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Topic - Age of the Universe

Pero - on 26 Feb 2013
I was watching the BBC cosmology programmes with Brian Cox a few weeks ago and he described how to estimate the age of the universe by using the rate of the universe's expansion (75km/s/MParcsec).

If something had travelled from Earth at 75km/s for about 13 billion years, then it would be about 1 MParsec away now. Which was the basis of the calculation that concludes that the universe is 13 billion years old.

But, it seems to me now that this is purely a coincidence. A point in space that is now 1 MParsec away has not been moving away at a constant 75 km/s. When it was closer, it would have been moving more slowly.

Finally, if the universal expansion is constant (75km/s/MP), then the calculation will give a constant 13 billion years regardless of how old the universe is.

Are there any physicists who can explain this?
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