EXCERPT FROM CHAPTER 8 "MAPPING SPACE: THE CRADLE" FROM WHEN MAPS BECOME THE WORLD
How and why did the Cosmic Microwave Background (CMB) appear? Cosmologists widely accept that an extremely brief period of exponential expansion of space (/inflation/) in the very early universe explains the near-perfect isotropy of the observed CMB (uniformity to one part in one hundred thousand). Inflation ended after a very small fraction of a second, and the energy of inflation was converted into a thermal bath of elementary particles at very high temperature. At that time, the universe was a rapidly expanding, high-energy plasma soup made up of leptons of various kinds such as electrons and baryons (protons and neutrons). This is a /radiation dominated/ era during which all particles were hot – that is, the particles were traveling at nearly the speed of light. As the temperature dropped, protons and neutrons stopped being free particles and become non-relativistic. Free neutrons do not survive indefinitely, but decay into protons.
Nucleosynthesis lasted from approximately 3 to 20 minutes after the Big Bang, as those protons and neutrons that had not decayed were able to bind, first creating the isotope 2H (deuterium), then initiating a chain reaction producing isotopes of the other light elements, 3He, and 4He. Trace abundances of other light elements (chiefly 7Li) also formed, but the only nuclei produced in any significant abundance were H (because there were not enough neutrons around for all protons to bind with) and 4He, the most stable light nucleus. Even so, today there are about 14 times more H than 4He.
As the era of radiation dominance came to an end (after about 50,000 years), only photons and neutrinos were still relativistic, traveling at or close to the speed of light. The universe then evolved into a /matter dominated/ period in which the energy density in non-relativistic particles (including so-called /dark matter/) exceeded radiation energy – and it still does. Initially temperatures were still too high for stable atoms to form. Free electrons caused typical photons to undergo frequent scattering. This kept the universe opaque, in a state of near-perfect thermal equilibrium.
The universe continued expanding for hundreds of thousands of years, and kept cooling down. At roughly 220,000 years old, the universe had a temperature of approximately 4000 Kelvin, and protons and electrons started /recombining/ or /combining/ as mainly hydrogen atoms. This process took about 150,000 years, until at about 3000 Kelvin almost all charged particles were bound. As electrically charged particles became increasingly absent, a typical photon no longer scattered but traveled freely. The universe somewhat slowly went from opaque to transparent. This process, through which photons were /decoupling/ from matter, produced a /last scattering surface/ or, more precisely, /last scattering layer/. Photon and matter temperatures were no longer in equilibrium, and so photons and matter could evolve separately. Today, we detect this last scattering layer as CMB coming to us from all directions.
After 370,000 years, and with the universe now transparent, the cosmic microwave background continued becoming redshifted from its original almost ideal black body infrared radiation peaking at about 3000 K to its current black body distribution of microwave frequencies peaking at 2.725 K. This is because it has traveled to us as if from an ever-expanding diffuse shell projection very far away, and increasingly receding (almost at the edge of the observable universe), and almost as old as the universe itself. The CMB is the signature or mark of the photons suffusing the entire universe when it was 370,000 years old, and the radiation will not “run out” as it was—and is—everywhere. Indeed, per the cosmological principle, think of the universe as having its center everywhere, and its circumference nowhere. Following this principle, the CMB map will look the same from anywhere in the universe.
How is space itself imagined on this map? CMB maps may teach us about the features of our early universe, but they also raise philosophical questions about representation. Recall Tom Van Sant’s map (Chapter 1). Despite claims to the contrary, this map was not the ultimate map. It was caught in history (as is Google Maps). Based on satellite imagery, it required multiple types of technical processing. Furthermore, it shows the entire Earth as if it were high noon, everywhere. CMB maps scrub the data observationally—for example, by abstracting out microwave radiation emanating from our galaxy’s disk, and other irrelevant, localized sources (Figure 1). Moreover, statistical protocols are required for data analysis and presentation. Such processual maps are complex achievements, not naïve, simple snapshots of the world.
Although we have learned that the map is not the territory (MINT), it is tempting to identify the CMB scalable map with what it represents. In these maps, the object of detection and representation is precisely and interestingly the same: photons. The actual material emanating from the universe’s birth is detected, abstracted, and represented. While forests, seas, and cities are not miniaturized and captured in the pixels of Van Sant’s composite satellite map (Figure 2), CMB photons are tiny remnants of the early universe that traveled over 13 billion years, smashed into detectors on satellites, and were transformed and represented, by and for /Homo cartograficus/, on the CMB map, on photon-emanating screens and paper. There is a kind of map-territory immediacy here. Unlike vapor trails of a jet engine or DNA electrophoretic bands—or so many other data types in science—these photons are not mere indicators, tracks, or indices. Bluntly put and while wishing to avoid pernicious reification of the CMB map, this map is a copy, a mirror, and a scaled-down representation of the actual birthing universe, as fed through statistical and computational procedures. [Footnotes suppressed.]