On Earth we know that people at opposite ends, such as UK and New Zealand, exist at the same time, because we can speak to them . The time to get to them for a physical face to face depends on speed of transport, but we both age by this time factor.
Does it mean that a hypothetical being in the cosmoscould exist at the same time as us but separated by immense distances, so that the space-time rules would prevent us from reaching the being in time to meetthem?
"The same time" really isn't. You hear each other with a tiny delay, relative to each other. The standard way to represent this is the distance, e.g. 15k km, /c
(speed of light: ~300km/h), so about 0.05 secs, for the signal to "travel" between you. The truth is that time slows down to effectively zero at c, so it's not actually "travel time" of the signal, but the time difference between you.
That said, there is no one universal line of time. Every point in the (infinite) universe sits on the tip of its own time cone. c
defines the edge of this cone. Your correspondent will be 0.05 secs behind you in your
timeline, but you will be 0.05 secs behind them in theirs
. (The same applies between you eye & your thumb, but with a much tinier time difference: your relative time-cone is almost flat at that scale.)
Anything outside our time cone (such as, perhaps, extra-terrestrial life?) will remain forever
invisible to us.