Giant 3-D Map of 1.2 Million Galaxies Will Shed Light on ‘Dark Energy’

Scientists have developed the largest-ever three-dimensional map of distant galaxies to measure one of the universe's most mysterious forces.

An international team of astronomers from the Sloan Digital Sky Survey III (SDSS-III) worked together to create the 3-D map of 1.2 million galaxies.

The map was used to make one of the most accurate measurements yet of "dark energy," which is the force behind the accelerated expansion of the universe.

"We have spent a decade 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," Dr. Jeremy Tinker of New York University, co-leader of the scientific team, said in a news release.

The new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of SDSS-III. Shaped by a continuous tug-of-war between dark matter and dark energy, the map allows astronomers to measure the expansion rate of the universe and determine the amount of matter and dark energy that make up the present-day universe.

The expansion rate is measured by determining the size of the baryonic acoustic oscillations (BAO) in the 3-D model of the galaxies. The original BAO size is determined by pressure waves that traveled through the young universe up until it was only 400,000 years old (the universe is currently 13.8 billion years old), at which point they become frozen in the matter distribution of the universe, scientists said.

By measuring the distribution of galaxies using this time frame, astronomers can make precise measurements on how dark matter and dark energy have competed to govern the rate of expansion of the universe.

"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," Dr. Florian Beutler of the University of Portsmouth's Institute of Cosmology and Gravitation who was involved in the study, said in a report in Phys.org.

Much of what astronomers know about the relative contributions of dark matter and dark energy comes from the leftover radiation from the Big Bang theory, which is called the cosmic microwave background (CMB).

But the new survey allowed scientists to measure dark energy from before the previously defined 5 billion years, starting from 7 billion years ago up to near the present day, 2 billion years ago.

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. The observed amount of infall is explained well by the predictions of general relativity. The agreement supports the idea that the acceleration of the expansion rate is caused by a large-scale phenomenon, such as dark energy, and not a breakdown of our gravitational theory.

A detailed description of the results of the study will be published in the Monthly Notices of the Royal Astronomical Society.