14 Nov

Today, we are leaving for a trip. Our destination ? Matter. This travel will bring you towards fondamental principles of matter to finish with misteries of dark matter. Fasten your belts, let’s go !


Introduction about matter


Everything that exists is composed of matter. Matter can be found in differents states : solid, liquid, gas or plasma. The fondamental property of matter is to have a mass. The matter that we all know is composed of baryions, this is why it is called baryionic matter. A baryon is a caterogy of particles having a non neglectible mass. The most known baryons are neutrons and protons. Since they also are hadrons, they are composed of three quarks.


Non baryonic matter


This expression refers to other kind of matter that are not composed of baryons. One of these form of matter is known as dark matter. Why this name ? Because this matter is invisible… this means that it is impossible for us to detect it. But then how did we come to this hypothesis in which dark matter could exist ? While measuring galaxies’ masses, scientists discovered an incoherence. Indeed, it seems that dark matter plays a role into galaxies’dynamics and in galaxies heap’s dynamics without emitting anything which could allow us to detect it.

Dark matter : composition


Different hypothesis are competing nowadays. It is possible that dark matter is composed of elementary particles which are not on earth. Another possibility is that dark matter is actually composed of molecular clouds or black holes that are really difficult to detect due to their lack of emissions. The problem is that estimations that were made on universe density and on the nomber of particles inside, need the existence of unknown particles this led to the hypothesis of a non baryonic kind of matter. What is surprising is that dark matter seems to be everywhere. In order to make simulations about galaxies masses become right, dark matter should be representer five times more than baryonic matter.


How the hypothesis about missing matter : dark matter was born ?


The first researcher who noticed this incoherence was the swiss Fritz Zwicky in 1933. Zwicky wanted to study a group of galaxies located in the heap of « Bérénice’s hair » (you can see the poetry in the given name) to calculate the mass by watching the dispersion of speed. Indeed the speed difference between heap components are linked to the mass inside the heap. Zwicky compared then the dynamic mass (deduced from its gravitational influence) to the light mass, which means the mass deduced from the amount of light coming from the heap. The scientist noticed that dynamic speed was 400 times higher than the speed coming from light. However, nothing was that surprising since measurements were not that precise. Moreover, black holes and molecular gas could participate to the measurement error.


In 1970, the american scientist Vera Rubin wanted to study spiral galaxies rotation. Once again, the idea was to compare the mass coming from stars’ light composing the galaxy to the dynamic mass of these stars. For the mass coming from light, Rubin made the assumption that the galaxy was made exclusively out of stars and estimated this mass based of stars distribution inside the galaxy. The scientist observed then that stars located at the periphery of the galaxy were moving at a speed that was surprisingly too high because it was not dinimishing besides the fact that they are very far from the center of the galaxy. The hypothesis that was then formulated was the existence of a huge quantity of invisible matter.



Recent results


In 2006, a team announced having observed dark matter for the first time. This was performed while observing the Balle heap. This heap results from a collision between two neighbouring heaps. Researchers wanted to determine the mass distribution into the heaps pair. They compared this mass distribution with the one of ordinary matter given by the light observation. The light measurement was obtained by studying the X-rays coming from the very hot gas of the heap resulting from the collision. Dark matter was not located at the same place than baryonic matter. This led to the conclusion that dark matter was not involved during the collision.


Unfortunately, to really confirm dark matter existence, one should prove it by a direct observation such as the interaction between dark matter particles with our detectors or the creation of one of these particles in a accelerator such as the LHC (see the link to get more information).


For dark matter’s nature, two theories arised : hot dark matter and cold one. These theories differ in the speed supposed for the particles which should compose dark matter. In the case of hot dark matter, particles should approach the speed of light whereas in the case of cold matter, particles could be really slow. Neutrino is the perfect candidate for the hot dark matter. But for the cold dark matter, it would be the WIMP or the MACHO. Nowadays, the theory which is taking the advantage is the cold dark matter because a lot of researcher agree with it.


This is the end of this article. Future will tell us more about this dark matter since this field of research is improving faster and faster. Stay informed on 2 Steps From Science !


« The pig : organ donor vocal cord graft, a myth? Not sure ! »


PhD Student in Biophysics

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *