When we look out in the sky, we're actually looking backwards in time. Light from more distant objects take longer to reach us and thus we are observing now how they appeared in the past. We can see back a few billion years with the light of galaxies. The microwave light of the background shines from long ago in an infant universe 300,000 years old (the epoch of "last scattering") and illuminates the particle soup that existed before this time. This soup has a very smooth consistency and is composed of fundamental particles like electrons, protons, helium nuclei, neutrinos.

The obvious questions are: how did the universe go from a smooth particle soup to a complex system of galaxies and large scale structure. Can we use the fact that we're seeing the surface of this soup in the microwave background to help us understand this question.

If we have small wrinkles or hills and valleys early on in the universe, matter will tend to fall into the valleys, eventually producing dense regions that become the sites of galaxies.

We represent these wrinkles by a sort of "top view" where the color coding refers to the density of matter (dark regions have more matter, light regions less).

Needless to say, this is a bit of an idealization for illustative purposes. Cosmologists actually run computer simulations to track how matter collects into valleys. For example, here is a simulation running forward in time which shows how particles collect and enhance small initially small wrinkles.

One question that remains unanswered is what is the origin of such large scale wrinkles in the first place. Inflation theory is that a period of rapid expansion takes very small scale fluctuations at the level of the particle soup and stretches them to cosmic proportions.

Here the blue bands are snapshots of the wrinkles in the density of the universe at various times. As time goes on, matter falls into these wrinkles and starts to build heavier and heavier objects. The crucial period when this process of gravitational attraction and infall can occur is related to an important concept in cosmology called the horizon. Like the horizon on the earth, it is the point beyond which we're unable to look. Unlike the earth's horizon, this distance is increasing with time because light from more distant regions has had more time to reach us. Heuristically, if there is a large clump in the universe we only know to fall toward it once it comes into the horizon.

A useful property of the microwave background is that when we look out across widely separated angles, we're looking at wrinkles on such large scales that this process of infall hasn't yet begun. We're looking at the primordial wrinkles themselves.

Small variations in the temperature of the background radiation from point to point on the sky are called anisotropies. These anisotropies were first detected by the COBE satellite in 1992. The current MAP version of the CMB is:

COBE and MAP then has told us what the large scale ripples in the background radiation temperature look like. However there is much to be gained by examining the fine details of the ripples. Recall that on the large scales, the temperature ripples reflected the primordial ripples themselves. That is because on scales that are larger than the horizon there hasn't been enough time for matter to collect in the valleys and the process of structure formation to start. When we look at smaller scales than the horizon, we see the process of structure formation at work.

The goal of the current generation of experiments is to understand this process in detail by looking at the small scale ripples in the background radiation temperature.

What we see on small scales is actually sound. The photons behave as a gas just like air. Ordinary sound waves are just travelling compressions and rarefactions of the gas which we hear as sound as they strike our ear drum. The photons also carry sound waves as gravity tries to compress the gas and pressure resists it. The reason why we see it rather than hear it is that when we compress the gas it becomes hotter. We see the sound waves as hot and cold spots on the sky.

The result is a spectrum of sound waves that are useful in determining the origin, evolution and fate of objects in the universe.

We think that fluctuations may have originated from a period of rapid expansion called inflation. Whether or not this actually happened can be "heard" in the microwave background. The fundamental tone of a musical system is related to its physical size - here the horizon size at last scattering.

There is also a pattern of overtones at integer multiples of the fundamental frequency.

In music, the pattern of overtones helps us distinguish one instrument from another: it is a kind of signature of the instrument that makes the sound. In the same way, the pattern of overtones in the sound spectrum of the microwave background ripples acts as a signature of inflation. Inflation's signatures are that the overtones follow a pure harmonic series with frequency ratios of 1:2:3...

COBE told us what the large-scale fluctuations in the background look like, but cosmologists today are more interested in the small-scale fluctuations. Astronomers divide up the sky into angular degrees, so that 90 degrees is the distance from the horizon to a point directly overhead. COBE measured temperature ripples from the 10 degree to 90 degree scale. This scale is so large that there has not been enough time for structures to evolve. Hence COBE sees the so-called initial conditions of the universe. At the degree scale, on the other hand, the process of structure formation imprints information in the ripples about conditions in the early universe.

Since the COBE discovery, many ground and balloon-based experiments have shown the ripples peak at the degree scale. What CMB experimentalists do is take a power spectrum of the temperature maps, much as you would if you wanted to measure background noise. The angular wavenumber, called a multipole l, of the power spectrum is related to the inverse of the angular scale (l=100 is approximately 1 degree). Recent experiments, noteably the Boomerang and Maxima experiments, have show that the power spectrum exhibits a sharp peak of exactly the right form to be the ringing or acoustic phenomena long awaited by cosmologists: