I have long wanted to review the process of measuring distances in deep space. It is not as simple as might be believed...
On earth, we measure short distances with a glance. A baseball player knows just how far it is to first base, and how hard to throw the ball. A car driver has a little more trouble, dealing with greater distances and higher speeds.
Look at an airplane in the sky. Is it big, fast and far - or small, slow, and near? Look at a picture of two airplanes with only sky around. Are they about to collide? Or miles apart?
We can measure these things using triangulation. Taking two measurements a known distance apart, and calculating the differences in the angle and length.
What about when the object is in space?
The largest "known" distance we can use is the diameter of the Earth's orbit (2 AU). This is fairly small in the grand scheme of things, so our triangle collapses to a line.
There is an article on Science Daily discussing the standard method for measuring large distances in space - the "Type 1a Supernova".
This method is dependent on many assumptions. Namely, that our models of stellar evolution are correct (models which rely on undiscovered dark matter).
I'm not certain that these measurements are wrong, but they are open to interpretation and error. I will need to pull some data from star catalogs, where you can see the distance to stars shifts - due to revising the underlying calculations.