What's the deal with dark matter

[Bill McKeeman]

My question is whether the (too fast) speed of peripheral galactic stars is observed to vary by galactic longitude as well as distance from the galactic center? Is there any data on this?

 

[ohwilleke]

What is "galactic longitude"?

 

[Bill McKeeman]

Galactic longitude, 360 degree measure of position about center.
http://www.thinkastronomy.com/M13/Manual/common/galactic_coords.html

 

[Gary A Gordon]

The existence of dark matter is not surprising if viewed as another indication that the universe that we find ourselves in is just one of an arbitrarily large number of universes formed from some pre-existing more basic "material". The properties of the parts of our universe that we interact with non-gravitationally are selected "anthropically". That is, they are consistent with the emergence of the objects and interactions that could produce the observed past, present, and possible futures of the universe that we find ourselves in. We do not propose that our own emergence was in some way the "goal" of this universe variant. Only that the negation of this supposition is not consistent with the facts of our current existence. One can suppose that the network of available basic "material" that is connected to the non-gravitational interactions that have and will occur in our universe during its existence only includes a small part of that pre-existing basic "material", with the unconnected part forming the "dark matter" that only interacts gravitationally with the rest. I have seen some articles here that disparage such anthropic explanations for things. That forces one to explain in other ways why the various values for material and interaction parameters have the specific values found. Other than future job security, it is hard to understand the justification for this bias.

 

[ohwilleke]

My question is whether the (too fast) speed of peripheral galactic stars is observed to vary by galactic longitude as well as distance from the galactic center? Is there any data on this?

There is quite a bit of data and I am pretty certain that the answer is no.

 

[SKHanson57]

Frame dragging is just too tiny an effect compared to what would be needed.

Possibly yet the frame drag of a rapidly spinning black hole could affect the orbit of a galaxy if the black hole is super massive in combination with an extreme magnetic field is present as well. ??

 

[SKHanson57]

Doing so for the black hole of a similar mass to the one at the center our galaxy and figuring the framing dragging effect at, say, 50,000 ly from the center, it works out to being the equivalent of an additional 7.4e-54 km/sec. Besides, frame dragging falls off with distance from the mass, so any effect it would have would be stronger near the BH than it is further, But stellar speeds nearer the center of the galaxies aren't the problem, it's the ones on the outskirts.

If no consideration to a magnetic field that may be interacting with said frame drag. I admit it can be calculated based on current observational assumptions to be small and non pervasive however I’d like to add that we have yet to observe a black hole and to this point in time have only observered what could be explained as the result of a black hole. The inferences do get better but still no observations.

 

[ohwilleke]

If no consideration to a magnetic field that may be interacting with said frame drag. I admit it can be calculated based on current observational assumptions to be small and non pervasive however I’d like to add that we have yet to observe a black hole and to this point in time have only observered what could be explained as the result of a black hole. The inferences do get better but still no observations.

The latest measurements of magnetic fields around black holes show them to be surprisingly small. And, a supermassive black hole is a tiny fraction of the total mass of a galaxy, although it is fascinating how strictly corollated central supermassive black hole size and total galaxy size are empirically.

 

[Drakkith]

If no consideration to a magnetic field that may be interacting with said frame drag. I admit it can be calculated based on current observational assumptions to be small and non pervasive however I’d like to add that we have yet to observe a black hole and to this point in time have only observered what could be explained as the result of a black hole. The inferences do get better but still no observations.

What do you consider to be an "observation" of a black hole?

 

[Chronos]

What we know is the universe is awash with regions of apparently empty space that behave as if occupied by large quantities of gravitating mass or pockets of the stress energy tensor. These regions tend to occur in the vicinity of large amounts of otherwise detectable mass, like galaxies and galactic clusters. Attempts to identify or detect constituent amounts of anything that may account for this phenomenon have been inconclusive. That pretty much sums what we can say with confidence about the current state of dark matter.

 

[ohwilleke]

What we know is the universe is awash with regions of apparently empty space that behave as if occupied by large quantities of gravitating mass or pockets of the stress energy tensor. These regions tend to occur in the vicinity of large amounts of otherwise detectable mass, like galaxies and galactic clusters. Attempts to identify or detect constituent amounts of anything that may account for this phenomenon have been inconclusive. That pretty much sums what we can say with confidence about the current state of dark matter.

I would say we know quite a bit more than that. We have a lot of data about the inferred distribution and dynamics of dark matter if that is what it is, in relation to visible matter, which make it possible to rule out myriad models for it. We don't know just what is causing this phenomena, but we know pretty definitively that lots of possibilities that have been considered are not the answer.

One of the really encouraging things about the search for the explanation of dark matter phenomena, unlike so many other unsolved problems in physics, is that we have rich data with regard to this one and both that data and our ability to analyze it are growing rapidly.

This is very different from, for example, the search for experimental evidence of supersymmetry or other high energy "new physics" where we have nothing but a bunch of null results, anomalies that haven't panned out, and a huge area of the parameter space at high energies that it will be impossible as a practical matter to explore experimentally for decades or even ever.

For example, observations by RAVE of Milky Way stars outside the plane in which most of the Milky Way's stars are found, can directly confirm or falsify lots of hypothetical solutions that otherwise reproduce the overall rotational dynamics of the Milky Way.

Similarly, observations of colliding galaxies like the Bullet Cluster, similarly impose really meaningful constraints on models that can be cross checked against other similar colliding galaxy systems.

The very systemic differences in this phenomena among galaxies that are of a particular type, and between different kinds of galaxies, and in galaxy clusters, all of which we have large data sets of, provide both overwhelming evidence that some kind of new physics or other is going on and constrains what the new physics can be.

By dint of simple hard work and cumulative data collection by many independent investigators, this is one unsolved problem upon which we are making real progress on a regular basis, even if it doesn't always seem like that.