The origin of galaxies has remained such an unsolvable mystery because technology has not yet progressed to such a point that it can satisfactorily answer this question. It was advancements in technology that helped clarify other contentious aspects of the universe, such as the theories of star formation. Once the next big leap in technological improvements is realized perhaps our perception of the universe will be extended far enough to give us the answer to the question of galaxy formation. Until then, there are several theories regarding galactic origins that have merit, two of which are the top-down model and bottom-up model of galaxy formation.
In the top-down model, we must go back almost to the beginning of the universe. Early on, after matter had gained dominance over radiation, there was little more than dispersed, if massive, gas and dust clouds. These gas clouds then began condensing to form the early galaxies known as quasar galaxies. This model is called the top-down model because it suggests that a single, large gas cloud would have yielded several smaller “daughter” galaxies. The clouds may have contracted due to ripples in the early universe propagated by the Big Bang, though nobody knows the cause for certain. However, computer simulations running this event showed that there was not enough time for the galaxies to form given the age of the universe, which is postulated to be around 12 billion years old. With only the fluctuations of the early universe, galaxies would have needed many billions of years more to form than the universe has been in existence. Therefore, some other component must have been in effect to accelerate the process of galaxy formation.
This “something” is theorized to be dark matter, a classification of matter which astronomers have only been able to detect by its gravitational effects on the cosmos. One theory is that the dark matter clumped together first, before the “normal” matter had been able to do so. This clumping of the dark matter then gravitationally attracted the “normal” matter in the cloud, causing it to collapse much faster than it would have with just the ripples. The presence of dark matter in the early universe would explain how the galaxies were able to form much faster than originally predicted.
Another prevalent theory is the bottom-up model. Unlike the top-down model, this theory argues that small gas clouds merged due to gravity and then condensed to form the galaxies, instead of one gas cloud breaking apart and forming many galaxies. James Binney and Michael Merrifield in their book Galactic Astronomy use the Milky Way as an example to support this theory1. It is well known that the Milky Way has satellite galaxies called the Magellanic Clouds. These satellites are considerably smaller than the Milky Way, and they are caught in our galaxy’s gravitational field. Models have shown that the Magellanic Clouds are destined to merge with the Milky Way because the force of gravity will cause them to collide1.
In this theory, the galaxies started out small, probably around the size of the Magellanic Clouds and other irregular galaxies observable today. They then grew by merging with other galaxies in close proximity through mutual gravitational attraction. The Hubble Space telescope is able to see galactic mergers that provides strong evidence for this phenomenon2. In fact, the Milky Way is predicted to collide with the Andromeda Galaxy in four billion years. If humanity is still around at that time, we will have a clear view of one of these merging events.
Of the models presented, the top-down model seems to be the most accurate, due to the ease with which the theory flows with the rest of cosmic evolution. The theory’s main premise of galaxies evolving from large gas clouds that then condensed into the galaxies matches well with both stellar and planetary evolution. In both the stellar and planetary models, the objects in question are formed from the accretion of a large amount of small, dispersed objects. Since this is also how the top-down model describes galaxy formation, it would be aesthetically pleasing for this model to be the most accurate one, if only for the sake of consistency. After all, if nature exhibits certain behaviors in not one but two major areas of astronomy, who is to say that the same cannot ring true for galaxies?
That being said, there are convincing pieces of evidence that favor the bottom-up model. Since the primary method of galaxy formation involves the merging of many smaller galaxies to form larger galaxies, one would expect to see much variation in the size of galaxies. This variation has been observed. For example, some larger elliptical galaxies are up to twenty times larger than some spiral galaxies. On the other side of the spectrum, some irregular galaxies are mere shadows of the larger galaxies, containing only a million or so stars compared to the billions found in spiral galaxies. Such variation in the sizes of galaxies could be easily explained by the bottom-up model—the size being dependent on how many merges a galaxy has undergone. However, the difference in size could also be explained in the top-down model because there is nothing in the top-down model that implies that the galaxies need to be similar in size. A large gas cloud could have fragmented into smaller condensing regions of many different sizes, some yielding the larger elliptical galaxies and some yielding the comparatively small irregular galaxies, depending on the size and mass of the initial condensing region. This evidence, therefore, is not singularly beneficial to the bottom-up model of the universe.
Both the top-down model and the bottom-up model have been promoted time and time again as the leading theories of galaxy formation. These theories share similarities, but they differ on some critical points: (1) whether the galaxies formed from a few large gas clouds or from many small gas clouds and (2) whether the merging of galaxies was a major factor in early galaxy formation. The gas cloud issue has already been addressed. As for the issue of galactic mergers, quite frankly, we cannot know for sure. We certainly can see galaxies interacting with one another in our observations of the universe, which is clear evidence that these galactic interactions happen; in fact, they happen quite frequently. The problem is, we do not know how often, if at all, they were interacting at the time of the early universe. Perhaps the merging of galaxies started only after they had fully formed, which would mean that the galactic interactions would only be an appreciable factor regarding galactic evolution, not necessarily galactic formation. We will not know the answer until we are able to directly observe those early years of the universe.
This topic is woefully theoretical. Though it seems that the top-down model is a more accurate reflection of reality than the bottom-up model, it could easily be that the bottom-up model is in fact the correct model of galaxy formation; at this point, nothing is beyond the realm of probability. Until our telescopes and detectors become advanced enough to see what lies in the early universe, any theory regarding galaxy formation could be proven true. Only time will tell which one, if either, is correct.
- Binney, James and Michael Merrifield. Galactic Astronomy. Princeton, NJ: Princeton University Press, 1998.
- Hubble Spies Energetic Galaxy Merger. Space.com. Tech Media Network. Last updated October 13th, 2009. Retrieved October 11th 2012.
- Gribbin, John. Galaxies: A Very Short Introduction. Oxford ; New York: Oxford University Press, 2008.
- Strobel, Nick. Astronomy Notes: Galaxy Origins. Last updated June 9th 2010. Bakersfield College. Retrieved April 15th, 2012.
- Goldsmith, Donald. Astronomy and Astrophysics In The New Millennium. The Origins of Stars and Planets. Taken April 21st, 2012.
- Eales, Stephen. Origins: How The Planets, Stars, Galaxies, and the Universe Began. London; Springer, 2007.
- Windows to the Universe. Solar System Formation. National Earth Science Teachers Association (NESTA). Taken April 21st, 2012.
- Image credit (Creative Commons): Argalla, Luca. The Orion Nebula. Flickr. Taken December 17, 2011.
Dylan Paré is currently a homeschooled high school senior. He plans on majoring in astrophysics while also continuing to pursue his interests in foreign languages and writing. Follow The Triple Helix Online on Twitter and join us on Facebook.