Dark Matter and Dark Energy
Today, researchers estimate that only around 5% of the universe consists of visible matter, meaning matter that we can perceive. Added to this, 26.8% of so-called dark matter and 68.3% of dark energy exist.
Dark matter amplifies gravity
Fritz Zwicky, the Swiss astrophysicist, was one of the first researchers to use the term dark matter. He observed the Coma galaxy cluster in 1933. He estimated the total mass of the galaxy cluster based on the visible galaxies and was thus able to determine the force of attraction - gravity - that prevailed there. He then determined the speeds of individual galaxies and realised that some galaxies were travelling so fast that they should actually overcome the gravity of the cluster and move away. Just as a rocket that is fast enough can overcome the gravitational pull of the Earth. However, these galaxies do not move away, but remain in the galaxy cluster. From this contradiction, Zwicky concluded that the gravity and, consequently, the mass of the galaxy cluster must be much greater than he had estimated. As this mass was not visible, he coined the term dark matter. This is matter that contributes to gravity but is not discernible to us.
Gravitational lenses aid the exploration of dark matter
According to the general theory of relativity, very massive objects deflect electromagnetic radiation and, consequently, also visible light. They therefore act on light in a similar manner to optical lenses. Researchers therefore call them gravitational lenses. On the one hand, these gravitational lenses provide clues to the existence of dark matter while, on the other, they could also be used to learn more about it.
Theoretically, every mass deflects light, but it only becomes relevant for observations in astronomy where magnitudes such as galaxies, galaxy clusters or black holes are involved. Objects that are behind the gravitational lens as seen by the observer may then appear in duplicate, distorted or amplified. When researchers count the galaxies in a galaxy cluster and compare them with the deflection effect that this galaxy cluster exerts on light from other objects, they realise that the mass of the visible galaxies is not sufficient to cause this. This is a further indication of the existence of dark matter. Researchers hope to use gravitational lenses to gain more insights into how dark matter is distributed, or even what it might consist of.
Dark energy explains the behaviour of the universe
Dark energy is a term cosmologists use to explain two phenomena.
- Dark energy is the reason for the accelerated expansion of the universe. In the 1990s, researchers were able to prove through measurements that the universe is expanding at an accelerated rate and, as a result, were awarded the Nobel Prize in Physics in 2011. Supernovae that are far away, and which we therefore see as they were in the past, since the light takes a long time to reach us, move away from Earth more slowly than supernovae that are close to us and whose light is “less old”. The fact that the universe is expanding at all was not surprising. The researchers explained this with the Big Bang theory, as the entire mass of today's universe was concentrated at one point and scattered in all directions during the Big Bang 13 billion years ago. Classically, one would expect the expansion to decelerate the longer it goes on, as the objects attract each other through gravity, thus gradually slowing expansion down again. Observations of supernovae disprove this, and researchers explain this acceleration of expansion with the existence of dark energy which acts in opposition to gravity and drives the universe apart.
- Another indication of dark energy is, very roughly speaking, the fact that Euclidean geometry prevails in our universe. This is the classical geometry we learn at school. Physicists say that the universe is flat. The visible matter and the estimated dark matter are not enough for the universe to be flat. Only the presence of dark energy can explain this fact.
