Kiana Shaffer

Elementary Astronomy

Jennifer Noon

December 1, 2012

ePortfolio Essay: Dark Energy & Dark Matter

            The Universe is a very interesting subject when it comes to astronomy, because it’s still unknown to what it’s made of, or how long it will last before its collapses. It’s no secret that gravity is the controlling force of the Universe. But in the 1990’s they wondered about the expansion of the Universe if it might have an abundance of energy density that it would stop its expansion and recollapse, or if it has little energy density that it would never stop expanding.  In 1998 the Hubble space telescope confirmed that, it observed that the Universe was expanding a lot slower than it is today. It is actually accelerating due to gravity but no one knows exactly what is causing this reaction. Throughout this paper it will discuss how dark matter effects dark energy from expanding the Universe.  

            There have been theorists and astronomers that have come up with three reasons. First reason could explain what was causing the expansion, was a result of Einstein’s theory of gravity that contained what was called a “cosmological constant”. Second they figured that there was some sort of strange kind of energy-fluid that filled space. And third that there was something wrong with Einstein’s theory of gravity and maybe there was a new theory that could include some sort of field that results in a cosmic acceleration. Theorists and astronomers haven’t discovered the real explanation about Universe expansion but they came to the conclusion that it is called dark energy. The chart below shows the rate of expansion due to dark energy. The description below the table explains it.

This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.

                Dark Energy is defined as a property of space, and Albert Einstein was the first person to realize that empty space is not nothing. Astronomers know how much dark energy there is in the Universe because they know how it affects the expansion. Besides that dark energy is a mystery. It turns out that “roughly 70% of the Universe is dark energy… and 25% is dark matter, everything else which is 5% they call normal matter” (NASA para 3). Dark energy has many different properties that are just beginning to understand the expansion of the Universe.

            The first property that Einstein discovered says that it is possible for more space to come into existence. The version that discusses cosmological constant makes a second prediction that empty space can possess its own energy. As this theory occurs and more space becomes into existence, more of this energy-of-space would appear. This would result in a form of energy that causes the Universe to expand faster and faster. The second property comes from the quantum theory of matter. In this theory “empty space is actually full of temporary (“virtual”) particles that continually form and disappear” (NASA para 5). But when physicists tried to calculate this energy they came out with too big of an answer and it was wrong by a lot. The mystery still continues. The third explanation of dark energy is that it is some sort of dynamical energy fluid or field that makes up all of space but as an opposite effect on the expansion of the Universe of matter and normal energy. They have named this theory “quintessence” after the Greek philosopher. They still cannot predict what it is like, how it interacts, or why it exists. The last possibility is that Einstein’s theory of gravity is incorrect and there may be a new theory out there that not only explains the expansion of the Universe but how clusters of galaxies behave. The factor that is needed to decide between dark energy possibilities is a property of space, a new dynamic fluid, or a new theory of gravity, and better data.

            Dark energy and dark matter make up the composition of the Universe, remember that 25% of the Universe is dark matter. What is dark matter? Well first of all dark means that it is not in the form of stars and planets that we see. Secondly “it does not form from dark clouds or normal matter, it is made up of particles called antimatter” (NASA para 8). Lastly we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. Some astronomers believed that dark matter was related to Baryonic matter because they thought it to be tied up in brown dwarfs or in small dense chunks of heavy elements. It turns out that “dark matter is not baryonic at all, but that it is made out of other exotic particles like anxions or WHIMPS (Weakly Interacting Massive Particles)” (NASA para 10).

Dark Matter Core Defies Explanation

This image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The result could present a challenge to basic theories of dark matter.

The mystery still continues today. We have begun to understand these two words and their properties related to the expansion of the Universe. Other theorists have said that as the Universe expands dark matter declines faster than the density of dark energy, so dark energy is dominating. It’s dark energy that seems to affect the expansion of the Universe, and they think they can say that confidentially because the rate of expansion is accelerating and is due to the unchanging quantity of dark energy. They continue on to say how “if dark matter is real than its effect would be that it could slow down gravity or even stop expansion caused by dark energy” (Dark Matter para 5). While there are many theories out there that could be true or proven wrong we know there is something in our Universe that is causing that and that could be dark energy and dark matter.

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