This entry presents the third and final installment on the Cosmic Microwave Background (CMB), all of which are part of a larger series I've been writing on the Big Bang and inflationary cosmology. Part III discusses research that followed the CMB's discovery in the early 1960s and continues to the present.
As reported in this 2003 University of Chicago news release on the WMAP research mission, images of the CMB provide a “baby picture” of the universe, "as it looked 380,000 years after the big bang, some 200 million years before any stars or galaxies had formed."
Writes Simon Singh in his book, Big Bang:
It might not be immediately obvious that observing the CMB radiation is equivalent to looking back in time, but exactly the same thing happens when astronomers observe a distant star... ...if the CMB radiation was released billions of years ago and has taken billions of years to reach us, then when astronomers eventually detect it they are effectively sensing the universe as it was billions of years ago, when it was only 300,000 years old (p. 446).
WMAP (alluded to above) and its predecessor mission, the Nobel Prize-winning COBE, were the first to find tiny variations in the CMB across different areas of space. (WMAP's images were designed to be 35 times sharper than COBE's, as seen here in comparative images of the two.)
Prior to the WMAP and COBE missions, observations of the CMB radiation showed it to be absolutely uniform and homogeneous across space (Singh, p. 447). This was problematic, because our present-day universe is obviously not absolutely uniform; some areas of space have larger and denser aggregations of matter than do other areas. In other words, there would need to have been at least some tiny variations in the CMB so that gravity could trigger accretions of matter over billions of years to produce the various kinds of astronomical objects we see today.
The Smoot Cosmology Group -- Smoot being a co-recipient of the Nobel Prize for the COBE discoveries -- discusses the importance of finding even slight non-uniformities in the CMB (scientists use the term "anisotropy" for a non-uniformity; the way I think of it, "iso" means "same," as in the word "isomorphic" which means the same shape, so the "an" in front of "iso" would signify "not the same"). According to the Smoot Group's website:
These anisotropies, or "ripples" in the temperature map, correspond to areas of various density fluctuation in the early Universe. Eventually, gravity would draw these fluctuations into even denser ones. After billions of years, these minute ripples in the early universe evolved, through gravitational attraction, into the planets, stars, galaxies, and clusters of galaxies that we see today.
Dan Hooper's book Dark Cosmos, which I reviewed previously, also provides some nice discussion of the CMB, COBE, and WMAP.