A hundred years ago scientists believed the universe was steady and unchanging. Einstein invented the cosmological constant to expand the fabric of space-time after his own equations for general relativity wouldn't allow for the cosmos to remain static as expected in a steady state universe.
Soon after, astronomer Edwin Hubble discovered the universe was actually expanding, consistent with Einstein's original general relativity theory. Einstein then removed his cosmological constant describing his failure to predict an expanding universe in theory before it was proven by observation, as his biggest blunder.
In 1998, astronomers studying distant exploding stars called a Type 1A supernovae discovered that not only was the universe expanding, but that the rate of expansion was accelerating due to some type of unknown force or dark energy. This bore a striking resemblance to Einstein's cosmological constant. Either that, or our theory of gravity is incomplete. Answering this question is one of the foremost challenges in 21st century cosmology.
This new measurement from BOSS is significant because that time frame - five to seven billion years ago - is the era when dark energy "turned on." The BOSS findings will help physicists figure out the exact nature of whatever is causing our universe to accelerate in its expansion.
But in order to do that, they must first gain a more complete understanding of the history of that expansion.
The discovery that led to the theory of dark energy relied on studying the red shifts of bright light from supernovae. BOSS, in contrast, looks at something called baryonic acoustic oscillation (BAO).
This phenomenon is the result of pressure waves (sound, or acoustic waves) propagating through the early universe in its earliest hot phase, when everything was just one big primordial soup.
Those sound waves created pockets where the density differed in regular intervals or periods, a "wiggle" pattern indicative of oscillation, or vibration. Then the universe cooled sufficiently for ordinary matter and light to go their separate ways, the former condensing into hydrogen atoms. We can still see signs of those variations in temperature in the cosmic microwave background (CMB), thereby giving scientists a basic scale for BAO.
BOSS is designed to measure those oscillations as a means of determining how far away the most distant galaxies really are, by looking at the angles of those peaks where galaxies are most densely clustered. Within the vast network of cosmic structure, those density peaks repeat with a good degree of regularity, making them an excellent "standard ruler" to measure the geometry of the universe.