If we could only see as far as our galaxy cluster, what and how would we know about anything beyond it? A silly question that occurred to me while watching one of those "scale of the universe" videos. 
If our observations were only limited to our galaxy cluster - maybe because the expansion of the universe was really far along and light from anything beyond could no longer reach us OR maybe the light from beyond hasn't had time to reach us yet - would our knowledge of physics in this hypothetical scenario (specifically general relativity) be the exact same as our actual knowledge?
Would we have good reason to assume that the galaxy cluster is the "observable universe"? More specifically, whatever we actually believe about our actual observable universe, would be believe the same for the galaxy cluster in the hypothetical scenario? (expansion and big bang and CMB, etc.)
If not, what would be different? I'm trying to understand the barriers to assuming that our actual observable universe is possibly (like the galaxy clusters) a cosmological structure among several others. 
 A: In this "island universe" scenario we would still have general relativity, since that was found considering nearby astrophysical phenomena (rockets, planets and stars) rather than the full cosmology (that came in later, as a natural application of the theory). 
If the scenario is in the remote future of the universe the CMB would be redshifted beyond detection, and there would not be any clear evidence for the Big Bang (no expansion moving remote galaxies, no remote and early galaxies showing dynamical change, and primordial nucleosynthesis would had been masked by stellar nucleosynthesis). Careful measurements of stellar halo motions and satellite galaxies might show evidence of expansion, but these effects would be small and non-obvious. 
I think the main lesson here is that the location and era one finds oneself in does affect what can be scientifically discovered. Some kinds of evidence (CMB, element abundances) do disappear over time. Other kinds of insights can be deduced independently (like GR). Theories like our current cosmology would be more obscure and less supported in a setting like this.
