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Last modified: January 26, 2018

Big Bang Model
--------------

The chronology of the Big Bang model is as follows:

Planck Epoch:       0 s - 10-43 s  
                    T = 1032 K.
                    No theories.

Grand Unification:  10-43 s - 10-36 s  
epoch               Gravity separates.

Electroweak epoch:  10-36 s - 10-32 s
                    T = 1028 K.
                    Strong Force separates.

Inflationary epoch: 10-36 - 10-32 s
                    Rapid expansion.
                    Universe become filled with a uniform quark–gluon plasma.

Quark epoch:        10-12 s - 10-6 s
                    Weak and electromagnetic forces separate.
                    Quarks continue to appear but cannot bind.
                    Appearance of the Higgs Field - elementary particles 
                    get mass.

Hadron epoch:       10-6 s - 1 s
                    Quarks combine and hadrons, including baryons such 
                    as protons and neutrons form.

Lepton epoch:       1 s - 10 s
                    Hadrons and anti-hadrons annihilate each other leaving 
                    leptons and anti-leptons to dominate the mass of the 
                    universe.

Photon epoch:       10 s - 380,000 years
                    Leptons and anti-leptons annihilate each other leaving 
                    photons to dominate the mass of the universe.  
                    
Nucleosynthesis:    3 mins - 20 mins
                    Protons (hydrogen ions) and neutrons begin to combine 
                    into atomic nuclei in the process of nuclear fusion.  All 
                    neutrons are absorbed into Helium nuclei. Ratio of H to 
                    He is about 3:1.

Matter domination:  70,000 years +
                    Matter dominates, allowing for gravitational collapse to 
                    amplify the inhomogeneities left by cosmic inflation, 
                    making dense regions denser and rarefied regions more 
                    rarefied. 

Recombination:      377,000 years
(decoupling)        Electrons get captured by the H and He ions, forming 
                    electrically neutral H and He atoms. Most of the protons 
                    are now bound up in these neutral atoms leaving photons 
                    to travel freely.  These are the photons that we see in the 
                    Cosmic Background Radiation after being greatly cooled 
                    by the expansion of the universe.  The universe becomes 
                    transparent.   Baryogenesis occurs.

According to this model the universe at early times was a nearly uniform 
expanding collection of high energy, high temperature particles.  As it 
expanded and cooled, small inhomogeneities were amplified by gravity 
and collapsed to form the structures we see today.

The big bang model is in perfect agreement with general relativity.  which 
predicts that a homogeneous universe would expand and cool in exactly 
this way. In addition, there have been many observational confirmations 
of the model. These include the apparent motions of distant objects relative 
to us and the cosmic microwave background radiation. 

It is tempting to try and extrapolate the big bang model all the way back 
to the time of the actual big bang.  This iimplies that we could run the 
equations of general relativity backwards to earlier times and higher 
densities.  Unfortunately, there is a problem.  When we try and describe 
regions of spacetime whose density exceeds the Planck density of roughy 
1093 g/cm3, which correspnds to a Planck time of approximately 10-43, 
quantum fluctuations in spacetime become important and quantum mechanics 
and general relativity start to disagree and we have no solid theories to 
describe this situation.  

As successful as the Big Bang theory is at explaining the universe from
the time after the Planck density, there are certain drawbacks.  While 
on a 'local' scale the universe is 'lumpy' and we need Einstein's General 
Theory to explain things, on a grand scale, the universe is measured to 
be homogeneous, isotropic and almost flat.  However, under Big Bang 
cosmology, for a closed (positive curvature) universe,  curvature grows 
with time.  Recall from the FRW equation:

H2 = 8πGρ/3 - k/a2

Rearranging we get:

k/a2 = 8πGρ/3 - H2
     
As time progreses, the H2 term get smaller and smaller and at some point
will become zero.  The expansion stops and the universe begins to 
contract.  Up until that point, however, k/a2 will grow. 

Another issue is that distant regions of space in opposite directions are 
so far apart that they could never have been in contact because the light 
travel time between them exceeds the age of the universe.  Yet the 
uniformity of the cosmic microwave background radiation (WMAP) tells 
us that these regions must have been in contact with each other in the 
past.  This is referred to as the 'horizon' problem.

Current theories of particle physics predict that in the extraordinary hot
and dense conditions that existed during the earliest stages of the 
universe, various kinds of 'relic' particles such as magnetic monopoles 
would be produced.  Big Bang cosmology predicts that we should be able 
to see these particles.  However, magnetic monopoles have never been 
observed in nature. 

Finally, to get us to the present day, the Big Bang model would require 
that the curvature of the universe at the time of the Planck density  
could not exceed one part in 1059.  If it were slightly more curved 
than this (closed), it would have recollapsed long ago.  If it were 
slightly less (open), it would have flown apart so quickly galaxies 
would never have formed. The probability that the curvature was exactly 
right for the universe to survive to later times is considered to be 
highly improbable  

To see how these things might be resolved it is necessary to introduce
the INFLATION THEORY.

 

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