Anthonares

In my undergraduate cosmology class my professor introduced this satellite as having brought about quantitative cosmolgy. While that’s probably just a bit of an exaggeration, this little craft definitely revolutionized the science. Prior to the launch of the Wilkinson Microwave Anisotropy Probe (WMAP) (and to a much smaller degree its predecessor, COBE), cosmology, the study of the evolution of the universe, was a mess of theories and ages. But after a series of papers published in 2003 detailing the first year of data collection from WMAP, the universe suddenly had a definite age, 13.7 billion years, and was shown to be dominated by dark energy.

Since then, WMAP has been the darling of the astronomy community and has undoubtedly led hundreds of eager young undergraduates into cosmology. The science community knew that WMAP was still collecting data, but I’m not sure that anyone really expected the news released last week (discussed at Cosmic Variance and at Bad Astronomy, here are the technical publications). The age of the universe was left unchanged at 13.7 billion years, but the date of first star formation was moved to 400 million years in better agreement with theory. Also, the first true evidence for cosmic inflation was presented as well.

It was an exciting announcement from a team that has already done so much for the field of cosmology. To better understand just why the WMAP mission is so important, I’ve put together this relatively brief summary of the study of the Cosmic Microwave Background in the last decade. These three missions have changed our prospective and refined our vision. Cosmology is now a field of true quantitative prediction that bears little resemblance to the unorganized conjecture of two decades ago.

The Cosmic Microwave Background

WMAP was launched in 2001 with the goal of mapping the Cosmic Microwave Background (CMB) radiation more accurately than had been done before. The CMB, as you may have heard, is the “afterglow of the big-bang.” More accurately, it is the tremendously red-shifted radiation emitted by the universe before it first became transparent.

Immediately after the big-bang, the universe was far too hot for nuclei to stably bond electrons. This meant that the universe was filled with a cloud of electrons that absorbed and re-emitted any and all light; in other words, the electron gas caused the universe in those first few moments to be entirely opaque. But when it was approximately 372,000-387,000 years old, the first hydrogen atoms formed and the universe became transparent for the first time. The Cosmic Microwave Background is the radiation emitted by that electron gas immediately before it disappeared. That radiation has been traveling for nearly 13.7 billion years becoming more and more red-shifted as the universe literally expanded beneath it.

The fact that the universe before the CMB formed was opaque means that we cannot directly take pictures of the vital first moments of creation. It does not mean that we are unable to learn about that early universe, however. The CMB preserves the state of the universe at the moment the CMB formed. Thus, areas of higher density of electrons emitted more energy and appear brighter now in the CMB. By studying the relative positions of enhanced density, cosmologists are able to calibrate their theories.

COBE: Cosmology Gets Serious

In 1989, NASA launched the first-ever satellite dedicated to cosmology: the Cosmic Background Explorer (COBE). In 1992, COBE produced, for the first time ever, a map of the sky in microwaves. These microwaves corresponded to a temperature of 2.73 Kelvin (or 2.73 degrees above absolute zero, this is the “ambient temperature” of the universe). Importantly, COBE showed that the CMB was not a smooth field. It contained lumps where the temperature was slightly higher or lower than other areas. These lumps recorded the ancient remnants of density variations in the early universe that would later go on to result in the distribution of galaxies, clusters, superclusters, and other mega-structures we observe today.

BOOMERANG: The Next Step

Very soon, it became apparent that the tantalizing glimpse offered by COBE would not satisfy the rapidly advancing field of cosmology. In the early 1990s, the launch of the Hubble Space Telescope promised to finally pin down the value of the Hubble Constant which would then fix the age of the universe. Dark matter was now largely accepted to dominate the universe. And, in 1998 studies of Type Ia supernovae revealed that the universe was not only expanding, but that the expansion was itself accelerating.

In 1999, a balloon-lofted imager experiment called BOOMERANG floated above Antarctica and captured this image of the CMB. Notice the refinement over the earlier COBE map. The additional detail allowed the BOOMERANG scientists to announce in 2000 that the universe was flat. The question of the curvature of the universe was at that point very much an open one, and one which COBE’s myopic vision had been unable to resolve. However, because it could not map the entire sky BOOMERANG was incapable of providing the data needed to pin down the exact age of the universe, or resolve unequivocally the relative distributions of normal matter, dark matter, and dark energy.

WMAP: The Start of a New Era

Technology had greatly advanced in the fifteen years since COBE was built. When WMAP launched in 2001, it carried an imaging system far more capable than was carried on COBE or BOOMERANG. Within a year, it had created several maps of the entire sky in a range of microwave wavelengths. These data were then combed to produce several important results: 1) the universe was 13.7 billion years old, plus or minus about 200 million years, 2) it is composed of 4% matter, 22% dark matter, and 74% dark energy, 3) the Hubble Constant is 71(km/sec)/MPc +/- 0.04, not 50 or 100 as some researchers had suggested, and 4) the universe is flat (similar to what the BOOMERANG craft had seen).

In order to reach those conclusions, the the density spectra of the CMB maps were analyzed. This is not the simplest of concepts, but perhaps it can be understood as follows: the CMB map on the right can be mathematically produced by summing an infinite series of disks of various sizes. Areas with blue colors have fewer disks while redder colors indicate more disks added to create those areas. The sizes and numbers of those disks used to create the map can then be plotted as is done in the graph on the left.

In this plot, the number of disks lies on the vertical axis while their sizes are plotted on the horizontal axis. The largest disk sizes are on the left of the horizontal axis while smaller sizes lie on the right. This plot is from the latest WMAP data release (more on that below). Data measured by WMAP are the black lines with error bars, while predictions from cosmological models produce the red curve and purple shaded regions. Notice the excellent agreement between models and data at most of the scales. COBE produced a similar graph but because it could not resolve the finer structures in the map, it could only resolve disks with a multipole moment of about 500 (if my memory serves correctly here). This meant that COBE could not distinguish between different cosmological models because many did not differ greatly until higher multipole moments.

If that paragraph did not make too much sense, don’t worry. I really just included it because I found it really cool to see exactly how cosmological models and the WMAP results are compared. The important lesson is the bottom line: we live in a lambda-CDM universe. Lambda means that the universe is dominated by dark energy that produces the expansion acceleration known as Einstein’s cosmological constant. CDM states that the dark matter is “cold” rather than “hot” dark matter. Lambda-CDM is not a single cosmological model, but rather is a framework for trying to understand the universe. Choosing a single model requires data from other sources, though as I mentioned above, WMAP does a pretty good job nailing down the relative fractions of dark energy, dark matter, and normal matter.

Why WMAP is Again in the News

After the WMAP scientists released their results in 2003, the satellite simply went on collecting data. Over two additional years, the returned data increased the signal/noise ratio over the first-year data, and it provided the first comprehensive measurements of the polarization of the CMB. The enhanced signal/noise ratio allowed for another concrete prediction: the first stars formed about 400 million years after the Big Bang (the first-year results suggested 200 million years, too early, according to theory). They also allowed for the first indication that the idea of cosmic inflation may not be all just a bunch of hot air.

The Big Bang has a whole host of theoretical difficulties (see the Wikipedia article) that became apparent as researchers developed the theory starting in the 1930s. The most pressing of these difficulties was that the CMB is startlingly and amazingly smooth. Those maps I showed you above are exaggerated by about 100,000 times. This degree of smoothness meant that the Big Bang was either astoundingly uniform in all directions, or that the entire universe had been in “communication” after the initial creation. However, this was also a problem because the speed of light is far too slow to have allowed the entire universe to communicate and smooth out any density variations. These two problems are known as the horizon and the flatness problems, in cosmology speak.

To solve these problems, researchers suggested that very early in its history the universe had expanded by a factor of about 10^43 in almost no time whatsoever. This idea came to be known as “inflation.” The expansion of the universe took place much more quickly than the speed of light, but this is possible without violating the laws of physics because it was the very fabric of the universe that expanded, and the speed of light simply measures the movement of mass past that fabric. The cause of inflation is not yet known, but some physicists think that it has to do with the idea of zero-point energy. As the universe expanded and cooled, it found itself in the awkward position of no longer being in the lowest possible energy state. The shift of vacuum state down to the lowest energy level released the unthinkable amount of energy needed to drive inflation. But again, that is still pretty much just a theory.

The latest WMAP results refined some of the points at both ends of the spectrum (shown above). The more accurate locations of these points then tell physicists something very interesting: there is more “spectral energy” at the left hand side of the spectrum than at the right. You don’t need to go up and try and deduce that from the graph. Suffice to say that non-inflationary universes would not have this disparity, there would be equal amounts of spectral energy at all scales. But the measured ratio between the power at both ends is about 0.95, slightly less than the 1 of a non-inflationary universe. Inflationary theory predicts this disparity (though not this exact value). So, the WMAP results provide the first suggestion (still not concrete evidence) that inflation really did take place. This is a remarkable accomplishment considering that inflation took place in the first 10^-34 seconds of the Universe’s existence.

Some Perspective

This revolution of an entire field and of the public’s understanding of the origins of the universe largely stands due to one small spacecraft. Compared to large observatories like Hubble or Spitzer, WMAP cost very little, yet has returned so much. History has taught us that whenever we open a new eye to the universe we learn not just new facts. What we learn alters our concept of ourselves and our role in the universe. The Earth does not hold the answers to our evolution or the formation of the universe, but Space does. There we have already discovered so much yet we can only just glimpse how little we truly know.

Also, because the very idea of a Big Bang is so inimical to many biblical literalists, they would seek comfort in mistranslations and lineages to state that the universe is but 10,000 years old. To make this assertion they ignore most facts and grossly distort most others. Science and cosmology, on the other hand, has accepted strange and seemingly non-intuitive facts, like the accelerating expansion of the universe, and adapted to further refine our understanding. This is the crucial distinction between psuedoscience and science. When the facts are against them, scientists will eventually abandon old theories and adopt new ones that bring us closer to the ultimate truths.