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Episode 92: Spiral Galaxies and a Young Universe

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Recap: A classic young-Earth creationist claim is that spiral galaxies would wind themselves up and become a uniform disk if they had been around for millions or billions of years. In this episode, I go through this basic claim and why a basic misunderstanding of spiral galaxy arms leads to its conclusion.

Puzzler for Episode 92: There is no puzzler in episode 92.

Answer to Puzzler from Episode 91: There was no puzzler in episode 91.

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Transcript

Claim: It's been awhile since I did a young-Earth creationism claim, so I wanted to take aim at one of the top-ten claims, literally. Dr. Russell Humphreys, Ph.D., is a young-Earth creationist and astronomer, and many years ago he assembled a top-ten list of reasons that young-Earth creationism is true. The very first item on that list is, "Galaxies wind themselves up too fast."

The claim, to really quote verbatim, goes like this:

The stars of our own galaxy, the Milky Way, rotate about the galactic center with different speeds, the inner ones rotating faster than the outer ones. The observed rotation speeds are so fast that if our galaxy were more than a few hundred million years old, it would be a featureless disc of stars instead of its present spiral shape.

Yet our galaxy is supposed to be at least 10 billion years old. Evolutionists call this ‘the winding-up dilemma,’ which they have known about for fifty years. They have devised many theories to try to explain it, each one failing after a brief period of popularity. The same ‘winding-up’ dilemma also applies to other galaxies.

For the last few decades the favored attempt to resolve the dilemma has been a complex theory called ‘density waves.’ The theory has conceptual problems, has to be arbitrarily and very finely tuned, and lately has been called into serious question by the Hubble Space Telescope’s discovery of very detailed spiral structure in the central hub of the ‘Whirlpool’ galaxy, M51.

About Spiral Galaxies

Spiral galaxies 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, the arms emanating from the central bulge or bar.

The feature in question in creationist circles is these subjectively beautiful spiral arms themselves. How are they there still there if the universe is old?The trick is that these arms are not “solid.” It's not like you have a length of string on a table that starts out straight, you put your fingers in the middle and twist it so the ends wind up around it. That's not how spiral arms form. Rather, they are a visual manifestation of another process, like water spiraling down a drain. Just as with water spiraling down a drain, where any given water molecule may be part of a density wave at one moment and not at the next, 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 is to think of 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 despite you traveling at the same speed. 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, they self-perpetuate.

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 (which 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.

For what it's worth, it should be mentioned that this is something that comes naturally in simulations of spiral galaxies and other kinds of rotating disks. For example, when I'm doing simulations of Saturn's rings, I start out with a random, uniform distribution of particles. After running the simulation in orbit around Saturn, after about 6 orbits, density wakes have formed with very few particles between them. At any given time step, one particle may be a member of a density wave, but a few timesteps later it may have left the density wave. Galaxies are probably a bit more complicated than Saturn's rings, and you have other complications like gas drag, but the basic idea remains.

Creationist Claim, Redux

Now that I've effectively explained why the creationist claim is wrong, let's get back to it and see what else can be learned. To quote from a source other than Russell 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, this is simply not the case. Galaxies are not like figure skaters with solid arms. You also don't have to fine-tune density wave physics to get them to actually happen, contrary to Humphreys' claim.

Another problem is the timescale. 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 I just quoted, 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). Even if you accept their faulty understanding of the way the arms work, there is no way that 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 Refutations of the "Naturalistic" Refutes

On the Creation Wiki website, they have a page that is attempted to be a refutation of the Talk Origins page on spiral galaxies that is a refutation of the creationist model. I'm going to refute the refutation of the refutation of the claim.

(1) 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, and I think most people who listen to this podcast are very familiar with the, "but evolution is just a theory!" argument. A scientific theory is when a hypothesis has withstood all attempts to falsify it, and all data are explained by it. So even the creationists 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.” Which is not the case here.

(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 when it simply is not. An argument from final consequences is where you start by saying your answer is true, therefore your premise or evidence or whatever you need to say to back up your already-known answer is true. That is not the case here.

(3) The final claim part of their refutation, is "Hubble images of [the] Whirlpool Galaxy [AKA M51] and others show that they are too tightly wound near the core to be explained by the Density Wave Theory. On the other hand[,] the Windup Model predicted this tight winding perfectly."

This 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 as opposed to the CreationWiki site. 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.

Wrap-Up

By way of wrap-up, this is a fairly classic young-Earth creationist claim, at least dating to the 1980s, that gets brought out from time-to-time. It rests on a key misunderstanding of what spiral arms are, and even if it were correct, the timescales of how long it takes a galaxy to rotate once would still easily allow for a universe at least several hundred million years old, if not over a billion.

Provide Your Comments:

Comments to date: 2. Page 1 of 1.

Stuart R.   Lyons, CO, USA

11:29am on Saturday, December 7th, 2013

Richie,

I don't know if you're a creationist or not, but your comment displays an incredibly common tactic employed by YECs: "Hundreds of Millions ≠ 13.7 Billion, Therefore 6000." It's a form of false dichotomy where you have effectively said that because I gave a number of less than 13.7 billion (you quoted "several hundred million years old, if not over a billion"), then 6000 years as a maximum is plausible.

Not only that, but you have quote-mined or at least misrepresented what I said. The quote you pulled had the context where I spent the entire episode explaining that the YEC concept that spiral arms are solid, static structures like string as opposed to density waves like water, is wrong. But that IF the creationist model were right, which it is not, then it would still easily allow for an age for spiral galaxies on the order of a billion years. That is not a "margin of error" (1 billion vs. 13.7 billion), that is an approximate, allowable age given a WRONG model... read more »

Richie Cunningham   M51

9:18am on Friday, December 6th, 2013

Interesting take. Not going to argue/debate the entire article but I want to point out that your very last sentence should be a problem for you (or believers of a 13.8 billion year universe).
"would still easily allow for a universe at least several hundred million years old, if not over a billion"
If a billion-ish is your maximum, that's a big problem to try and account for 13.8 billion. (over 13 times greater)... that's a large margin of error.

It is not a problem for young-earth creationists as this allows for human error and assumptions to create inflated dates. 6,000+ years is within the maximum range.

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