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“Dark Matter”
Valerie Wilson
Dark matter. The most mysterious and exotic particle among us. Not only was it just
newly apparent in the 20th century, but, interestingly enough, we’ve never actually found or
seen any dark matter. What could we possibly know about something so good at hiding? Not
nearly as much as scientists want and hope to someday know. We do, however, know it exists.
Not only that it exists but that dark matter combined with dark energy makes up 96% of all
matter in our universe. The matter that makes up you and me and the grass and trees is only
about 4% of all matter. How can it be possible that such a tiny percentage can appear so
massive while a percentage of matter that is so large, we can’t see at all? And how is it we know
it exists?
It began, like so many things in science do, with Einstein. In 1917, Einstein was applying
his new theory of general relativity to space and time. According to the theory of general
relativity, mass affects the shape of space and the flow of time. Gravity results because space is
warped by mass; the greater the mass, the greater the warp. However, at this time scientists
did not know that the universe was expanding. It was assumed that after the big bang over 12
billion years ago, that our universe would continue to expand but eventually slow down and
stop. He found that his equations didn't quite work for a stationary universe and he gave up on
the idea.
In the early 20th century astronomers still believed the universe was made up of just
our Milky Way. Then in 1922 astronomer Edwin Hubble, using the Hooker Telescope, identified
a type of star called a Cepheid variable. He used this to measure distances to nebulae. What he
found was that there were nebulae far too distant from us to be a part of our galaxy meaning
they must belong to other galaxies. Not only did he discover the existence of other galaxies but
also that the farther away these galaxies were, the faster they moved away. This could mean
only one thing; the universe is expanding. The idea of the expanding universe revolutionized
astronomy. The idea that our universe used to be smaller led to the Big Bang theory, meaning
the universe began as a tiny point that suddenly and swiftly expanded to create everything we
know today. How does all this tie into dark energy though? The expanding universe is actually
how we define dark energy. Dark energy differs from dark matter in that dark energy is the
antigravity. It pushes outward on the expanding universe.
Now that we know the universe is in fact expanding, scientists could reevaluate
Einstein’s theory of general relativity. In the 1930’s, astrophysicists started to hypothesized dark
matter due to a lack of mass. In early studies, scientists couldn’t figure out why stars and other
galaxies were moving at such fast speeds with such a small amount of mass. In 1932 Jan Oort
measured the motions of stars in the Milky Way and was the first to find evidence for dark
matter when he found there must be more than the mass of the material that can be seen.
At this same time Fritz Zwicky was calculating the gravitational mass of the galaxies
within the Coma Galaxy cluster and found the gravity was 400 times greater than expected
from the galaxy's luminosity, which means that most of the matter must be dark. Zwicky called
this dark matter.
Now that we’ve discovered it, how is it that we prove it? Dark matter's existence is
purely based on gravitational effects on visible matter and gravitational lensing around
background radiation. Gravitational lensing is fairly simple but very difficult to explain. When
we look out at a galaxy, the light emitted from that galaxy is distorted due to dark matter
blocking our view. The light emitted from the galaxy bends around the dark matter so we see a
ring. It’s similar to looking through a magnifying glass. Because we can see these rings or this
gravitational lensing, we can “see” where dark matter is.
For years now scientists have been trying to figure out just what dark matter is. We
know of it being a ponderous substance, it’s not moving at the speed of light meaning we can’t
see it, it doesn’t interact with you and me or interact with any ordinary matter except through
gravity, and it can move through other matter undetected. There were possibilities that it could
be the MACHO or Massive Compact Halo Object which has a high mass, emits very little light,
and is present throughout the galaxy. Another idea is that dark matter is brown dwarfs or failed
stars which are bigger than Jupiter, massive in weight, and emit very little light. There was also
the idea of axions which were created at the time of the Big Bang, again, emit very little light,
and there are millions in the universe. But axions were dismissed because they can change to
protons while dark matter is stable. Others were high energy particles, neutrinos, which can
pass through ordinary matter and have low light. And the most closely related particles are
WIMPs or weakly interacting massive particles. Scientists have found that a combination of
MACHOs and WIMPs could be making up dark matter. But until proven, scientists have claimed
dark matter as a new exotic particle.
Scientists today are desperately working to further prove the existence of dark matter.
In Minnesota, The Soudan Underground Laboratory, located 2,341 below in an old iron mine, is
the leading deep underground science and engineering laboratory in the United States.
Scientists from around the world have been working at Soudan for 25 years trying to answer
basic questions about the Universe in the Cryogenic Dark Matter Search. They use very cold and
super-sensitive detectors called CDMS detectors. The CDMS detectors are very pure
hockey-puck sized disks of germanium. They put these detectors in a sort of freezer that keeps
temperatures at -490 degrees Fahrenheit. Because it is so cold, there is very little motion.
Atoms and other particles slow down so that these detectors don’t pick up much activity. The
idea is that if a dark matter were to pass through the detector and hit a germanium nucleus it
would cause the entire detector to vibrate and raise in temperature. If these detectors were at
room temperature on the surface above ground, the disks would detect every moving particle it
encountered.
The big question is, have we discovered anything with these detectors? Have we
encountered any dark energy and been aware of it? The answer is no. Not yet. But if dark
matter is ever detected it may be able to explain how the universe behaved directly after the
Big Bang and ultimately what our universe may become in the distant future.
Works Cited
Aguirre, Lauren. "Dark Matter." Nova. PBS, 25 June 2008. Web. 30 Nov. 2013.
<http://www.pbs.org/wgbh/nova/physics/dark-matter.html>.
"15: Patricia Burchat Sheds Light on Dark Matter." Cohen, June. TEDTalks: Space Trek. TED
Conferences, LLC. 1 Feb. 2008. Television.
"Dark Matter." Cort, Julia. NOVA ScienceNow. PBS. 25 June 2008. Television.
"Cryogenic Dark Matter Search: What is CDMS?." University of Minnesota: Soudan Underground
Laboratory. University of Minnesota, 25 Jan. 2012. Web. 30 Nov. 2013.
<http://www.soudan.umn.edu/cdms/Index_2.shtml>.
Hewitt, Paul G.. "11: The Atomic Nature of Matter." Conceptual physics. 11th ed. Boston, Mass.:
Pearson, 2012. 209. Print.
"Hubble Discoveries - Dark Energy." HubbleSite. N.p., 1 Jan. 2008. Web. 30 Nov. 2013.
<http://hubblesite.org/hubble_discoveries/dark_energy/>.
McManus, Emily. "Patricia Burchat: Shedding Light on Dark Matter." TED: Ideas Worth
Spreading. TED Conferences, LLC, 30 Nov. 2013. Web. 30 Nov. 2013.
<http://www.ted.com/talks/patricia_burchat_leads_a_search_for_dark_energy.html>.
""Dark Matter/Dark Energy/20"." Tarantino, Louis, and Douglas Cohen. The Universe. A&E
Networks. 15 Jan. 2008. Television.
“What I Learned”
Valerie Wilson
Dark matter and energy are far too complex for my feeble mind. I basically didn’t know
what to write my paper on so I asked my friend who majored in physics at Weber State “What
should I write my final paper about for physics?” and he mentioned that dark matter is
interesting. I knew nothing of dark matter… at all. I think I’ve heard people talk about it in
passing or in text books but if someone had asked me “What’s dark matter?” all I’d have said is
“Matter… that is dark?”
I was shocked to find out that scientists don’t really know that much about it. I mean,
they know roughly what it does and where it is but when you look at the major details of it,
dark matter is a pretty foreign particle. I love it though, things that are mysterious. People get
frustrated and go into serious depth and detail about different philosophies they have on dark
energy about how people don’t believe it exists or that dark energy is actually God and has
everything to do with the rapture. One person’s blog even discussed that aliens may be the
source of dark energy.
It’s pretty interesting when you discover an entire new subject and realize there’s
hundreds of people studying something you knew nothing about. There are so many people
involved with the Cryogenic Dark Matter Search and working tediously to figure out what this
invisible substance is. It’s pretty incredible.
I’d love if I were still around when they finally discover a dark matter particle. It would
be such a huge moment in history.