'Dark energy measured with map of 1.2 million galaxies'

Researchers have constructed the largest-ever, three-dimensional map of 1.2 million distant galaxies to make one of the most precise measurements yet of the dark energy of the universe.
"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 Jeremy Tinker from New York University in the US.
"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," said Tinker.

These new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) programme of the Sloan Digital Sky Survey-III.
Shaped by a continuous tug-of-war between dark matter and dark energy, the map created by BOSS 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, researchers said.
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, researchers said.

The end result is that galaxies have a slight preference to be separated by a characteristic distance that astronomers call the acoustic scale, they said.
The size of the acoustic scale at 13.7996 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen, researchers said.
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 have made the largest map for studying the 95 per cent of the universe that is dark. In this map, we can see galaxies being gravitationally pulled towards other galaxies by dark matter," said David Schlegel from BOSS.

"And on much larger scales, we see the effect of dark energy ripping the universe apart," said Schlegel.
"We can now measure how much the galaxies and stars cluster together as a function of time to such an accuracy we can test General Relativity at cosmological scales," added Shirley Ho from Carnegie Mellon University in the US.
To measure the size of the ancient giant waves to such sharp precision, BOSS had to make an unprecedented and ambitious galaxy map, many times larger than previous surveys.
The findings were published in the journal Royal Astronomical Society.