The Hubble Constant is the ratio of speed of recession of a galaxy due to the expansion of the universe. The problem with measuring the Hubble Constant is that it is extremely difficult to get an accurate number (there are lots of variables we cannot control that will affect the value). However, over the years, due to scientific instruments that are more accurate, we are gaining a more accurate value for the Hubble Constant. Over the years, the Hubble constant keeps decreasing in value. At the moment, the value of Hubble’s Constant is roughly 70 km/s/Mpc.**This can be quite difficult to understand. Therefore, if you want a implied and shorter version of this article, scroll down to the bottom and look for ‘Hubble’s Law and the Age of the Universe’.**

_{0}d. Therefore:

- t = d / H
_{0}d which equals 1 / H_{0}.

_{0}d. Therefore:

- Time for the galaxy to reach us at the speed it’s travelling away from us = d / H
_{0}d. - t = 1 / H
_{0}. This is the age of the universe.

**upper**limit because we have assumed a constant velocity where the average velocity of the galaxy would have been faster because in a reversed universe where everything is falling together, gravity would accelerate their velocities.

A bigger H

_{0}–> faster expansion –> less time to get to present size –> younger universe.

Taking the value of H_{0} nowadays of 70 km/s/Mpc…

- 1 Mpc – 3.10×10^19 km.
- H
_{0}= 70~~km~~/s / 3.10×10^19~~km~~(km cancel each other out). - H
_{0}= 2.26 x10^-18 - 1 / H
_{0}= t_{universe}_{ }= 4.43×10^17 seconds which is 14 billion years.

## Summary

- The graph with Hubble’s Constant as the gradient makes clear that the further away a galaxy is from us, the faster it will be moving away from us. This supports the big bang theory because the graph proves everything is expanding outwards from each other.
- v = H
_{0}d where v is the velocity of the galaxy and d is the distance the galaxy is from Earth. - We can measure the age of the universe with 1/ H
_{0}. This is an upper limit because we are implying that the velocity of the galaxies are staying constant. - The age, at the moment, of the universe is 14 billion years although there are galaxies still older than this.

## Hubble’s Law and the Age of the Universe

- Hubble found that the further away a galaxy is, the larger its red shift.
- He interpreted this to mean that distant galaxies are receding from us.
- For a galaxy a distance ‘d’ from us, Hubble wrote v = H
_{0}d where v is the speed of a galaxy away from us and H_{0 }is a constant called the Hubble constant.

t = d/v = d / H

_{0}d = 1 / H_{0}

This time is independent of d and v and tells us how long ago the Universe was a single point – this is the age of the Universe.

Strictly, in a reversed Universe, the galaxies accelerates as they fall together so that the ‘Hubble time’, 1 / H_{0}, gives an upper limit for the age of the universe.