What's up y'all? Here is a paper I'm submitting for my special project in my Astronomy 101 class. It's on dark matter--a really cool, unknown substance that is believed to make up the bulk of the mass in the universe. Anyway, I just wrote this rough draft and I wanted to post it to get your feedback. It's an introductory class, so it's not rocket science. It's pretty simple, straight-forward stuff. What I need you to do is read through it for me for spelling, grammatical errors, consistency and to ensure that the paper flows fairly smoothly, and that it sounds professional. I just wrote it, so don't be surprised if you find a lot of errors. Please let me know what you think! I have to submit it by the end of the day tomorrow (Monday, 26th of Nov). Thanks...OH YEAH! I almost forgot--I didn't write an official conclusion paragraph, because I felt like the last paragraph did a good enough job of summing it all up and ending the paper. However, let me know if you agree or disagree....
Dark Matter: A Little Light on the Subject
As humans it is easy to forget how dependent we our on our senses as catalysts to our human experiences. Take the sense of sight as an example. More often than not, the average person's assumption (perhaps even at a subconscious level) is that all that there is in physical existence is that which can be seen. In addition to that, it is easy to forget that our site is merely a complex, physiological 'interpretation' of light as it is received through our eyes and processed in our brains. It is hard to blame the average person for such assumptions, which would only pose a problem if there were found to be forms of matter with properties that did not interact with light and were therefore invisible to our eyes. Unfortunately there seems to be just such a form of matter in existence that permeates space, and for years scientists have been blind to it because they succumbed to the same assumptions as the average person. As a result it seems their understanding of the universe has suffered so severely as to cause them to miss nearly twenty-two percent of the total mass-energy believed to be contained in the universe. This matter is now known as dark matter (another seventy-four percent of the total mass-energy is believed to be comprised of dark energy, energy which is at least as mysterious and difficult to detect. However, it is beyond the scope of this paper, which will focus solely on dark matter. It is interesting to note, however, that this means approximately ninety-six percent of the total mass-energy in the universe is virtually undetectable by human senses). (Wikipedia)
So how was this dark matter discovered? Well, strictly speaking it is a hypothetical substance that has yet to be directly identified, at least to a degree that is without significant controversy among astronomers. It was first proposed by Swiss astrophysicist Fritz Zwicky in 1933. He estimated the total mass of a galaxy cluster he was observing based on the gravitational movements of the galaxies it contained, and compared it to the mass he observed visually within the cluster. The discrepancy between the two figures was astounding: the estimate based on the galactic movements was some 400 times that of the visually-based estimate. Given the speed of the galactic orbits, and the required amount of mass it would take to reach such a speed (which far exceeded the amount of mass actually observed), Zwicky came to the conclusion that there must be a tremendous amount of matter present that could not be observed. Thus, the concept of dark matter was born, though no further advancements were achieved in studying or confirming it for nearly forty years. (Wikipedia)
Seventy-four years later, the theory has only grown stronger among astronomers. Galactic motion is still the preferred method to study dark matter, however the strongest evidence for its presence its found in a property other than basic gravitational movement: galaxy-rotation curves. In theory, stars and gas in galaxies should spin slowest at the center, then speed up progressively as they extend outward to about the mid-range orbits of the galaxy. Once past these orbits, star orbits should again become progressively slower. In reality, galaxies do not behave in this way. Instead, they start out with slower orbits in the center, speed up progressively as they approach the mid-range orbits, then they level off and maintain similar speeds out to the outer limits. In order for this to be the case, there would have to be a significant amount of invisible mass present in the outer reaches of these galaxies in order to provide the gravitational strength to maintain such high orbit velocities so far out from the center. (Arny 487)
Recently, in addition to the inductive reasoning described above pointing to the presence of dark matter, scientists believe they have identified large amounts of dark matter through what is called gravitational lensing. This is a phenomenon in which light from distant objects (stars, luminous gas, etc.) passes by other objects (in this case, dark matter) and is 'bent' by the gravitational pull of the matter. As a result, the image is distorted as it passes by the dark matter, and scientists are able to use these distortions to calculate the amount of dark matter, as well as its gravitational force. (Than) An example of this is that of the Bullet Cluster, where two galaxy clusters collided. A comparison between a map of the cluster based on gravitational lensing with a map of the cluster derived from X-rays of the hot gases found that the hot gases gathered to the center, whereas the dark matter is found in the outer regions. (Wikipedia) NASA has declared that this observation is direct proof of the existence of dark matter (Hupp), though there are still alternative theories held by some astronomers that counter belief in its existence. We will briefly explore some of these alternatives now.
There is one main alternative proposed by astronomers to counter arguments for the existence of dark matter. This alternative proposes the possibility that scientists are still ignorant of some aspects of gravity, specifically with regard to great distances and weak fields. One method that astronomers subscribing to this view have developed to offset this alleged ignorance is called the Modified Newtonian Dynamics, which as the name might suggests corrects Newton's laws to account for the inconsistencies between the mass that is observed and the mass that is calculated based on gravitational movements. This view fails to refute the evidence for dark matter provided by gravitational lensing, however, and it fails to explain it otherwise as well. (Wikipedia)
Incidentally, it should be noted that there are other alternatives proposed by astronomers that appeal to quantum mechanics that are far beyond the scope of my education, let alone this paper. (Wikipedia) Suffice to say that despite such alternatives, dark matter is the prevailing theory among astronomers to explain the gap between directly observed mass and the calculated mass based on gravitational movement. I leave it to the reader to pursue the matter of quantum mechanics alternatives if it so suites their interests.
Now that astronomers have lead us to consider the possibility of the existence of such a mysterious substance, can they provide some idea of what it might be made of? As could be expected, ideas and theories abound, but that is all that we have at present. They range from known materials, such as planetesimals (moon-sized, planet-shaped, high mass bodies), low-mass cool stars, dead white dwarfs or neutron stars, black holes, or neutrinos hypothetical materials such as photinos or WIMPS (weakly interacting massive particles). (Arny 487) Significant progress in this area is required in order to adequately answer this question. For me, it is this most elusive aspect of the overall subject of dark matter that sustains my interest.
Works Cited
Arny, Thomas T. Explorations: An Introduction To Astronomy. 4th ed. New York, NY: McGraw Hill Higher Education, 2006.
Than, Ker. "Radio Telescopes Could Make Dark Matter Visible." Space.com. 03 Jan 2007. 22 Nov 2007. <http://www.space.com/businesstechnology/070103_tw_radiomap.html>
Hupp, Erica, et al. "NASA Finds Direct Proof of Dark Matter." NASA.gov. 21 Aug 2006. 22 Nov 2007. <http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html>
