First part in a series of four on Dark Matter
Some of you may have heard of dark matter, this mysterious type of matter that no one can see but makes 27% of the content of the Universe while visible matter (you, me, all stars and galaxies) accounts for only 5%. How do we know it really exists? In fact, its existence is confirmed in many different ways.
Fritz Zwicky, a Swiss astronomer, was the first to suspect the existence of dark matter in 1933. He was trying to measure the mass of a galactic cluster (a group of several galaxies) using two different methods. He tried to infer this mass from the speed of the galaxies. Just like kids on a merry-go-round have to hold on to avoid being ejected, galaxies are held together in a spinning galactic cluster by the gravitational force provided by the matter it contains. If there were not enough matter to create this force, the galaxies would simply scatter.
He then compared his result with the mass evaluated from the light the galaxies shed. He realised that there was way more matter in the cluster than what was visible. This matter of an unknown type generated a gravitational field without emitting light. Hence its name, dark matter.
Velocity curves of spinning galaxies
But it was not until the 1970’s that an American astronomer, Vera Rubin, measured the speed of stars in rotating galaxies accurately enough to convince the scientific community. She observed that stars in spinning galaxies were all rotating at roughly the same velocity, no matter their distance to the galactic centre. This is in contradiction with Kepler’s law that describes the rotation of planets around the Sun.
A planet located further from the Sun rotates slower, following the curve labelled A in the graph below. However, Vera Rubin showed instead that stars in a spinning galaxy followed curve B. This was as if the stars were not rotating around the visible centre of the galaxy but around many unknown centres, all providing additional gravitational attraction. This could only happen if huge amounts of invisible matter filled the entire galaxy and beyond.
One striking dark matter detection technique is called “gravitational lensing”. It is based on the way that large concentrations of matter (visible or dark) create gravitational fields strong enough to distort space.
Imagine a stretched bed sheet where we toss a ping-pong ball. The ball will simply roll following the surface of the sheet. But if you drop some heavy object in the middle of the sheet, the ball will still follow the sheet surface but will now move on a curve.
Light behaves the same way in space. An empty space, void of any matter is just like a stretched sheet. There, light moves in a straight line. In the presence of a massive object such as a star or a galaxy, the space is deformed and light follows the curvature of the distorted space.
(Adapted from Pat Burchat’s TED talk)
Light coming from a distant galaxy will bend when passing near a massive clump of dark matter as shown above. The galaxy will appear shifted, as if coming from different places (images on top and bottom). In three dimensions, all diverted light will form a ring as seen on the photo below taken by the Hubble telescope. In case the galaxy and the observers are not perfectly aligned, only small arcs form.
(Photo credit NASA)
This technique is now powerful enough to produce maps of the dark matter distribution in the Universe.
Cosmic microwave background
Astrophysicists can even infer how much dark matter exists by studying the cosmic microwave background. This is relic radiation dating back to when the Universe was barely 380,000 years old. This fossil light has been travelling around for more than 13 billion years and now reaches us coming from all and no direction in particular.
The map of the Universe below was drawn using data taken by the Planck satellite. It shows hotter spots corresponding to where first dark matter then visible matter started forming lumps, providing the seeds for galaxies. Nowadays, scientists believe dark matter acted as a catalyst in galaxy formation.
(Photo credit Planck experiment)
The microwave background radiation can be decomposed just like sound from a musical instrument can be broken into harmonics. From the features of its “power spectrum”, i.e. the amount of radiation associated to each frequency, astrophysicists can calculate the quantity of dark matter contained in the Universe.
Today, we have numerous and convincing proofs of the presence of dark matter but see it only indirectly through its gravitational effects. How about direct evidence? This will be my next topic. But beware: there’s hot debate in the scientific community on how to interpret the various direct detection results.
Second part in a Dark Matter series: Getting our hands on dark matter
Third part in a Dark Matter series: Cosmology and dark matter
Fourth part in a Dark Matter series: Can the LHC solve the Dark Matter mystery?
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For more information:
Hangout with CERN: The Dark Side of the Universe
TED Ed clip: Dark matter: The matter we can’t see
TED talk by Pat Burchat: Shedding light on dark matter