What is Hubble's Law? What is it used for?

Question

Ivy Allen · Accepted Answer

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 #v = H_0 xx D# where #H_0# is the Hubble constant. The field of cosmology was revolutionized (or at least allowed to begin) by this equation, which demonstrated that galaxies outside of our local group (where gravity binds them) were all moving away from us. This implied that at some point in the past, the galaxies were closer together, meaning that the universe started in a hot, dense state known as a Big Bang. The value of #H_0# thus gives us a measure of the age of the universe and has been one of the most hotly contested (and variable) constants we have! We can determine the distance to galaxies that appear to us to exist on a flat plane - there is no sense of depth in the night sky - because we can measure the recession velocity of galaxies (the speed they are moving away) very accurately, with very little uncertainty. We measure this velocity using the red-shift of the light we receive. This law effectively provided us with (a) a sense of time and (b) a sense of scale in cosmology—not bad for a three-term equation that elementary school kids can understand!

Layla Anderson · Answer

undefined Hubble's Law, which is used to calculate the distances to galaxies and measure the rate of universe expansion, states that the recessional velocity of a galaxy is directly proportional to its distance from Earth. It describes the relationship between the distance to a galaxy and its recessional velocity.