On a clear night, some regions of the sky, such as the Milky Way, have higher densities of stars than others.
But instead of visible light, suppose you had the ability to see only low-energy microwaves. The sky would look very different. Astronomers routinely examine the sky with a variety of telescopes, some of which look only at microwave radiation.
The first such observations showed the intensity of microwaves to be the same in all directions. No structure could be seen. Eventually, the energy spectrum of the microwaves and some other subtle properties of this radiation gave important clues about the universe before it was even 400,000 years old.
The energy spectrum looked like something physicists call "black body" radiation. This is radiation given off by matter, and is characteristic of its temperature. This was truly phenomenal. It also reflected a temperature of just 2.725 degrees Kelvin (above absolute zero). This was very strong evidence in support of the Big Bang model for the beginning of the Universe. Pretty cool!
Just one microsecond (millionth of a second) after the Big Bang started, the universe consisted of a very hot plasma made up of photons (light of varying energies), electrons and baryons (protons and neutrons). As it expanded, the universe cooled.
After about 400,000 years, the temperature had dropped to about 3,000 degrees Kelvin. At this lower temperature, electrically charged protons and electrons combined to form neutral hydrogen atoms.
Photons were now being scattered, or deflected, by atoms. This is in contrast to being absorbed upon colliding with charged particles. Consequently, it is said that radiation and matter had become decoupled. This is equivalent to saying the universe had become transparent to photon radiation (light).
Since the universe is about 13.75 billion years old, the decoupling event had occurred very early in its existence. At this stage, the universe still had no structure. The extremely uniform cosmic microwave background radiation is a relic from this "surface of last scattering".
How could this radiation appear to come from such a cold (2.725 degrees K) source if the temperature of the universe was still several thousand degrees when it became transparent? The answer comes from the continuing expansion of the universe, a central part of the Big Bang model. As space expands the energy of CMB photons is diminished, shifting toward the red end of the energy spectrum.
The Big Bang model predicts the relationship between the amount of redshift and the energy of the CMB. Both the observed uniformity and energy spectrum had proven to be strong confirmations of the model.
There is more to the story. As astronomers looked with ever more sensitive instruments, extremely small deviations from uniformity were discovered. These effects were very subtle and did not upend the earlier findings, but could not be ignored.
The very early universe was a very hot gas occupying a very small space. Under these conditions, inherently small quantum fluctuations are expected. Such fluctuations became magnified as the universe expanded. Enhanced models have been successful at accounting for these irregularities.
Also, there were some characteristic peaks in something called the spatial power spectra of the CMB radiation. The door was opened for cosmologists to piece together a more robust model for development of early universe.
The peaks proved to be a wealth of information. They are thought to have arisen from competing processes between photons and baryons before decoupling occurred. Better understanding of these processes has implications for the curvature of the universe, the density of baryons and the density of something called dark matter.
It has also been observed that a component of CMB radiation has small variations in its polarization. Light glare off a shiny surface is polarized, and the effects can be minimized with sunglasses having polarizing lenses.
The polarization is the result of CMB radiation traversing regions of space having a substantial density of free electrons. Those electrons arose from re-ionization of atoms by ultraviolet light sometime after the decoupling event. Mapping the distribution of this polarized light gives information about the distribution of matter after decoupling.
The history of the Big Bang model and its supporting data provides a good illustration of how science works. Scientific knowledge is always tentative. When new data is developed, if it doesn't support existing models, those models must be abandoned and replaced or revised.
This can either be a big deal or not. Usually, it just results in a more robust model. The old model typically proves to be a special case of the model replacing it. The old model may work under limited circumstances, while the new model accounts for behavior over a wider range of conditions.
Other kinds of telescopes are designed to view the cosmos across the whole spectrum of energies. Each lends a new perspective on the kind of matter making up the universe, its distribution, and what the implications are for the history of the Universe.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.