|MadSci Network: Astronomy|
Dear Ricardo, It is true that Hubble's law predicts velocity = Ho x distance, where Ho is the value of the Hubble constant, and if we were to insert a large enough distance into this formula the corresponding velocity would indeed be greater than the speed of light. However, this relation is only true locally (for small distances), so we never actually have to tackle that problem and the velocity will never exceed the speed of light! You were right to think of the velocity in question as not really referring to the motion of a galaxy: it is instead related to the expansion of spacetime itself. When we refer to a galaxy's 'recessional velocity', strictly speaking we are talking about the galaxy being carried along with the expanding spacetime rather than moving through space at some velocity (although in practice you will often hear statements like 'the galaxy is moving away from us at 95% the speed of light'). For more mathematical details, you could consult some astronomy textbooks, e.g. Chapter 3.3 "Common Big Bang Misconceptions" in Cosmological Physics by John Peacock or Chapter 25.2 "The Expansion of the Universe" in Modern Astrophysics by Bradley Carroll and Dale Ostlie. I hope this is helpful! Meghan
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