Cosmology and the Universe: The Big Bang, Dark Matter and Dark Energy 259
of the interstellar gas. It is the interstellar gas and dust, which provides
the material for the formation of new stars within the galaxy. The space
between the galaxies also contains gas and dust particles with densities
believed to be about one millionth of their interstellar counterparts
within a galaxy.
Interstellar and intergalactic space is filled with dust and gas and with
isotropic fluxes of low energy photons, neutrinos and cosmic rays, which
consist primarily of high-energy protons and electrons. These elementary
particles do not interact very strongly with the interstellar dust or the
stars and play little or no role in the formation of stars or galaxies. They
do provide information, however, regarding the origins of the universe.
In addition to all of these observable forms of energy and matter we will
discover that there is also two mysterious substances in the universe that
we will learn more about later in this chapter. They are dark matter and
dark energy.
The Expanding Universe
The vastness and the complexity of the universe is a source of mystery.
One wonders how the universe came into existence and how structures
such as clusters, galaxies, stars and planets were formed. Perhaps the
most mysterious feature of the universe to be explained is its expansion,
which recent observations seem to indicate is actually accelerating.
Before attempting this awesome task, let us first understand how the
expansion of the universe was determined phenomenologically. This
involves understanding how the distances to stars, galaxies and clusters
are determined and how the velocities of these objects are measured.
The determination of the velocities of astronomical bodies in the
direction along the line of sight of the object either towards or away from
Earth is quite simple. The motion transverse to the line of sight is more
difficult to determine especially as the distance to the object increases.
Stars and interstellar gas are composed of exactly the same atoms found
on Earth, such as hydrogen and helium. The frequency of light emitted
by atoms is, therefore, known. If a star is moving away from or towards
us, the frequencies it emits will be Doppler shifted. This is similar to the
Doppler shift of a train whistle. Just as the frequency of the train whistle
increases as the train enters the station and decreases as it leaves, the
frequency of the star’s electromagnetic radiation decreases or increases
depending on whether it is moving away from or toward the Earth.