This post is about dark matter and is the latest in my series on cosmology, the study of the origin and evolution of the Universe as a whole. As readers of my previous posts will recall, dark matter makes up about 27% per cent of the mass of the Universe.
Evidence for dark matter
Our solar system contains the Sun, eight planets with their moons and various minor bodies such as dwarf planets, comets and asteroids. If we plot the speed that each planet orbits the Sun against its distance from the Sun, then we get the curve shown below.
The graph above shows the speed at which the planets orbit the Sun in kilometres per second, plotted against their distance from the Sun in astronomical units (AUs). 1 AU is just under 150 million km and is the average distance between the Earth and the Sun.
The way that the speed of the planets’ orbits falls off with their distance from the Sun indicates that nearly all the mass of the Solar System is concentrated in its centre at the Sun. The further away a planet is from the Sun, the weaker the Sun’s gravitational pull and the more slowly it orbits. See Note 1.
Jupiter, the most massive planet, has only 0.1% the mass of the Sun. In fact the total sum of the masses of all the planets, their moons, dwarf planets (like Pluto), asteroids and comets in the Solar System is less than 1% of the mass of the Sun. This means that the effects of gravity caused by the other bodies in the Solar System on the speed of the planets’ orbit are insignificant.
The Sun belongs to the Milky Way galaxy, which contains about 400 billion stars (Cain 2013). If you were to look at the Milky Way from a great distance, it would look as shown below.
What the Milky way galaxy would look like from outside
All the stars in the Milky Way rotate around its centre, and the Sun rotates at a speed of 782,000 km/hour (Cain 2008). However, the distance between the Sun and the galactic centre is so great (nearly 30,000 light years) that it takes around 230 million years to complete a full revolution. This vast period of time is sometimes called a cosmic year.
Most of the stars in the Milky Way are concentrated near its centre. So if, like the Solar System, most of the matter in the Milky Way were in the form of stars, then it too would be concentrated at its centre. We would expect that as we get further from the centre of mass, then the stars would revolve around the centre of the galaxy more slowly in the same way that the planets orbit more slowly as we get further from the Sun. We would expect a rotation curve (a plot of the speed that a star orbits the centre of the galaxy against its distance) in which the orbital speed falls off with distance from the galactic centre, similar to A in the diagram below.
In fact the orbital speed of a star around the centre of the galaxy does not fall off with its distance from the galactic centre. The rotation curve for our Milky Way galaxy is actually like B in the diagram above. The only way that these results can reconciled with our existing laws of physics is for there to be a large amount of matter in the outer regions of our galaxy which is not in the form of stars. The pull of gravity due to this matter means that the rotation curve does not fall off with distance. Because it does not emit light it is called dark matter and, to produce the flat rotation curves observed for our galaxy, most of its matter must be in the form of dark matter.
In the 1970’s astronomers measured the rotation curves of other spiral galaxies. It became clear that all spiral galaxies had rotation curves in which the speed at which a star orbits the centre of the galaxy does not decrease as a function of the distance from the centre of the galaxy. An early pioneer of this work was the American astronomer Vera Rubin (1928- ) pictured below.
In an influential scientific paper presented in 1980 she and her colleagues presented observations of the rotation curves of a large number of spiral galaxies (Rubin et al 1980). All of these showed rotation curves similar to the Milky Way. To explain her observations, spiral galaxies would need to be surrounded by an invisible dark matter halo which would, in general, have about five times the amount of matter that is held in the galaxy in the form of stars.
This diagram shows a typical spiral galaxy surrounded by an invisible dark matter halo. The bright centre of the galaxy is shown in white and the outer regions of the galaxy are shown in light brown. The dark matter halo, which although shown in blue is invisible, is not flattened in a disk like the galaxy and extends to about 3 times the galaxy’s radius.
Since Rubin’s pioneering work, it is now generally accepted that most of the mass of galaxies is in the form of dark matter.
Clusters and groups of galaxies.
In general, galaxies are found in groups and clusters, the largest of which contain thousands of galaxies. The speed at which these galaxies are moving with respect to each other in these groups and clusters is often very high. For large clusters, such as the one shown below, individual galaxies can be moving at speeds of over 1000 km/s (3,600,000 km/h) relative to each other. To prevent the galaxy groups and clusters from flying apart, something must be holding them together. The most widely accepted explanation of this is that there must be a great deal of dark matter in most galaxy groups and clusters, and it is the force of gravity due to all this dark matter which binds the cluster together.
Part of the Virgo Cluster, a large cluster of galaxies about 50 million light years from Earth
Other Evidence for dark matter
Other evidence for dark matter comes from gravitational lensing, where the strong gravity from clumps of dark matter which lie between a very distant object and Earth actually form a “gravitational lens” and bend light rays causing two images of a very distant object to be seen.
A gravitational lens caused by the large amount of dark matter around a cluster of galaxies (D) causes two separate images (B and C) of a very distant galaxy (A) to be seen.
I won’t say any more about this in this post, but if you would like to know more about gravitational lensing caused by dark matter click here for an interesting article from the phys.org website.
In addition, cosmologist believe that clumps of dark matter were the seeds of galaxy formation. Without dark matter there wouldn’t be enough matter for galaxies to form. How galaxies form is such a huge topic that I could write several posts about it so I will come back to this at a later date.
What is the nature of dark matter?
As you will recall from my previous post ordinary matter is made up of atoms. Some dark matter may be in the form of ordinary matter in objects such as brown dwarfs. These are objects which are midway in size between the lightest stars and large planets such as Jupiter. Because they emit little or no light, brown dwarfs are extremely difficult to detect.
However, for reasons which I’ll discuss in a future post, cosmologists believe that although some dark matter is in objects such as brown dwarfs, most dark matter isn’t made up of atoms. Instead it is made up of an entirely different kind of matter altogether. Various candidates have been suggested for the particles which make up dark matter but none has ever been detected by astronomers or in any particle physics experiment. The nature of dark matter is one of the great unsolved problems in physics.
This post is the fifth in my series about cosmology. The other posts are:
(1) The Universe Past, Present and Future. This describes what is meant by the Universe and gives an overview of its origins, evidence for its expansion and discusses its ultimate fate. To view this post click here.
(2) A brief history of the Universe. This gives a history of the Universe from just after the big bang until the current date. To view this post click here.
(3) Dark Energy. This post gives the reasons why cosmologist believe dark energy exists and why it makes up nearly 70% of the mass of the Universe. To view this post click here.
(4) Dark Energy over Time. This post discusses how the amount of dark energy in the Universe has varied over time and its implications on its future evolution. To view this post click here.
1 It is fairly easy to show, using high school physics, that if all the matter in the Solar System is concentrated in the Sun, then the speed of a planet’s orbit is proportional to the inverse square root of its distance from the Sun.
Cain, F (2008) Sun orbit, Available at: http://www.universetoday.com/18028/sun-orbit/(Accessed: 19 February 2015).
Cain, F (2013) How Many Stars are There in the Universe?, Available at:http://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/(Accessed: 19 February 2015).
Rubin, V. C.; Ford, W. K. & Thonnard, N. (1980) Rotational properties of 21 Sc galaxies with a large range of luminosities and radii, from NGC 4605 (R = 4kpc) to UGC 2885 (R = 122 kpc). The Astrophysical Journal, Vol. 238, 471–487