The Thermal History of the universe - 1

The Thermal History of the universe
As T = (1 + z)T0 the universe gets hotter when we go further back in time. The average energy of the particles increases with the higher temperature. There is a real chance of more interactions can happen. Therefore, at early times all possible particles were relativistic. If they were interacting strongly then the particles would have remained in thermal equilibrium. In the following figure the thermal history of the

universe is shown up to seconds.

(courtesy Imperial Collage - Cosmology Lectures 2012)

As mentioned earlier time equal to zero is accepted as the big bang. However, the earliest we can start talking where classical general relativity gains control of the known universe as a whole is cosmic time of

known as the Planck time. Hawley and Holcomb (1997) state that, the characteristic length-scale of the universe known as the Plank length at this time, was

There is nothing at the moment we can say about the time, from the beginning until the Plank time
which is called Planck epoch. It is accepted that all four fundamental forces of nature, namely gravitational, electromagnetic, weak nuclear interactions and strong nuclear interactions composed a single force during this time. We think at the end of this Planck epoch, the gravitons fell out from equilibrium with the other particles, and gravity decoupled from the other forces. It is believed that the cosmic background
of gravitational waves formed from the gravitons streamed out through the universe.

Hawley and Holcomb (1997) further explain that, the temperature was so high from the Planck time till about unified epoch. During this stage electromagnetism, the weak interaction, and the strong

and we have very little understanding of the nature of the matter under such conditions. This interval is called the interaction were uni ed, and they made up a single indistinguishable force. The theories support that notion are called grand uni ed theories (GUTs). There are other incomplete theories exist that apply to conditions during the uni ed epoch as well.

Cosmologists believe sometime before the end of the uni ed epoch, universe entered a period of exponential expansion called in ation. If that occurred, it must have taken place sometime around

 after the big bang according to Hawley and Holcomb (1997). As noted by Hawley and Holcomb (1997) the most signi cant remnant of the uni ed epoch is the excess matter remaining after the epoch end. However, it is assumed that the universe consisted of a brew of highly relativistic particles, including quarks and more exotic particles.

Hadrons and Leptons are elementary fermions. We can categorise the type of particles of the universe like this:

• Leptons - fundamental particles which participate in weak interactions, electrons, muons, taons and neutrinos are leptons

• Hadrons - made up of quarks which participate in strong interactions.

Hadrons have two subfamilies

•  Baryons: Fermions made up of three quarks, for example Proton and neutron.

• Mesons: Bosons made up of two quarks. Examples are pions and kaons.

Hawley and Holcomb (1997) noted further that particles created from pure energy in ordinary processes must always be created in matter anti-matter pairs known as pair production. When the particle and anti-particle collide, they destroy one another, converting their rest mass to photon energy. The conservation of baryon number rule should be observed here like baryons anti particle should be negative baryons. However in most grand uni ed theories this conservation rule no longer holds. Reactions like transforming quarks into leptons and vice versa can occur, thus violating the baryon conservation. These particular reactions can occur in such a way that tiny excess of matter can remain. This process by which matter was
preferred over antimatter is called baryogenesis which would have created the material that our current universe consist of.

We think the end of the unified epoch came atand this was followed may be by quark epoch where universe consisted of free quarks and gluons, other carrier particles of combined electromagnetic and weak force, more exotic heavy particles and anti-particles. The weak and electromagnetic forces were unified as the electroweak interaction during most of the quark epoch.

The four fundamental interactions:

1. Electromagnetic interaction: these are responsible for the forces between electrons and protons in atoms, and for the emission and absorption of electromagnetic radiation, such as light. A small leftover electromagnetic interaction of the electrons and protons in atoms allows atoms to bind together to make molecules and so is responsible for chemistry.

2 Strong interactions : these provide the (very) strong force between quarks inside protons and neutrons. A small residual strong interaction between quarks binds protons and neutrons together in the nuclei of atoms.

3 Weak interactions : these are responsible for processes, such as radioactive beta-decay, that involve both quarks and leptons.

4 Gravitational interactions : these make apples fall, maintain planets in their orbits around stars, and control the expansion of the Universe. However, they are negligible within the atom. But when matter aggregates into huge (and electrically neutral) lumps, such as planets and stars, gravity holds sway. You will see that it also holds some surprises, undreamt of by Newton.

Reference:
1. J. Hawley and K. Holcomb. Foundations of Modern Cosmology. Oxford University
Press, USA, 1997. ISBN 9780195104974. URL http://books.google.co.uk/
books?id=eBFawfP8ak8C.
2. How the universe works - Andrew Norton Page 62