Monday, April 13, 2015

After the Big Bang: the Formation of Galaxies

Astronomers in the latter half of the 20th century were able to make groundbreaking discoveries that expanded our understanding of the universe. We now have achieved a better understanding of how the universe may have came into existence and of the broad outlines of the Universe's evolution. However, despite these great discoveries, fundamental questions like how the first galaxies came about still remain unanswered.

Although we do not know how the universe came into existence, we do know from the Big Bang theory that 10-6 seconds after the "bang" that brought space and matter into being, protons and neutrons began to form, and twenty minutes later the universe was filled with hydrogen and helium ions. During the first 50,000 years after the "bang," the universe experienced a radiation-dominated era, where photons dominated matter and kept structures from forming. For several hundred thousand years immediately after, the universe was too hot for elements to form. The universe existed as a mix of subatomic particles and radiation. The first hydrogen and helium atoms did not form until the universe cooled to the point where the matter became transparent to the radiation. Waves of radiation stretched and diluted until they made a faint glow of microwaves which filled the entire universe. Images taken by NASA's Cosmic Background Explorer show maps of the slight differences from the mean temperature from one location to another that are now believed to be the first signs of structure. However, we do not know whether these subtle variations are the beginnings of the formation of the first galaxies.

Many theories have been proposed about the formation of galaxies. Many astronomers used to believe that the universe broke into large clumps that contained enough building materials to make structures like great walls and sheets of millions of galaxies. Continuous fragmentation would have produced smaller and smaller clumps which would finally "give birth" to individual galaxies such as our Milky Way and neighboring galaxies like the Large and Small Magellanic Clouds. Such clumps have been photographed by the Hubble Space Telescope and were speculated to be possible explanations to the formation of modern galaxies. However, simulations of the universe's evolution have shown that this "top-down" theory of structure formation is not plausible. Instead, the vast majority of astronomers now suppose the universe was formed by small pieces of gas clouds and star clusters that merged over time to form galaxies and clusters of galaxies. The first structures to form were quasars, luminous regions around super-massive black holes, and stars that were completely made out of hydrogen and helium. Then star clusters merged to form the bulges of galaxies, which then accumulated more gas and dust to form flatten disks with spiral arms. Smaller galaxies merged with bigger ones; gravity attracted galaxies to the groups, clusters, and even superclusters we observe today.

- Alice Zhang