Following Lemaitre’s article in 1931 regarding the expansion of the universe, several theories involving the implication of such an expansion emerged. One of these theor-ies was "The Origin of Chemical Elements" by Alpher and Gamow. The same year, following this article, Alpher and Robert Herman published a prediction of a "relic ra-diation" as an effect of the expansion of the universe which at present time should have a temperature of 5◦ K. This signature radiation would not be discovered until over a decade later. In an attempt of removing interference from a receiver at Bell Labs in New Jersey, Arno Penzias and Robert Wilson found a microwave signal coming from all directions. Unable to remove the noise signal, Penzias and Wilson came into contact with Princeton physicist Robert Dicke who, having done research on this radiation, informed them what they had actually found. As a result of their findings, Penzias and Wilson received the Nobel Prize in physics 1978 for their discovery of the CMB.
After the discovery of the CMB, the hunt for anisotropies in the CMB signal began.
According to theory, large scale structures in the universe such as galaxy clusters and filaments originated from small perturbations which grew to modern day size due to gravitational instability. The measurements of these fluctuations remains an ongoing endeavor. Already in the 1980s their upper boundary was set to (∆T /T .10−4). This value is too low for baryons to alone explain the large scale structures we see in the universe today. Remember from section 1.6 that during the early universe baryons were coupled and could not collapse under gravity to form structures. The time baryons have had to collapse is simply insufficient to account for the large structures we observe today. To explain these structures we need particles that could start to collapse under gravity before the baryons, such that after recombination baryons could fall down into these already existing gravitational halos. These unseen non-interacting particles are called dark matter and the search for them continues to this day. Additional proof of dark matter include the direction of the CMB dipole and the distribution of mass in rotating spiral galaxies.
Since its discovery, many different experiments have been carried out to measure the CMB anisotropies and polarization. Three of the most notable are "The Cosmic Background Explorer" (COBE),The Wilkinson Microwave Anisotropy Probe (WMAP)
and Planck.
2.2.1 COBE
COBE was NASA’s first cosmological satellite, and its goal was to measure the CMB radiation. Active between 1989 and 1993, COBE is often regarded as the "inception of cosmology as a precise science" (The Royal Swedish Academy of Sciences, 2006).
Installed on COBE were three data gatherings instruments,The Differential Microwave Radiometer (DMR), The Far-InfraRed Absolute Spectrophotometer (FIRAS) and The Diffuse Infrared Background Explorer (DIRBE).
The DMR instrument was designed to map the fluctuations in the brightness of the CMB radiation. One of the great successes of COBE was the DMR detection of CMB anisotropies on all observed angular scales. This detection provided support to the Big Bang theory which had begun to receive criticism due to the previous failure to detect these fluctuations [25].
The FIRAS instrument was a spectrophotometer installed to measure the CMB spectrum. The FIRAS measurements of the monopole and the dipole provided a CMB spectrum that fit a blackbody spectrum so precisely that it was determined that less than 1 part in 10 000 of the total energy of the CMB was released more than 1 year after the Big Bang [26].
Figure 2.1: The cosmic background data from COBE with the blackbody curve fit at 2.74 K. Figure from 2.1
The main goal of the DIRBE instrument was to map dust emissions from distant
galaxies. These emissions are referred to as Cosmic Infrared Background (CIB). CIB originates from redshifted, absorbed or re-emitted radiation from the earliest galaxies and stars. CIB is one of the foregrounds of CMB and through a detailed mapping over a wide frequency range this component could theoretically be subtracted from CMB.
CIB is however contaminated with dust emissions from both our own solar system (the zodiacal cloud) and from dust grains in the Interstellar Medium (ISM). The difficulty of separating the dust-emission foreground from our galaxy to the CIB foreground has led the treatment of both as a single foreground,thermal dust.
WMAP
After the release of the COBE data there were still several questions that needed to be answered. Some of these included
• Do we live in a flat or curved universe?
• How old is the universe?
• Will the universe keep expanding forever?
• How old are the oldest large structures?
• What is dark matter and how much of it is there?
COBE, which studied the CMB with an angular resolution of 7◦ across the sky, did not have the resolution, sensitivity or accuracy to answer these questions. A new, more sensitive experiment was needed, and from 2001 until its decommission in 2010 theWilkinson Microwave Anisotropy Probe (WMAP) did just this. WMAP managed to constrain the shape of the universe to flat within 0.4%and has determined the age of the universe to be 13.77 billion years to within 0.5%. It managed to determine that baryons only make up for 4.6%of the universe. 24%is made out of dark matter and the remaining part consists of dark energy. Due to the apparent flatness of the universe the WMAP team also concluded that the universe is undergoing an accelerated expansion, and that it will continue to expand forever [27].
Planck
The Planck satellite was active from 2009 to 2013. Its motivation was a once and for all measurement of the intensity and polarizations of the CMB anisotropies. Having exact measurements of the CMB allowed for precise calculations of cosmological parameters which in turn enables realistic models of how the universe works. Among other scientific goals, the Planck mission also studied the Milky Way to map the cold dust distribution along the spiral arms, catalogued galaxy clusters by measuring theSunyaev-Zel’dovich (SC) effect and searched for gravitational waves [28]. In order to achieve the necessary precision and resolution of the angular power spectrum the Planck experiment had an angular resolution corresponding to three times smaller scales than for the WMAP mission. In addition, Planck observed in nine frequency bands compared to the five
bands on WMAP yielding much more information regarding foreground models than previously accessible.
The final results from the Planck experiment shows strong support for the Λ-CDM (dark energy and cold dark matter) model which is the dominating cosmology model today. Some of the parameters the Planck experiment measured included, the age of the universe (13.797 billion years), that 68.47%of the universe consists of dark energy and that 31.53%consists of dark matter and baryons.
The Planck satellite was deactivated in 2013. The experiment officially ended in July 2018, when the final science paper from the Planck team was published [29].