Astronomy Question - Hubble Constant

Question

The hubble constant can be expressed in two ways (Km s-1 Mpc-1 and s-1) but how to work it out?

What is 50km s-1 Mpc-1 in s-1?

Answer 1

Mpc - megaparsec

Look up what a parsec is in km, multiply by a million, substitute that in your equation and cancel out the km.

You will be left with an extremely small number, which is the reciprocal of the estimated age of the universe in seconds.

Answer 2

The expansion rate is how much space expands linearly (a speed) per unit distance of space.  The number is like 72 ± 8 km/s/Mpc.  So km is a distance and Mpc is a distance.  So the final units are really 1 divided by time.  But they use kilometers and megaparsecs to give a visualization of how small a number it is.  72 kilometers is to a few towns over, and a megaparsec is about 3 million light years - about double the distance to the Andromeda galaxy.  So the number, 72, is really small in real terms.  Convert megaparsecs to kilometers, and take the reciprocal, and you have an approximation to the age of the Universe.