FlatEarth":ixhr4ffm said:
1) The Big Band Theory is about our universe in total and not about a portion of an even larger universe. Such a universe is not predicted by the BB. That's an adjunct to the theory and is really just speculation (Sorry, Michio Kaku.) That is why I thought you were talking about the only theory I know of that predicts a bigger universe: String Theory.
Sorry, but I think we are talking across each other here, referring to different concepts. I am not referring to any kind of universe that is not our own. All I am referring to are the parts of the universe we have never received a photon from.
No evidence exists to suggest that the boundary of the observable universe represents the size of the physical universe. If the physical universe has a boundary (which most doubt), and that boundary was at the edge of our observable universe, it would imply that the Earth is actually at the centre of the universe, in violation of the cosmological principle.
Consider the CMBR. These CMBR photons were released throughout the universe, around 400,000 years after the Big Bang. We detect them today, and as all photons travel at c, we assume the CMBR photons we currently detect in all directions were originally emitted at the same distance in all directions. Their emission points form a conceptual sphere around us, known as the
particle horizon.
The Big Bang theory states that the CMBR photons we currently detect were originally emitted only around 40 million light-years away, and the place that they were emitted from has receded with the expansion of the universe to a current distance of around 46 billion light years away. This is what represents the edge of our observable universe.
We assume that CMBR photons were released around here too, and those photons would now be 46 billion light-years away. So, just as there are galaxies around here but 13.7 billion years ago CMBR photons were emitted here, we assume there now are galaxies 46 billion light-years away in the region of the universe where the CMBR photons we detect were originally emitted from.
Now then, as the CMBR was released throughout the universe, we can assume that CMBR photons have been hitting the Earth throughout history. We can also assume that we will continue to detect CMBR photons in the future, photons that were originally emitted further away than the CMBR we currently detect.
So, unless we suddenly stop detecting the CMBR, we can assume that there are now galaxies further away than 46 billion light-years. Only if we suddenly stop detecting the CMBR, we will know the universe is finite and that we have detected photons from every part. Until then, we can assume we will keep on detecting the CMBR, which means the universe is currently larger than our observable part of it.
Cosmic Inflation (which is part of the mainstream Big Bang model) estimates the size of the physical universe to be at least 10^23 times the size of the observable universe.
This is all mainstream Lambda-CDM cosmology, nothing to do with string theory or the parallel universes that Kaku posits.