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The Observable Universe is generally said to contain all space that could "in principle" have had a causal impact on Earth, but the exact limits of the "in principle" causal interaction go unspecified. Wikipedia notes some astrophysicists distinguish the Visible Universe, all space that was in our past light cone at recombination, from a broader Observable Universe, all space that was in our past light cone at the end of the inflationary epoch. While obviously the first definition has more practical importance in cosmology, the latter seems to be much truer to the meaning of "in principle".

Is this latter definition known to be "final"? In other words, are there theoretical reasons to believe that there actually was no causal influence between the OU and its neighboring regions of space during the inflationary epoch, or that any such interactions had no causal impact on post-inflationary dynamics? If so, what are they?

Intuitively, it seems like if you define $OU(t)$ as the Earth-centered ball of present-day space that was in our past light cone $t$ seconds after the Big Bang, the size of this ball grows without limit as $t$ approaches 0. This implies the entire Universe, even if it is infinitely large, is "in principle" causally connected to Earth. At what point does this intuition go wrong?

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I will address this:

In other words, are there theoretical reasons to believe that there actually was no causal influence between the OU and its neighboring regions of space during the inflationary epoch, or that any such interactions had no causal impact on post-inflationary dynamics.

The inflationary period was invented because of the great uniformity of the Cosmic Microwave Background radiation, which shows the radiation detected here, which happened, in the present model, at about 380000 years after the Big Bang at the decoupling of photons from masses . It fits the black body radiation curve better than any laboratory measurements have shown for matter, and this implies that a thermodynamic process homogenized the original soup before the decoupling of photons. The problem is that at that time, there were regions in the universe which could not communicate thermodynamically with the other regions, due to being in different regions of the light cone.

This basic discrepancy in the BB model was resolved by introducing quantum mechanics in the beginning of time for the BB evolution. Quantum mechanics with its probabilistic solutions is not constrained by light cone considerations and the inflaton particle beats up the energy content at times before 10^-32 seconds and homogenizes it, the quantum mechanical very small inhomogeneities from the inherent probabilistic nature become the seeds of the tiny inhomogeneities observed in the CMB and the later generated clusters of galaxies etc.

So there exists a causal connection from the inflationary period to the present universe, the seeds of an homogeneity from it. The causal impact happened at times before 10^-32 seconds. After 380000 years the galaxies started forming in their separate causal regions

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  • $\begingroup$ As you are correctly pointing out, all of this only matters if the beginning of this universe is actually a quantum mechanical process. If we modify general relativity very slightly then all of this goes into the wastebasket very quickly because we never ever get close to the quantum regime and the universe is already homogenized at the time of $T_{cosmological}\approx 0$. $\endgroup$ – CuriousOne Feb 12 '16 at 7:29
  • $\begingroup$ @CuriousOne well, quantum uncertainties give also the seeds of galaxies reasonably. Why are you averse to a quantization of gravity bringing it in smoothly and unifying all the forces ? $\endgroup$ – anna v Feb 12 '16 at 7:39
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    $\begingroup$ I am not averse to anything, including "not flavor of the day" models. Let's not pretend that there is a quantum theory of gravity. We both know there isn't and it's certainly not needed to homogenize the universe. Einstein-Cartan can do it nicely. What's the difference between Einstein and Einstain-Cartan? The same as between Einstein with and without cosmological constant.. an "Ooops... maybe I should have left that in there, after all." $\endgroup$ – CuriousOne Feb 12 '16 at 8:01

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