Distant type Ia Supernovae indicated the existence of Dark Energy.

This research received the Nobel Prize in Physics 2011, which awarded to Saul Perlmutter (50%), Brian Schmidt, and Adam Riess. (25% each.)

                    

                            Saul Perlmutter

Brian Schmidt

Adam Riess

                               














In early 20th century, Albert Einstein calculated the evolution of the universe according to his theory of general relativity. The final equation told that the universe was expanding. Hence, he put another strange term in his equation, the term contained the cosmological constant in order to gave the stable universe.

Several years after that, Edwin Hubble accidentally found redshift; the change of measured light's wavelenght in longer way which tells us that the object is moving out of us (See: Doppler Effect of Light), of galaxies which are far enough from the Solar System. Furthermore, galaxy's equivalent velocity is proportional to its distance from us. This relation was called "Hubble's Law" and it told us that the universe is expanding. Hence, physicists, even Einstein, believed that Einstein's equation without cosmological constant could explain the universe correctly.

The graph plotted according to Hubble's Law.

We will stop the problem about the evolution of the universe here for a moment. Now, I would like to briefly explain the evolution of stars. When a star has burned all of its nuclear elements out via nuclear fusion reaction, it will change in 2 different ways; collapse to be a white dwarf and explode to be a supernova. The way stars will go depends on their mass, if their mass is more than Chandrasekhar Limit, which is approximately 1.4 solar mass, the supernova will occur.

A white dwarf, compared with the Earth.

              

There is an interesting case; a binary star system contains two stars, when the heavier one, which must die first, dies and becomes a white dwarf. White dwarf can attract the mass from another star to be a part of white dwarf's mass. One day, white dwarf's mass reaches the Chandrasekhar Limit, so, a supernova occurs. The supernovae those occurred like this are called supernova type Ia. They have a special property in that they always occur with object which its mass is around Chandrasekhar Limit, so, its luminosity, the energy radiated per second, is always the same. Since sky objects' brightness depends on luminosity and distance, we can use supernovae type Ia as the distance indicator for distant galaxies. Some astronomers called distance indicators as Standard Candle.

The mass attracting of a white dwarf.

Our three nobel-prize-physicists discovered the recessional velocity of distant galaxies which are much farther than those studied by Hubble. The physicists used the supernovae type Ia to know the distance to those galaxies. Finally, physicists calculated the recessional velocity of those galaxies and found that the universe has expanded with positive acceleration for about 4.7 billion years.

This discovery made the recovery for the cosmological constant, but not in the amount that makes the universe stable. No one knows what makes the cosmological constant nonzero. We just call it "Dark Energy". Hence, this research assured the validity of dark energy in the universe. The universe's dark energy must be much more than matter and radiation in order to make the universe expanding with positive acceleration. Further researches concluded that dark energy dominates about 70% of the overall universe's mass and energy.

Estimated composition of the universe.

References
http://countinfinity.blogspot.com/2010/12/what-is-white-dwarf.html
http://en.wikipedia.org/wiki/Brian_Schmidt
http://en.wikipedia.org/wiki/Saul_Perlmutter
http://newswatch.nationalgeographic.com/2011/10/12/nobel-prize-in-physics-2011/
http://scienginespace.blogspot.com/2011/09/measuring-cosmic-distances.html
Brian Greene (2004). The Fabric of the Cosmos.