Rasmus Grønfeldt Winther

How and why did the Cosmic Microwave Background (CMB) appear? Cosmologists widely accept that the near-perfect isotropy of the observed CMB (uniformity to one part in one hundred thousand) is explained by an extremely brief period of exponential expansion of space (inflation) in the very early universe. As inflation ends after a very small fraction of a second, the energy of inflation is converted into a thermal bath of elementary particles at very high temperature. At this 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 in which all particles are “hot” (i.e., travel at nearly the speed of light). As the temperature drops, protons and neutrons stop being free particles and become non-relativistic. Free neutrons do not survive indefinitely but decay into protons.

Nucleosynthesis lasts from approximately 3 to 20 minutes after the Big Bang, as those protons and neutrons that have not decayed are able to bind first creating 2H (deuterium), initiating a chain reaction producing the other light elements 3He and 4He. Trace abundances of other light elements (chiefly 7Li) are also formed but the only nuclei produced in any significant abundance are H (because there aren’t enough neutrons around for all protons to bind with) and 4He, the most stable light nucleus. Even so, there are about 14 times more H than 4He.

As the era of radiation dominance comes to an end (after about 50,000 years), only photons and neutrinos are still relativistic. The universe then evolves into a period of matter domination in which the energy density in non-relativistic particles (including so-called dark matter) exceeds radiation energy. Initially temperatures are still too high for stable atoms to form; a typical photon undergoes frequent scattering off free electrons. This keeps the universe opaque, in a state of near-perfect thermal equilibrium.

The universe continues expanding for hundreds of thousands of years, and keeps cooling down. At roughly 220,000 years old, the universe has a temperature of approximately 4000 Kelvin, and protons and electrons start recombining or, more aptly semantically, combining as mainly hydrogen atoms. This process takes 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 travels freely. The universe somewhat slowly went from opaque to transparent. This photon decoupling from matter produced a last scattering surface or, more precisely, last scattering layer, today detected as CMB coming to us from all directions. Photon and matter temperatures were no longer in equilibrium, and could evolve separately.

After 370,000 years, and with the universe now transparent, the cosmic microwave background continues 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 (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.

Original Image

Original Image