What is Hubble's Law? What is it used for?
Hubbl’s law is one of those deceptively simple, but revolutionary equations in Physics.
It simply states that the distance to a galaxy is proportional to its recession velocity (the speed it is moving away from us.) It can be stated as
This equation revolutionised (or allowed the initial development of) the field of cosmology as it showed that galaxies beyond our local group (where they are bound to us by gravity) were all moving away from us. This in turn implied that in the past they were closer together i.e. the universe began in a hot, dense state - astronomer speak for a Big Bang.
The value of
Because we can measure the recession velocity of galaxies (the speed they are moving away) very accurately indeed we can infer the distance to galaxies that all appear to us as existing on a flat plane - there is no sense of depth in the night sky. We measure this velocity using the red-shift of the light we receive and the uncertainty is very low.
In effect this law gave us (a) a sense of scale and (b) a sense of time in cosmology. Not bad for a three-term equation that you can explain to primary school students!