There are two main ways of detecting or, better to say, two ways how dark matter was detected. The first is orbital velocity and the second is gravitational lensing.
The orbital velocity of a celestial body depends on gravitational pull and distance from an object or another celestial body, which exerts gravitational attraction. Just to recall that gravitational force between two bodies is inversely proportional to the squared of the distance between these bodies. Therefore, the closer the object is to the object of gravitational pull, the faster it rotates around this object. Mercury rotates faster around the sun than Venus. Venus rotates faster than Earth. The further away a planet is from the Sun, the slower it rotates around the Sun. Pluto has the slower rotation around the Sun.
Now, let us look at it in a slightly different way and imagine Mercury rotating around the Sun at a constant speed higher than any other planet due to a strong gravitational pull from the Sun. It stays in orbital motion due to a strong gravitational pull from the Sun. But now, try to imagine that the mass of the Sun has suddenly decreased or almost disappeared. Then there would be nothing to keep Mercury in such orbital motion. Instead, it would continue to go away from the solar system in a straight line.
Another good example is if you have experience with fishing. In the place where I come from, people used to catch fish by using a fish hook which was attached to a very long piece of string. It was no proper fishing device and was composed only of a long piece of string with a fishing hook at the end. The fishing was usually done from the shore. It was important to throw away the fish hook as far as possible into the sea. The best way to do that was to rotate the piece of string in the air with the fish hook at the end of it. Once they achieved good rotational movement of the fish hook, they would let go of the piece of string from their hand and the fish hook would fly away to sea.
Picture 4.01
This is very similar to the gravitational pull of the Sun or any object keeping another object in orbital motion around it. Once this gravitational pull disappears, like letting go of a piece of string in the above figure, the rotating object will fly away, as the fish hook did. With this logic in mind, we can understand how dark matter was discovered.
In fact, this is how the presence of dark matter was first detected in the early part of the 20th century.
It was Fritz Zwicky, Swiss astronomer, who discovered dark matter in 1933 when he studied the Coma cluster of galaxies. (This is the cluster of galaxies located in the constellation of Coma Berenices. The Coma Berenices constellation is next to the Leo constellation which can be found in the sky when we look at night from March to May. This refers to the Northen hemisphere, which means only part of the sky visible from the north part of Earth, Europe, North America, for example. See Puiture 4.02.)
Picture 4.02 The Leo constellation can be easily seen with a head looking like a reverse question mark. Once you find the Leo constellation, it is easy to find the Coma Berenices constellation.
The Coma Berenices constellation has the Coma cluster (blue dots) next to it. Coma Berenices in Latin means Berenece’s hair. It is mythology about Berenice II (born 269-died 221 BC) who married Ptolemy II Eurgetes, Macedonian King of Egypt, around 245 BC. When Ptolemy went to revenge the murder of his sister in Syria, Berenice promised to cut her long hair for his safe return. He returned safely but, as her hair was not found where it was stored following cutting, the court astronomer said that the goddess was so pleased that she decided to put her hair in the sky where it formed the Coma Berenices constellation.
Zwicky calculated the speed at which galaxies rotate around in clusters and the amount of matter available as seen by the luminosity of present stars. He found out that there is a huge discrepancy between the speed and the amount of available stars. It was only around 10% of visible mass, which if it was the only mass available, it would not hold galaxies to go around at such speed. In other words, if there was not some 90% invisible mass present, then there would not be enough gravitational pull and galaxies would fly away from each other.
In 1970, Vera Rubin and W. Kent Ford observed the same phenomena where mass of the stars in a galaxy only had 10% of mass needed to keep them orbiting at the speed they did around the centre of the galaxies. It is 90% of invisible mass which kept them going with such speed or otherwise, these stars would fly away.
Gravitational lensing is another important phenomena, which happens as the result of the gravitational pull large mass has on light. Einstein made calculations and accurately predicted the degree of banding of light, which depends on the amount of mass of the object the light passes near to on its way.
These phenomena can be observed when a massive object is between stars and us, which are far away. The light travelling from the star can be seen although it is behind a massive object and this is due to bending of the light due to gravitational pull as the Picture 4.03 shows.
Picture 4.03 Bending of the light due to a star (star is the circular object). Position B shows where the star light from a distant star would be seen if there was no massive star on the way on the light.
Position A is where this star should not be seen when a massive star (circular object) is in front. However, due to light binding as it is pulled by gravitation of the star, that star is seen displaced at a place shown by the interrupted line.
Astronomers have observed that a number of galaxies in the distance are subject to gravitational lensing in some parts of the sky with no visible massive object between the galaxies and observer. It was therefore the only possible conclusion for such phenomena that the gravitational lensing was caused by invisible mass, which will be dark matter.
With current technology, experts and people working in that field are able to identify, thanks to gravitational lensing, the exact place where dark matter is situated in the universe. The final result is amazing. It shows the arrangement of dark matter as a web throughout the universe. Whenever there is dark matter, there is a number of galaxies. Dark matter serves as scaffolding upon which ordinary matter is organised in stars and galaxies. It looks like the whole universe is built up in the way everything is built up around us, starting from single cell organisms to mammals, including us. If you research cell structure, you will find out that a cell has its own cytoskeleton (numbers of microtubules), which is the cell skeleton around which the cytoplasm and cell are organised and built. Animals, including us as the most developed species, have their skeleton, around which muscles and all the rest of the body is built. If we look at any building, we see that, in order to be built and maintain its structure, it needs to have a strong support of still wire or its own skeleton. The same logic can be applied to the universe. Galaxies are built around a skeleton of the universe, which is presented with dark matter.
My own illustration below is my attempt to show how this looks. As stated in the preface of this book, there are beautiful images and illustrations which are easily accessible online. I was tempted to use some of these as examples in my own book. However, as I do not know how to obtain permission to use them, I decided to make my own illustrations, but I would encourage you to have a look at the beautiful examples available online.
Dark matter like a spider’s web with galaxies (blue dots) built on it. Pleas look at the Internet for these images, which are beautiful there.
Picture 4.04
Over the last two decades, astrophysicists have focused their attention more on the existence of dark matter. They have tried to find out what the dark matter is made of. As far as I am aware, nobody has managed to identify the nature of dark matter. There are existing theories, which I will outline here with no intention of going into too much detail.
Ordinary matter (making up planets, stars, and galaxies, involving us) is made of baryons. Baryons are protons and neutrons. Electrons are around 2000 times smaller than protons and are somehow dismissed. That is why ordinary matter is usually referred to as baryonic matter. O
rdinary matter makes up around 4 % of the universe.
The existing theories regarding the nature of dark matter are that it can be made of:
1.Baryonic matter
2.Non-baryonic matter, which again can be divided into
Hot dark matter
Cold dark matter
1.Baryonic matter can make up to 15 % of ordinary matter. It is suspected to be a cloud of helium within galaxies.
MACHOs or massive compact halo objects are also suspected of being a baryonic part of dark matter. It was suspected to be a primordial black hole created during the Big Bang, faint red star or brown dwarf. I will give some details about the characteristics of each of these objects, or rather celestial bodies, in Part 2 of my book.
2.Non-baryonic matter is a different kind of matter which does not interact with other particles in the sense of the four fundamental interactions which exist among particles of ordinary matter, except for gravitational interaction.
We know that the temperature of the system or a particular matter depends on kinetic energy or speed of movement of molecules or particles within the system or a particular matter. The quicker molecules, particles move, the higher the temperature of the system or a particular matter is.
With this logic in mind, we can understand further division of non-baryonic dark matter into hot dark matter and cold dark matter:
a.Hot dark matter should be composed of particles which move fast, achieving relativistic speed (close to the speed of light). It is suspected that this kind of matter is made of neutrinos.
b.Cold dark matter should be composed of massive particles, which move slowly.
WIMPs or weekly interacting massive particles are suspected to be the nature of cold dark matter. Within this, there is the theory of the existence of so-called super symmetric particles. It is popularly called SUSY. Namely, according to the Standard Model theory, particles have partners that differ in spin of ½. A photon will have its partner photino, for example, which could be a particle of dark matter.
These are current theories or speculations as to what might possibly be the nature of the dark matter, but so far there is no evidence to support or clearly identify its nature.
5
DARK ENERGY
Around 73 % of the universe is composed of dark energy. Roughly speaking, dark energy can be defined as an increase in space between galaxies, and that increase is accelerating. It is the repulsive force, which counteracts gravitational force.
It was Albert Einstein who first acknowledged the existence of dark energy but for the ‘wrong reason’. Namely, at the time when he worked on the theory of relativity, the universe was considered to be static. It was supposed that the universe was not expanding, but not contracting either.
When we have gravitational force keeping the matter of the universe together in the shape of galaxies and clusters of galaxies, then it is difficult to imagine that the universe will remain static as gravitational force will pull matter together and the universe will collapse on itself. The universe was at that time considered to be static, so Einstein introduced the cosmological constant, in 1917.
It is repulsive force and the opposite of gravity with the exact same force as gravity but in the opposite direction or expansion rather than contraction due to gravity. As there is the exact amount of force but in the opposite direction then it is easy to understand that in such a case the universe will remain static. In every book or documentary regarding this topic, you can read or hear how Einstein commented on the introduction of the cosmological constant as his ‘biggest blunder’, once he found out that the universe is expanding. As we all know, it turns out instead to be one of his biggest achievements and predictions.
However, it was not until 1998 that dark energy was discovered thanks to work by two international teams, one involving Adam Riess and Brian Schmidt and the other involving Saul Perlmutter. They all shared the Nobel Prize in 2011 for discovery of dark energy.
Both teams used luminosity of type 1a supernovas, thanks to which they were able to establish distances between galaxies.
Type 1a supernovas have the same intensity of light or luminosity when they explode whenever they happen in the universe. Because of the exact amount of luminosity it is possible to determine the distance between galaxies when compared to the degree of brightness type 1a supernovas have at the particular galaxy whose distance is known and the brightness of supernovas in faraway galaxies.
How do they do that?
Very roughly speaking, it is known that brightness of light decreases with distance and that the decrease takes place gradually and proportionally to the increase in distance. The equation for it is as follows:
Brightness is flux or amount of light per unit of space while L is the whole light emitted in all directions, which travels away from the source as a sphere. This is actually formula which can be used to calculate the distance of light from us or distance of a particular type of star from us.
So far, a number of different stars have been identified which emit light with particular intensity. According to the degree of brightness a particular star produces, they are categorised or classified in several or precisely seven type of stars. A particular letter is given to a type of star with a particular brightness in order of decreasing temperature, and decreasing brightness as: O, B, A, F, G, K, and M.
O and B type of stars are very bright but uncommon while M stars are common but dim.
You can easily memorise the types of stars according to their brightness from higher to lower intensity by using the mnemonic ‘Oh be a fine girl, kiss me’.
So if we find a G type star, as an example, in a faraway galaxy, but her brightness is less than that from a G type of star, which is closer to us with known distance, then we can calculate the distance of this star from us thanks to the degree of reduction of her brightness. When we know the distance of this star, then we know the distance of this galaxy, as this star is located in this particular galaxy.
Once we know the distance of particular galaxies then we will have the exact intensity of light emitted by type 1a supernovas occurring in this galaxy. Now when we know the distance of this galaxy and intensity of light of the supernova at that distance, we can compare this brightness with the same type of supernovas observed in a galaxy far away. Depending how much fainter this supernova from the closest one with a known distance is, we can calculate how far away is this galaxy.
Using this method, both teams of astronomers concluded independently that galaxies with supernovas at the distance when the universe was two-thirds of the current size are much further than they should be. It was because light from type 1a supernovas was fainter than it should be.
Type 1a supernovas are very helpful in measuring distances between galaxies. This was very helpful in the discovery of dark energy. Basically, by measuring distances between galaxies using this method, it was clear that universe expansion is accelerating which was due to the repulsive force of dark energy.
Type 1a supernova is used as a standard candle for measuring distances. Independent of this, another technique for measuring distances in space can be used, and that is baryonic acoustic oscillation.
Baryonic acoustic oscillation is used as a standard ruler to measure distances. The distance, which it applies to, is a constant and is around 470 million light years or 490 million light years. I find different figures for it in different books.
I do not think it is very important to know precisely the value of this distance. What is important is to get a rough idea that this distance is a constant and is the largest yardstick used in astronomy so far, almost close to half a billion light years but not quite.
Type 1 a supernovas are those which are believed to have originated from a binary system consisting of a moderately massive star and a white dwarf. A white dwarf is a dead star with a large mass density. As such, it sucks into itself mat
erial or mass from its massive companion. The reason for this is that its companion is too close to white dwarf to escape its gravitational pool. However, getting the matter from a massive star, white dwarf is increasing in size. Once its mass grows above the Chandrasekhar limit of 1.44 solar mass, it will end in a massive thermonuclear explosion or type 1a supernova.
Subrahmanyan Chandrasekhar is an Indian-born American astrophysicist who won the Nobel Prize together with William A. Fowler for key discovery in relation to later evolutionary stages of massive stars.
He determined that any star remnant with a mass bigger than 1.44 times the mass of the Sun cannot exist as a white dwarf but instead blows off in a supernova explosion.
Baryonic acoustic oscillations is a notion referring to quite exciting progress made relatively recently in the field of astronomy or astrophysics.
Two Russian astrophysicists Rashid Sunyaev and Yakov Zeladovuch predicted their existence in 1970 but the oscillations were not seen until 2001 when balloon-based microwave detectors were constructed.
Baryonic acoustic oscillations refer to acoustic waves or ripples, which were created and travelling through baryonic matter at a very early stage of universe creation.
They were a result of opposite forces of gravitational attraction due to the enormous mass density of baryonic and dark matter, which was pulling matter inwards and the pressure of photons and electrons pushing it outwards. Dark matter, however, interacts with other particles only through gravitational fundamental interaction. It is subject only to gravitational force and therefore stays in the centre of the sound wave. Baryons and photons due to pressure created travel in the shape of a spherical sound wave outwards.
Journey Through Time Page 9
