Accordingly to the Big Bang cosmology, the last scattering surface is a 2dim (topological) sphere S2. The sphere is a TWO SIDED surface. On the inward side of the last scattering surface was, and after 14 billion years of inflation, is the universe.
What was on the outward side of the last scattering surface then and what is on the outward side of the sphere of 14 billion years radius?
Everything that could have been observed of the space-time region where recombination takes place by a hypothetical 13. 7 * 10^9 years old com-moving observer, i.e. everything of the recombination era that is in causal contact with us. Lame ascii art of a space time diagram (i.e. 1 space and 1 time dimension) that I treid to get inline, but failed in a post below.
The "surfaces" of last scattering are the intersection of the light cone and the era of recombination.
You did not get my question. Let put it more clear. What was on the other side of the last scattering surface. Inside was early universe. What was outside??? Other words what was that "something" into which the Big Bang universe expands???
@J. Breski,
Oh sorry, yes I see I misread your question. One can only speculate, but we are seeing a new part of the universe everyday and it is therefore only natural to assume that there _is_ a universe outside of the observable universe which (in a statistical sense) looks just like the observable universe (i.e. the piece we happen to be seeing) and has (to good approximation) the space time shape of a Friedman Lemaitre Robertson Walker universe with small cosmological constant. What is on the outside of the sphere of last scattering then is just the rest of the "era" of recombination of which we are seeing an ever increasing part (which I guess one could see changing if one could image the CMB on the scale of lightyears and follow it for years, but I would be deeply impressed if that happened during my lifetime)
There is nothing that the universe evolves "in".
See also this long thread: https://www.researchgate.net/post/The_flatness_problem_or_why_do_we_think_the_observable_universe_is_big
Here is a lame ascii art spacetime diagram showing the era of recombination (on cosmic scales instantaneous, and therefore a space like hyper surface) and backward light cones of observers intersecting in the surface(s) of last scattering
It seems to me that you have some problems with elementary geometry
Perhaps, but there is no need to be offensive and I don't actually think so.
The S^2 of the surface of last scattering we see is the intersection of our backward 3D lighcone and the essentially 3D space like hypersurface of the era of recombination intersecting in a two dimensional surface inside a 4D space time. That is what is depicted in the lame ascii picture, except that two codimension one hypersurfaces intersect in an S^0, because the picture is two dimensional.
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