Dark Matter, what is it? - NRC Publications Archive

NRC Publications Archive
Archives des publications du CNRC
Dark Matter, what is it?
Tapping, Ken
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. /
La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version
acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien
DOI ci-dessous.
Publisher’s version / Version de l'éditeur:
http://doi.org/10.4224/23000792
Skygazing: Astronomy through the seasons, 2016-09-27
NRC Publications Record / Notice d'Archives des publications de CNRC:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=8e1661b7-1984-4225-8502-39e62bb12cc9
http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=8e1661b7-1984-4225-8502-39e62bb12cc9
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/copyright
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/droits
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Questions? Contact the NRC Publications Archive team at
[email protected]. If you wish to email the authors directly, please see the
first page of the publication for their contact information.
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez
pas à les repérer, communiquez avec nous à [email protected].
DARK MATTER, WHAT IS IT?
Ken Tapping, 27th September, 2016
The Earth, other planets, and all the stars and
galaxies we can see add up to about 16% of the
material making up the universe. The other 84% is
invisible. However, it’s there, or something is,
because we see its gravitational influence almost
everywhere. If we measure the orbit of the Moon,
we can calculate its mass. When we measure the
orbits of stars around their galaxies, and do the
same calculation, we end up with several times
more mass than we can see. Someone started
referring to this invisible stuff as “Dark Matter”, and
the name stuck. However, nobody is happy with
the idea of a mystery ingredient that makes our
calculations work but is otherwise undetectable, so
the quest is on to find out what Dark Matter is. One
possibility is that our ideas about gravity are not
correct, but could they be incorrect to the extent of
84% of the mass of the universe?
So far our knowledge about gravity has worked
very well. Even though we still don’t know exactly
what it is, we can use it to navigate our spacecraft
around the Solar System and model the collapse
of clouds to form stars and planetary systems. We
have also used it in searching for new planets, by
measuring their gravitational tugging at known
bodies. We are fairly reluctant to accept – yet –
that something is grossly wrong with our
gravitational calculations. What else is there?
One possibility is an unknown particle, that is
heavy enough to create strong gravitational
effects, but otherwise does not react much with
anything else. These have become known as
WIMPS (weakly interacting massive particles).
This might sound like the fudge factor we started
with – an undetectable something that makes our
calculations work. However, according to our
understanding of how elementary particles work,
WIMPS, if they exist, should be detectable.
If 84% of the universe is made up of WIMPS, there
are lots of them around. Even if they only interact
with our kind of matter occasionally, there should
still be enough interactions for us to detect some.
That is the objective of the LUX experiment. Lux is
Latin for light, and is also the acronym for the
Large Underground Xenon experiment.
Deep in a mine in South Dakota there is a large
tank containing 350 kg of liquid xenon. The tank is
lightproof, and inside the tank are many sensitive
light detectors. The device is deep in a mine to use
the Earth to screen out any cosmic rays and
“strongly interacting particles” from space. WIMPS
would easily penetrate down to the detector, and
possibly collide with one of the xenon atoms,
producing a flash of light, which will be detected.
Xenon atoms are particularly big targets. The
project has been running now for about three
years and as yet has detected no WIMPS. These
particles are either even more wimpy than
predicted, or they don’t exist at all.
However, the idea that Dark Matter is made up of
some sort of particle is still the most popular
among scientists, so there are plans to make an
even bigger detector, containing 7 tonnes of
xenon. Experience so far suggests that patience
will be an important component in this search. How
long will it take to produce a credible negative
result? Since the idea that Dark Matter is made up
of particles is largely in agreement with our current
understanding of particle physics, this is the most
widely accepted idea. If these detectors don’t find
any WIMPS or other Dark Matter particles, how
long will it take to establish a solid negative result?
If no WIMPS turn up, our ideas about the nature of
space and the dynamics of the universe could be
in for another upset. Is there something wrong with
our understanding of gravity? Stay tuned!
Venus lies very low in the after-sunset glow, but
might be visible as a bright, starlike object. Mars
and Saturn lie very low in the southwest after dark.
Mars is on the left and Saturn on the right. The
Moon will be New on the 30th.
Ken Tapping is an astronomer with the National
Research Council's Dominion Radio Astrophysical
Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]