Dark Matter Teacher Guide - Perimeter Institute

The amount of dark matter Mdark is the difference between the total
mass M gal and the luminous mass Mstars
We can calculate M gal as follows
The amount of dark matter is
(Note: Final answer obtained by using all of the digits present in intermediate
answers and not rounding off.)
4. a) We are given the following information
As the collision is elastic, we can calculate the energy of the
germanium nucleus from the change in energy of the WIMP using the
principle of conservation of kinetic energy.
Before the collision
After the collision
Therefore, the amount of energy transferred to the germanium
nucleus is
b) As we need 1 x 10 -7 J of energy to lift a grain of sand by 1 mm
So, the amount of energy transferred to the germanium nucleus is
40 million times less than the energy needed to lift a grain of sand by
1 mm.
Therefore, scientists need a highly sensitive detector if they hope to
ever detect a WIMP.
5. A good answer to this question might include points such as the
following:
• One of the main pieces of evidence for dark matter comes from
observations of stars in galaxies. The stars are moving at higher
than expected speeds if galaxies only contain visible mass.
• There are many ways to detect objects other than by seeing them.
Sight is only one of our five senses.
• There are many examples of scientists coming to accept the
existence of things they could not see, eg. radio waves, atoms,
magnetic fields.
Examples of things from everyday life we believe to exist
but cannot see:
• Air: although we cannot see it, we can detect its movement
(i.e., wind).
• X-rays: we cannot see them but they allow doctors and dentists to
obtain images of various parts of the body.
• Microwaves: although we cannot see them, they heat our food in
microwave ovens.
41