Hundreds of scientists from the Sloan Digital Sky Survey (SDSS-III) collaboration worked together to produce this largest-ever, extremely shiny, three-dimensional map of the sky. And it’s not even the whole sky. Performed as part of the Baryon Oscillation Spectroscopic Survey (BOSS) program of SDSS-III, the image above represents a just a single slice: 3% of the volume they mapped. Each point of color stands for an entire galaxy. The color of each galaxy gives its distance from Earth, ranging from yellow on the near side of the slice to purple on the far side. The gray places are gaps in the data. And, of course, any map like this is also looking back in time, showing us what was happening when the light we see was emitted up to billions of years in the past.
In the image below, the rectangle on the far left shows a cutout of 1000 square degrees in the sky containing nearly 120,000 galaxies, or roughly 10% of the total survey. The spectroscopic measurements of each galaxy — every dot in that cutout — transform the two-dimensional picture into a three-dimensional map, extending our view out to 7 billion years in the past. The brighter regions in this map correspond to the regions of the Universe with more galaxies and therefore more dark matter. The extra matter in those regions creates an excess gravitational pull, which makes the map a test of Einstein’s theory of gravity.
In total, the map encompasses a volume of some 650 billion cubic light years. It shows that galaxies are organized in trailing superclusters shot through the void, a superstructure of baffling scale that came into being as matter was flung out into the cosmos in the first fraction of a second after the Big Bang. SDSS elaborates:
BOSS measures the expansion rate of the Universe by determining the size of the baryon acoustic oscillations (BAO) in the three-dimensional distribution of galaxies. The original BAO size is determined by pressure waves that traveled 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 are preferentially separated by a characteristic distance, that astronomers call the acoustic scale. The size of the acoustic scale at 13.48 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen.
SDSS added that measuring the distribution of galaxies since that time lets astronomers measure how dark matter and dark energy have competed to govern the rate of expansion of the universe.
Building the volumetric map naturally involved a lot of associated observations and insights, the sum of which produced several associated reports that the BOSS team submitted this week to the Monthly Notices of the Royal Astronomical Society. The reports deal with ideas like the way that the trailing clusters of galaxies inform us about how the universe is changing through time.
The astronomers also use their observations to zero in further on the nature of dark matter and dark energy. They calculated the amount of matter and dark energy that make up the present-day universe — and the net impact of all that dark matter and energy on the present state of the cosmos, over a period of billions of years. “Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the Universe’s expansion rate,” said collaborator Dr. Ariel Sanchez. Dr. Florian Beutler, another collaborator, said, “If dark energy has been driving the expansion of the Universe over that time, our maps tells us that it is evolving very slowly, if at all. The change is at most 20 per cent over the past seven billion years.”
For further, more scientifically toothsome reading, including PDF versions of the reports related to this cosmic map, check out what the Max Planck Institute for Extraterrestrial Physics had to say about the project.
Or read from ExtremeTech: What is dark matter? and What is dark energy?