In the epoch following the Big Bang the universe was opaque to electromagnetic radi-ation, due to the Thomson scattering of free electrons with very short mean free path.
As the universe expanded, the energy of these electrons decreased to the point where free electrons could bind to protons, creating the first neutral hydrogen. During this process, the amount of free electrons rapidly decreased until the remainder could travel freely without collisions. This event is referred to as Recombination, and the last time photons collided (scattered) is calledthe surface of last scattering(SLS) which occurred
1Hans Bethe is not mentioned on purpose.
approximately 380 000 years after the Big Bang. As the electrons, now bound to pro-tons to form neutral hydrogen, settled into more stable energy states, phopro-tons were released. These decoupled photons continued travelling freely and can be seen today as the CMB radiation. The study of the CMB is a whole field within cosmology by itself, and provides fundamental insights regarding the early universe. The CMB provides us with the earliest information regarding the statistical properties of the universe . 1.6.1 Dark ages and Epoch of re-ionization
Following Recombination our universe entered what is called the "Dark ages". The Dark ages are so called since all photons are travelling freely within the universe, and no new photons are generated since stars have yet to form. We therefore have very few means of collecting information from this time period. However, as the age of the universe grows, the baryonic matter in the form of dense molecular clouds, falls further down into the DM-halos potential wells. Eventually the baryonic matter collapses under gravitational pressure, resulting in the creation of the first luminous objects. At 400 million years (z ≈ 15) after the Big Bang the first galaxies started to evolve. These galaxies contained population III2 and population II stars, as well as black hole driven sources such as mini-quasars. The ultraviolet radiation emerging from these galaxies starts to ionize the surrounding gas. This is known as the beginning of the epoch of reionization. The Inter Galactic Medium (IGM), being neutral, ionizes and heats up the universe, ending the dark ages. When a sufficient amount of radiative sources has formed the IGM becomes completely ionized, which is the situation today to a very good approximation. Recent evidence suggest that the epoch of ionization occurred at redshift z= 15 to z = 6, where z = 6 corresponds to the time when the IGM is completely ionized [22]. As this ionizing radiation propagates the IGM it leaves behind radiative fingerprints, which we can analyze. These are i.e. the 21cm line, CII lines, Lyα lines and CO lines. These signals can act as tracers of star-forming regions and are thus a very important research topic in modern day cosmology.
2Population III stars have as of yet not been detected
Observational cosmology
Ever since Karl Jansky detected the first radio signal from outside our own solar sys-tem in the 1930s, observational astronomy has played a big role of modern cosmology.
In observational astronomy, we either focus on individual sources of radiation or the statistical properties of a wider field. Galaxy surveys, solar studies and exoplanerary studies are some examples of experiments focusing on the study of individual sources while the CMB and the CIB experiments are examples of wider field experiments. In this chapter we aim to summarize the main features of modern observations with the hope of both providing a broad picture of the history of cosmological experiments as well as highlighting their shortcomings. We will place extra focus on the Cosmic Mi-crowave Background (CMB) and galaxy surveys as they are the most complementary for intensity mapping (IM).
2.1 Space-based vs ground-based experiments
The space-based experiments can be divided into two types, orbital experiments (satel-lites) and interplanetary experiments (ie. "New Horizons"). However, since interplanet-ary missions yield little information in terms of cosmology my main focus in this section will be on satellites.
The biggest advantage with satellites is the absence of light pollution and atmo-spheric pollution which ground-based experiments have to deal with. Although very effective experiments, there are significant draw-backs concerning satellites efficiency.
Satellite missions have an unfortunate historic tendency to cost more than originally planned. One example of this is the James Webb Space Telescope (JWST). Named after the former NASA administrator James Webb, the JWST was originally planned as a low-cost experiment. In 1997 the JWST (then named NGST) had a budget of 0.5 billion USD and a preliminary launch date in 2007. After a multiple budget corrections and launch date extensions, in 2011 the US House of Representatives effectively can-celed the whole project by withdrawing funding to NASA. This decision was fortunately reversed by the US congress later that same year. Congress set a new budget limit at 8 billion USD for the planned launch in 2018. Latest updates from NASA [23] estimates
a launch in March 2021 and have as of now exceeded the congress approved budget by 800 million USD.
Although the JWST is somewhat of a extreme example in terms of satellite budgets it does highlight some important factors for long term projects. First of all, long term projects depending on state funding can greatly suffer due to a change in policy. Other examples of this are the Hubble Space Telescope (HST) and the COBE satellite which suffered delays following the Challenger disaster [24] in 1986. Second, since no mech-anical adjustments can be made once the satellite is launched it is important that the technology on board is up to date. The last factor especially is a common source of delays for space based missions.