Register  /  Login      

The dark side of the Universe: the Dark Matter – PhotoNick

By CoperNick

 Alice Gaggero

 

February 10, 2020

We all have listened at least once to someone talking about the Dark Force referring to the Star Wars’ Movies, but this article has nothing to do with it.

Instead, I would like to introduce you to the “real” dark side of the Universe: the Dark Matter.


Fig 1: Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter.

We all have knowledge of the visible part of the Universe, made by particles which creates the stars and the planets, but in the 20th century scientists have posited that the universe could not consist only of visible matter. This is how they began talking about the Dark Matter.

The initial issue was that the galaxies rotate at such speed that they should fall apart if they were made only of visible matter, because the gravity force would not be able to hold them together.


Fig 2: Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the ‘flat’ appearance of the velocity curve out to a large radius.

So scientists began to propose that something, which we cannot see, should provide the extra matter that galaxies need not to fall apart: they named it Dark Matter.

Studies and observations suggest that Dark Matter constitutes about the 27% of the universe.

The attribute “Dark” refers the fact that this kind of matter does not interact with the electromagnetic force: basically, it does not absorb, reflect or emit light, making it difficult to individuate.

Now, what is the Dark Matter composed of?

For now we are only able to list what does not compose dark matter; the rest is still a hypothesis.

We know that dark matter is not planets or stars – at least, as we know them: besides the fact that visible matter interacts with the electromagnetic force, observations prove that there is too little visible matter in the universe to make up the 27% required.

Second, we know that dark matter is not dark clouds of “normal matter”, made up of particles called baryons: indeed, we would be able to detect baryonic clouds by their absorption of the radiations traversing them – which proves that they interact with the electromagnetic force.

Third, we know that dark matter is not antimatter, indeed we do not see the unique gamma rays that are produced when the antimatter annihilates with “normal matter”.

Last, we know that dark matter is not large-galaxy black holes, as we do not see enough gravitational lenses (the high concentrations of matter bending light when passing near them) for them to cover the 27% of universe’s composition.

 

Fig 3: 3-D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.

 

Regarding what it is that constitutes Dark Matter, there are at least two different theories:

The first one considers MACHOs (MAssive Compact Halo Objects): they are objects raging in size from small stars to super massive black holes, made of ordinary matter (protons, neutrons,…), and they can be a) neutrons stars, black holes or b) brown dwarfs.

  1. The former two are the results of a supernovae of a massive star, and supporting this thesis is the fact that they can be both dark when isolated; on the other hand, resulting from a supernovae, they are not common object and there is no evidence that they occur in sufficient numbers in the halo of galaxies to cover the 27% of universe’s composition as the dark matter should do.
  2. Brown dwarfs have a mass that is less than 8% of the Sun’s mass, for this reason they cannot produce the nuclear reactions that make stars shine. Scientists have observed objects that are brown dwarfs or large planets around others stars, but for now those detected in our galaxy are not enough to represent all the dark matter of the Milky Way.

The second theory consider WIMPs (Weakly Interacting Massive Particles) which are subatomic particles that do not consist of ordinary matter. They are called “weakly interacting” because they can pass through ordinary matter without any effects, and “massive” because they have mass.

In this case, the candidates are a) neutrinos, b) axions and c) neutralinos.

  1. The existence of neutrinos was hypothesized by physicists in the early 20th century, and then proved at the end of the century. They were thought to be without mass, but in the 1998 one type of neutrinos with mass was observed. Scientists think that the neutrinos present in the universe are not enough to contribute significantly to the dark matter because they have a really “small” mass.
  2. The axions are particles theorized to explain the absence of an electrical dipole moment of neutrons. They have not been observed yet, nonetheless they are supposed to have a “small” mass, but to be present in large number in the universe. They should have been produced by the Big Bang.
  3. The neutralinos are a type of particle considered by the theory of Supersimmetry: an attempt to unify all forces under one theory. They are supposed to be massive particle, with a mass between 30 and 500 times the mass of protons. Like the axions they have not been detected yet.

Theoretically, there is the possibility that massive subatomic particles, created in the first moments after the Big Bang, in the right amounts and with the right properties, could constitute the dark matter of the universe. However, only neutrinos have been detected and there is not enough of them in the universe. Some scientists think that the axions and neutralinos could be produced at LHC and be detected by the energy and momentum missing after the collision because they would pass through the detectors without interact with them.

As you can see, there is still plenty of questions about dark matter without an answer, but astronomers and physicists around the world are working to answer all of them.

Bibliography:

Picture credits:

READ MORE ARTICLES AT THIS LINK