Cosmology The Origin and Future of the Universe Part 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

Particle Experiments  Temperatures of around K have been achieved in particle accelerators and this temperature amounts to that present about s after the Big Bang

Particle Experiments  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 10 4 K nuclei can not hold on to their orbiting electrons and nuclei move around in a ‘sea’ of electrons – this is known as a plasma

The Basic Principles  At temperatures in excess of 10 7 K 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 K these quarks can no longer hold together as protons and neutrons

The Basic Principles  At higher temperatures, the four forces which exist in the Universe become less distinguishable from each other  At K the Weak and Electromagnetic forces merge and at higher temperatures the Strong and Gravitational forces can no longer be identified as separate forces

The Basic Principles  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 K

The Basic Principles  At temperatures greater than K 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 c 2

The Basic Principles  At temperatures which are higher than K 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

Why Does Matter Exist in the Universe?  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

Why Does Matter Exist in the Universe?  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?

Why Does Matter Exist 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

The Future of the Universe  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

The Future of the Universe  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

The Future of the Universe  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

The Future of the Universe  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

The Future of the Universe  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

The Future of the Universe  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