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Dark Energy Confirmed: How ancient sound waves shaped the entire universe

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Just 30,000 years after the Big Bang, the universe started singing. Vast soundwaves rang out and expanded through the primordial cosmos, their ripples determining the universe's large-scale structure. And this all fits perfectly with one particularly theory of dark energy.

The Baryon Oscillation Spectroscopic Survey, or BOSS, has just completed a massive survey of a whopping 327,349 galaxies. These galaxies are on average about six billion light-years away, which was quite possibly the most momentous time in the universe's history since the Big Bang itself. Six billion years ago, the universe reached a tipping point, where the matter in the universe became spread out enough that the force of gravity could no longer slow down the universe's attraction. Instead, the repulsive force of dark energy took hold, and the universe has been speeding up its expansion ever since.


By studying these hundreds of thousands of galaxies, all dating back to right about the time dark energy emerged as the dominant force in the universe, cosmologists can hopefully learn more about this mysterious...something. Dark energy is everywhere - it likely accounts for about 73% of all the mass-energy in the universe - and fifteen years of astronomical observations tell us that it's absolutely essential to explaining the behavior of the universe.

And yet, it's difficult to even conceptualize what it is even in the vaguest of terms, in part because we may not even have the physics yet to explain what it is. (In case that makes you skeptical of dark energy's very existence, I'd recommend our own Dr. Dave Goldberg's spirited defense of the stuff.) The hope is that the data we get from BOSS - which includes their distances from each other and the universe's age and rate of expansion relative to each galaxy - can help eliminate some dark energy models while illuminating others.


The good news from BOSS is that it really does look like astrophysicists are on the right track when it comes to dark energy, and one model in particular is coming away looking very good. Central to these findings are baryon acoustic oscillations. These are the primordial soundwaves I mentioned earlier. These acoustic waves were formed just 30,000 years after the Big Bang, as regular matter started collapsing around dense dark matter.

The resultant pressure forged these waves, which oscillated outwards for about 350,000 years, tracing out the future structure of the universe as they went. By the time the universe had cooled enough to stall these waves, matter had clumped around the center and edges of the wave, causing more galaxies to form in these areas than elsewhere.

That's the theory behind these baryon acoustic oscillations, or BAOs, and it turns out all the galaxies spotted by BOSS are exactly where they should be according to the BAO model...assuming dark energy was also there to direct how these countless galaxies cluster together. Speaking to BBC News, Professor Will Percival of the University of Portsmouth explains how all the measurements confirm both BAOs and dark energy:

"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."


In particular, the BOSS survey found a number of pairs of galaxies that were separated by a distance of some 500 milllion light-years. That's the precise distance predicted by the cosmological constant. This constant is simply the idea that the amount of repulsive energy - dark energy, in other words - is uniform across all space, and this amount is proportional to the size of the universe.

The cosmological constant isn't necessarily an elegant solution, insofar as it introduces a new arbitrary constant that is apparently hardwired into the structure of the universe just because. And yet all this BOSS data suggests that it's the cosmological constant at the heart of dark energy, even if that simply shifts the big mystery back one step. Still, for anyone looking for real, tangible evidence of dark energy's existence, look no further than BOSS.


And there's still plenty more where that came from. The survey itself is still only about a third complete, and the more galaxies cosmologists have to play around with, the more we can restrict with models of dark energy work and which do not. There are dark energy tests that won't work with just a few hundred thousand galaxies that could reveal major breakthroughs if we have millions to work with.

That's why the European Space Agency's Euclid mission, slated to launch around 2019, is so exciting - it is set to measure the precise positions of some 50 million galaxies going back over 10 billion years. Dark energy is officially running out of places to hide.


arXiv via BBC News.

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