the first galaxies

The Beginning of the Universe

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

After the epoch of recombination, the universe continued to cool and expand. The universe then was filled with a equidensity mix of hydrogen, helium, lithium, and beryllium. Every place in the universe had almost exactly the same density of hydrogen atoms as every other place. However, the universe was still not perfectly uniform. Some regions of the universe were ever so slightly denser than other regions. Though the differences were not large (just one part in one hundred thousand), little by little gravitation forces worked to gradually pull matter from the surrounding areas onto these overdensities and after a few hundred million years, enough matter had been drawn onto these overdensities to form the first stars.

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.

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. next...