cosmology the origin and future of the universe part 2 from the big bang to today

CosmologyThe Origin and Future of the UniversePart 2

From the Big Bang to Today

Basic Principles

The basic principles of the Big Bang theory are: The Universe was initially exceedingly

small and exceedingly hot Since the Big Bang the Universe has

been continuously expanding and continuously cooling

Big Bang

It is believed that all of the hydrogen and helium from which stars have evolved was produced in the first few minutes of the life of the Universe

A million years later the temperature had cooled to 3 000K and the hydrogen and helium combined with electrons to produce atoms

Big Bang

Later, pairs of atoms combined to form molecules

When electrons had been trapped in atoms, the photons no longer collided with them and consequently travelled further unimpeded – the Universe became transparent

Big Bang

A million years from the Big Bang was the start of the period of time during which gravitational forces have been prevalent in the development of the Universe

Any clumping of atoms produces larger masses which have greater gravitational forces and which attract more matter

Big Bang

Hence the formation of the stars and galaxies!

The Universe could not have been uniform or else the clumping would have all happened in one place

These imperfections in the uniformity of the Universe are present today in the small variations in the microwave background radiation

Big Bang

In fact, variations corresponding to variations in temperature of the order of three hundred thousandths of one Kelvin have been detected by the Cosmic Background Explorer (COBE) in 1992

After this discovery all attention turned to the first few minutes of the Big Bang

Particle Experiments

High speed particle accelerators have given scientists the ability to investigate fundamental particles

As temperature is the measure of the kinetic energy of particles then finding out what happens at high speed informs scientists about what happens at high temperatures


Temperatures of around 1012K have been achieved in particle accelerators and this temperature amounts to that present about 10-3s after the Big Bang


However, particle accelerators are very expensive to construct and, the faster particles are made to move the heavier their mass becomes.

This makes it even more difficult to make the particles accelerate

The Basic Principles

At normal temperatures, atoms are the building blocks of matter

At temperatures greater than 104K nuclei can not hold on to their orbiting electrons and nuclei move around in a ‘sea’ of electrons – this is known as a plasma


At temperatures in excess of 107K the nuclei themselves are no longer stable and split into component protons and neutrons

Protons and neutrons are made from fundamental particles called quarks – at temperatures above 1013K these quarks can no longer hold together as protons and neutrons


At higher temperatures, the four forces which exist in the Universe become less distinguishable from each other

At 1015K the Weak and Electromagnetic forces merge and at higher temperatures the Strong and Gravitational forces can no longer be identified as separate forces


Particles which are known as ‘messenger’ particles are believed to be responsible for all of the four types of forces

W & Z bosons are the messenger particles for the Weak Force and evidence for these was discovered by colliding protons and anti-protons at speeds corresponding to 1015K


At temperatures greater than 1015K physicist believe that high energy particles without mass, viz. photons, are constantly being converted into particles which do have mass

The mass of these particles is given by E = Δm c2


At temperatures which are higher than 1015K the photon energy is converted into particles of even greater mass

Why Does Matter Exist in the Universe?

There is evidence that when a photon produces a particular particle of matter then a corresponding anti-particle is produced

If the particle and the anti-particle were to meet again then they would annihilate each other and produce a photon of energy

In the early stages of the Universe then there would have been enough energy to produce very large X-particle/anti-particle pairs


Below a certain temperature, photons will no longer produce particle/anti-particle pairs although such pairs will still annihilate each other when they meet

This means that all conversions would then be mass > energy


As particles and anti-particles were originally created in equal numbers then why have all of these not annihilated each other?

Why is there matter still existing in the Universe?


It is thought that there was an asymmetry in the decay of massive X-particles and their anti-particles during the very early, hot stages of the Universe

This may have resulted in an extremely small excess of matter over anti-matter

This would explain the small amount of matter remaining in the Universe after the mutual annihilation of matter and anti-matter

The Future of the Universe

The rate at which galaxies are moving apart must decrease with time due to their mutual gravitational attraction

What happens in the future will depend on the size of the gravitational forces compared with the rate of expansion


If the average density of the Universe is smaller than a critical density then the gravitational forces will be too small to stop the Universe expanding

Consequently the Universe would be open or unbounded


If the average density of the Universe is greater than the critical density then the Universe would start to contract again until it produced the Big Crunch – the opposite to the Big Bang

This would mean that the Universe would be closed or bounded


If, however, the average density was equal to the critical density then the Universe would approach a definite limit to expansion

The Universe would be flat or marginally bounded


Scientists need to know the average density of the Universe if the future is to be predicted – the critical density can be calculated from the Hubble Constant

It is difficult to estimate the density of the visible matter in the Universe, but there is also the question of how much dark matter there is – some scientists suggest up to 90% of the Universe is dark matter


It is estimated currently that there is little difference between the average density of the Universe and the critical density

This suggests that the Universe may be flat or, at least, makes it difficult to decide whether it is open or closed


It is only just becoming possible to determine which of the three outcomes of the Universe is likely by measuring the rate at which the Hubble constant varies with distance

Also, knowing the exact relationship between average density and the critical density is important when determining the age of the Universe

cosmology the origin and future of the universe part 2 from the big bang to today

Documents

big bang slide

big bang theory

matter slide

transparent slide

plasma slide

cooling slide

basic principles

high temperatures