wiki Word:  DARK ENERGY

Definition: Dark energy explains the greatly increasing acceleration of the universe.

Dark energy is not very dense, and it only interacts with gravity and no other force. It is thought to fill up 70% of space since it uniformly fills it in places where no matter exists. We don't know exactly why its there.

Dark energy is thought to exist throughout space and be spread out evenly. It is said to have what is called a "negative pressure" which is that instead of attracting things to it, like gravity does, it repels things, and is so strong and evenly spread that it spreads stars and galaxies apart. As these galaxies spread apart, the universe expands rapidly.

This leads to a theory of the fate of the universe called the "Big Rip" in which the dark energy becomes the single most dominant force in the universe and is so strong that it rips apart all gravitationally bound structures like stars and solar systems until it becomes so strong that it even rips apart atoms.

There are two models of dark energy: the cosmological constant, and quintessence.

In context to dark energy, cosmological constant is a very, very large reservoir of energy that increases as the universe expands. It distinguishes the effects on the universe from matter against those of dark energy. There is a problem with this theory, however, as the amount of energy required to have this reservoir is very great, and would make the universe expand so fast that stars and galaxies could not even form.

Quintessence explains that the universe expands rapidly because of a dynamic field that can change in space and time.

Dark energy was first discovered when people noticed that galaxies were actually moving away from each other. They saw this in the change in infrared light as they moved away from us. The farther they go, the lower their wavelength is. An astronomer named Edwin Hubble created Hubble's law, which shows a proportionality in galaxies' distances with their redshift. This led to the acceptances of previous observations by other scientists about the concept of dark energy and an expanding universe.

 This chart can be  debatable, as some sources say that dark energy makes up about two thirds of the universe and others say that it makes up about seventy percent of the universe, like this chart shows. What we do know is that dark energy makes up most of the universe and we hardly know or understand anything about it.

This term tells why the universe is expanding and accelerationg at such a rapid rate. This term also needs a lot of reasearch, as dark energy has little evidence that shows it exists and more studying needs to be done. It is one of the more interesting subjects of space science that most people do not know about. It would be interesting to learn about it.

1. "Dark Energy." Wikipedia. 11 May 2008. Wikimedia Foundation. 11 May 2008 <http://en.wikipedia.org/wiki/Dark_energy>.

 1.  "Dark Energy." Wikipedia. 12 May 2008. Wikimedia Foundation. 15 May 2008 <http://en.wikipedia.org/wiki/Dark_energy>.

2. Caldwell, Robert R.. "Dark Energy." Physicsworld. 30 May 2004. Institute of Physics. 15 May 2008 <http://physicsworld.com/cws/article/print/19419>.

3.  "Dark Energy." Goddard Space Flight Center. 27 Sep 2004. NASA. 15 May 2008 <http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html>.

http://en.wikipedia.org/wiki/Dark_energy

 4. Pasachoff, Jay M PhD. Science Explorer-Astronomy. Boston, MA: Pearson Education, 2005. Pg. 152

2. Caldwell, Robert R.. "Dark Energy." physicsworld.com. 30 May 2004. Institute of Physics. 11 May 2008 <http://physicsworld.com/cws/article/print/19419#container>. 

3. "Dark Energy." NASA Goddard Space Flight Center. 27 Sep 2004. NASA. 11 May 2008 <http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html>.