Last week I read this article in Science Daily about BOSS and its recent most accurate measurements of the universe to date. http://www.sciencedaily.com/releases/2012/03/120330081844.htm In describing the experiment to measuring the accelerating expansion of the universe the author says “Baryon acoustic oscillation measures the angle across the sky of structures of known size, the peaks where galaxies cluster most densely in the network of filaments and voids that fill the universe. Since these density peaks recur regularly, the angle between appropriate pairs of galaxies as precisely measured from Earth reveals their distance -- the narrower the apparent angle, the farther away they are.” My question is how can this be a viable test to base an accurate measurement if the target itself could be moving for other reasons than expansion? I think I understand the procedure they are attempting which appears to be the opposite of the parallax effect and instead of measuring from two opposite points it is done from one point, that being the apex. The way I see it these clusters of galaxies and their peak density points may be known distances (apart from each other) but what if the points are gravitating toward each other. I assume they would be hence the cluster. If these points move toward each other (reducing the angle) wouldn’t BAO incorrectly interpret this as expansion?