“If you think this Universe is bad, you should see some of the others.” –Philip K. Dick
When it comes to measuring the expansion history of the Universe, the concept is simple enough: take something you know about an object, like a mass, a size, or a brightness, then measure what the mass, size or brightness appears to be, and suddenly, you know how far away that object has to be.
Add in a measurement of the object’s redshift, and you can figure out not only what the expansion rate of the Universe is today, you can figure out the entire expansion history, and therefore what makes the Universe up. For practically all of the 20th century, we used brightness — or standard candles — exclusively. But new developments in both galactic surveys (like SDSS) and our understanding of dark matter and inflation has enabled us to use a new technique: baryon acoustic oscillations, or a standard ruler, for this task.