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Beyond Verses

Saturday, May 21, 2011

Dark Energy Confirmed Again!

"We all need to look into the dark side of our nature - that's where the energy is, the passion. People are afraid of that because it holds pieces of us we're busy denying." -Sue Grafton

No, not the dark side of our nature, just the dark side of nature! Because if all our Universe were made out of were atoms and photons, we wouldn't get a Universe that looks like ours.

What do I mean? Let's take a look.

(Image credit: MPA Garching and Volker Springel.)

The Universe starts off as a very smooth place, where regions that are denser or less densethan average are only something like 0.003% away from average. To put it in economic terms, if the average salary were $50,000 a year, the richest person would make about $3 extra, and the poorest person would make about $3 less than average.

But that's the early Universe. Over time, those richer, denser regions attract more and more matter, growing in size and scope.

(Animation credit: Center for Cosmological Physics, National Center for Supercomputer Applications, and Andrey Kravtsov (U. Chicago) and Anatoly Klypin (NMSU).)

Today, we see the very densest regions as being the places with the greatest concentrations of galaxies, and the least dense regions are devoid of almost all matter.

We can even still find galaxies merging today; evidence of the legacy of these small gravitational inequalities from when the Universe was billions of years younger than it is

(Image credit: NASA and The Hubble Heritage Team (STScI).)

But it isn't like we just get giant clumps of galaxies that fall in to one another, and collapse under gravity. As much as we like to pretend that gravity is the only thing that matters in the Universe, there's another force that's often just as important.

Don't believe me? Then think about the biggest, brightest, close object to you in the Universe. 300,000 times as massive as the planet Earth, the Sun's gravity is absolutely tremendous.

(Image credit: NASA/CXC/M.Weiss.)

And yet the Sun is less dense than the Earth is! That's because there's something holding the Sun up that the Earth doesn't have: radiation pressure!

All the nuclear fusion happening at the core of the Sun produces a gargantuan amount of energetic photons. It's so powerful that, at the Sun's surface, the outward pressure is enough to counteract the force of the Sun's gravity, and that's why the Sun doesn't contract or collapse.

And then we come to the Universe.

(Image credit: CCA Zurich.)

Sure, today, the Universe looks a lot like this. The pressure from radiation is totally negligible except in the hottest and densest of objects, like stars.

But go back in time, to when the Universe was very young, and radiation was of incredibleimportance! In fact, for the first 10,000 years of the Universe or so, it was even more important than matter! Which means, during this time, if you were matter trying to collapse under the influence of gravity, radiation pressure would bounce you back out!

(Wish I could find the image credit for this one!)

This is part of why the early Universe is so smooth, for certain. But this feature -- matter trying to collapse under the influence of gravity vs. radiation pressure pushing it back out -- creates a "wave" or "ripple" like you see above. You can see a dense region at the center, you can see it get sparser and sparser as you move away, and then it gets denser once again.

This feature, created by normal atoms (baryons) and radiation (photons), is known as anacoustic oscillation. Why? Because it's a pressure wave, just like sound is! The only difference is, this type of wave determines how galaxies group together.

(Image credit: Chuck Bennett and Nature.)

So if we go and measure how far apart the galaxies in the Universe are spaced from one another, we can figure out:

What percentage of the matter is normal matter,
What percent is dark (non-baryonic) matter, and
How quickly the Universe has expanded since, or what percent of the Universe is dark energy

It's a very clever way to do it, and it's a brilliant way to check our laws of gravity. If we're doing it correctly, and General Relativity is right, we should, by measuring these galaxies, see how the Universe has expanded from the Cosmic Microwave Background over billions of years to form galaxies, and then from those galaxies to our eyes.

(Image credit: NASA's WMAP and the Sloan Digital Sky Survey.)

Well, the WiggleZ team from Australia just released their results this week: the most comprehensive survey -- of 200,000+ galaxies -- designed to measure dark energy by this method.

Their results are a spectacular confirmation of the best prediction of our Universe: one where 70-75% of the energy is dark energy, and where the total amount of baryons is only about 4-5%, with the rest being dark matter. They also found, to the best of their measurements, that dark energy is, in fact, a cosmological constant, with no change over time and the correct equation of state. (I.e., it gives the right pressure/energy density combination to be a cosmological constant.)

Led by Warrick Couch and Michael Drinkwater, and also with scientists such as Chris Blake and Karl Glazebrook, the WiggleZ dark energy survey has been a smashing success, and I'm happy to report that the team appears to have done everything impeccably. Some good press coverage is available here and here, as well as the full press release here.)

The full WiggleZ team (as best as I can find) is pictured below.

And I'd like to say, on a personal note, I'm really impressed, and I'm really happy for all of this!

Most of you don't know this, but before I turned the bulk of my energies towards communicating and teaching science, I was offered a job at Swinburne, working with the WiggleZ team. (Check it out on the old job rumor page.) They were a great group, and they made the job decision an extremely difficult one for me, because I could tell they were doing exciting, top-notch science, and it's great to see it come to fruition like this, even if I wasn't a part of it. Every once in a while, I let my imagination wander to the life I would have had if I had taken it, and all I know is that it would be vastly different than the one I'm living now.

But who knew, more than four years ago when I made that decision, that I'd be telling thousands of you about their smashing success?!

This is another great success for dark matter, dark energy, and the standard big bang picture of the Universe, and yet another tremendous challenge for alternative theories to explain. All you have to do is measure how far apart galaxy pairs are, how that distance changes as the Universe expands, and that's one new way for you to measure dark energy,completely independently of supernovae!

Congratulations to the entire WiggleZ team, and to all of you for learning about the latest, greatest confirmation of the most mysterious force in the Universe! 

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