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Our future, our universe, and other weighty topics

Monday, June 13, 2016

A Tenfold Worsening of the Spiral Galaxy Explanation Problem?

One of the most amazing facts about the large-scale universe is the very large number of beautiful spiral galaxies. Most of the larger galaxies in the local universe are spiral galaxies. In my previous post The Unsolved Mystery of Why So Many Galaxies Are Beautiful Spiral Galaxies, I discussed the inadequacies of current attempts to explain the existence of so many spiral galaxies. 

The Whirlpool Galaxy (Credit: NASA)

Galaxies rotate, taking about 200 million years to rotate. For spiral galaxies this rotation leads to what is called the winding problem. This is the problem that the rotation of spiral galaxies should cause the spiral arms of galaxies to be ruined after a few rotations (in less than a billion years), due to a “winding up” effect. But somehow the spiral arms of spiral galaxies have apparently persisted for more than 10 billion years. Scientists have attempted to explain the persistence of spiral arms using a theory called the spiral density wave theory. But that theory is not in good shape, and is not well-supported by evidence. A scientific paper with the phrase "A case against density wave theory" in its title mentions “further negative evidence for density wave spirals.”

When I wrote my original post, I had never heard of “super spiral” galaxies. In the spring of 2016, news articles started to appear on this extra-large type of spiral galaxies. A NASA press release was entitled “Scientists Discover Colossal 'Super Spiral' Galaxies.” It said that the newly discovered type of spiral galaxy was as big and as bright as the biggest and brightest galaxies previously known.

The new type of “super spiral” galaxy is ten times more massive than our own galaxy. This apparently makes the spiral galaxy explanation problem ten times worse than it previously was.

Page 4 of this paper shows 53 of the “super spiral” galaxies. They look pretty much like spiral galaxies we are used to seeing in photos of distant space. Referring to a major attempt to simulate galaxy evolution with a computer simulation, section 7.2 of the paper says, “"Even the largest galaxy evolution simulations to date, such as the Illustris simulation...are not big enough to manufacture a significant number of super spirals."

The Illustris project was the largest attempt to simulate the universe using a supercomputer, and used 8000 CPU's running in parallel. I searched all 4 scientific papers published by the Illustris team, and found no evidence that their simulation had produced an outcome in which a large fraction of the galaxies are spiral galaxies. The authors made no attempt to categorize how many of their simulated galaxies were spiral galaxies. One of the papers claims that the simulation produced “ a reasonable population of ellipticals and spirals,” but from that statement we cannot tell whether the number of spiral galaxies was 10%, 1%, or .0001%.

I also tried using the “Infinitely Scrolling Galaxy Explorer” of the Illustris project, at this location. This allows you to scroll through simulated galaxies produced by the simulation. Very few of the simulated galaxies had clear spiral arms like that of the Whirlpool galaxy. Almost all the galaxies shown looked like elliptical galaxies or irregular galaxies or disk-shaped galaxies with random concentrations of stars but not spiral arms. It seemed that less than 2% of the simulated galaxies were spiral galaxies, and the number could have been less than 1%. In our universe ring galaxies are rare, but in the Illustris simulation there seemed to be many times more ring galaxies than spiral galaxies. In the Illustris simulated galaxies, in the rare cases in which there was something what looked like a spiral arm, there was almost always just one spiral arm, rather than the two or three spiral arms we see in real spiral galaxies.

I therefore find the way in which the Illustris project reported its outputs to be misleading in regard to the issue of whether the project was able to produce a universe in which a large fraction of the galaxies are spiral galaxies. Here is what an MIT press release of the project stated (a press release reproduced on the Illustris web site):

With this model, we are able to get agreement with observational data on small scales and large scales,” says Mark Vogelsberger, an assistant professor of physics at MIT and first author of a new paper in the journal Nature that describes the modeling effort. While modeling 41,416 galaxies in all, the simulation closely matches the rate at which certain types of galaxies develop across the universe as a whole. “Some galaxies are more elliptical and some are more like the Milky Way, [spiral] disc-type galaxies,” Vogelsberger explains. “There is a certain ratio in the universe. We get the ratio right. That was not achieved before.”

But this statement does not match what you see when you scroll through the galaxies produced by the project, using the “Infinitely Scrolling Galaxy Explorer” on the Illustris web site. While a large fraction of the simulated galaxies are disk-shaped, only a tiny percentage (perhaps as few as 1 percent) look like spiral galaxies with one or more spiral arms (spiral galaxies have 2 or 3 spiral arms). Who should we blame here for this misleading statement? Given the fact that the MIT press release writer has inserted in brackets the word “spiral” (a word Vogelsberger apparently did not use), we perhaps cannot directly blame Vogelsberger. But we can fault him for failing to correct the modified version of his statement. With the insertion of the word “spiral,” the reader is left with the very misleading impression that the Illustris simulation “got the ratio right” by creating a simulated universe in which the number of spiral galaxies was similar to the ratio in the known universe. The simulation did no such thing

Another press release of the Illustris project claimed that the project created “a realistic mix of spiral galaxies like the Milky Way and giant elliptical galaxies.” That phrase (repeated by many other news sources that used the press release) is not accurate in light of the fact that in our universe a large fraction of the galaxies are spiral galaxies, but in the Illustris simulation only a tiny fraction are spiral galaxies (as little as 1% or less). 

It seems that our scientists do not actually have a credible explanation for the high occurrence of spiral galaxies in the universe. The recent discovery of spiral galaxies ten times bigger than any previously observed underscores this explanatory shortfall.