Astrophysicists Nikhil Padmanabhan of Princeton University and David Schlegel of Lawrence Berkeley National Laboratories led a team of international colleagues that painstakingly surveyed the color and redshift of 10,000 of these unique galaxies. Using data from the Sloan Digital Sky Survey in New Mexico and from a telescope in Australia, the researchers were able to map a fan-shaped slice of the cosmos that covers a tenth of the sky in the Northern Hemisphere. They then applied these measurements to the broader sample to create their three-dimensional map.
"There's statistical uncertainty in applying a brightness-distance relation derived from 10,000 red luminous galaxies to all 600,000 without measuring them individually," Padmanabhan admits. "The game we play is, we have so many that the averages still give us very useful information about their distribution."
With such a measure of the distribution of matter, the researchers could test a proposed ruler based on regular variations in the grouping of ordinary matter every 450 million light-years or so. "Unfortunately, it's an inconveniently sized ruler," Schlegel says. "We had to sample a huge volume of the universe just to fit the ruler inside."
By showing such regular variations, the map confirms theories about dark energy--a mysterious force that accounts for the acceleration of the universe's expansion. As expected, its influence is outsized, making up nearly 75 percent of the universe's density. "By looking at where density variations were at the time of the cosmic microwave background and seeing how they evolve into a map that covers the last 5.6 billion years, we can see if our estimates of dark energy are correct," Padmanabhan notes. Now the astronomers just need to make even more precise maps--hard to do when so much of the matter is in the dark. The research is slated to appear in an upcoming issue of Monthly Notices of the Royal Astronomical Society.