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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - I.07. How can the Big Bang (or inflation) be right? Doesn't it violate the idea that nothing can move faster than light?

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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
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Next Document: I.08. If the Universe is only 10 billion years old, how can we see objects that are now 30 billion light years away? Why
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 	(Also, can objects expand away from us faster than the speed
	of light?)

In the Big Bang model the *distance* between galaxies increases, but
the galaxies don't move.  Since nothing's moving, there is no
violation of the restriction that nothing can move faster than light.
Hence, it is quite possible that the distance between two objects is
so great that the distance between them expands faster than the speed
of light.

What does it mean for the distance between galaxies to increase
without them moving?  Consider two galaxies in a one-dimensional Big
Bang model:
                          *-|-|-|-*
                          0 1 2 3 4

There are four distance units between the two galaxies.  Over time the
distance between the two galaxies increases:

                       * - | - | - | - *
                       0   1   2   3   4

However, they remain in the same position, namely one galaxy remains
at "0" and the other remains at "4."  They haven't moved.

(Astronomers typically divide the distance between two galaxies into
two parts, D = a(t)*R.  The function a(t) describes how the size of
the Universe increases, while the distance R is independent of any
changes in the size of the Universe.  The coordinates based on R are
called "co-moving coordinates.")

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