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# What is the Hubble's Constant Tweet

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1. What is the Hubble's Constant
Hi,

I know that the Hubble's constant is worked out by measuring the distance and velocity of different galaxies away from us and plotting a graph of velocity against distance. Hubble deduce that the further a galaxy is away from us the fast it's traveling since that the line is linear and goes through the origin. This could mean that all of the distant galaxies all originated from one point in space and our universe is expanding. Also, the time of the universe can be worked out by the gradient of the graph.

However, my questions are:
1) What if the distant galaxies had different velocities in its past? (The universe was expanding at a faster rate initially but then slowed down due to gravity. Then, 'dark energy' overcomes the forces of gravity and it is discovered that galaxies are accelerating away from each other.)
2) The hubble's constant is obviously not a constant, why do we call it one?
3) The hubble's constant is a function of time itself, how are we able to deduce the time taken for the expansion (age of the universe) by using V = H*d (H, Hubble's constant; V, Velocity; d, distance).
4) If the distant galaxies did have changing velocity, shouldn't the graph plotted by Hubble be a curve instead of a stright line?

Chungy
Thank you!
2. What is the Hubble's Constant
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3. Re: What is the Hubble's Constant
1/H is only the age of the universe if the motion of the galaxies at modelled by V=d/t, that there is a constant motion. But the galaxies did not have uniform motion though. So how can we say 1/H is the age of the universe? if we dont know the motion of the galaxies in the past?