Oh here is another interesting property of Hubble’s law that I don’t think I talked about last time. You can use it to set a minimum on the age of the universe. So to review, Hubble discovered that the universe is expanding and he found that the rate at which stuff (stars nebula etc.) is moving away from us is proportional to the distance they are away from us. The further they are away the fast they are receding. And then he graphed his results and found a line. The equation goes something like this: (the speed at which something is moving away from us) = (the Hubble parameter) X (the distance from us to them). This Hubble parameter is sometimes called the Hubble constant but it’s not really constant. It changes with time, that’s why we use the word parameter. There is another really cool aspect of the Hubble parameter, we can use it to tell how old the universe it.
(the Hubble parameter)= (the speed at which something is moving away from us)/ (the distance from us to them)
In math terms this is
H=S/d so if we invert this we get
1/H=d/S
and if you take any distance (say the distance that a car has travel) and you divide it by the speed that it went at they you know the time it took to get there. So from the inverse of the Hubble parameter give you the time that it took the stuff (stars nebula etc.) to get where is at, thus putting a minimum on the age of the universe. The current accepted value for H is 70.8 ± 4.0 (km/s)/Mpc which give the age of the universe to be 13.8 billion years.
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