[AGarvin]

From what I understand there is still considerable observation going on to try and fine tune and calibrate the tools we use to determine these distances. This is why astronomers have been chasing down Type 1a supernovas. Type 1a SN are believed to be white dwarf stars that accrete mass from a companion star that has gone red giant. When they reach 1.44 solar masses (called the Chandrasekar limit after the physicist who discovered it), they detonate in what is basically a carbon bomb. Because they all detonate at the same mass and are pretty much made of the same stuff, they will all have pretty close to the same absolute brightness. Using this, we can then infer their distance by applying the inverse square law using their apparent brightness. I think it was through Type 1a SN observation that the acceleration of the universes expansion was first discovered...hello dark energy, but don't hold me to that one.

When Type 1a are discovered, they can then be used to help calibrate other tools such as Hubbles redshift distance law. This law basically relates distance and velocity to redshift. It uses what is called the Hubble parameter which is currently estimated at around 72 metres per second per megaparsec. In other words, for every megaparsec away an object is, its recessional velocity increases by 72 metres per second under cosmological expansion....and yes, local motion through space, known as peculiar motion should be taken into consideration, but hey, when something is 5 billion ly away, what's a few million among friends .

Andrew.