Hubble’s Law and measuring distance
Measuring distances
Standard methods, such as using cepheid variables, were are used to measure large distances in astronomy. Hubble and his co workers made the key observation that the faster the galaxy is moving away from us the further it is from us. The red shift was used to calculate velocity.
The graph below, plotting the velocity against distance from Earth, is a straight line, showing that the two are directly proportional. The gradient of the graph is constant and that value is called Ho - Hubble’s constant, so:
V= HoD
The current best estimate of Hubble's constant is about:
Ho = 71 kmS-1Mpc -1
Where the velocity away from us is measured in kilometres per second and the distance of the galaxy is measured in megaparsecs. This is the value suggested as a good reflection of recent measurements taking into account their reliability.
The best explanation of “red shift” in the spectra of distant galaxies is that:
★The universe is expanding and has done so since the big bang, this is just like the expansion of a balloon being blown up but imagine that the galaxies are spread through the inside of the balloon, not just on the surface.
★As it expands the distances between the galaxies increases.
★Because of gravitational attraction holds them together, individual galaxies do not expand.
★The radiation traveling between the galaxies stretches and the further it has to travel the longer it takes so the more it stretches.
★The change in the wavelength is directly proportional to the distance the radiation travels, that is, the distance away from us of the galaxy we are looking at.