The night sky is dark. Bar the presence of a few distant stars and our lunar companion, we look out into a void of overwhelming darkness. In 1823, astronomer Heinrich Olbers published a paper discussing how strange it was that the sky is dark and not uniformly bright with starlight, coining the term Olbers’ paradox. The paradox has since been resolved; the explanation is related to the expansion of the Universe preventing a large amount of starlight from reaching us.
A Sea of Microwaves
Whilst the night sky is certainly scarce in light at visible wavelengths, it is positively glowing with microwaves. Like visible light, microwaves are a form of electromagnetic radiation, but with much longer wavelengths, ranging from around 1mm to 30cm. These microwaves form an ensemble that exists almost uniformly at all positions in space and is known as the cosmic microwave background (CMB).
Nature of the CMB
The CMB has an average temperature of around 3 Kelvin or -270 degrees Celsius and thus has a very low energy associated with it. Different wavelengths within the CMB make different contributions to the overall energy. The CMB can be well modelled using a black body radiation distribution, with the largest contribution to the background energy coming from radiation with wavelengths of around 1mm, as shown below.
Whilst the energy of the CMB is small, the individual microwaves themselves have very low energies and thus the density of CMB radiation is fairly high. There are around 400 million CMB waves per cubic meter anywhere in the Universe, orders of magnitude higher than the average for matter particles such as protons or neutrons. The CMB also represents over 90% of the total radiation content of the universe; radiation from other sources such as stars is often ignored during cosmological calculations.
Detecting the CMB
The CMB was discovered serendipitously by a pair of American radio astronomers, Penzias and Wilson, in 1964. The pair were confused by the presence of a low level of excess noise which prevailed no matter what direction their radio antenna was facing. After an exhausting effort to try and remove the noise, including shooing away a group of pigeons that had roosted in the antenna, the astronomers were informed by a colleague that they had likely discovered the recently hypothesized CMB.
Confirming the expected properties of the CMB is difficult, even with modern technology, due to its low energy. Measurements are often made from high-altitude balloons to access colder temperatures and reduced noise. Data was taken recently from space by the Planck satellite, more than 1.5 million km from Earth.
Origin of the CMB
Albeit accidentally, Penzias and Wilson discovered the most convincing source of evidence for the hot big bang model of the universe that we have to date. Observations that the universe is expanding imply that the universe was much smaller, denser and hotter in the distant past. The CMB, it turns out, is leftover radiation from the big bang, the remnants of the birth of the universe.
We can shed light on the origin of the CMB by considering the universe in its infancy. Extreme temperatures meant the universe contained only high energy electromagnetic radiation and a sea of protons, neutrons and electrons, prevented from forming atoms by constant ionisation.
There are several important cosmic events to be considered to understand the transition of the aforementioned EM radiation into the CMB. The epoch of recombination is the time at which the density of EM radiation became low enough for the rate of ionisation to decrease to a level where atoms could form. The epoch of radiation decoupling is the time at which the rate of electron-radiation collisions dropped below the rate at which the universe was expanding, allowing radiation to move large distances without interacting with matter. Shortly after came the time of last scattering, the point at which the density of matter became low enough for radiation to flow outwards freely without interaction.
Following the epoch of last scattering, the high energy EM radiation produced in the fires of the big bang flowed outwards without obstruction and filled the universe. This is considered to be the birth of the CMB. Over the course of over 13 billion years, the temperature of the radiation has continued to cool with the expansion of the universe to the low temperature we detect today.
As discussed, the CMB has an incredibly uniform spatial distribution. There are, however, small deviations from the average temperature of around 0.001%. The size of these deviations is related to density and velocity fluctuations at the time of last scattering; their existence suggests that the early universe was not completely homogenous. CMB temperature fluctuations can therefore be used to infer information about the properties of the early and present-day universe.
The extreme uniformity of the CMB certainly sports an element of cosmic elegance, however it poses a problem for cosmology. Consider two points on opposite sides of the visible universe; the CMB is approximately the same temperature at each point suggesting opposite sides of the universe have reached thermal equilibrium. However, the distance between these points is so large that there should not have been enough time for information about each point to be communicated to the other. This is known as the horizon problem. Alan Guth proposed the idea of inflation in 1981, a short period of accelerated expansion at the beginning of the universe, which is currently the most satisfactory solution to the horizon problem.