Sunday, 23 March 2014

The Thermal History of the universe - 2

Continuing from previous post: about the Thermal history of the universe. When the time is around
and a temperature of
the weak interaction (see previous post)  thought to be decoupled from the electromagnetic force. Now all four forces mentioned earlier were separated. During the transition the carrier particles of the uni fied electroweak force were transformed (hypothetically) into 4 new particles.
Three of them are called bosons
which acquired mass and the other one is massless photon. 

To discuss this further, according to Phillips (1994), it is generally accepted that, within the first nano seconds the universe was filled with a gas of fundamental particles
like leptons, anti-leptons, quarks, anti-quarks, neutrinos, ant-neutrinos, gluons and photons. We assume that quarks, anti-quarks and gluons annihilated and transformed to less massive particles when the temperature fell below . However, the number of quarks very slightly exceeded the number of anti-quarks. The small number of quarks remaining were thought to be responsible for the present number of protons and neutrons of the universe. when the temperature decreased further the heavier leptons and anti-leptons were annihilated as well.

When the cosmic time was
quarks formed neutrons and protons while 
Therefore between a millisecond to a second after the big bang the universe was consisted of electrons, positrons, neutrons, protons, neutrinos, antineutrinos and photons. At about 1s when
neutrinos started to decouple.

Soon after this, all of the positrons and most of the electrons were removed by annihilation of electron-positron pairs. This seems to have occurred when cosmic time was approximately 4 seconds and
. Phillips (1994) further states that, when 
t~3min and 
neutrons combined with protons to form light nuclei - Helium and other light particles, which lead to a universe with approximately 75% of its mass consisting of hydrogen and 25% of helium. 
After around 300,000 years later
and the temperature was around 4000k, it was a low enough temperature for the formation of stable atoms, and photons to decouple. Hydrogen and helium nuclei combined with electrons and

formed neutral hydrogen and helium atoms which lead to photons stopping to interact strongly with matter. The universe became transparent to electro-magnetic radiation which cooled down to about 3k at present time because of the expansion of the universe. This is the so-called cosmic microwave background detected by Penzias and Wilson. Olive (1999) claimed that, the connection between the BBN and the CMB is a key test to the Standard Big Bang Model.

About Penzias and Wilson: The accidental discovery of cosmic microwave background radiation is a major development in modern physical cosmology. Although predicted by earlier theories, it was first found accidentally by Arno Penzias and Robert Woodrow Wilson as they experimented with the Holmdel Horn Antenna. The discovery was evidence for an expanding universe, (big bang theory) and was evidence against the steady state model. In 1978, Penzias and Wilson were awarded the Nobel Prize for Physics for their joint discovery.

1. J. Hawley and K. Holcomb. Foundations of Modern Cosmology. Oxford University

Press, USA, 1997. ISBN 9780195104974. URL
2. A. Phillips. The Physics of Stars. Manchester Physics Series. John Wiley &
Sons, 1994. ISBN 9780471941552. URL
3. K. A. Olive. Primordial big bang nucleosynthesis. ArXiv Astrophysics e-prints, Jan.
1999. URL

No comments: