Armagh Observatory: School Student Essays

The Big Bang

Laura Wilson, Friends School Lisburn, 6 April 2001

20 billion years ago, cosmic expansion began and the universe was born. The early Universe was very hot (approximately 10 billion K), very dense and very unstable. According to the big bang theory, huge amounts of energy were released, giving rise to matter, antimatter and dark matter.

This theory is based on a number of assumptions:

• The fundamental laws of physics do not change with time;
• Einstein's theory of general relativity correctly describes the effects of gravity;
• The early universe was filled with a gas of expanding and intensely hot elementary particles in thermal equilibrium;
• Any changes in matter or radiation have been so gradual or insignificant, they haven't affected the evolution of the universe.

The Big Bang Theory has enabled scientists to make certain predictions. Firstly, as the universe expands, the galaxies move further away from each other with a velocity proportional to the distance between them. Hubble's study of red shifts backed up this prediction. Secondly, there should be a background of microwave radiation in the universe, left over from the big bang. Robert W Wilson confirmed this prediction in 1964. Thirdly, during the first few minutes after the big bang, light atomic nuclei were released from protons and neutrons. This has also been proved.

In ancient times, atoms were thought to be the smallest pieces of matter. (The word atom means indivisible in Greek) Since then, smaller particles have been discovered: neutrons, protons and electrons. They, in turn, are made up of quarks. It has been estimated that there are 1 x 10⁸⁰ particles in the region of the universe we can observe. These particles are the building blocks of matter, antimatter and dark matter.

Antimatter

Antimatter is made up of antiparticles: elementary particles of opposite charge but otherwise identical to their partners. For example, the antiparticle of an electron is a positron.

This idea suggests that the matter we are composed of is only half of a symmetrical whole. If a true symmetry of matter and antimatter exists, when any particle meets its antiparticle, they are both an annihilated and energy is emitted in the form of gamma radiation. The amount of radiation released by these collisions can be calculated using Einstein's famous equation E = mc² (where E is energy, m is the combined mass of the two particles, and c is the speed of light).

It is thought that the first basic particles emerged when a large concentration of energy coagulated into matter and antimatter. A perfect creation would have lead to equal quantities of these two types of matter. This in turn would have lead to a universe destroyed within an instant of its creation. The conclusion drawn from this reasoning is that our universe arose because it is quintessentially asymmetric.

According to mainstream cosmology, creation was barely completed when something intervened, breaking the symmetry between matter and antimatter. After this great annihilation, a small amount of matter was left over. The universe as we know it is made up of this left over matter, suggesting that we are just a fraction of the result of an even greater creation than we had previously thought.

When ordinary matter is created, and equal amount of antimatter is created too. The idea that antistars, antiplanets or even intelligent beings constructed from antimatter could exist is perhaps more science fiction than science fact.

Dark matter

Dark matter is matter known to exist because of its gravitational influence but so far undetected by other means. Our galaxy must contain much more mass than had previously been suspected. The stars we can see and the dust and gas we know of constitute less than 10% of our galaxy. The remaining 90% appears to be distributed spherically about the centre off the galaxy, reaching out far beyond the most distant visible stars.

The gravitational influences between galaxies also provide evidence of the existence of dark matter. The galaxies within galactic clusters are gravitationally bound to each other more tightly than their total mass would predict.

Dark matter is sometimes known as 'missing mass'. The only things missing from this mass are non-gravitational evidence of its existence and detectable electromagnetic radiation, so the term 'dark matter' is more appropriate.

Missing hydrogen

Only half the hydrogen from the early universe ended up in galaxies like the Milky Way - the rest went missing. Scientists in New Jersey recently claimed that they have found some of this missing hydrogen. It appears to be in superhot clouds hiding in intergalactic space. One explanation for the disappearance of the hydrogen is that it became so hot, its atoms lost their electrons. Since atoms cannot emit or absorb light without electrons, the hydrogen became invisible. This could theoretically happen if the hydrogen was shockheated to over 100000°C.

Explanation of the Big Bang

The Big Bang Theory is only a theory and so there is no concrete evidence to suggest that it is entirely correct. Two of the problems associated with the theory are the lack of explanation of the conditions necessary for the Big Bang and the possibility of the existence of monopoles. These are exotic particles which would have dramatically altered the evolution of the universe, resulting in events which would oppose the way we see the universe now.

There are many theories as to how the universe was created, and similarly, how it will end. Some scientists believe that the universe could end just as it started: in a state of infinite density, known as the Big Crunch. At present, we do not fully understand the Big Bang or the Big Crunch. Hopefully if we learn more about dark matter and find out what it was that upset the balance of matter and antimatter we will be at least part of the way there.