Creating the Microwave Background

The figure shows why we see a microwave glow when we look far, far away, and therefore look way back in time, almost to the Big Bang:

Here's a brief description to go with the diagram, as it unfolds:

1. The Universe begins its life hot & bright & dense & foggy. It is bright because it is so hot, and foggy because it is too hot for atoms to form – we say the gas is ionized. Within ionized gas, electrons are free to scatter light – like water droplets in an Earth-bound fog.

 
2. As the Universe expands, it also cools. After about 400,000 years, when the temperature drops below 3000 K (5000 °F), electrons finally get caught by protons to make atoms. With no free electrons, the fog clears and the Universe suddenly turns transparent – like the rapid clearing of a morning mist.

 
3. Thus, when we look out through the transparent Universe, back in time, our gaze finally meets a bright wall of glowing fog.

 
4. However, as the light from this wall crosses the Universe on its way to us, the Universe's expansion stretches the light waves a thousand-fold, and they finally arrive as microwaves, visible to microwave telescopes, not to our eyes.

The CMB is Young and Far

Astronomers are lucky, our telescopes are like time machines. Looking out into space, we see distant objects as they were when their light set out on its long journey. Telescopes, therefore, look down time-tunnels, at the end of which sits the Big Bang.

In the 14 billion year timeline which stretches from the Big Bang to us now, the moment at which the fog cleared is very early indeed. Compared to an 80 year human lifespan, it occurs just 12 hours after conception. Amazingly, we can actually witness this remote time.

This sketch illustrates the immense distance of the CMB:

The top two lines show a few choice locations/moments along the cosmic distance/time line: The Hubble Telescope (HST) can study teenage galaxies; its successor (JWST) will study child galaxies. Microwave telescopes witness, written on the foggy wall, the cosmic DNA of the first cell.

In terms of distance: traveling out along a 26 mile marathon race, the wall of fog lies just 4 feet from the finishing tape. How frustrating! The Big Bang moment is forever hidden behind the fog – so close and yet so far.

The Big Bang is all around us!

Here's the most bizarre aspect of all: since looking in any direction looks back to the foggy wall, then we see the wall in all directions. Thus, the entire sky glows with microwaves coming from the hot gas of the early Universe: the flash from the Big Bang is all around us!

Although our eyes can't see them, there are as many microwave photons falling from the sky as there are photons of light falling from a full moon. With microwave sensitive eyes, at night we could see our surroundings and cast shadows by the light of creation.

Observing the Microwave Background

The history of observing the microwave background is fascinating and has been told many times. Here is an extremely brief summary:

Penzias & Wilson (1963)	COBE (1992)	WMAP (2003)

The CMB was discovered, essentially by accident, in 1963 by Arno Penzias and Robert Wilson, for which they rightly earned a Nobel prize. Since then there have been many attempts to measure both the CMB spectrum and its patchiness. For our purposes, there were two very important observations. First was NASA's 1992 COBE satellite. This measured a pure "thermal" spectrum, confirming that the CMB came from hot gas. This satellite also measured, for the first time, slight patchiness in the emission brightness – variations from place to place by about 1 part in 100,000 – equivalent to the height of a bacterium sitting on a bowling ball. COBE's map, however, was blurred by ~7° (about a fist at arm's length), and a much sharper all-sky map was made in 2003 by NASA's WMAP satellite (resolution ~¼°, or a pea at arm's length). The next major step forward will be ESA's Planck satellite, to be launched in 2009.

All-sky maps of the CMB

Here are four depictions of the all-sky microwave background. The first three are as "Hammer-Aitoff projections", and so look oval. The final two are spherical.

	The complete surface of the spherical Earth can be shown as a flat oval map (a Hammer-Aitoff projection). The full sky panorama is also a spherical surface, and its appearance (in microwaves) can be shown using similar ovals.
	The CMB is highly uniform, as illustrated here. This means the young Universe is extremely smooth. Click images for more details.
	But not completely: COBE's 1992 map showed patchiness for the first time. Red to Blue = tiny differences in brightness. Resolution 7°.   Movie fades between COBE and WMAP (176 kB).
	WMAP's now famous 2003 map of CMB patchiness. Resolution ¼°.   Movie zooms and pans the image (3 Mb).

Spherical versions of the raw and stretched WMAP all-sky image.   Movie spins the ball around (2.8 Mb).

Making the microwave background maps is an extremely difficult and technically challenging process. Most details are irrelevant to our story – but here are two that are more interesting: link.