New enigma in physics: Dark Energy

Kyshalee Vázquez

Common beliefs concerning the universe are that it is stationary and mainly composed of everything we can perceive with our senses. Actually, when we think about the universe what comes to mind may be stars, planets, galaxies, perhaps black holes and nebulae. However the National Aeronautics and Space Administration states that these celestial bodies only compose less than five percent of the universe, a miniscule portion of such a grand cosmos. Then, what is the remaining portion of the universe made off? And if the universe isn’t stationary, what is causing its motion?

After the Big Bang the universe started to expand, and all the matter was close enough to be pulled together by gravitational forces to form the universe we study today. Astronomers knew the universe was still expanding from the energy of the Big Bang, but it was believed that this expansion was decelerating. This deceleration was attributed to the fact that the universe had enough mass that would pull on each other by gravitational forces, and therefore slow down the process of expansion. However the rate of this deceleration was unknown and consequently two groups of astronomers searched for answers. 

The idea was to find Type Ia supernovae because they reach the same luminosity when they undergo a thermal explosion. By determining the brightness and measuring the redshift it is possible to know the distance of the supernova and how long ago they occurred. Of course this process took a few years because Type Ia supernovae are hard to come by, for their brightness only lasts a few weeks. With the information gathered after this long research process it was evident that the stars were farther away than they should be, if the universe were slowing down. These two groups of astronomers were confused since this information indicates that the expansion of the universe must be accelerating and not decelerating.

As it is usual in science, another question emerged. Why is the expansion of the universe speeding up if it has enough mass to slow it down? A new factor had to be added to this quest that was not considered by these astronomers at first, but it is believed that it was considered by Albert Einstein when he introduced the cosmological constant in is general relativity theory.  Astronomers thought that this new factor, called dark energy, must be a kind of repulsive force that keeps separating the galaxies farther away expanding the universe. 

Although dark energy keeps separating these immense groups of celestial bodies from each other, we shouldn’t worry about it separating Earth from the Sun, because there is yet another presence that like dark energy we cannot perceive with our senses, called dark matter. Dark matter is held responsible for the formation of galaxies, because there is so much dark matter around the celestial bodies creating a gravitational attraction additional to the gravitational pull they already have as masses of normal mater. It is not likely that dark energy will separate the planets or stars in a galaxy itself due to this dark matter. Both dark energy and dark matter are relatively new to our knowledge and very little is known of them.

Finally after this series of discoveries, scientists have concluded that dark energy and dark matter were created in the Big Bang along with normal matter. Because the universe was smaller, all the matter was closer and the force of gravity was greater than dark energy, so the expansion of the universe was slower. As the universe kept slowly expanding dark matter and gravity formed the galaxies, continuously segregating them farther apart. When dark energy overwhelmed the gravitational force, due to the growing distance, the universe started accelerating in its expansion.

Illustration shows the changes in the rates of expansion since the universe’s birth.

Bibliography

Netting, Ruth. "Dark Energy, Dark Matter." NASA Science Astrophysics.    Web. 15 April 2011.

Smale, Alan. "Dark Energy." Imagine the Universe. 29 October 2009.     Web. 15 April 2011.

Villard, Ray and Riess, Adam. "Refined Hubble Constant Narrows Possible Explanations for Dark Energy." HubbleSite. 7 May 2009. Web. 15 April 2011.