The history and description of Dark Energy

1. Dark Energy
Billions of years ago, the universe was nothing but an infinitesimally small
particle. Then, in less time than the blink of an eye, the universe expanded and
increased in size by a factor of 1050[1]. Expansion eventually began to slow
down, allowing galaxies, star clusters, and so on, to form. Theoretically,
expansion should still be slowing down; but to the contrary, expansion is in fact
accelerating. Some scientists theorize that an unknown force, called Dark
Energy, may be the cause of this accelerated expansion, while others disagree.
For some time, exploding stars, or supernovas, were used as a "cosmic
measuring stick". That is, scientists used these supernovas to calculate the age
of the universe. In 1998, two groups of astronomers surveyed supernovae in
very distant galaxies[2]. These supernovae were much dimmer than expected to
be, and calculations proved that the stars were over ten billion light years away,
much farther away than they should be had the universe been expanding at a
slowing rate, or even a constant rate, as previously theorized. This discovery
proved that the cosmos are not expanding at a slowing or a constant rate, but
instead they are expanding at an accelerated rate. Since this discovery, scientists
have been trying to uncover what it is that accounts for this accelerated
expansion.
Scientists have calculated the density of the cosmos, and they have also
calculated the total mass of all visible galaxies. However, the galaxies make up
less than one-third of the density needed to satisfy the current calculations of the
early universe. Simple logic tells us that there must be something else in the
universe, with some kind of mass, which accounts for over two-thirds of the
density of the cosmos. This was first deduced by Fritz Zwicky in 1933 from
application of the Virial Theorem to the Coma cluster[1]. He showed that the
cluster contains ~ 4000 times the amount of mass in visible stars. The new
theory incorporates a different force called Dark Energy. At first, scientists did
not know how Dark Energy works or what it is physically made up of. Some
proposed ideas of Dark Energy are: a cosmic field associated with inflation, a
low-energy field called "quintessence," and the cosmological constant, or a
negative pressure, as suggested by Albert Einstein[1].

2. In July of 2003, scientists confirmed that Dark Energy exists, but they still
cannot truly explain it. They do know that Dark Energy is different from every
other kind of energy found. Some say it is a negative gravity, while others say
that it does not necessarily act opposite to gravity, but, instead, it acts more like
a negative pressure. Scientists do know for sure that Dark Energy moves space
apart, causing the rate of expansion of the universe to increase. Physically, Dark
Energy is invisible and of an unknown form, but accounts for 65 percent to 75
percent of the makeup of the early universe[2]. However, scientists have no way
of measuring Dark Energy with the current technology, as it affects only the
universe over very large distances, as opposed to gravity which affects the
universe over both large and small distances.
Einstein's Theory of Relativity allowed for the existence of a force such as Dark
Energy. He spoke of a "cosmological constant"[1] [2] that left open the possibility
that even empty space has energy. Also, according to his theory, this energy
must have some kind of mass since energy equals mass multiplied by the speed
of light squared, or
. While most of Einstein's theory makes sense,
some say the amount of Dark Energy decreases with time, as opposed to
remaining constant. It is quite possible that Dark Energy is controlling the
expansion of the cosmos, so understanding the nature of Dark Energy is vital to
the prediction of the fate of the universe. In order to do so, scientists must create
more advanced technology to measure density, pressure, changes with time, and
so on, in galaxies billions and billions of light years away.
There are also those scientists who do not believe in the theory of Dark Energy.
Some physicists claim that they can explain the expansion of the universe
without having to factor Dark Energy into the equations. These scientists say
that gravity is, in fact, the force causing the cosmos to expand. They have added
a term to Einstein's equations in his theory of relativity to support their ideas,
which will not affect the early universe, but will instead affect the universe after
billions of years.
Of course, there are many other theories involving and excluding Dark Energy.
For example, an additional theory is that gravity slowed the expansion of the
universe after the "Big Bang" until the universe was half of its current age.
Then, an opposing force, Dark Energy, consumed gravity and it began pushing
the galaxies away at an accelerated speed. Others say that Dark Energy conceals

3. other dimensions that scientists have yet to find; we know of a first, second and
third dimension, but where are the fourth, fifth, sixth, and so on, dimensions
hiding? (Maybe perhaps String Theory will prove it universally). Some even say
that in order for Dark Energy to be real, space must be flat as opposed to
curved.
Scientists say that Dark Energy could also be responsible for the end of our
universe. Some other theories of the end include the "Big Crunch" and the "Big
Chill" which deal with either an abundance of gravity or a lack of gravity.
However, a new theory, called the "Big Rip,"[2] says that if dark energy
continues to accelerate, it will eventually rip apart stars, solar systems, galaxies,
and even atoms.
In short, scientists are very uncertain as to the nature of Dark Energy. Which
theory is correct? Is Dark Energy just an excuse for the miscalculations of the
density of the cosmos? Could empty space really just be empty space? Will the
universe indefinitely continue to expand, leaving the Earth and the human
species alone for eternity?
Bibliography
[1] B. Carroll, D. Ostlie- An Introduction to Modern Astrophysics
[2] http://en.wikipedia.org/wiki/Dark_energy