Dark Matter
Does it exist?
For hundreds of years people have wondered about the Universe. They have sought for
answers about its nature, how it began and what is made of. Some of the greatest minds in
science and astronomy have answers to some of these questions and solid theories based on time
tested science. Based on our current understanding of the laws of gravity, motion, and physics
we have a good grasp on the topic of this paper, dark matter. Dark matter is a controversial
subject that some scientists disagree over and seek to explain a few things such as what makes up
the bulk of mass in the universe and what keeps galaxies together, by means other than dark
matter. This paper will discuss dark matter and then I will provide evidence in support of the
concept of dark matter’s existence in the universe.
Dark Matter Defined:
It is important to understand a few ideas about dark matter before we jump into
discovering its existence. Dark matter is an invisible substance that neither emits light nor
reflects light, this is where the “dark” in dark matter comes from. NASA said about dark matter,
“If you drove into a wall made of dark matter, you wouldn’t crack a headlight or inflate an
airbag. You wouldn’t even know it happened.” (NASA.gov)
The particles that make up dark matter don’t conform to normal matter standards,
consisting of atoms that include neutrons, protons and electrons; this form of matter is also called
non-baryonic matter. According to Alan MacRobert, of Sky and Telescope magazine; “There’s
excellent evidence that most of this dark matter exists in the form of exotic particles not yet
discovered in any physics lab” (skyandtelescope.com).
According to Bennett, Donahue, Schneider and Voit, of the textbook The Cosmic
Perspective, like the neutrino, non-baryonic dark matter most likely has no electrical charge and
does not emit light as does the matter that makes up “bright” galaxies, solar systems, stars and
planets. As mentioned above, this explains the “dark” in dark matter. Unlike neutrinos, dark
matter is heavier, which causes them to move much slower and collect in large groups. This
collecting behavior forms large masses to develop and along with mass comes our friend gravity
(Bennett, Pg.690-692).
Understanding the make-up of dark matter is essential to understanding how it behaves
and how evidence of its existence has been discovered.
Gravitational Lensing and the Additional Mass:
Gravitational lensing is the idea that gravity from massive
objects can bend light. This bending can distort, enlarge and
create multiple images of objects behind the lens. The picture to
the right displays two galaxy clusters that have collided; they are
visible and bright in the center of the photo. Also displayed in the picture are several blue
colored galaxies just outside the perimeter of the galaxy clusters. These blue images are the
same galaxy positioned directly behind the cluster, but the gravitational lens (which is the two
colliding clusters) has created several images.
Using mathematics and Einstein’s general theory of relativity, they can calculate the mass
of the clusters based on the amount of distortion and bending that is occurring. Bennett, again in
his text says, “Einstein’s general theory of relativity tells us how massive these clusters must be
to generate the observed distortions” (Bennett, Pg.689)
After scientists had determined the mass of a galaxy or galaxy cluster, they noticed that it
contained for more mass than should be there based solely on the mass of stars found within
them. Bennett says “up to 50 times that of the combined mass of the stars in the cluster’s
galaxies”. So what is creating all the extra mass? Dark matter is the answer. It’s this mass and
the gravity produced that keeps galaxy clusters together.
Gravitational lensing isn’t the only tool used to validate the existence of dark matter.
Next I’ll discuss the Chandra X-ray observatory discovery.
Chandra X-ray Observatory:
In August of 2006, the Chandra X-ray observatory made an amazing discovery that
proves the existence of dark matter. An x-ray photo of the colliding clusters mentioned above
showed some interesting facts about hot gases that are being produced by the clusters. The mass
of the gas was much larger than the mass of the stars; therefore, dark matter would need to be
present to make up the difference, so as to keep the gas from flying off into space. NASA’s
press release had this to say about the discovery:
“These observations provide the strongest evidence yet that most of the
matter in the universe is dark. Despite considerable evidence for dark
matter, some scientists have proposed alternative theories for gravity where
it is stronger on intergalactic scales than predicted by Newton and Einstein,
removing the need for dark matter. However, such theories cannot explain
the observed effects of this collision” (NASA.gov, PR 06-297).
It’s the extra mass from the dark matter that holds the stars and gases together in these galaxies.
Further evidence resulted from the Chandra discovery and additional tools have been
used to prove the existence of dark matter. The use of mathematics, Newton’s laws and
Einstein’s theories in conjunction with gravitational lensing and x-ray spectral discoveries, as
I’ve mentioned in this report, provides ample proof that dark matter exists. Bennett has stated
that either “Dark matter really exists and we are observing the effects of its gravitational
attraction [or] there is something wrong with our understanding of gravity that is causing us to
mistakenly infer the existence of dark matter” (Bennett, Pg.690). It is my position that we are
observing gravity correctly and by this reasoning and my evidence presented above, that dark
matter does exist.
Work Cite
Bennett, Jeffery, Donahue, Megan, Schneider, Nicholas & Voit, Mark; The Cosmic
Perspective, Custom Edition for SLCC; Pearson Custom Publishing; 2008; Print
MacRobert, Alan M.; “Dark Matter Gets a Reality Check”; Sky and Telescope; E-copy;
2003; http://www.skyandtelescope.com/news/3306826.html
Unknown Author; NASA.gov; “NASA Finds Direct Proof of Dark Matter” 2006; E-copy;
Press Release 06-297; http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297
_CHANDRA_Dark_Matter.html
Unknown Author; NASA.gov; “Hubble Finds Ghostly Ring of Dark Matter”; 2005; E-copy;
http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html