Dark Matter

Ken Tapping, May 27th, 2015

In the sky this week…

  • The western sky after sunset is dominated by Venus (lower and brightest) and Jupiter (higher and less bright).
  • Saturn lies low in the southeast.
  • The Moon will reach First Quarter on the 25th.

The two things that have turned our vision of the universe upside down over the last few years must certainly be "Dark Matter: and "Dark Energy". We'll look at the second of these topics next time.

We normally think exotic things result from exotic experiments, followed by arguments among scientists about data interpretation, and then a growing acceptance of the results. The discovery of Dark Matter was nothing like that. It came from relatively clear and repeatable measurements using physics set down by Isaac Newton.

When one object, like our Moon, orbits another, like the Earth, that orbit is completely defined by the mass of the body about which the object is moving, how far it is from that body, and its velocity. If we know two of those quantities, we can calculate the other. It's all described by Newton's ideas about gravity. This basic physics approach was used to measure the masses of distant galaxies, and produced almost incredible results.

Stars are also in orbit; they are following paths around the centres of their galaxies. For example the Sun and the rest of the Solar System lie about 30,000 light years (a light year is the distance light travels in a year – just under 10,000,000,000,000 km) from the centre of our galaxy, the Milky Way, and orbiting at about 230 kilometres a second. Since we know a distance and a velocity, we can calculate the mass of our galaxy. Moreover, if another galaxy is close enough to us for us to distinguish individual stars, and we are viewing it more or less edge on, we can do this measurement for other galaxies too.

When we view a distant galaxy edge on, we see stars on one side of the galaxy moving towards us as they pursue their orbits, and those on the other side moving away. The distance the star is from the centre of the galaxy is something we can measure, as is the speed at which stars are approaching or receding from us. So we then just plug the speed and the distance into our Newtonian formula and out comes the mass of the galaxy. That's when we get a big surprise. The masses come out far too large for the number of stars and amount of gas and dust we can see in that galaxy.

When you make one measurement and get a funny answer, obvious questions to ask oneself are "am I doing the measurements properly?" or "am I making a mess of the calculations?" However, when other researchers get similar results, at some point we have to accept that we are seeing something real. Since mass is a measure of how much matter an object contains, galaxies must contain a lot more matter than we can see. This invisible material has been named "Dark Matter". Current estimates are that about 83% of the matter in the universe is invisible. The Earth and everything we can see through our telescopes are made up of the remaining 17%.

This means the nature of the universe is largely related to stuff we cannot see - hence the great efforts being made to establish what dark matter is. This has led to a very diverse set of theories. One is that dark matter is made up of heavy particles called WIMPS (weakly-interacting massive particles). Cool acronyms are essential in modern science. WIMPS are particles that have enough mass to explain our measurements but are otherwise hard to detect. Although our theories allow WIMPS to exist, nobody will accept something that was introduced to get the "right answer" until some totally independent evidence is uncovered.  Because these particles interact only weakly with other matter, magnetic and electric fields, we are going to need exotic tools, such as the Large Hadron Collider to search for them.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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