Hi. I don't know much about physics but I'm very interested. This question perhaps belongs in the borderland between philosophy and physics.
What does physics/theoretical physics have to say on the nature of the first ever periodic oscillation? What exactly oscillated and against what? In what way would it have been periodic given that nothing oscillated before that?
I understand this isn't the type of question that seeks a definitive answer so I'd settle for your thoughts on the subject. Thank you very much for this service.
> the nature of the first ever periodic oscillation?
> What exactly oscillated and against what?
First thing to understand is how science works: specifically, that before anyone can say FOR CERTAIN that something happened, there must be experimental, observable evidence to support the idea. When observations to test a hypothesis are unavailable or ambiguous, we can not say what happened exactly.
For example, we know that there is (almost) infinitely more matter than anti-matter in our Universe -- this is an observed fact. However, we can't say WHY this is the case. There are various ideas on why, in the first 10^-12 of a second of our Universe, matter became dominant. However, no experiment has (as of yet) allowed us to sort out which of these hypotheses has more merit. Thus, we can't say exactly WHEN anti-matter became such a small fraction of our Universe, because we can't say WHAT caused this to happen.
Similarly, we can't say exactly when the first periodic interaction occurred in our Universe, because a lot of the early interactions are speculative at best. We CAN say, "During this time period, early in our Universe's expansion, oscillations did occur", and have a quite good observational evidence to back up that claim. However, we eventually reach a point where we are forced to say, "There MAY have been oscillations going on earlier than this point, but we can't be certain at this point."
With that in mind, the earliest time that oscillations must have been occurring was during the Hadron Epoch, simply because the existence of hadrons requires that quarks have joined together under the influence of the strong force, and are thus undergoing oscillations in their color. A schematic of this change in quark color is shown in this animation
These oscillations occur at a very rapid pace, but I have not been able to find the frequency.
> In what way would it have been periodic given that nothing oscillated before that?
As I noted before, there MAY have been oscillations before this, or they may not have been. With the experimental evidence we have now, we can't say either way.
However, if oscillations of quarks within the hadrons, just formed in the first fraction of a second after the end of Inflation, were (indeed) the first oscillations, they would have started simply because of the nature of quark gluon interactions. Given a Universe that (like ours) has that kind of interactions, then the joining of quarks into hadrons will inevitably result in quark oscillations.