Cosmologic philosophy: dark matter is or isn't? IS? (Introduction)

by David Turell , Monday, July 17, 2017, 01:30 (2687 days ago) @ David Turell

The latest set of calculations is very supportive of the theory:

https://www.livescience.com/59814-is-dark-matter-real.html?utm_source=notification

"... dark matter theories make predictions for how fast galaxies are rotating. But, until now, measurements made of the detailed dark matter distribution at the center of low mass galaxies didn't line up with those predictions.

"A recent calculation has changed that. The calculation helps resolve the conundrum of the Tully-Fisher relation, which compares the visible, or ordinary, matter of a galaxy to its rotational velocity. In very simplified terms, scientists have found that the more massive (and therefore brighter) a spiral galaxy is, the faster it spins.

"But if dark matter exists, how "big" a galaxy is should be determined not just by its visible matter, but also by its dark matter.  With a huge piece of the equation — the amount of dark matter — missing, the Tully-Fisher relation shouldn't hold. And yet it does. It was hard to imagine any way to reconcile this relationship with existing dark matter theory. Until now.

***

"there is the theory of dark matter: That a type of matter that doesn't interact with light at all, yet exerts a gravitational pull, permeates the universe.

"Were the galactic rotation measurements the only data we have, it might be hard to select between these different theories. After all, it might be possible to tweak each theory to solve the galactic rotation problem. But there are now many observations of many different phenomena that can help identify the most plausible theory.

"One is the speed of galaxies within large clusters of galaxies. The galaxies are moving too quickly for the clusters to stay bound together. Another observation is of light from very distant galaxies. Observations of these very distant ancient galaxies show that their light is distorted by passing through the gravitational fields of more nearby clusters of galaxies. There are also studies of small non-uniformities of the cosmic microwave background that is the birth-cry of the universe. All of these measurements (and many more) must also be addressed by any new theory to explain galactic rotation speeds.

***

"The Tully-Fisher relation is a tough challenge for dark matter models. The rotation of a galaxy is governed by the total amount of matter it contains. If dark matter truly exists, then the total amount of matter is the sum of both ordinary and dark matter.

"But existing dark matter theory predicts that any random galaxy may contain larger or smaller fractions of dark matter. So, when one measures the visible mass, you could potentially be missing a huge chunk of the total mass. As a result, visible mass should be a very poor predictor of the total mass (and thereby rotational speed) of the galaxy. The galaxy's mass could be similar to that of the visible (ordinary) mass or it could be much larger.

"Thus, there is no reason to expect that the visible mass should be a good predictor of the rotational speed of the galaxy. Yet it is. 

***

"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 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."

Comment: More evidence for dark matter. It is truly amazing how much we can learn about the workings of the universe.