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Chapter 7 - Current Theories of Dark Matter |
This chapter of the video
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discusses the possibility that there is no dark matter and that, instead,
we need to modify our existing laws of
gravity.
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describes how most physicists think that dark matter consists of an
as-yet-undetected type of subatomic
particle.
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discusses the two leading candidates for this new particle, weakly
interacting massive particles (WIMPs) and
axions.
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interviews a number of dark matter researchers on their opinions.
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discusses some of the experiments worldwide trying to detect dark
matter directly.
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Planets
All of the dark matter theories mentioned in Chapter
6 of the video involve objects that have been detected
experimentally. Their failure suggests one of the two
following possibilities:
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Dark matter is made of objects that have never been detected in experiments.
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Dark matter does not exist. Much of the evidence for it comes from effects related to gravity (e.g., the orbital
speeds of stars, gravitational lensing). Thus if our current laws of gravity do not apply on a galactic scale, this
evidence is undermined.
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WIMPs
 Having ruled out all experimentally detected types of
matter as making up the bulk of dark matter, many
physicists turned to undetected varieties. One of the most
popular ideas is that dark matter is made of hypothetical
subatomic particles called "weakly interacting massive
particles" or WIMPs, as in Figure 20.
WIMPs are many times more massive than a proton
and have no electric charge. Electromagnetic radiation
is produced by charged particles, so since WIMPs are not
charged, they do not emit electromagnetic radiation of
any frequency and thus appear dark. Many physicists
are confident that dark matter is made of vast clouds of
WIMPs travelling rapidly in all directions.
Axions
A second theory involving undetected particles is that dark
matter is made of hypothetical subatomic particles called
"axions". Axions are many times lighter than electrons
and have no electric charge. One of the main differences
between WIMPs and axions is their mass. Thus, the
difference between the two theories (WIMPs or axions) is
that dark matter is either made of a large number of light
particles (axions) or a smaller number of heavier
particles (WIMPs).
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 Searching for Dark Matter on Earth
Earth lies within the Milky Way galaxy which is dominated
by dark matter. This means that if dark matter is made of
WIMPs or axions, billions of unseen particles are passing
through your body each second, as in Figure 21. Physicists
should be able to detect a tiny fraction of these particles
(if they exist) using highly sensitive experiments. Thus, numerous groups worldwide have set up a number of
such experiments, with some of the most promising ones
taking place 2 km underground in a working nickel mine at
SNOLAB in Sudbury, Ontario, Canada.
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PICASSO Dark Matter Experiment
One of the experiments at SNOLAB is the PICASSO
(Project in Canada to Search for Supersymmetic Objects)
experiment (Figure 22), which is highlighted in the video.
It consists of millions of tiny droplets of superheated
liquid Freon (C4F10 ) suspended in a gel. There is a very
small chance that a WIMP passing through the experiment
will collide with a fluorine nucleus within one of the
droplets. When this happens, energy is transferred to the
droplet, causing the liquid to vapourize and a tiny bubble
to form. The bubble then rapidly expands, sending out a
shock wave that physicists detect using acoustic sensors.
ICE CUBE Dark Matter Experiment
Another dark matter experiment is located at the South
Pole. The ICECUBE experiment consists of a vast array
of sensitive light detectors located in 1 km deep holes
in the ice. If dark matter is made of WIMPs, then dark
matter that is gravitationally trapped within the Sun and
Earth should, indirectly, cause light to hit the detectors in a
distinctive pattern.
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CERN and the LHC
Yet another experiment that might detect dark matter is
taking place just outside Geneva, Switzerland at CERN, the
world's largest particle accelerator (Figure 23). Using the
Large Hadron Collider (LHC), physicists hope to actually
create dark matter particles (WIMPs) via extremely high energy
collisions between subatomic particles. If they
succeed, this will provide evidence for the WIMP theory
of dark matter.
Modifying Newton
A small minority of physicists advocate a radical solution
to the mystery of dark matter: modifying Newton's theory
of universal gravitation on the scale of a galaxy (or larger).
One theory is called Modified Newtonian Dynamics
(MOND) and it can explain the mass discrepancy between
the Orbital and Brightness Methods. MOND does this
by altering the relationship between the magnitude of the
gravitational force F and distance r from
for very large distances.
Although MOND can explain the evidence supporting the
existence of dark matter within galaxies, it cannot explain
the evidence for it from gravitational lensing. In addition,
changing a basic physical law is a very rare occurrence in
physics and most physicists do not believe that this is the
solution to the mystery of dark matter.
Conclusion
The race to be the first to detect dark matter here on Earth
is intense. Whoever succeeds first will, for the first time
ever, have directly observed the particle that makes up, on
average, 90% of the mass of every galaxy in the universe.
They are almost certain to win a Nobel Prize.
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