Portrait of the Universe as a young Universe (380K years): how ancient is the light?

[RTFirefly]

I’m reading the Washington Post story about the pictures that have revealed the structure of the Universe at age 380,000 years.

What the space telescope has seen clearly isn’t light of that vintage, even though

The image was created from the most sensitive temperature measurements ever made of a faint glow that is dissipating through space, the last remaining light from the blinding flare of the big bang.

Since

One of the biggest surprises from the new data is that the first generation of stars to shine in the blackness of space ignited just 200 million years after the big bang.

OK, so what exactly is our space telescope seeing, how long ago did the light leave its source, and how do we extrapolate back from there to get a pic of the Universe at such an incredibly early point in time?

Assuming there’s an explanation that can be grasped by a reasonably intelligent layman, of course.

[rsa]

You might want to check out this similar thread from today.

http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=162554

But, I think the answer you are looking for is 13.7B years. The WMAP is measuring the Cosmic Microwave Background radiation at a temperature of 2.73 degrees above absolute zero.

[bernse]

From what I read, its “heat” left over from the bang, not heat from stars.

[RTFirefly]

Cosmic microwave background radiation, actually. (I was confused by the Post’s use of the term ‘light’ as opposed to ‘radiation’. Since the Washington Post isn’t a scientific publication, I took for granted that they were using ‘light’ in the lay sense, i.e. spectrum visible to the human eye.)

Thanks, rsa, for pointing me to the right thread. I just didn’t pick up on the ‘WMAP’ acronym. The explanations by Achernar and bryanmcc were exactly what I was looking for.