How can spacetime be expanding faster than the speed of light when the speed of light is the speed limit of the universe?
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2$\begingroup$ Duplicate? physics.stackexchange.com/q/60519 $\endgroup$– AllureCommented Aug 17, 2021 at 0:37
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1$\begingroup$ Does this answer your question? Can space expand with unlimited speed? $\endgroup$– PM 2RingCommented Aug 17, 2021 at 4:03
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2 Answers
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The speed of light is the speed of causality. It is defined by the geometry of spacetime itself, and every observer will agree on its value (except for experimental issues that may arise when measuring it).
However, when trying to measure velocities between distant observers you must compare vectors (the four-velocity in this case) that live in very distant regions of spacetime, and there is no intrinsic way of comparing them. The way we usually assign meaning to this comparison is by defining a coordinate system that is centered around us and to compare the coordinate motion of objects with respect to our notion of velocities.
This is a rather intuitive way of doing these comparisons, both from a physical side and from a geometrical side, but it does allow objects to move "faster than light" with respect to this notion. However, it is important to remark that light emitted by them would be seen (by them) travelling at the exact same speed.
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The idea that space expands faster than c is a fundamentally flawed concept. The expansion of space is measured in units of m/s/Mpc. A quantity measured in those units cannot be compared to a quantity measured in m/s. There is no sense in which a greater than or less than comparison is even valid between such incompatible quantities.
