How is expansion manifested physically?

[PeterDonis]

Is the visible universe being pushed from the inside, or pulled from the outside?

Neither. It is expanding because of inertia; no push or pull is required.

(Strictly speaking, the acceleration of the expansion due to dark energy can be thought of as a small "push" that is added on to the inertia I just described; but that "push" is exerted equally everywhere so it can't be restricted to "inside" or "outside" the visible universe.)

There are many alternative theories for the origin of the universe, some of which support 'pulled from the outside'.

Do you have any references?

are you suggesting that the mysteries of dark energy have been solved?

We don't have to "solve all the mysteries" of dark energy to know how it affects the expansion of the universe. We just have to know how it appears in the equations that describe the dynamics of the expansion, and we do--it's a cosmological constant, with a value that we can, and have, measured.

 

[Chronos]

The concept of 'outside' the observable universe is problematic. Such a region would not be causally connected to the observable universe, thus could not exert causal effects on the observable universe, therefore the idea of 'pulling' from the 'outside' is unphysical

 

[Drakkith]

Our current understanding is based on a theory that has mislaid more than 95% of the mass/energy in the universe, cannot explain the anisotropy of stellar phenomena, has no explanation for the anomalous orbital speeds of the outer stars of spiral galaxies, does not allow for the unification of GR and QM, etc...

Several of these claims are unjustified. The universe is isotropic at the largest scales. It is only on the smaller scales that anisotropy can be seen and this is easily explained by gravity acting on the varying density of matter that arose from quantum fluctuations in the very early universe.

The big bang theory has no role in our ability to unify GR and QM. Any theory that unified the two would be used to further expand the big bang theory's predictions.

Given the many inconsistencies and anomalies that are extant in our current understanding, should we not be questioning the fundamentals of that understanding?

We are always searching for new fundamentals, so your question is moot.

 

[Finny]

Ash: You are on the right track judging from your posts so far.

Regarding 'proper distance' and other cosmological measures.

It's useful to keep in mind that the the best fit cosmological model (the 'standard' one everybody uses) is called Lambda CDM [LCDM] cosmological model. If you use this model, and that's the standard in these forums, then 'distances' and 'times' can be compared with others without the ambiguity of unconstrained general relativity by using the specific assumptions in the model.

The LCDM model is the fine-tuned version of the general FLRW [Friedman...et al ] where the parameters are chosen to get the best possible fit to our universe—that is, to match observational data.

That standard cosmological model uses some standard assumptions, like homogeneity and isotropy, already mentioned in previous posts, and standard inputs to provide answers which people calculating with it can use to compare results and discuss 'distances' and 'age of the universe' without unnecessary confusion. For example, as noted by Powell, the single frame of reference is agreed upon the be that of the CMB...being at rest at each end of the 'distance' with respect to local cosmic microwave background in both places. Oddly, it seems, when you first hear this, because who measures 'distances' here on earth, for example, with both ends moving relative to each other.

When you read things such as: " The present is year 13.7 billion of the expansion of the universe and we are receiving CMB from hot matter that was 42 million light years from us when emitted ..." you are using the standard conventions of the standard model. If you are not at rest with respect to the CMB, time and distance will not be the same.

Regarding the balloon analogy: just don't make any analogy between the rubber material and empty space...focus on the dots changing distance as the radius expands. Nor can you make an analogy that if you decrease the radius of the balloon to zero everything started from a single point in space. That's not part of the analogy. It's an analogy not a perfect model over all conditions.

And that reminds me, neither is the standard cosmological model a perfect model over all conditions...Assuming homogeneity and isotropy enables solving the Einstein field equations; the model starts after inflationary expansion concludes...so we have two models 'glued' together at the front end. And before that,as far as I know, nobody has a generally agreed upon model for the big bang itself.