The Beginning of the Universe
A Timeline of the Universe
This diagram shows a short timeline of the universe. At very early times (< 10-32 seconds), it is thought that the universe had this very rapid inflationary phase, where quantum fluctuations were blown up to macroscopic scales. After this brief period of exponential growth, the universe then transitioned into a more gradual phase of expansion and cooling. At a time of 400,000 years after the Big Bang, protons first recombined with the available electrons in an event known as recombination, leaving photons to wander the universe freely. The universe then entered a dark age for a period of several hundred million years, until the first stars and galaxies formed. Subsequent to this time, galaxies have been merging and coalescing to become larger and more mature systems.
It is now widely agreed that the universe started in a hot big bang. The quantum fluctuations which were present shortly after the Planck time were blown up onto macroscopic scales by a short period of highly rapid growth during the first 10-32 seconds of the universe. The early period of rapid growth is called inflation and is thought responsible for the remarkable degree of homogeneity seen in the early universe as well as the small amounts of structure present then.
Then, what followed this early inflationary phase of the universe was a more gradual, normal evolution, where the universe continued to cool and expand. During this cooling phase, the first protons and neutrons condensed out of the mix, and then hydrogen, helium, lithium, and beryllium froze out. Eventually, when the universe was 400,000 years old, the universe had cooled to the point where the ambient photons in the universe were no longer energetic enough to keep hydrogen ionized. This resulted in one of the first truly great phase changes in the state of elemental hydrogen during our universe's history. Hydrogen — which up to this point existed in the fully ionized form as free protons — recombined with the available electrons to form neutron hydrogen. At the same time, the ambient radiation — now too low energy to ionize hydrogen — were simply left free to wander the universe. This background radiation is still with us today in a much cooler form and provides us with one of the first and powerful pieces of evidence for existence of the hot Big Bang. The significance of this epoch is such that it has earned the special name "recombination" to highlight this significance.
The Dark Ages/First Light
A Timeline of the Universe
This timeline of the universe demonstrates how far back the current generation of deep images bring us. For example, galaxies seen in the Hubble Ultra Deep Field (the deepest image ever taken by human kind) date to as early as 700-900 million years after the Big Bang (5-7% of the age of the universe). Since the first galaxies and stars are thought to form just 200-300 million years after the Big Bang, current observations are now bringing us very close to first light.
As time went on, more matter and stars were pulled onto these overdensities, resulting in the formation of the first galaxies. These galaxies then merged with other galaxies to form larger galaxies — and the whole process of galaxy buildup had begun.
The formation of the first stars and first galaxies brought about several fundamental changes in the rapidly evolving universe. For one, these stars provided the universe with its first source of elements heavier than beryllium. While conditions early on in the big bang were not appropriate for the generation of the heavier elements, such elements could easily be generated at the cores of massive stars and then later expelled into the surrounding universe when these stars explode as supernovae. The second profound change brought about this first generation of stars was the introduction of high-energy photons into the universe. Previously, the last the universe had seen of such high-energy photons was during the first 400,000 years of the universe when the universe was still very hot and hydrogen still existed in an ionized state.
A Timeline of the Universe
Here is a cosmological simulation showing the evolution of galaxies from very early times to the present day. Galaxies start out very small in the universe and then progressively merge to form larger and larger systems. Time is demarcated in the simulation as the cosmic redshift "z." Redshift 30 ("z=30") corresponds to a period roughly 100 million years after the Big Bang, while lower redshifts correspond to later times ("z=0" corresponds to the present day).
These high-energy photons started what was likely a rather extended process by which hydrogen in the universe was reionized. This reionization process apparently began several hundred million years after the Big Bang and finished some 600 million years later. Currently, there is great interest in understanding the properties of the first galaxies because they likely played a key role in this process.
The first generation of small galaxies was likely well in place 400 million years after the Big Bang. Following this initial phase of galaxy formation, galaxies then went through an extended phase of merging and coalescence with other galaxies, whereby they built up from masses of several thousand solar masses to billions of solar masses. This buildup process extended until the universe was roughly two billion years old. Then, due to some feedback process -- now predominantly speculated to be AGN feedback -- it is thought that this buildup process halted and gas accretion and star formation in the most massive galaxies halted and galaxies underwent a much different form of evolution. This later evolution continues to the present day.