N Ryan Burk

The Mystery of Dark Energy

By N Ryan Burk

Atoms, electromagnetism, gravity, and other such constants are concepts which many of us are taught in Physics 1010. We have no problem in accepting these things as the building blocks of our Universe. These are things which can be tested, observed, and measured. It is then reasonable to assume that everything else out there is just empty space. Likewise, we may assume that the galaxies, planets and stars revolve throughout the universe in uniform motions, like unchanging and predictable parts of a clock in an unchanging universe. Yet with modern scientific observances, we have learned that this is simply not the case.

This discovery began with the question, “How far away from us are the nearest neighboring galaxies?” To answer this question, Astronomers relied upon a special kind of Supernova, which allow them to use light in order to measure distance. “…All type 1a supernovae have approximately the same intrinsic brightness, and thus can be used as ‘standard candles’ enabling us to tell how fast distant objects are moving away from us. With that data, we can map out the expansion history of our Universe…” (Hooper, 2006 – pg. 214)

The Supernovae were observed, and strange patterns were found. Not only were Astronomers and Astrophysicists able to find a distance, but they also were able to find that other galaxies were moving away from us, and at an increasing rate. Recently, this same observation was found as stated in a 2011 CBS News report: “The researchers… [observed] a special class of exploding stars called type 1a supernovas, which are useful because they always release the same amount of light. Astronomers compare this intrinsic brightness to their measured brightness - which varies depending on how far away from Earth they are - to judge cosmic distances. The new measurements confirm astronomers' growing consensus that the universe is not only expanding like a balloon, but picking up speed in doing so.” (Moskowitz, 2011)

This finding was fundamentally unsettling to Astrophysicists, because it conflicts with previously set ideas of the cosmos. Idealistically, after the big bang, the expanding universe will continue to grow, but at a slower and slower rate. After all, the highest speeds found in an explosion should be at the center and origin of the explosion. In addition, we can’t forget gravity. Gravity pulls matter and energy together, so if the edge of the universe was shifting, it should logically be shifting inward. This new found data suggested that something else in the universe was causing it to grow and expand faster as more time passed. “The discovery in 1998 that the Universe is actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing.” (Smale, 2009)

Was there something Scientists missed? What could possibly be out in the vastness of space that could cause this expansion? Scientists constructed a new term, “Dark Energy” to describe the new phenomenon. But what is it? Although this new energy in space is mainly a mystery, we do know some things that it can’t be, and what it should be. Dark Energy “…is the extra ingredient that cosmologists require in order to balance the density of the universe and account for why its expansion is accelerating.” (Nicolson, 2007 – pg. 139)

Defining Dark Energy is difficult because it must be an invisible, intangible energy. One thing we can do, however, is measure everything else in space which we do know about. When we take into account all the known matter, dark matter and energies in the universe, a certain void or silhouette is left as a result. The math comes out leaving a tremendous burden on Dark Energy. “It turns out that roughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe.” (Netting, 2010) Not only is this mystery changing the shape of our universe, it is also the major player in it!

For the math to work out, Dark Energy itself becomes a stranger concept. Because it pulls against gravity, it cannot group together like all other matter. Light, stars and galaxies have no effect on it. In order to have such a strong effect on the universe, it must be smoothly distributed throughout all space. It cannot be like anything else we know, in order to fill-in the astrophysical blanks. “Dark Energy… Has to make up for the shortfall in density between the critical density and the actual density of matter and radiation, and it has to provide the repulsive influence that is driving the accelerating expansion of the universe.” (Nicolson, 2007 – pg. 139). Therefore, Dark Energy must fundamentally have positive energy density and negative pressure. This means that it must complete the matter-energy and void ratios in the universe, but it cannot add to the pressure of the universe. So, theoretically, if you were to fill a balloon with Dark Energy, it would not expand, but collapse in on itself due to the negative pressure property.

Albert Einstein, like most physicists of the 20^th century, held the idea of a static universe which kept its general size. He believed that, although there was an “antigravity” energy which naturally existed in space, the pull of gravity and other forces balanced everything out. This resulted in the size of the universe staying constant. Einstein called this antigravity force, the “Cosmological Constant.” Since it has recently been found that the universe is actually expanding, this concept was re-visited. It was possible that Einstein was on to something. According to Einstein’s theory, "empty space" can possess its own energy. “Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster.” (Netting, 2010) The concept sounded like a solution to the mystery of Dark Energy, yet there was one big problem. Einstein’s theory of the “Cosmological Constant” could not account for the magnitude of Dark Energy and its influence. The math simply didn’t work out, and the theory had to be tossed aside.

This mathematical error is also found in a more modern view of a cosmological constant dubbed “Vacuum Energy.” This is thought to be an extra form of energy which adds more density to the universe. “When theoreticians try to calculate the expected density of vacuum energy, they obtain rather startling results. According to the simplest quantum physics calculations, the energy density of the vacuum should be about 10¹²⁰…times greater than it actually is.” (Nicolson, 2007 – pg. 141) This margin of error simply isn’t acceptable in the scientific community. The truth is, a constant in the universe cannot account for the increasing, accelerating, and other mysterious properties of dark matter.

Since the big bang, Astrophysics have calculated that the universe has gone from being radiation and energy dominant, to matter dominant, and then to Dark Energy dominant. In order for our other theories and laws to hold, this ratio must have changed in that order. If Dark Energy was as powerful in the beginning as it is today, then all energy and matter would have been pulled apart before our galaxies could have formed. The universe would have expanded far too quickly. Therefore, Dark Energy must be a non-constant, changing element of the universe. “In order for dark energy to be the major component of the cosmic mass-energy budget now, but an insignificant one in the past, its density must either remain constant or change much more slowly than the density of matter.” (Nicolson, 2007 – pg. 139) A new approach to Dark Energy which examines this changing ratio is called “Quintessence.” Quintessence is named after the fifth element in medieval philosophy, describing the strange perfect essence of the heavens. This theory explores the possibilities the strange idea of Dark Energy may have more variables and “curve-balls” than anyone would have ever expected. “Other possibilities being explored are topological defects, time-varying forms of Dark Energy, or a Dark Energy that does not scale uniformly with the expansion of the Universe.” (Smale, 2009) Quintessence is a leading approach to Eark energy today, yet the prospect of a massive force on the universe that varies significantly through time is an unsettling one. This leads Scientists to a view of a universe which has an expiration date.

Dark Energy (an invisible, weightless, positive energy and negative pressured essence) constitutes 70% of what makes up the cosmos. So what would happen to the universe if it fluctuated dramatically? Three logical results would occur. First, if Dark Energy’s influence decreased in the universe, then it would lead to a “Big Crunch.” This would be the opposite of the “Big Bang. In this case, the universe would slow down, stop, and then pull back in onto itself in an epic implosion, ending all space and time. Secondly, Dark Energy can continue to cause the universe to expand at increasingly higher rates until all forces that keep matter and energy intact would be ripped apart. This is the “Big Rip” theory, and would stretch the universe so thin that nothing we now know would continue to exist. The third result explores the possibility that the universe will grow steadily. At that rate, more space will appear between galaxies, but gravity and other forces will still keep matter intact. The truth is, mankind simply does not possess the data to determine which (if any) of these will be the fate of the universe.

Today, the mystery continues. As the next wave of Scientists rise to new challenges, new discoveries are made and new possibilities are explored. In the case of Dark Energy, who is to say when the puzzle will be solved, or who will solve it? It is a relatively new mystery, and one that will probably take some of the greatest minds of our future to finally answer. Just as “Cosmological Constant” and “Vacuum Energy” hit their snags, many skeptics today are beginning to question each aspect of the Dark Energy approach. Yet, just as the Scientists, Skeptics still only have a few pieces of the puzzle to examine. “It wasn’t until 1998 that the Supernova Cosmology Project at the High-Z Supernova Search Team first announced their discovery that the Universe was expanding at an accelerating rate. It was only with this observation that most cosmologists became convinced that dark energy exists. Dark energy, as a concept, is still in its infancy... Along with the discovery of dark energy, skepticism regarding dark energy’s existence is in its infancy as well.” (Hooper, 2006 – pg. 213) Just as Dark Energy itself is hard to define, the future of its prominence in Astrophysics is also difficult to estimate. Yet the findings in the night sky still keep us wondering, and still motivate our research. “What is Dark Energy? More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery.” (Netting, 2010)

Bibliography

Hawking, S. W. (1988). A Brief History of Time: From the Big Bang to Black Holes. New York, NY:

Bantam Books.

Hooper, D. (2006). Dark Cosmos: In Search of our Universe’s Missing Mass and Energy. New York, NY:

HarperCollins Books.

Moskowitz, C. (2011, March 15). New Measurements Boosts Bizarre Dark Energy Theory. CBS News.

Retrieved from http://www.cbsnews.com/stories/2011/03/15/scitech/main20043535.shtml

Netting, R. (2010, August 19). NASA Science Astrophysics: Dark Energy, Dark Matter. Retrieved from

http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

Nicolson, I. (2007). Dark Side of the Universe: Dark Matter, Dark Energy, and the Fate of the Cosmos.

Baltimore, MD: Canopus Publishing Limited.

Smale, A. (2009, Oct 29). Dark Endergy – Introduction. NASA Goddard Space Flight Center. Retrieved

from http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html

N Ryan Burk

Thursday, May 19, 2011

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