Wilkinson Microwave Anisotropy Probe

Enter the Wilkinson Microwave Anisotropy Probe. WMAP was conceived as a way of pushing to a new level of precision and a new set of tests of the big bang theory. Most of those tests involve looking at anisotropies in the radiation, small variations in tem­perature from one part of the sky to another.

The all-sky map of microwave radiation has to have foreground emission from the Milky Way removed before it can be interpreted. There is a temperature gradient across the sky caused by the mo­tion of the Solar System relative to the universe as a whole at a speed of about 360 kilometers per second. The microwave sky is 0.00335 K warmer toward the direction of our motion and the same amount cooler in the direction opposite to our motion. That small signal is also modeled and subtracted out.28 What’s left is a mottled pattern of very low-level variations. COBE had enough sensitivity to detect the variations statistically, but with an angular resolution of 7 degrees (the angle of your outstretched fingers at arm’s length) it could not say much about the detailed structure of the radiation.

COBE was a small satellite that traveled in a 900-kilometer high orbit of the Earth. The instrument that measured temperature variations had two horn receivers pointing in different directions, with the satellite rotating every 70 seconds so they could sweep across the sky. WMAP was a much larger and more sophisticated satellite, even though it was a third the mass of COBE. It collected microwaves with a pair of 1.5-m dishes and its receivers detected the radiation in five frequency bands. It rotated every 130 seconds and made a complete map of the sky every six months. The satel­lite was launched in 2001 and sent to a Lagrange point (where the gravity of the Earth and Moon balance) 1.5 million kilometers from Earth, where the contaminating radiation is much lower than in low Earth orbit. As a result, WMAP was forty-five times more sensitive than COBE and it was able to resolve thirty-five times smaller regions on the sky, or two times smaller than the angle of the full Moon in the sky (plate 21). The large difference is com­parable to the gain of the Hubble Space Telescope over a one-foot diameter telescope of the ground.29

WMAP operated flawlessly for ten years, and the exciting re­sults of COBE and WMAP generated the momentum for a third – generation microwave satellite called Planck, named after the Nobel Prize-winning German physicist. Planck is primarily a Eu­ropean mission. Launched by ESA in 2009 to a location at the same Lagrange point as WMAP (L2), it improves on WMAP in both sensitivity and angular resolution.30