Most Precise Measurement of Scale of the Universe

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Physicists on the Baryon Oscillation Spectroscopic Survey (BOSS) have announced the first results from their collaboration, revealing the most precise measurements ever made of the large-scale structure of the universe between five to seven billion years ago. They achieved this by observing the primordial sound waves that propagated through the cosmic medium a mere 30,000 years after the Big Bang.

And so far, the data supports the theory that our universe as flat, comprised of roughly a quarter cold dark matter, and four percent ordinary matter, with the rest made up of a mysterious force dubbed “dark energy.”

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A hundred years ago scientists believed the universe was steady and unchanging. Einstein invented the cosmological constant to expand the fabric of space-time after his own equations for general relativity wouldn’t allow for the cosmos to remain static as expected in a steady state universe.

Soon after, astronomer Edwin Hubble discovered the universe was actually expanding, consistent with Einstein’s original general relativity theory. Einstein then removed his cosmological constant describing his failure to predict an expanding universe in theory before it was proven by observation, as his biggest blunder.

In 1998, astronomers studying distant exploding stars called a Type 1A supernovae discovered that not only was the universe expanding, but that the rate of expansion was accelerating due to some type of unknown force or dark energy. This bore a striking resemblance to Einstein’s cosmological constant. Either that, or our theory of gravity is incomplete. Answering this question is one of the foremost challenges in 21st century cosmology.

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This new measurement from BOSS is significant because that time frame — five to seven billion years ago — is the era when dark energy “turned on.” The BOSS findings will help physicists figure out the exact nature of whatever is causing our universe to accelerate in its expansion.

But in order to do that, they must first gain a more complete understanding of the history of that expansion.

The discovery that led to the theory of dark energy relied on studying the red shifts of bright light from supernovae. BOSS, in contrast, looks at something called baryonic acoustic oscillation (BAO).

This phenomenon is the result of pressure waves (sound, or acoustic waves) propagating through the early universe in its earliest hot phase, when everything was just one big primordial soup.

Those sound waves created pockets where the density differed in regular intervals or periods, a “wiggle” pattern indicative of oscillation, or vibration. Then the universe cooled sufficiently for ordinary matter and light to go their separate ways, the former condensing into hydrogen atoms. We can still see signs of those variations in temperature in the cosmic microwave background (CMB), thereby giving scientists a basic scale for BAO.

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BOSS is designed to measure those oscillations as a means of determining how far away the most distant galaxies really are, by looking at the angles of those peaks where galaxies are most densely clustered. Within the vast network of cosmic structure, those density peaks repeat with a good degree of regularity, making them an excellent “standard ruler” to measure the geometry of the universe.

Measuring the angle between pairs of galaxies will tell scientists how distant there are — the narrower the angle, the greater the distance. And once you know the distance, you can deduce an object’s age, thanks to the telltale redshift of light as it travels across the universe, stretching proportionally in such a way as to give physicists a peek at how the universe expanded since the light left its source.

Redshifts aren’t uniform, however. This is where BOSS is most helpful, since it can statistically analyze the redshifts of literally hundreds of thousands of galaxies in its dataset. With that large a sample, the variations in redshifts can be taken into account, while still achieving a precise measurement of distance.

Here’s what the University of Portsmouth’s Will Percival told BBC News about BOSS’s first results:

“Because you can trace this pattern all the way through the Universe, it tells you a lot about its content. If it had a different content – it had more matter, or it had less matter, or it had been expanding at a different rate – then you’d see a different map of the galaxies. So, the fundamental observation is this map. “What we find is everything is very consistent with Einstein’s theory of general relativity, coupled with the cosmological constant that he put into his equations. He put it in originally to make the Universe static, and then took it out. But if we put constant in with the opposite sign, we can get acceleration. And if we do that, we find equations that are perfectly consistent with what we’re seeing.”

Eventually BOSS will have cataloged over a million galaxies. These are just the initial results, based on less than a quarter of the expected data that will be amassed by the time the survey ends in 2014, at which point the European Space Agency’s Euclid mission will take over, launching in 2019.

While BOSS’s findings are consistent with the cosmological constant model for dark energy, so far it’s based on limited data. “The cosmological constant may be the simplest explanation,” David Schlegel, a member of BOSS at the Lawrence Berkeley National Laboratory, has said. “But we’re just beginning to explore the times when dark energy turned on. If there are surprises lurking there, we expect to find them.”

Image Credits: (top) Graphic: Zosia Rostomian. Images from Eric Huff, the SDSS team, and the South pole Telescope team. (middle) Don Lang, Apache Point Observatory. (bottom) The 2.5m Sloan telescope at Apache Point Observatory, courtesy SDSS.

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