Cosmology and the Universe: The Big Bang, Dark Matter and Dark Energy 271
total energy of the non-dark universe. This radiations field does not
interact with matter. It is spread evenly throughout the whole universe, a
remnant of the initial explosion, which set the universe on its present
course.
Before turning to the nature of clusters, galaxies and star formation in
the next chapter we must first examine the role of dark energy in
explaining the accelerating expansion of the universe and the role of dark
matter in the transition of the universe from a homogeneous fluid into its
current state of lumpiness consisting of superclusters, clusters, galaxies,
nebulae and stars.
Dark Energy and the Accelerating Universe
When the Big Bang theory was first proposed it was believed that the
universe had been expanding and was expanding at the same constant
rate. In 1998 this all changed when the observation of a Type 1a
supernovae indicated that the universe was expanding at an accelerating
rate. A Type 1a supernova is a white dwarf star made up of carbon that
suddenly resumes the fusion process when its temperature increases
above the threshold due to the accretion and gravitational condensation
of gas from its surroundings. The supernova acts as a standard candle
that allows an accurate determination of the distance to the galaxy
in which it resides. The conclusion that the expansion of the universe
is accelerating has been since corroborated by a number of other
observations including cosmic microwave background (CMB) radiation,
a more accurate determination of the age of the universe, improved
measurements of highly red shifted supernovae and the x-ray properties
of galaxy clusters.
An explanation of the acceleration of the universe’s expansion
requires that much of the energy in the universe consist of a component
with large negative pressure, which is identified as “dark energy”. Dark
energy is also used to explain that the Universe is very nearly spatially
flat, and therefore according to General Relativity the Universe must
have a critical density of mass/energy. But from the observed
gravitational clustering of the observable mass of the universe a great
chunk of matter/energy is missing that is needed to explain the nearly
spatial flatness of the universe. The missing mass/energy is believed to
be made up of dark energy, which is believed to permeate all of the space
in the universe. The exact nature of dark energy is not understood. Some