MadSci Network: Astronomy

Re: How can I see a faster than light movement?

Date: Thu Jun 8 08:39:52 2000
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 956946499.As

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,

Chapter 3.3 "Common Big Bang Misconceptions" in Cosmological Physics
John Peacock


Chapter 25.2 "The Expansion of the Universe" in Modern Astrophysics
Bradley Carroll and Dale Ostlie.

I hope this is helpful!


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