Dark Energy Confirmed

How ancient sound waves shaped the entire universe
BY ALASDAIR WILKINS APR 3, 2012

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.

Planck telescope reveals ancient cosmic light

By Jonathan Amos
Science correspondent, BBC News

This is the extraordinary place where we all live - the Universe.

The picture is the first full-sky image from Europe's Planck telescope which was sent into space last year to survey the "oldest light" in the cosmos.

It took the 600m-euro observatory just over six months to assemble the map.

It shows what is visible beyond the Earth to instruments that are sensitive to light at very long wavelengths - much longer than what we can sense with our eyes.

Researchers say it is a remarkable dataset that will help them understand better how the Universe came to look the way it does now.

"It's a spectacular picture; it's a thing of beauty," Dr Jan Tauber, the European Space Agency's (Esa) Planck project scientist, told BBC News.

Dominating the foreground are large segments of our Milky Way Galaxy.

The bright horizontal line running the full length of the image is the galaxy's main disc - the plane in which the Sun and the Earth also reside.

In the way

This is where most stars in the Milky Way form today; but because this picture records only light at long wavelengths (microwaves to the very far infrared), what we actually see are not stars at all.

Rather, what we see is the stuff that goes into making stars - lots of dust and gas.

Of particular note are the huge streamers of cold dust that reach thousands of light-years above and below the galactic plane.

"What you see is the structure of our galaxy in gas and dust, which tells us an awful lot about what is going on in the neighbourhood of the Sun; and it tells us a lot about the way galaxies form when we compare this to other galaxies," observed Professor Andrew Jaffe, a Planck team member from Imperial College London, UK.

But as beautiful as the Milky Way appears, its emission must be removed if scientists are to get an even better view of its mottled backdrop, coloured here in magenta and yellow.

This is the famous cosmic microwave background (CMB) radiation, and a key target of the Planck mission.

The CMB is the "first light". It is the light that was finally allowed to move out across space once a post-Big-Bang Universe had cooled sufficiently to permit the formation of hydrogen atoms.

Before that time, scientists say, the cosmos would have been so hot that matter and radiation would have been "coupled" - the Universe would have been opaque.

Super-cold detectors

Researchers can detect temperature variations in this ancient heat energy that give them insights into the early structure of the Universe and the blueprint for everything that came afterwards.

A major quest for Planck is to find firm evidence of "inflation", the faster-than-light expansion that cosmologists believe the Universe experienced in its first, fleeting moments.

Theory predicts this event ought to be "imprinted" in the CMB and its detail should be retrievable with sufficiently sensitive instruments.

Planck is designed to have that capability. Some of its detectors operate at a staggering minus 273.05C - just a tenth of a degree above what scientists term "absolute zero".

Planck is already in the process of assembling a second version of the map. It has funding to acquire at least four versions.

"We know that eventually as the data get better and better, what you end up getting to are the limitations of what you know about the instrument," explained Professor Jaffe.

"And so, by running Planck for longer we can learn a lot more about the instrument itself and thereby remove a lot of the contaminating effects that are just because of the way it produces its noise."

Patient analysis

The project team will need a while to analyse all the data and assess its significance. A formal release of fully prepared CMB images and scientific papers is not expected before the end of 2012.

However, such has been the anticipation for Planck data that one or two groups have already tried to make unauthorised interpretations simply from the images released to the media like the one on this page.

But Dr Tauber says this activity is pointless.

"The CMB is certainly visible but the image itself is colour-enhanced so you couldn't do any science with that," he explained.

"We have also reduced the resolution of the image to something which is more manageable for people to look at. Otherwise it would just be too big."

Planck is a flagship mission of Esa. It was launched in May 2009 and moved to an observing position more than a million km from Earth on its "night side".

It carries two instruments that observe the sky across nine frequency bands. The High Frequency Instrument (HFI) operates between 100 and 857 GHz (wavelengths of 3mm to 0.35mm), and the Low Frequency Instrument (LFI) operates between 30 and 70 GHz (wavelengths of 10mm to 4mm).