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The flat FRW metric is given by:

$$ds^2=-c^2dt^2+a(t)^2dr^2$$

If we take $dt=0$ then we get:

$$ds=a(t)\ dr$$

Thus we find that space expands.

If we take $ds=0$ to find the null geodesic followed by a light beam we get:

$$c\ dt=a(t)\ dr$$

Does this imply that cosmological time expands along with space?

John Eastmond
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1 Answers1

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Well, yes. "Expansion" of time as you call it is the cosmological age. The "direction" of progression of the time defines cosmological arrow of time.

user3394040
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