06Apr2012

The Birth Of Dark Energy

An artist's depiction of the GRAIL twins (Ebb and Flow

Just when we thought the Sloan Digital Sky Survey (SDSS-III) couldn’t deliver any more, it announced the most accurate measurements to date of the expansion of the Universe.  Scientists from the University of Portsmouth and the Max-Planck Institute for Extraterrestrial Physics presented their findings in six related papers – the culmination of more than two years of work by the team of scientists and engineers behind the Baryon Oscillation Spectroscopic Survey (BOSS), one of the SDSS-III’s four component surveys.

“There’s been a lot of talk about using galaxy maps to find out what’s causing accelerating expansion,” says David Schlegel of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, the principal investigator of BOSS. “We’ve been making a map, and now we’re using it — starting to push our knowledge out to the distances when dark energy turned on.”

“The result is phenomenal,” says Will Percival, a professor at the University of Portsmouth in the United Kingdom, and one of the leaders of the analysis team. “We have only one-third of the data that BOSS will deliver, and that has already allowed us to measure how fast the universe was expanding six billion years ago — to an accuracy of two percent.”

The last twenty years of astronomy has given us tremendous discoveries – and the most incredible of all has been recognized with the 2011 Nobel Prize in Physics.  The Universe is expanding and accelerating.  But that’s not all.  Galaxies are moving farther apart and at an ever faster rate.  Just what is responsible for this “accelerating expansion”?  Some believe that Einstein’s Theory becomes repulsive at large distances, but the most popular explanation is an unseen property of space known as “dark energy”. Is it modified gravity or is it dark energy?  The initial stage of answering this question is to obtain the most accurate distances to as many galaxies as possible.  Through these observations, astronomers can map the history of expansion.

  Enter the BOSS…

By employing a unique spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico, BOSS is giving us the most detailed map of the Universe created to date.  Over the next six years, it will analyze the spectra of more than a million galaxies – some so far away their light has had to travel more than six billion years to reach us.  That’s almost half the age of the Universe!  The BOSS is producing maps which reveal galaxies and galaxy clusters which are “clumped together into walls and filaments, with giant voids in between them.”   It is hypothesized these formations originated from slight density variations in the early Universe, and show “baryon acoustic oscillations” – pressure driven waves.

Even though an incredible amount of time has passed, these oscillations can still be detected within our Universe.  “Because of the regularity of those ancient waves, there’s a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million,” says Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics, director of SDSS-III and a pioneer in baryon oscillation surveys for nearly a decade.  

In their graphic simulation, “the number of galaxy pairs by separation of distance” and the peak is 500 million light years.  The distance which matches the peak is dependent on the presence of dark energy – but the point is moot unless the initial distance to the galaxies is perfectly accurate.  That’s where BOSS comes in. “We’ve detected the peak separation more clearly than ever before,” says Nikhil Padmanabhan of Yale University, who along with Percival co-chairs the BOSS team’s galaxy clustering group. “These measurements allow us to determine the contents of the universe with unprecedented accuracy.”

However, accurate galaxy distances aren’t all the BOSS is capable of.  It’s also testing General Relativity, explains Beth Reid, a NASA Hubble Fellow at Lawrence Berkeley National Laboratory. “Since gravity attracts, galaxies at the edges of galaxy clusters fall in toward the centers of the clusters,” Reid says. “General Relativity predicts just how fast they should be falling. If our understanding of General Relativity is incomplete, we should be able to tell from the shapes we see in BOSS’s maps near known galaxy clusters.”  Reid led the analysis of these “redshift space distortions” in BOSS. 

By taking into account the effects of dark energy, the team was able to determine that the formation of galaxy clusters was consistent with Einstein’s predictions. “We already knew that the predictions of General Relativity are extremely accurate for distances within the solar system,” says Reid, “and now we can say that they are accurate for distances of 100 million light-years. We’re looking a billion times further away than Einstein looked when he tested his theory, but it still seems to work.”

The new results are absolutely fascinating – and all six of the papers are consistent with their findings. “All the different lines of evidence point to the same explanation,” says Ariel Sanchez, a research scientist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and lead author on one of the papers. “Ordinary matter is only a few percent of the universe. The largest component of the universe is dark energy — an irreducible energy associated with space itself that is causing the expansion of the universe to accelerate.”

But this is just the beginning, says BOSS principal investigator Schlegel. “For the past 13 years, we’ve had a simple model of how dark energy works. But the truth is, we only have a little bit of data, and we’re just beginning to explore the times when dark energy turned on. If there are surprises lurking out there, we expect to find them.”

Written by Tammy Plotner for Astro Space  News

 

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