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Big Bang Cosmology

The universe is believed to have began from an immense explosion known as the big bang. Cosmologists observe that the universe is expanding, and has been since the beginning. The big bang does not explain why the universe exists, but describes its evolution since it began 13.8 billion years ago. The cause of the big bang remains one of physics' unsolved puzzles. Below is a chronology of the big bang, walking through the stages of the universe's development.

big bang cosmology
The Planck Epoch: The instant of creation. It is not known how or why this began. It can only be observed that it did begin. There is no sense in asking what happened before, because time itself may have began at that instant since time is a property of the universe itself. The big bang does not describe matter expanding to fill empty space. Space itself is what is expanding. All of space, at this moment was extremely small (smaller than the nucleus of an atom) and energy was distributed within it. Because the energy, or mass (E=mc2) of the universe has been constant since this time, the universe was at an unimaginable energy density. This energy was in the form of photons and exotic particles. Normal matter or even more familiar subatomic particles could not have existed during this scorching moment.
Inflation: Inflation theory is a modification to the original big bang theory. But it is now widely accepted as part of the standard big bang model. This theory postulates that the universe underwent a period of rapid expansion, soon after it was born, increasing at an exponential rate. In the first 10-32 of a second, the universe grew by a factor of at least 1026. It is believed that this period of inflation is what caused the visible universe to be extremely uniform as it would have stretched small isothermal regions far beyond the cosmic horizon. This would also have caused the universe to be extremely flat on scales that can be measured. All of this rapid expansion could have made the universe too smooth for things like stars and galaxies to form. But quantum fluctuations during inflation would have been stretched to galactic scales. These slight density variations would provide the seeds of galaxy formation in the eons to come.
Baryogenesis: The universe continues to expand at a slower rate after inflation ends. As it expands it cools, by the same laws of thermodynamics that exist today. The conditions at the Planck epoch and during inflation are quite speculative, but the physics of this era are well explored in particle accelerators of our time. Once the temperature is low enough, protons and neutrons become stable. Neutrons form at the rate of one for every 10 protons, resulting from the value of the Boltzmann factor (e(mass(n)-mass(p))c2)/kT.
Nucleosynthesis: After continued expansion, the protons and neutrons cool (slow down) enough to form atomic nuclei (75% hydrogen, 25% helium, and very trace amounts of Lithium and Beryllium. These are the first four elements of the periodic table.) The proportion of these elements predicted from theory agree closely with the observed relative abundances of the elements in the universe. This phase begins between 100 and 300 seconds after the Planck epoch and lasts for only a few minutes. The nuclei exist as ions, since they are too hot to combine with electrons. Light is continually absorbed and re-emitted by the ions, making the hot plasma opaque.
Decoupling: After about 380,000 years the universe cools to 3000 Kelvin. At this temperature, neutral hydrogen and helium atoms are stable. Once neutral atoms form, light can pass through freely. This is the time when the cosmic microwave background (CMB) was emitted. It has continued traveling through space until this day.
Galaxy Formation: Slightly denser regions of gas begin to collapse under the pull of gravity. These gas clouds form small galaxy systems from which the first stars form. The universe would be a few hundred million years old before these new lights begin to form. Because only hydrogen and helium exist, these stars were believed to be unusually massive. These stars burned hot and fast fusing hydrogen into successively heavier elements. A very large star will continue to fuse elements until it reaches iron. Once the iron core reaches 1.44 solar masses, the star will go supernova, creating all of the natural elements heavier than iron. The blast from the supernova spreads the newly manufactured elements into the galactic medium. These elements are then available for the formation of future generations of stars and planets.
Coalescence: Many generations of stars are born and die. Further enriching the universe with heavy elements. Since the beginning until now, stars have converted about 2% of the universe’s hydrogen into other elements. Small galaxies coalesce into larger galaxies. Groups of galaxies are drawn together by gravity to form clusters and super-clusters of galaxies (thousands or millions of galaxies). All the while the universe is still expanding, so these massive clusters are getting farther away from each other. This process continues today (13.8 billion years after the Planck epoch).

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updated Jun 17, 2013
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