7.2. What is the CMB?

The observable universe is expanding and cooling. Therefore in the past it was hotter and smaller. The cosmic microwave background (CMB) is the after glow of thermal radiation left over from this hot early epoch in the evolution of the Universe. It is the redshifted relic of the hot big bang. The CMB is a bath of photons coming from every direction. These are the oldest photons one can observe and they contain information about the Universe at redshifts much larger than the redshifts of galaxies and quasars (z 1000 >> z few).

Their long journey toward us has lasted more than 99.99% of the age of the Universe and began when the Universe was one thousand times smaller than it is today. The CMB was emitted by the hot plasma of the Universe long before there were planets, stars or galaxies. The CMB is thus a unique tool for probing the early universe.

One of the most recent and most important advances in astronomy has been the discovery of hot and cold spots in the CMB based on data from the COBE satellite (Smoot et al. 1992). This discovery has been hailed as "Proof of the Big Bang" and the "Holy Grail of Cosmology" and elicited comments like: "If you're religious it's like looking at the face of God" (George Smoot) and "It's the greatest discovery of the century, if not of all time" (Stephen Hawking). As a graduate student analyzing COBE data at the time, I knew we had discovered something fundamental but its full import didn't sink in until one night after a telephone interview for BBC radio. I asked the interviewer for a copy of the interview, and he told me that would be possible if I sent a request to the religious affairs department.

The CMB comes from the surface of last scattering of the Universe. When you look into a fog, you are looking at a surface of last scattering. It is a surface defined by all the molecules of water which scattered a photon into your eye. On a foggy day you can see 100 meters, on really foggy days you can see 10 meters. If the fog is so dense you cannot see your hand then the surface of last scattering is less than an arm's length away. Similarly, when you look at the surface of the Sun you are seeing photons last scattered by the hot plasma of the photosphere. The early universe is as hot as the Sun and similarly the early universe has a photosphere (the surface of last scattering) beyond which (in time and space) we cannot see. As its name implies, the surface of last scattering is where the CMB photons were scattered for the last time before arriving in our detectors. The `surface of last screaming' presented in Fig. 9 is a pedagogical analog.

Figure 9. The Surface of Last Screaming. Consider an infinite field full of people screaming. The circles are their heads. You are screaming too. (Your head is the black dot.) Now suppose everyone stops screaming at the same time. What will you hear? Sound travels at 330 m/s. One second after everyone stops screaming you will be able to hear the screams from a `surface of last screaming' 330 meters away from you in all directions. After 3 seconds the faint screaming will be coming from 1 km away...etc. No matter how long you wait, faint screaming will always be coming from the surface of last screaming - a surface that is receding from you at the speed of sound (`vsound'). The same can be said of any observer - each is the center of a surface of last screaming. In particular, observers on your surface of last screaming are currently hearing you scream since you are on their surface of last screaming. The screams from the people closer to you than the surface of last screaming have passed you by - you hear nothing from them (gray heads). When we observe the CMB in every direction we are seeing photons from the surface of last scattering. We are seeing back to a time soon after the big bang when the entire universe was opaque (screaming).