The oldest light we can observe
The cosmic microwave background
The cosmic microwave background, Britannica says, is electromagnetic radiation filling the universe that is a residual effect of the big bang 13.8 billion years ago. Britannica The cosmic microwave background, NASA says, is the oldest light we can observe in the universe. NASA
We can see the heat that was there at about 380,000 years after the expansion of the universe began 13.8 billion years ago (which, in that Q&A, is what is referred to as the Big Bang). NASA This heat covers the entire sky and fills the universe. It still does. NASA
It would be some 380,000 years before the universe cooled enough for these nuclei to capture electrons, forming the atoms we are familiar with today. As they became part of atoms, the captured electrons slowed down, releasing some of their energy that we detect as cosmic microwave background radiation today. NASA The cosmic microwave background, a NASA Physics of the Cosmos page says, originated just 380,000 years after the Big Bang, when the Universe was dense, hot, and opaque. NASA
NASA says WMAP measured the cosmic microwave background radiation over the full sky. This microwave radiation was released approximately 375,000 years after the birth of the universe. NASA The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. NASA
About one million years later the universe was sufficiently cool for atoms to form. The radiation that also filled the universe was then free to travel through space. Britannica
380,000 years, 375,000, or a million?
A NASA Hubble page, a NASA Physics of the Cosmos page, and John Mather’s NASA Q&A all put the cosmic microwave background at about, or just, 380,000 years after the Big Bang (or after the expansion began). NASA’s WMAP overview says that radiation was released approximately 375,000 years after the birth of the universe, and that the afterglow light WMAP saw was emitted about 375,000 years after inflation. Britannica’s big-bang model article says that about one million years later the universe was sufficiently cool for atoms to form. These are not the same figure, and the “one million years” wording is the outlier among the pages we checked.
We observe that same light today, stretched by the expansion of the universe to a cold 2.7K glow. NASA This remnant is the cosmic microwave background radiation — the “three degree” (actually 2.728 K) background radiation — discovered in 1965 by Arno A. Penzias and Robert W. Wilson. Britannica WMAP measured small variations in temperature. For example, one part of the sky has a temperature of 2.7251° Kelvin, while another has 2.7249° Kelvin. NASA
Precise measurements made by the COBE satellite launched in 1989 determined the spectrum to be exactly characteristic of a blackbody at 2.735 K. Britannica
How cold is the afterglow?
A NASA Physics of the Cosmos page calls it a cold 2.7K glow. Britannica’s expanding-universe article also uses 2.7 K. Britannica’s big-bang model article writes “three degree” (actually 2.728 K). Its cosmic microwave background article says the 1965 papers gave about 3 K, and that COBE, launched in 1989, found a blackbody at 2.735 K. NASA’s WMAP overview illustrates tiny differences around 2.7251° and 2.7249° Kelvin. The figures cluster, but they are not copies of one another.
At a redshift of about 1,090, radiation would have decoupled from matter when hydrogen recombined. These photons from a redshift of 1,090 form the cosmic microwave background. Britannica
It was mapped with satellites NASA and ESA built: COBE, WMAP and Planck. The universe at that point was extremely smooth, with only tiny ripples in temperature. NASA COBE, WMAP and Planck all saw further back than JWST, though JWST will see farther back than Hubble. JWST was designed not to see the beginnings of the universe, but a later period, perhaps hundreds of millions of years after the one those missions were built to see. NASA