Chapter 11: Dark and Exotic Matter

The Universe contains matter, which evidently, we know exists. However, there is visible matter and invisible matter or dark matter. There is evidence that establishes this “missing matter,” but it is impossible for us to know what this matter is, or the form that it takes. Two ideas have been proposed over the last 30 years. The first proposal was a halo, which consists of heavy clumps ordinary matter that form spherical covers around galaxies. The other type is a particle that does not emit or interact with radiation.

Dark matter, unlike most objects in the Universe, does not shine, but there are many other objects in the Universe that don’t shine like planets, asteroids, comets, and rock masses. Interestingly enough, these objects don’t shine because they don’t give off energy like a star. They simply reflect light from the Sun, which allows them to be seen. The problem with dark matter is that we cannot identify it form. Maybe it is in such an obvious form that we’re looking too hard for it and can’t identify it. Dark matter accounts for such a large mass that small objects such as planets and other small masses combined could not account for all of the dark matter in the Universe. At the same time, they can’t be too heavy because if they are, they will ignite and become stars.

Objects that have a mass that is equal to or less than 80 per cent of the Sun’s mass have been called Massive Compact Halo Objects or MACHOs for short because they are massive and could be used to explain the mass of halos.

An example of a MACHO is a brown dwarf. So far in my studies in science, I have never heard of a brown dwarf. Brown dwarfs are very much like planets, and have masses a few times greater than Jupiter’s mass. Although their masses are greater than Jupiter’s, they are actually smaller than Jupiter because they are so dense from the principles of quantum mechanics. Planets are held together by electromagnetic forces, but brown dwarfs follow Heisenberg’s Unicertainty Principle that accounts for the movement of particles as they try resist gravitational force. Brown dwarfs are very heavy, and because they account for such a great amount of mass, they can clearly be considered to be dark matter.

Black holes also possess characteristics of MACHOs. When light attempts to travel beyond its gravitational pull, it is restricted and pulled towards the centre of the gravitational force. A black hole is essentially an object with a radius equal to the Earth’s radius with the ability to restrict light. The point at which no light can escape is called the event horizon, causing objects to appear dark. Although it was believed that black holes were just a speculation- a mathematical speculation based on Einstein’s genera theory of relativity- there is evidence for their existence.

At a black hole’s surface, the gravitational pull is so powerful that it destroys the surrounding material, and separates it into constituents of energetic particles and radiation. Black holes are surrounded by light that can be viewed with x-ray telescopes. If this is so, can we correctly categorize black holes as MACHOs?

Brown dwarfs and black holes are definitely dark, but gravity makes it possible to see them- gravity works as a lens. Take, for example, a star shining in the distance, and a MACHO is passing in front of it. It is expected that it would cause and eclipse as it passes in front of the star, however, this is not the case. Instead, the MACHO deflects the light and focuses towards the Earth. Just as the MACHO passes in front of the star, the light is beamed at us and we can see the light. This effect is called microlensing and effectively deflects dark objects.

Although effective, there are two drawbacks to this method of identifying dark matter. First of all, stars change their brightness and dimness regularly. It is possible that in some cases, their twinkle may be confused with the passage of a MACHO. Secondly, microlensing is incredibly rare. To find such a event, an observer must observe several million stars over years and be in the right place at the right time to catch one of them twinkle.

Researchers have even postulated the existence of a dark matter particle, however, such a particle would be too light to detect or to clump together in the Sun’s core. These particles could potentially collide at the centre of the galaxy. In doing so, they would produce pairs of electrons and positrons eventually producing beams of photons.