A team of scientists has put more than a million galaxies up to 6 billion light years away on the largest ever 3D map of the universe in an attempt to precisely measure Dark Energy. “We have spent five years collecting measurements of 1.2 million galaxies over one quarter of the sky to map out the structure of the Universe over a volume of 650 cubic billion light years,” said co-leader Jeremy Tinker of New York University. Tinker added, “This map has allowed us to make the best measurements yet of the effects of dark energy in the expansion of the Universe. We are making our results and map available to the world.”
These new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of the Sloan Digital Sky Survey-III. Shaped by a continuous tug-of-war between dark matter and dark energy, the map allows scientists to measure the expansion rate of the Universe and thus determine the amount of matter and dark energy that make up the present-day Universe. BOSS measures the expansion rate of the Universe by determining the size of the baryonic acoustic oscillations (BAO) in the three-dimensional distribution of galaxies. The original BAO size is determined by pressure waves that travelled through the young Universe up to when it was only 400,000 years old (the Universe is presently 13.8 billion years old), at which point they became frozen in the matter distribution of the Universe. The end result is that galaxies have a slight preference to be separated by a characteristic distance that astronomers call the acoustic scale.
The size of the acoustic scale at 13.4 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen. Measuring the distribution of galaxies since that time allows astronomers to measure how dark matter and dark energy have competed to govern the rate of expansion of the Universe. “We’ve made the largest map for studying the 95 percent of the universe that is dark,” noted principal investigator David Schlegel. “In this map, we can see galaxies being gravitationally pulled towards other galaxies by dark matter.
And on much larger scales, we see the effect of dark energy ripping the universe apart.” The map also reveals the distinctive signature of the coherent movement of galaxies toward regions of the Universe with more matter, due to the attractive force of gravity. Crucially, the observed amount of infall is explained well by the predictions of general relativity. “The results from BOSS provide a solid foundation for even more precise future BAO measurements, such as those we expect from the Dark Energy Spectroscopic Instrument (DESI),” said researcher Natalie Roe, adding “DESI will construct a more detailed 3-dimensional map in a volume of space ten times larger to precisely characterize dark energy and ultimately the future of our universe.” A collection of papers describing these results was submitted this week to the Monthly Notices of the Royal Astronomical Society.