The hot big bang model is currently the best explanation we have for the evolution of the universe, and it has become a key part of the standard model of cosmology. In this model we make an assumption that, the universe is homogeneous and isotropic on the largest scales.
We also make an assumption that the laws of physics, which have been verified in laboratory conditions are also valid in the early universe. Finally, we assume that the cosmological principle holds.
With the assumption of homogeneity and isotropy the evolution of the universe is governed by the Friedmann equations obtained from General relativity.
From these equations of motion, and our knowledge of the content of the universe today, a picture emerges in which a universe began in a hot dense state, and expanded and cooled into the one we see around us today. There are many observable relics from this hot dense origin for example the radiation we observe as the Cosmic Microwave Background (CMB).
The best evidence we have for the isotropy of the observable universe is the unifor- mity of the temperature of the Cosmic Microwave Background Radiation (CMBR). Today the CMBR study reveals that we have microwave radiation photons with 2.75 K temperature throughout the universe.
Distributions of galaxies give us also direct evidence of homogeneity.
As the universe cooled different physics was in operation and different particles were present. These different types of particles were baryons, electrons, photons, neutrinos, fermions and bosons and antiparticles. Baryon is comprised of three quarks and is not a fundamental particle. Baryons participate in strong interac- tions. The Electron is regarded as a fundamental particle. Historically neutrinos were thought to be massless particles and travel close to the speed of light. General acceptance is there are three types of neutrinos and they all interact weakly with other particles. Neutrinos denoted by symbol ν. However some recent experimental evidence indicates that neutrinos may have a mass.
The elementary particles are divided into two main groups depending on the amount of spin that they carry and they are referred to as Bosons and Fermions. All neutrinos and Baryons are fermions while the photon which has twice the spin of the electron is a boson.
Standard model of the Big Bang Nucleosynthesis (SBBN) describes the approxi- mately first twenty or so minutes of the evolution of the universe. It is the phase of evolution in which protons and neutrons, which previously had existed as separate particles combined to form atomic nuclei. Only once the energy scale has dropped sufficiently is this energetically favourable.
The abundance of light elements is an- other relic of the hot big bang, and armed with this theoretical knowledge astrophysicists and astronomers accumulated the abundances of the light elements thought to be produced just after the big bang.These elements are Helium 4 [4H e], Helium 3 [3He], Deuterium, Lithium 7 [7Li], Beryllium and Boron. With the predicted abun- dances at hand astrophysicists and astronomers aimed their telescopes to far ends of the universe and observed the abundances of those light elements and begin to compare the observed to predicted.
The standard Big Bang Nucleosynthesis model achieved significant success in predicting the light-element abundances produced during the nucleosynthesis that agrees well with the observations. It also helps us constrain the parameters of the standard model of cosmology, in particular the number of baryons with respect to photons.
New discovery announced today: http://www.bbc.co.uk/news/science-environment-26605974