The Milky Way Galaxy Just Got Bigger (at Least for a While)

in the late 20th century astronomers realized that most of the matter of the galaxy is invisible.

Perhaps the most startling development of our understanding of the Milky Way came in the late 20th century when astronomers realized that most of the matter of the galaxy is invisible. This dark matter, as it has come to be called, probably accounts for 90% of the Milky Way’s mass. What dark matter is made of remains a mystery.

Dark matter seems most concentrated away from the lighted portions of the galaxy, relegating dark matter to the outer regions and in the halo. Thus, astronomers speak of the Milky Way and other galaxies having dark matter haloes. And dark matter may extend beyond the obvious limits of the galaxy defined by what we see. If dark matter extends much beyond visible matter in the galaxy, then the Milky Way may be much larger than thought.

A recent study attempted to determine just how far the Milky Way’s dark matter may extend.

A recent study attempted to determine just how far the Milky Way’s dark matter may extend. The paper is technical, so many people may find a news report of the study more understandable. The researchers used three different models to simulate the origin and development of the Milky Way. All three models assume that the galaxy formed early in a big bang universe.

The simulations were done for galaxy formation in the environment of the local group, a local small collection of galaxies, of which the Milky Way is one of the two dominant members (the other being M31, the Andromeda Galaxy). In the simulations, the galaxy formed as matter condensed in the early universe. As the galaxy gained mass, its size, as defined by the largest orbits of its matter, increased. It is this maximum orbital size of constituents of the Milky Way that define the galaxy’s size. The researchers found this to result in a Milky Way 1.9 million light years across, about 15 times larger than currently thought.

In Theory . . .

Keep in mind that this is a theoretical result. However, it does make some predictions that might be testable.

Keep in mind that this is a theoretical result. However, it does make some predictions that might be testable. For instance, if the galaxy truly is this big, there ought to be at least a few stars orbiting the Milky Way nearly a million light years away (this is the orbital radius, half the diameter of the galaxy). Such stars would appear very faint, but searches are under way for these stars. If stars were discovered that far out and moving with the speed expected from such a distant orbit, then this would amount to confirmation of the prediction.

Another possibility is detecting orbital motion of very small dwarf galaxies. In addition to large galaxies, such as the Milky Way, there are many more very small galaxies that orbit the large ones. The Milky Way has many such orbiting dwarf galaxies, with more discovered frequently. A new search with existing and upcoming very large telescopes could reveal more of these tiny galaxies at greater distances than found so far. If they are found, and if their motions are consistent with the predictions of this paper, then they, too, would amount to confirmation of the models.

What are we to make of this? Keep in mind that this is a model, not (yet) based on actual observations. Furthermore, this model is contingent upon many evolutionary assumptions, such as how galaxies form and evolve, and even when and how the universe came into existence (via a version of the big bang model) and evolved. That is a lot of assuming. Even if some data eventually are found that conform to the expectations laid out in this paper of a much greater size of the Milky Way Galaxy, other very different models could just as easily explain the data. The biblical model doesn’t prescribe a big bang and billions of years of galactic origin and evolution. Instead, God miraculously made the heavenly bodies, including galaxies, on the fourth day of creation week (Genesis 1:14–19).