A: Gerard Bassols, I know a little bit about fundamental physics
That’s a good question since so many people imagine that the speed of light is very fast, when it is extremely, extremely slow in an astronomical context. And the only answer is, because the (observable) universe is already quite old and has grown to an incomprehensible size.
It is generally believed that the speed of light must have remained constant through the universe’s history (although some non-mainstream theorists challenge that view) so when the universe was very young, say around 1 billion years old when proto-galaxies were just forming, it would not take too much time for light from one proto-galaxy to reach a neighbor one. Astronomical structures were much closer to each other, much more densely packed.
But by now space has stretched a lot and it takes light a lot of time to reach anywhere but the closest galaxies. The surface of last scattering (the CMB radiation source) was only around (edit) 40 million light years away from “the location where the Earth sits now” when that light was emitted (if we can use that expression, the Earth did not exist yet, neither any star nor galaxy, it’s only for representing the rate of the space expansion since then), and yet is has taken 13 billion years to reach us, because as the light was trying to come to here, our space location was receding from that source surface at an enormous rate, and only now after 13 billion years has that light been able to catch up with that space expansion and finally reach us. Perhaps it’s worth clarifying that the CMB has always been bathing “our current location in space”, but with a different frequency and slightly different emission times. The earlier the epoch, the higher the frequency of the CMB which was reaching “our space location”.
Another factor is that our human size and time scales are so short. Things happen very fast at our tiny scale, in a human lifetime we perceive a lot of events, change is very fast. For much bigger structures change happens much more slowly. Perhaps if there could be a sentient system the size of a galaxy, his perception of the rate of change would be much slower, perhaps it could live for billions of years and light would proportionally seem to travel much faster. Obviously this is just wild sci-fi speculation!
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