The Cosmological Distance Ladder

The Ladder

Although the general public doesn't hear much about it, the cosmological distance ladder is a hot topic in astronomy.  The reason is that theories about the universe hinge on on how big the universe actually is, and the uncertainties at the highest steps of the cosmological distance ladder are large enough to leave room for debate over which theory is correct. Since each rung of the ladder is built on prior steps, it is critical that each step be as accurate as possible.  Unfortunately, each new step up the ladder brings about a little more uncertainty.  Each step up the ladder is a little more unsteady than the previous step.  You can imagine the ladder as in the figure below where the bottom rungs are secure and solid, but each step up becomes a little more uncertain, until at the top rung, there can be disagreements large enough to support different theories about the ultimate fate of the universe.

Let's get our bearings a little.  The first step is often considered to be something called stellar parallax.  I'm going to briefly describe what parallax is.  Then we are going to take a couple steps backwards because although parallax is very simple to understand, using parallax to measure the distance to the stars  was historically an enormous undertaking requiring a few preliminary steps as a foundation.  After explaining what parallax is, I will describe those preliminary steps up to stellar parallax, and from there, we will continue building the distance ladder out to the furthest reaches of the universe.