CMB Surface of Last Scatter
The cosmic microwave background radiation's "surface of last scatter" is analogous to the light coming through the clouds to our eye on a cloudy day.
When WMAP observes the microwave background sky it looks back to when there were free electrons that could readily scatter cosmic background radiation. This cosmic background "cloud surface" is called the "surface of last scatter". If there were any "features" imprinted in this surface of last scatter (i.e.- regions that were brighter or dimmer than average) they will remain imprinted to this day because emitted light travels across the universe largely unimpeded.
Site link: What WMAP Records
Credit: NASA / WMAP Science Team
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WMAP # 990053