Introduction
In preparation for a few public lectures I’ll be doing in the next 6 months, I wanted to address another one of the three main (that I’ve see) straight-forward young-earth Creationist claims about astronomy that “prove” we live in a young universe: Spiral galaxies “wind themselves up too fast.”
This is actually the #1 claim in Russ Humphrey’s treatise on “Evidence for a Young World” that you can find on sites such as Answers in Genesis or the Institute for Creation Research (e.g., this link).
Note that the #2 reason presented is that comets would disintegrate too quickly (which I’ve addressed here) and the #5 claim (#3 astronomy claim) is that the Earth’s magnetic field is decaying too quickly, which I will address in a future blog post.
About Spiral Galaxies
Spiral galaxies, such as the one above (M101), are generally medium- to large-sized congregations of stars. They have either a bulge in the center or a bar in the center. The bulk of the galaxy is a disk (much wider than it is thick) that contains spiral arms. For more basic information on galaxies, see this link.
The feature in question in creationist circles is these subjectively beautiful spiral arms themselves. The trick is that these arms are not “solid.” It is not the case that stars either always exist within a spiral arm or they always exist outside of an arm. Rather, the arms are constantly picking up stars and losing others. What the arms represent are just density waves.
The common analogy to think of is cars on a highway. You may be driving along with many dozens or hundreds of meters between you and the car in front of you. Then, for no apparent reason, you start to get much closer to the car in front of you. And then, for the next several kilometers, there are only maybe five to ten meters between you and the car ahead of you. Afterwards, traffic seems to thin out again and there’s a large distance between you and the next car.
What you have just experienced is a density wave. You are a star, traveling the road that is an orbit around the galaxy, and every now-and-then you find yourself in a density wave where you have to slow down.
The mechanism that perpetuates the density waves – why they don’t just dissipate – is that as a star approaches a density wave, it will speed up slightly due to the gravity of the stars there. And as a star is about to leave a density wave, it will slow down a little, again because of the higher gravity there. So they won’t just smooth out over time.
How did the spiral arms get there in the first place? The main idea here is that all you need is a disk of stars. Stars closest to the center of the disk will need to rotate around it faster than those near the edge, just like planets in our solar system (Mercury’s velocity around the sun is much faster than Earth’s). This can easily set up the initial differential rotation needed to start them.
In addition to this, stars do not orbit on circular paths, rather on elliptical ones (Kepler’s first law). When farthest from the center, their velocity will be at its slowest (Kepler’s second law). When you have just a few extra stars traveling a little slower in some parts of a differentially rotating disk, then you will get spiral patterns.
The Creationist Claim
To quote from a source other than Russ Humphreys: “Stars closer to the center of a spiral galaxy orbit the galaxy faster than stars farther away. Over many millions of years, the difference in orbital rates should wind the spiral tighter and tighter. We do not see any evidence for this in galaxies of different ages.” (This is from the Creation Wiki website.)
Problems with the Creationist Claim
The problem with this is that it rests upon the unstated major premise that density waves are physical parts of galaxies that contain a set of stars that is unchanging. That way, the differential rotation will cause them to “wind up” into a featureless disk. As I have already explained above, this is simply not the case. Galaxies are not like figure skaters.
Another problem is a timescale here. Russ Humphreys may be correct when he places a maximum age of “a few hundred million years” on his fallacious understanding of the theory of spiral galaxies. However, many others, including the one quoted above, will say “many millions of years,” or even “thousands of years.”
Those time scales are way too short. The sun takes about 250 million years to orbit around the galaxy once (as anyone who watches Monty Python knows). There is no way that – even given their faulty understanding of the model – galaxies would “wind up” within less than 1% the time it takes a star half-way from the center (about where we are) to complete a single orbit. This is actually a fairly good example (like comets) about how creationists often don’t understand the timescales involved with astronomical phenomena.
Creationist Refutes of the “Naturalistic” Refutes
(1) To quote again from the Creation Wiki website, the very first response to the explanation I have given is, “First of all this is a theory not a proven fact.” This is, to put it nicely, a bogus argument. As I have stated many times on this blog, a scientific theory is when a hypothesis has withstood all attempts to falsify it, and all data are explained by it. So even them saying it’s a theory is an admission of that. However, “theory” is often used in a derogatory manner by creationists because the colloquial definition is more along the lines of, “a vague idea.”
(2) The next response is, “Furthermore, it does not come from first principles, but is simply the latest in a series of theories designed to save the long age theoretical system from reality.” Well, yes it does come from first principles. Try running a computer model of spiral galaxies, and you’ll see it work pretty darn well after only plugging in “first principles” like gravity.
The next part of that, “series of theories,” is not as derogatory as they intend. Science progresses. If one theory has explained all the data to-date but then the next piece it can’t explain, then a new theory needs to be developed. This, of course, is in contrast to creationism where evidence that refutes their “theory” is simply tossed out the door.
The final part of that sentence, “series of theories designed to save the long age theoretical system from reality,” is simply an attempt to paint what I’ve presented as an Argument from Final Consequences (logical fallacy) when it simply is not.
(3) The final claims, that observations of M51 have shown that the arms in the center don’t fit with this theory – is a misreading of the technical literature. The reference given (Zaritsky et al. (1993), “Inner spiral structure of the galaxy M51,” Nature, 364) clearly states:
The coherence of the arms over this large radial range challenges current theories of spiral structure. We suggest that a combination of several mechanisms, such as the interaction of M51 with the neighboring galaxy NGC5195, forcing by the central ‘bar’, or distortions from density waves, is required to generate the observed structure.
No where does it “[call] into serious question [the spiral density wave theory] by the
Hubble Space Telescope’s discovery of very detailed spiral structure
in the central hub of the ‘Whirlpool’ galaxy, M51,” to quote Humphreys. Rather, it states that just using a simple model that I laid out that you would get in an introductory astronomy class does not tell the whole story. It tells a lot of it. But you do need other information in order to explain every detail of the observations.
Final Thoughts
I think that with this post, I’ve fairly tidily explained why creationist arguments for a young universe based on spiral galaxies are fallacious, generally falling subject to a gross misunderstanding of the theories involved, the technical literature, and various other, smaller, factors.
I also want readers to remember that I am not trying to undermine religion. Faith in a divine creator is a completely separate issue. It is only when people use that faith as a starting point to make testable, scientific claims that I will explore them and refute them if fallacious enough.
Exposing PseudoAstronomy 