The Cosmic Microwave Background

The Basic Idea

The cosmic microwave background (CMB) is an almost-uniform background of radio waves that fill the universe.  The CMB is, in effect, the leftover heat of the Big Bang itself - it was released when the universe became cool enough to become transparent to light and other electromagnetic radiation, 100,000 years after its birth.  At this time, the universe was filled with a hot, ionized gas.  This gas was almost completely uniform, but did have slight deviations - spots that were slightly (1 part in 100,000) more or less dense.  The slight changes in the intensity of the CMB across the sky (deviations of only than 1 part in 100,000) give us a map of the early universe.  The picture below such a map, measured by the Wilkinson Microwave Anisotropy Probe (WMAP), a space-based microwave telescope for studying the CMB. By studying this map, astrophysicists have learned an enormous amount about the evolution and composition of the universe. 

A Bit More Detail

The study of the CMB is an extremely rich subject which has revolutionized the study of cosmology. It's impossible to do it justice here - for more detail, see cosmology texts and the web references at the bottom of this page.

Why microwaves?

When the CMB was initially emitted it was not in the form of microwaves at all, but mostly visible and ultraviolet light. Over the past few billion years, the expansion of the universe has redshifted this radiation toward longer and longer wavelengths, until today it appears in the microwave band.

Why is it lumpy?

In its early days, the universe was extremely smooth and homogenous... but not quite perfectly so. At the time the CMB was released, for example, its density was constant to about 1 part in 100,000. It is believed this smoothness comes about because of inflation, a time of extremely rapid expansion in the first 10-34 seconds of so of the universe's existence. This rapid expansion smoothed out any lumpiness the universe may have initially had, but quantum mechanical fluctuations introduced new ones - tiny fluctuations of density at all length scales. These tiny fluctuations have grown with time due to gravity (slightly denser regions attract more stuff to become denser yet), eventually providing the seeds for the galaxies and galaxy clusters we see today.

This lumpiness affects the CMB largely because of gravitational redshifting. Radiation emitted from a dense spot in the sky has to fight against a bit of extra gravity as it heads toward our detectors. When it leaves that gravity well, the radiation will be a little less energetic than radiation emitted from a less-dense region, so that spot of the sky will appear to be a little colder. A map of the apparent temperature of the CMB across the sky thus gives you a map of the density of matter in the early universe.

How do we learn about dark matter from the CMB?

Most of the cosmological information we get from the CMB is found by studying its power spectrum, a plot of the amount of fluctuation in the CMB temperature spectrum at different angular scales on the sky. The upper plot at right shows measurements of the power spectrum as of 2003 - large angular scales are at the left of the plot, while smaller sky features contribute to the right of the plot.

The shape of this power spectrum is determined by oscillations in the hot gas of the early universe, and the resonant frequencies and amplitudes of these oscillations (which "notes" the universe likes to play!) are determined by its composition. Since we know the physics of hot gases very well, we can compute the properties of the oscillating gas by studying the positions and relative sizes of these peaks. The position of the first peak, for example, tells us about the curvature of the universe (and hence how much total stuff there is in it), while the ratio of heights between the first and second peaks tells us how much of the matter is baryonic (ordinary matter). In practice, there are many variables that affect all parts of the power spectrum, and detailed computer simulations (the red curve in the plot) are used to sort it all out.

Is there more to do with the CMB?

In the next decade or so, many new CMB experiments are planned. The Planck satellite is expected to study the CMB in even greater detail than WMAP was capable of. The main focus will be on measuring the polarization of the CMB, an early measurement of which is described in the lower part of the plot at right. Studying the polarization (particularly the "B-mode" portion) will give us new windows onto the physics of the early universe, perhaps even letting us learn about some of the details of inflation itself.

For more details, see pages by Wayne Hu and Martin White, particularly their recent Scientific American article.

Jeff Filippini, UC Berkeley Cosmology Group (August 2005)
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