told me that it is not considered to have started until after the tiny part of the first second which is the inflationary period.
More precisely, the first state of the universe that we actually have good observational evidence for is the hot, dense, rapidly expanding state that, according to our best current models, occurred at the end of inflation. This state is the one that is properly referred to as the "Big Bang" state in our actual cosmological theories--the term "Big Bang" is also used to refer to those theories themselves, the group of models that all contain the same hot, dense, rapidly expanding state and the same future development of that state into the universe we observe.
In articles for non-scientists, it is true that the term "Big Bang" is also used to refer to the "initial singularity" that appears in highly idealized models such as those that were developed in the 1920s as the first attempts to apply General Relativity to cosmology. However, practically nobody actually believes that those models are applicable before the end of inflation, i.e., before the "Big Bang" state described above. They are just notional models that are used to get across some basic ideas to non-scientists. (There are some speculative models that try to extrapolate the highly idealized models back to the Planck epoch, but AFAIK they haven't gotten any traction.)
Also, while our best current understanding is that what preceded the "Big Bang" state described above was an inflationary epoch, we actually don't know that with the same confidence that we know about the "Big Bang" state itself. It is possible that something else came before, and there are various speculative models that have been published in the literature. But they all have in common the key property that there is no "initial singularity". In inflationary models, the inflationary state is "eternal" in the past (some early inflation models weren't like this, but those have all been ruled out). In other speculative models, there are various versions of a "bounce", i.e., a previous collapse phase that got turned by some quantum process into the expanding phase we now observe. There is even a speculative model (the Hartle-Hawking "no boundary" proposal) in which the universe only has a finite extent in the past, but there is no initial singularity (quantum corrections "smooth things out" so that everything remains finite--yes, it's counterintuitive, but AFAIK nobody has challenged the mathematical consistency of this model).
In short, there is no current version of a "Big Bang theory", whether it's our best current understanding or the speculative alternatives, in which the theory starts "immediately after the singularity". The only model in which that is the case is the notional model described above, that nobody actually believes is applicable. That's why I objected to that particular phraseology.
Quantum321 said:
When ever someone disagrees with a published article and does not offer a competing publication I really don't give their opinion much credence.
Your "published article" is not a textbook or peer-reviewed paper; it is just an article for popular consumption on the NASA website. (At least, that's the "published article" you linked to in the exchange with phinds that led to this post by me.) You should not be using it as a source to find out what the Big Bang theory, the actual one used by cosmologists and described in textbooks and peer-reviewed papers, actually says.
If you want something from an actual paper, the Liddle paper that was linked to earlier has this on p. 10:
"Inflation does not replace the hot big bang theory; it is a bolt-on accessory attached at early times to improve the performance of the theory."
In other words, the inflationary epoch happened
before the events covered by the standard hot big bang theory. This is entirely consistent with what I described earlier in this post. Note also that that paper never even mentions an "initial singularity".
