Download speech on dark matter

I have been studying Physics for approximately four months, and if I have learnt
anything through my lectures and reading, it is that there are STILL some major
problems with our comprehension of the universe that we exist in. Before my studies
I (somewhat naively perhaps) assumed that after the existence and findings of great
minds such as Aristotle, Galileo, and Einstein, that we’d pretty much figured
everything out by now, and that a modern day physicist’s job was simply to tie up the
loose ends left over, and clean up. I have since learnt this to be far from the truth, and
that we have only just scratched the tip of the iceberg of scientific knowledge.
One term that seems to have been popping up in the media a lot recently, and once or
twice in our astrophysics course is DARK MATTER. The term intrigued me, as it
sounded quite sinister and sexy. So, when I learnt that I had to do an oral presentation
on something scientific, I thought it was a good opportunity to find out what this
DARK MATTER is, and today I am going to try and tell you guys a little bit about it.
Firstly, the term DARK MATTER is a loose one, referring to a substance that does
not emit or reflect enough electromagnetic radiation to be detected directly. In other
words, with our current technologies, we cannot see this matter, and thus the term
dark is merely a reflection of our lack of understanding of the matter, rather than any
reference to it’s colour.
So if we cannot see this so called DARK MATTER, how do we know it’s there?
Well, the existence of DARK MATTER can be inferred from observational evidence
of the motion of galaxies.
One example of this is the motion of a spiral galaxy, rotating in space. Newtonian
and Keplerian mechanics suggest that the velocity of an orbiting star is governed by
the same respective laws that govern orbits in our own solar system. These laws state
that the velocity of the orbiting planet (say Earth) is proportional to the mass of both
the Earth and the object that the Earth is orbiting (in this case the Sun), as well as
being inversely proportional to the distance between the two objects. Thus if you
increased the distance between the Earth and the Sun, you would expect it’s orbital
speed to decrease (hence Neptune has a slower orbital velocity than Earth).
Now, if we return to our spiral galaxy, we would expect an orbiting star to behave in a
similar way. HOWEVER, careful observational evidence has proven this NOT to be
the case, and that in-fact orbiting stars in such circumstances actually travel more or
less uniformly regardless of their radius, and at a much faster velocity than predicted
by the mathematics based on the amount of visible mass.
THUS, for such a star to maintain a high velocity orbit, there must be something else
out there, having a strong gravitational effect on the star’s orbit…This indicates the
existence of the mysterious DARK MATTER.
To explain the phenomena observed, we must rethink our model of the universe to
include this dark matter. One way of visualising this is to imagine the dark matter as
the skeleton on which the flesh of the universe is hung. This indicates that all visible
matter (including the earth) is surrounded by dark matter. In-fact, cosmologists now
believe that the MAJORITY of the universe is composed of DARK MATTER, with
the visible part only making up approximately 10% of what is out there! I find this
quite a startling concept, as we often look up to the heavens in amazement, but really
it is just some minor detail in the grand scheme of things, and it makes ones life and
problems seem particularly insignificant!
So, if we are to accept the existence of DARK MATTER, than what actually is it?
Well, we don’t actually know, although there are a number of competing theories in
cosmology which are yet to be experimentally proven. Some believe that the dark
matter is composed of baryonic matter (i.e. protons, neutrons, and electrons, the same
stuff that the visible matter is made of), and some believe that it is made of more
exotic non-baryonic particles which are yet to be discovered.
Of the baryonic matter, the belief is that the dark matter comes from massive dark
bodies, such as black holes, which do not emit light due to their strong gravitational
pull, and failed stars such as brown dwarfs. This type of dark matter is collectively
known as MACHOs which stands for MASSIVE COMPACT HALO OBJECTS, but
calculations suggest that they can only account for up to 20% of the predicted dark
matter in the universe. Thus the rest must be composed of non-baryonic matter.
This non-baryonic matter is referred to as WIMPs which stands for WEAKLY
INTERACTING MASSIVE PARTICLES, and is currently believed to contribute up
to 90% of the dark matter in the universe. WIMPs are theoretical particles, and have
never been detected. If they do exist, they are extremely difficult to detect with
current technologies as they do not interact with electromagnetism or the strong
nuclear force, and thus cannot be seen. There are currently numerous experiments
being conducted throughout the world in an attempt to discover WIMPs. Many of
these experiments are conducted deep underground, as it is believed that their
detection is hampered by interference from cosmic rays; WIMPs are believed to be
incredibly strong and passing through the Earth all the time, whilst cosmic rays are
halted by the atoms in the rock, making a deep mine the ideal place to try and detect a
WIMP.
So Why does this all matter? Does Dark Matter, Wimps, Machos, (not to mention
DARK ENERGY, and NEUTRINOS amongst others) etc have any significance in the
long run? Well the answer is yes, and one extremely significant role they play is in
aiding us to predict the fate of our universe.
At the moment there are two competing theories of the end: The Big Crunch, and The
Big Freeze. The Big Crunch is where the expansion of the universe slows down and
is overcome by the attractive force of gravity. All the matter in the universe will be
drawn together in an almighty black hole. The Big Freeze is where the universe
continues to expand and eventually becomes too cold to sustain life. Now, these two
fates are governed by the density of the universe, and thus if we have a better
understanding of the nature of dark matter, we can calculate the density of the
universe more accurately, and therefore make a more precise prediction of how it will
all end…
Well, thanks for listening to me, and if you have any questions I will do my best to
answer them.