By 1978 Rubin had analysed 11 spiral galaxies, all of which were spinning too fast to stay together according to Newton's laws. As a result the astronomer community had to accept that dark matter existed.
But what is dark matter? What is it made of? Three hypotheses have been
made:
1- Dark matter is made of dim ordinary matter that is made of brown dwarf
stars, neutron stars, black holes ... that are nearly invisible. Together
they are called "MACHOs". These objects are also called "Baryonic
matter" (made of protons and neutrons).
2- Dark matter is made of very hot "Non-baryonic matter", possibly
neutrinos. It is called "Hot Dark Matter".
3- Dark matter is made of a new type of matter called "Cold Dark Matter
or WIMPS". At the present time it is the leading candidate to explain
dark matter.
In 1916 Karl Schwarzschill found an exact set of solutions to Einstein's equations for a massive star. It could be used to calculate the gravity of any star. Einstein used them to calculate the sun's gravity. Schwarzschill then showed that around all massive stars there were "imaginary" magic spheres -now known as Event Horizons. Anybody or object crossing these spheres would be attracted by the stars' gravity without possibility of escaping. Not even light could escape! He was also able to calculate the radius of these spheres -called the Schwarzschill's Radius (3 km for the sun; 1 cm for the earth). This means that if the sun could be compressed to a sphere of 3 km radius, it would become a black hole. Herman Weyl showed that there could be another universe on the other side of that black hole.
In 1934 J. Robert Oppenheimer and Harland Snyder showed that black holes are not only made by a collection of particles collapsing under their own gravity. In fact all massive stars that have used all their fuel will implode under their force of gravity. Once they reach a radius equal to their Schwarzschill's radius, the stars collapse to form black holes. Black holes, they said, are the natural end-points for millions of dying giant stars.
Einstein showed that wormholes could exist within black holes. These black holes could be used, n theory, to reach other parts of our universe, or even another universe. However technical problems make the use of wormholes impossible today.
In 1963 the mathematician Roy Kerr from New Zeeland found another exact solution of Einstein's equations showing that a dying star is a spinning black hole. Following the conservation law of angular momentum, a star collapsing under gravity spins faster. Such a spinning star could collapse into a ring of neutrons that remains stable because the centrifuge force balances the gravity force. Kerr also showed that if a person fell into a Kerr black hole, he or she would not be crushed to death but would be transferred to another universe through an Einstein/Rosen bridge. However, this would be a one-way trip. Gravity would allow the escape from our universe but the same gravity would prohibit the return journey through the event horizon. To come back one would need a second Kerr black hole connecting the parallel universe to ours. It is not certain, however, if the Kerr black holes are stable. If they are not, the person trying to go through the bridge would destabilise the black hole and the bridge would close.
The concept of black holes has only been accepted by most scientists since about 1998 when astronomers identified hundred of them using the Hubble Space Telescope, the Chandra X-Ray telescope and the Very Large Array Telescope. Now they believe that most galaxies have at their centre a flat circular core, often one light year across inside which there is a big black hole in. All of them are rotating at big speed (up to a million miles per hour).
Black holes cannot be seen but one can see their "Accretion Disk" of gas surrounding them. The black hole event horizons lie at the centre of these accretion disks and inside the black holes themselves.
Two types of black holes have been found:
- Stellar black holes. They are the final result when gravity crushes of
dying stars that then implode into black holes.
- Galactic black holes. They exist at the centres of galaxies and quasars.
Their weight can be as high as billions of solar masses.
Mot black holes are very old (billions of years old) but now astronomers can see black holes being born from gamma ray busters. An US satellite "Vela" picked up signs of huge explosions in space. Astronomers realised that they were from gamma ray busters releasing huge amount of energy in space within a few seconds. It is now believed that they are the result of "Hypernova" creating massive black holes.
Scientists now believe that there are also mini-black holes. The energy required to create a black hole in laboratory is so high that it is impossible to do. However, if there is a parallel universe existing less than one millimetre from ours, then the energy required is much smaller and some could be soon produced, or instance by the Large Hadron Collider (LHC) being built at CERN, Geneva, Switzerland.
Normally black holes are produced when a large amount of matter is compressed to less that its Schwaezchill radius. But they can also be produce by compressing energy (according to Einstein's theories, mass and energy are equivalent). Scientists hope that the LHC will be able to produce some mini-black holes, about 1,000 times the mass of the electron and with a life of 10E-23 second. Cosmic rays from outer space may also contain some mini-black holes and the Pierre Auger Cosmic Ray Observatory is looking for them.
Finding some mini-black holes in cosmic rays or in laboratories would be
an indication that String Theory is correct.