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Rob Sheldon on a big current question: Is dark matter real?

Hubble image captures what dark matter is supposed to be.

As asked by Don Lincoln, Senior Scientist, Fermi National Accelerator Laboratory at Space.com, acknowledging the difficulties:

However, in a paper released in June, scientists have given dark matter models a significant boost. Not only does the new work reproduce the successes of earlier predictions of the dark matter model, it also reproduces the Tully-Fisher relation.

The new paper is a “semi-analytic” model, which means that it is a combination of analytic equations and simulation. It simulates the clumping of dark matter in the early universe that may have seeded galaxy formation but also includes the interaction of ordinary matter, including such things as the infall of ordinary matter into another celestial body due to its gravitational pull, star formation and the heating of infalling gas by starlight and supernovas. By carefully tuning the parameters, the researchers were better able to match the predicted Tully-Fisher relationship. The calculation’s key is that the predicted rotational velocity includes a realistic value for the ratio of baryons to dark matter in the galaxy.

The new calculation is an important additional step in validating the dark matter model. However, it is not the final word. Any successful theory should agree with all measurements. Failure to agree means that either the theory or the data is wrong, or at least incomplete. A few discrepancies between prediction and measurement still remain (such as the number of small satellite galaxies around big ones), but this new paper gives us confidence that future work will resolve these remaining discrepancies.

Dark matter remains a powerfully predictive theory for the structure of the universe. It is not complete and it needs validation by discovering the actual dark matter particle. So, there is still work still to do. But this most recent calculation is an important step toward the day where we will know once and for all if the universe really is dominated by the dark side. More.

Our physics color commentator Rob Sheldon replies,

There are two points of clarification that should be made that the particle physicist Lincoln perhaps finds hard to

“This odd matter is thought to be a new kind of subatomic particle that doesn’t interact via electromagnetism, nor the strong and weak nuclear forces. Dark matter is also supposed to be five times more prevalent in the universe than the ordinary matter of atoms. However, the reality is that dark matter’s existence has not yet been proved.”

In the 3 years, we are pretty certain that dark matter is NOT an exotic subatomic particle. Numerous attempts to pin down exotic particles have failed, the liquid Xenon LUX experiment, the gamma-ray annihilation galactic surveys, the LHC Tevatron searches, the axion telescopes, the emulsions from the NASA long-duration satellite, etc. There really is no viable theoretical model for dark matter right now. None.

As experimentalists, we must take negative results as seriously as
positive results, and the past 20 years have seen a large collection of negative results for the presence of exotic particles. This is a solid experimental result that must be acknowledged.

On the other hand, the gravitational effects of dark matter have been
observed better than ever before. Gravitational “Einstein lenses” can pin down the warping of space caused by invisible dark matter, so the dark matter in galaxy clusters has been imaged. Galaxy rotation curves can be computed for 1000’s of galaxies and now even dwarf galaxies to tell us exactly where the dark matter concentrates. We know where the dark matter is, where it accumulates, its effective viscosity and density. We are even tracking backward to its production in the Big Bang, and its dynamic evolution since then, though that becomes increasingly model-dependent.

So why does Don Lincoln say “its existence has not yet been proved”?

He may be thinking of MOND, “modified Newtonian dynamics”, which suggests that if the matter is a certain distance away and moving at a certain velocity, it tugs less than Newton suggested. This theory takes the galactic rotation curves (centripetal force) and rather than adding missing dark matter to get it to balance, subtracts Newtonian gravity from the equations.

The problem for MOND is that it doesn’t explain the Einstein lensing and images of dark matter, nor does it work for dwarf galaxies. And worst of all, it doesn’t provide a coherent explanation for why Newtonian gravity changes for specific distances and velocities.

It is a classic example of “ad hoc” data analysis, a Lakatos “degenerate science program”, or a Kuhn “paradigm shift” and will probably become a text book example of it in the near future.

But it tells you the desperation of the field that is willing to consider even such ugly solutions to a puzzling experimental result.

But if it is real, and it isn’t an exotic particle or a change in the laws of physics, what else can it be?

Ordinary matter: Protons, electrons, molecules, ice.

How come no one is considering that possibility? Isn’t it ruled out by

No. Just because something can’t be seen in a telescope doesn’t mean it isn’t there. Like black holes, for example, which is every astrophysicist’s favorite battery, cannot be seen in a telescope. Nor can they see comets outside the orbit of Mars. Nor most planets the size of Earth in the rest of the galaxy. Lot’s of things can’t be observed presently.

But what about the claim that dark matter must be “non-baryonic” or exotic?

That claim is based on a model called “Big Bang Nucleosynthesis” (BBN), which says that from a few milliseconds to 1000 seconds, the Big Bang made all the matter in the universe. And if we increase the amount to include dark matter, then the density goes up, and if the density goes up, the ratios of primordial elements Li/He/H gets all thrown out of whack in the model. Ergo, dark matter can’t be normal stuff.

But it’s just a model result. A model built in 1967 by Fred Hoyle who was trying to disprove the Big Bang. A model with many simplifying assumptions including homogeneity, isotropy, and a non-magnetic Big Bang. A model first written in FORTRAN, with most versions today still in FORTRAN. I like FORTRAN and wrote my thesis in it, but it does limit the kinds of modifications you can make to the model. There just really is no reason to dogmatize this simplistic result. Given the multiple $billions spent on dark matter searches (I’m pinning all the LHC upgrades on the belief in SUSY), surely a few million $ can be found to upgrade BBN models. At the very least we should add magnetic fields, and perhaps later we can afford the expense of 3-D.

But every one of the observational properties of dark matter discerned by astronomy can be explained by a huge population of galactic comets. It nicely explains all the MOND observations. It even explains the new Tully-Fischer results Lincoln cites (which are just an extension of the MOND data). For me, this is the most obvious place to look, and the answer Don Lincoln with his shrinking particle physics budget would rather not consider.

See also: Does dark matter really exist (at LiveScience)

As I understand it, the motion of the bodies in our solar system is explained, within observational error, entirely on the basis of the visible matter, which means that there is either no dark matter in our solar system or its distribution is identical to the distribution of ordinary matter here. An explanation as to a different distribution of dark matter vs. ordinary matter is in order.EvilSnack
July 18, 2017
05:19 PM

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