10 How the World Began
207
10.6 Cosmic Microwave Background
Now that we have established that our universe began with a Big Bang about
13.8 billion years ago, and has been expanding ever since, it is time to look
at some of the events that have occurred along that timeline. We will discuss
the earliest moments of the universe in the next Section, but first let us begin
our story with an important period, the so-called recombination era, about
380,000 years after the Big Bang, and in so doing resolve Olber’s Paradox. At
that time, according to the FLRW model, matter was in the form of a rarefied
plasma and the estimated temperature was around 3000 K. A plasma in a
terrestrial experiment is created when matter is heated until the temperature
is so high that atoms cannot exist, but are split by the thermal energy into
their constituents. Before the recombination era, charged particles (mostly
electrons and protons) could not bind into atoms because the thermal radiation they continuously emitted and absorbed had enough energy to prevent
the formation of stable bonds. The cosmic medium at that time resembled a
bright white fog.
Afterwards, as the temperature decreased below the threshold of 3000 K,
the average energy per photon became insufficient to break the atomic bonds
when they formed, so that hydrogen atoms started to appear. Also photons,
not having enough energy to ionise hydrogen, (i.e. strip its electron away
from the atom) could no longer be absorbed, but only scatter elastically and
the universe started to become transparent.
The light emitted from the hot plasma of the recombination era had a
distribution of frequencies, or a spectrum, that is characteristic of what is
called “black body radiation”. It may seem somewhat odd to call a whitehot, incandescent mass a “black body”. The terminology arises because when
cold, the object is a perfect absorber of incident radiation, i.e. it is black.
This is an ideal situation, because most objects reflect some of the incident
radiation, and are therefore not perfect absorbers. As we have seen in Chap. 5,
the attempt to explain black body radiation resulted in the birth of Quantum
Mechanics.
We expect light escaping from the recombination era and arriving at earth
today to display the type of black body radiation spectrum discussed in
Chap. 5. However, if the light is not absorbed, due to the expansion of
space it will undergo a red shift similar to the red-shift of light from stars
in distant galaxies, which we discussed in the previous Section. In this case,
however, the distance is so great that the radiation wavelength is stretched by
a factor of approximately 1100. What was initially visible light now appears
as microwave radiation, and is known as the Cosmic Microwave Background
207
10.6 Cosmic Microwave Background
Now that we have established that our universe began with a Big Bang about
13.8 billion years ago, and has been expanding ever since, it is time to look
at some of the events that have occurred along that timeline. We will discuss
the earliest moments of the universe in the next Section, but first let us begin
our story with an important period, the so-called recombination era, about
380,000 years after the Big Bang, and in so doing resolve Olber’s Paradox. At
that time, according to the FLRW model, matter was in the form of a rarefied
plasma and the estimated temperature was around 3000 K. A plasma in a
terrestrial experiment is created when matter is heated until the temperature
is so high that atoms cannot exist, but are split by the thermal energy into
their constituents. Before the recombination era, charged particles (mostly
electrons and protons) could not bind into atoms because the thermal radiation they continuously emitted and absorbed had enough energy to prevent
the formation of stable bonds. The cosmic medium at that time resembled a
bright white fog.
Afterwards, as the temperature decreased below the threshold of 3000 K,
the average energy per photon became insufficient to break the atomic bonds
when they formed, so that hydrogen atoms started to appear. Also photons,
not having enough energy to ionise hydrogen, (i.e. strip its electron away
from the atom) could no longer be absorbed, but only scatter elastically and
the universe started to become transparent.
The light emitted from the hot plasma of the recombination era had a
distribution of frequencies, or a spectrum, that is characteristic of what is
called “black body radiation”. It may seem somewhat odd to call a whitehot, incandescent mass a “black body”. The terminology arises because when
cold, the object is a perfect absorber of incident radiation, i.e. it is black.
This is an ideal situation, because most objects reflect some of the incident
radiation, and are therefore not perfect absorbers. As we have seen in Chap. 5,
the attempt to explain black body radiation resulted in the birth of Quantum
Mechanics.
We expect light escaping from the recombination era and arriving at earth
today to display the type of black body radiation spectrum discussed in
Chap. 5. However, if the light is not absorbed, due to the expansion of
space it will undergo a red shift similar to the red-shift of light from stars
in distant galaxies, which we discussed in the previous Section. In this case,
however, the distance is so great that the radiation wavelength is stretched by
a factor of approximately 1100. What was initially visible light now appears
as microwave radiation, and is known as the Cosmic Microwave Background