One of the fastest moving fields in contemporary astronomy involves getting some new tricks out of the oldest light in the Universe. The cosmic microwave background radiation, or CMB for short, is the light left over from the hot plasma that filled the Universe after the Big Bang. This glow fills all of space and encodes the state of the Universe at the early time when it was released. This information has been and is still being mined by space telescopes, the now long-in-the-tooth Wilkinson Microwave Anisotropy Probe and the newer Planck satellite. The data that we have gained from these experiments already have revolutionized our understanding of early-Universe cosmology.
In addition to teaching us about the Universe's earliest moments, it turns out the CMB can be used for other purposes. One of these is the finding and counting up of galaxy clusters that live between our observers and the distant screen it establishes on the boundaries of the sky. As a background, the CMB acts as a backlight to every object that formed or exists in the time since its creation. These CMB photons can thus be affected by the objects through and by which they pass.
Galaxy clusters are a perfect example of this. Galaxy clusters are the largest collections of matter in the Universe -- the gathering place of dozens to hundreds of galaxies and huge amounts of hot gas. There are actually two ways that clusters affect CMB photons. The more subtle is a phenomenon known as the Integrated Sachs-Wolfe (ISW) effect. As photons pass through a galaxy cluster, they are blue shifted as the "drop" into the well of the cluster's gravitational attraction, then red shifted as they climb out to escape. In a static or matter-dominated Universe, these shifts exactly cancel. But in our Universe, which has been accelerating in its expansion for the past few billion years, the red and blue shifts don't cancel out, leaving a characteristic effect on photons that flow through clusters. This effect has been detected and provides striking independent evidence for the Universe's accelerating expansion.
As nifty as the ISW effect is, the Sunyaev-Zeldovich effect may prove to be an even richer vein for observers to mine. This effect comes about from the direct interaction of the hot gas that lives in clusters with the CMB photons. Essentially, the CMB is "heated up" by interacting with the hot gas in a way that telescopes on Earth can detect. Because this effect is acting on CMB photons, it is one of very few observations that doesn't discriminate against clusters that are very distant from us -- a cluster so distant that its stars can't be seen directly can still be found by searching for this effect in the CMB light.
This is why the SZ effect has astronomers and cosmologists heated up themselves. For a start, clusters are extremely interesting objects on their own, but they are difficult to search for directly using either optical or X-ray methods, which are the traditional tools. In the optical, it's a lot of work to definitively prove that a grouping of galaxies are truly bound together by gravity, since galaxies that sit next to each other on the sky can be millions of light-years apart in the direction along our line of sight. The hot gas in cluster centers is a smoking gun for clusters, but X-ray telescopes can't easily scan the sky searching for clusters we haven't found through other means. Hence the ability for CMB observations to catalog cluster locations could make individual cluster finding a much more straightforward process.
In addition, cosmologists have long known that the number and sizes of clusters are an independent source of information about the Universe's laws and initial conditions. As the largest objects in the Universe, clusters probe the tail of the probability distribution for density fluctuations -- regions with so much extra stuff relative to the background are very rare events. Hence, an exhaustive catalog of these rarest events can tell us a lot about the nature of the probability distribution for cluster formation, much as super-tall NBA centers give us a notion of what nations and racial backgrounds have a genetic propensity for tallness. This, in turn, gives us a handle on what kind of physics was at play when that probability distribution was formed in the very first moments of the Universe's existence.
Right now, two major experiments to hunt for SZ clusters are underway: the South Pole Telescope (SPT), located, surprisingly enough, at the South Pole; and the Atacama Cosmology Telescope (ACT), located in Chile's high, dry Atacama desert, where it almost never rains, keeping the sky crystal clear for observing most of the year. Unlike the all-sky space telescopes used to look at the CMB before, these telescopes are designed for high-resolution work. This is because the primordial fluctuations in the CMB are from so early on in the Universe's history that they have been spread out by the Universe's expansion into large and easy to resolve shapes, even using the a small telescopes that can be carried into space. Clusters, however, are diminutive in comparison, requiring the larger collecting areas that ground-based telescopes can provide. These telescopes have already gathered and released their first data, confirming the promise of SZ observing by finding at least a few clusters that had never before been seen in any other way. However, there has also been a disappointing surprise: the telescopes have seen far fewer clusters than expected. This is still something of a mystery, but the most likely explanation is that the computer models used to calculate the size of the effect have, hitherto, inadvertently overestimated it. In any event, though, this field is expanding quickly, and will be teaching us a lot about clusters and many other things in the coming months and years.