New WMAP results: quantum fluctuations, galaxies, and the first stars

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Mar 16, 2006
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The Wilkinson Microwave Anisotropy Probe (or WMAP, to save me typing about 50 letters) was launched in 2001 to study the light that flooded the Universe from its earliest moments.

The birth of the Universe was hot, and so the light created in the event was extremely high-energy. But as the Universe expanded, that light lost energy. In a sense, the light used up energy to fight that expansion to get to us (sort of (but only roughly, not exactly!) like the way you have to use more energy to walk into the wind).

The exact details are a bit complicated, but basically, after more than 13 billion years, the light from the Bang has lost so much energy that it's now very low energy microwaves. WMAP was designed to look at this kind of light. Other satellites have done this before, but WMAP has better eyesight, if you will: it can see smaller features than ever before, as well as fainter features. By studying this light we can learn a huge amount about the early Universe.

The early results from WMAP were amazing, increasing our knowledge about the early Universe by a giant leap. But now, two years later, new results have been announced.

The big result is that the light left over from this early time matches the models of the Big Bang and Inflation very well. The Big Bang model says the Universe started in a single moment, and has been expanding since then. Inflation is this weird idea that for a teeny tiny fraction of a second, the Universal expansion accelerated hugely. Inflation explains a lot of problems that had cropped up in the Big Bang model as observations got better. What's nice about inflation is that it explains a wide variety of issues including problems in particle physics as well as cosmology without really changing what happened in the very early Universe, before inflation. In other words, the Big Bang and inflation are separate models which fit together to explain what we see in the sky.

The gas filling the Universe right before inflation kicked in was not perfectly uniform; it had small regions that were slightly denser than other regions. These differences were incredibly small, caused basically by fluctuations on a quantum scale, smaller than atoms. But then inflation suddenly happened, and these tiny fluctuations got amplified tremendously. In fact, these fluctuations are what grew into the galaxies and clusters of galaxies we see today in the Universe!

The inflation model predicts the way these amplifications occurred, and the really cool news is that the WMAP data matches these predictions very well. This is a tremendous confirmation of the models scientists have of how the Universe has behaved all the way down to 10-35 of a second after the Big Bang, when inflation started. As physicist Brian Greene put it in the press conference about this,

These observations are spectacular and the results are stunning… it is truly inspiring. Galaxies are nothing but quantum mechanics writ large across the sky.

Another piece of the new results just announced has to do with the way the light form the early Universe was polarized. Polarization is a funny property of light. Light is a wave. You can think of it being able to wiggle up and down, or left and right, or upper left to lower right. But when light reflects off a surface, some of it gets rotated a bit so that more will wiggle left and right than did before the reflection.

This is what happens when light reflects off glass, metal, and water. When you wear polarizing glasses, those glasses filter out the light that gets polarized a certain way, and that significantly reduces the amount of light you see. It reduces the glare from reflections. You can see this yourself: take your polarized glasses and look at light reflecting off a car hood or windshield. Then take the glasses and literally rotate them. You'll see the amount of reflected light you see get brighter and dimmer as you rotate the glasses, as the light wiggling one way is alternately blocked and let through by your glasses. If you had really good glasses, the amount of light let through or blocked would tell you quite a bit about the properties of the surface -- the metal or glass -- off which the light reflected.

When the Universe was younger, it was hotter and denser, and filled with gas. The light reflected off (technically, it scattered off) this gas, and was partially polarized. WMAP has detectors that can see polarized light, and measure how much of the light got polarized. This in turn tells scientists about the gas that filled the early Universe. Since this gas was strongly affected by the first giant stars that were born way back then, the polarization of the light reveals clues about that first generation of stars.

Scientists are now confident that the first stars were born 400 million years after the Big Bang. Study of WMAP's initial results, two years ago, indicated the stars formed earlier than that, around 200 million years after the BB, but the uncertainty in the data was pretty big. More data and more study since then have really beaten down the error bars in this result, and the confidence level is much higher now that stars formed 400 million years after the birth of the Universe.

This to me is perhaps the most amazing result of WMAP. I am fascinated by the Big Bang and inflation models, of course, but they're pretty esoteric and weird. But asking yourself, when was the first star born?, that's a question that is solid, something you can sink your teeth into. For all the history of mankind, that question has not been answerable with any confidence. It's almost like a fairy tale, a silly story to amuse yourself with, but not one that could ever be answered.

But that's changed now. Somewhere out there in the sky, the light from those first stars is reaching us, and it's been struggling against the expansion of space for 13.3 billion years. The light itself has not yet been unequivocally detected, but our telescopes and our detectors get better all the time. And as we learn more, we want to know more! We'll study the data more, and collect more, and build better and better machines to do it.

Like the Universe itself, our will to learn grows more every day.'