The probe has also given physicists their first clues about what drove that frantic expansion, and revealed that the cosmic "dark age" before the first stars switched on was twice as long as previously thought.
On Thursday, the WMAP team revealed the best map ever drawn of microwaves from the early universe, showing variations in the brightness of radiation from primordial matter. The pattern of these variations fits the predictions of a physical theory called inflation, which suggests that during the first split second of existence the universe expanded incredibly fast
That simple model should generate strong gravitational waves, which would leave their own distinctive imprint on the polarisation of the microwave background. If either WMAP or ESA's upcoming Planck satellite can detect a gravitational-wave signal, physicists will begin to learn why inflation happened, and that could have profound implications for fundamental physics.
Meanwhile, a slightly more prosaic puzzle has been solved. The original WMAP data suggested that the first stars started ionising gas after only 200 million years, seemingly leaving too little time for gas to gather into clumps and make stars. The new data show it did not happen for 400 million years – a long age of darkness, and plenty of time for the first stars to form.
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