First, we are not at the level of the standard model yet, we are just discussing cosmology!
Now, while there are efforts to create inflating universes that do not satisfy the BGV theorem, no counterexamples have been found. But, all of these attempts uses the classical spacetime, i.e. the laws of General Relativity, i.e. not quantum theory.
The interpretation of QM vary quite significantly, but importantly, the predictions and mathematical equations of every single interpretations are the same. Indeed, Quantum Field Theory (which is an application of the principles of quantum theory) is the most successful physical theory in the 20th/21st century in the sense that it has been confirmed empirically to the most significant digits of all the physical theories.
So, if indeed you want to do this:
The best theory that has been empirically verified is quantum theory.
Iâm not talking about the predictions and mathematical equations. Iâm referring to the inferences that are made from evidences of QM that may or may not reflect reality and which are beyond being directly accessed empirically to be verified and therefore not able to be relied upon with as much certainty as accessible empirically verified reality. Is there any empirical data from QM that would suggest there is no initial event of physical reality, or is it just ideas based on interpretations of QM that it could âpossiblyâ be the case.
Then Iâm not even sure what you are asking. In the end we donât have a theory of quantum gravity, so we canât say much. Neither can Vilenkin about the start of the universe as that is a quantum gravity problem.
I guess what Iâm saying is, going by empirically verified evidence of physical things having a beginning and a cause, the likelihood of the initial event of physical reality having a beginning, and a cause based on the evidence is pretty high.
By the way, I think a previous post of mine got moved to âWhat does BGV Theorem Sayâ and I was supposed to continue from there. Somehow I missed it and now realize that. So I apologize for the mistake.
I am not sure this probability can be stated rigorously, and even if it did, I would guess that itâs very dependent on priors. Regardless, this is a philosophical statement that is not supported by the BGV theorem. You are free to find support for it elsewhere.
Whatever your starting point is, may it be information, wavefunction of the universe, fields, etc, it boils down to the same thing: physics does not give you creation from true nothingness. This is really a tautological statement, as âphysicsâ itself is not nothingness.
So in QM a state of nothingness canât exist. There has to be something. Before time, energy, matter and space existed, there is just the vacuum field potential for a universe to be created of the nothingness, and it did. Pretty simple concept to understand.
All that same evidence also says that physical things are assembled from other physical things and are caused to assemble into what they are by physical forces of attraction and repulsion, so that would imply the universe must have been assembled from other physical things and caused to do so by the laws of physics.
It certainly seems highly plausible, donât you think? And for probability wouldnât you say going initially by our basic background knowledge of what we experience in reality that it would give a pretty high probability to start with? And wouldnât the BGV and the Penrose Hawking singularity theorems qualify as evidence to, if not significantly, at least marginally increase the probability? Of course this would not be a scientific, but a metaphysical probability, right?
Seems to me only if you ignore the fact that all the empirical evidence from directly observed physical events shows that physical events always have a beginning, as well as a prior cause, which would infer that the initial physical event would also have a beginning, and therefore logically could not have been assembled from existing physical matter.
It would also imply that the âlaws of physicsâ wouldnât exist since they wouldnât have anything physical to describe. Aside from that, since they are causally effete, I donât see how they would be relevant to causing a physical event in the first place. Seems to me you would have to make a highly unreasonable move to argue from that position since it wouldnât be consistent with âallâ that we know of physical events that are accessible to direct empirical confirmation.
All events that had a beginning had a beginning. But we do not have empirical evidence about events that were already underway before we started seeking evidence. Iâm not sure that we even have a clear meaning for âeventâ.
As I understand it, a physical event concerns a new instance of something physical, which, from what we know, would entail a cause and effect relationship. And what we do have is empirical evidence for all physical events that have been empirically verified to date, for which âallâ have a beginning as well as a cause.
Thatâs what we do know. Based on that it seems pretty safe to infer that these properties would apply to all physical event. There will always be things we donât know. Now Iâm not sure, but if the argument is based on what isnât known, isnât arguing from that position usually considered an argument from ignorance?
Again, I do not think it is highly plausible - I canât even start thinking of how plausible it is unless a rigorous formulation can be given. You need to give me something like P(model|data) before Iâll be convinced.
As I kept saying, the BGV (and actually also the Penrose Hawking) theorems just show that General Relativity breaks down at some point, under some conditions. Does this constitute as evidence that supports a beginning?
I am not certain, as we could have universes that does not have a beginning yet also have points where GR breaks down - after all, we know that the universe is fundamentally quantum.
Again, depends on what you mean by ânothingnessâ. Of course true philosophical ânothingnessâ is not allowed by QM, as the rules of QM itself is not ânothingnessâ. Further, you need to have a wavefunction of the universe on a superspace, a choice of Hamiltonian, etc. These again are not nothing.