Dusty_Matter:

It is the estimates of the amounts of molecular H2 that is speculative.

Just a couple of questions here, doesn't the math associated with the Recombination era of the Big Bang predict the hydrogen/helium proportions across the universe?  And doesn't current observation agree with that prediction?  If so, wouldn't this enormous abundace of molecular hydrogen skew that proportion?

Yes...

I think that there are several strands here for me to reply to.

 Firstly, I think that the people who have reported the galactic molecular hydrogen results would not readily agree with your comments about the accuracy/reliability of their estimates that are based on real, quantitative measurements. This work has involved a good many contributors, using very much up to date space telescope spectroscopic data. Results of this kind keep on appearing. I have just come across a 2007 paper in the much-respected journal 'Science' by F. Bournaud et al, 25 May 2007, pp1166-1169, entitled 'Missing mass in collisional debris from galaxies.' These workers were surprised to find 2-3 times the amount of 'dark matter' compared to visible matter in dwarf galaxies arising from the collision of larger galaxies. And, guess what - these people don't believe that the dark matter here is the mysterious 'non-baryonic' matter, but rather cold molecular hydrogen (H₂). Perhaps the famous Bullet Cluster work would repay a study of this kind as well -  clearly there is a lot of 'missing mass' here that reveals itself in gravitational lensing from which the familiar red/blue picture has been deduced.Secondly, why shouldn't our galaxy, the Milky Way, be very much the same as other spiral galaxies in which much of the missing mass has been accounted for in terms of molecular hydrogen, as I indicated in my last post.Thirdly, I have re-read the article on Dark Matter in the November issue of Astronomy magazine, and it seems to me that it is largely about the techniques that scientists are currently using to search for non-baryonic dark matter. There isn't much evidence from anywhere, as yet, of what non-baryonic dark matter really is. Most claims for it also end with the caveat that 'we think that this is the real stuff, but it might not be'.So, I will keep an open mind on this unfolding story and see what hard evidence is reported. Science is about testing hypotheses, looking at the evidence and amending a hypothesis in the light of the evidence etc. Now is not the time to be too dogmatic about anything in cosmology - theories can change!!

 

At one time we didn't know what stars were made of, but we knew they existed. You are quite correct, we don't know what dark matter is made of, but we know that it exists. It will probably be some time before we really figure it out, and you could probably look at these initial experiments of their trying to find out what it is, as being in it's infancy stage. We know it's not molecular H2, however. If the gravitational lensing, as shown in the pictures from these galactic collisions, was from this hydrogen, then we would have been able to identify it by it's spectral readings. We don't know what dark matter is, but we know it's not molecular H2, as it would have to make up over 80% of the mass of all the galaxies, including ours, and it doesn't.

I have always had a problem with the Dark Matter theory. In order to explain the unexpected higher velocity of the outer stars in a galaxy, it is theorized that there must be some unknown matter at the edge of those galaxies that causes the outer stars to orbit the center of their galaxy at a speed higher than what would be expected based on gravitational law. That doesn't pass the common sense test. Why would the dark matter always congregate at the outer edge of the galaxies? And why would it congregate in just the right amounts from galaxy to galaxy in order to maintain those higher outer star velocites? Well, the answer to the last question is it doesn't. I think dark matter may be just a property of space.

Recent work by Univ of Maryland astronomer Stacy McGaugh indicates the ratio of normal matter to dark matter decreases as the size of a galaxy decreases and the relationship is systematic with scale. So large galaxies appear to have less dark matter for their size than the small galaxies. How does McGaugh come up with the information to measure dark matter to normal matter ratios between the galaxies? He compares the expected outer star velocities to that based solely on graviational law and the discrepancy is then assumed to be dark matter and from there the amount of dark matter is calculated so as to explain the outer star velocities. McGaugh's results showed a linear variation from 17% normal matter for large galaxies to only .2% normal matter for small galaxies.  If one were to believe that dark matter is related to normal matter and is part of the development of a galaxy then those numbers should be fairly constant. But if "dark matter" is nothing more than a property of the continuum of space or a change in the property of gravity with distance then these numbers make more sense. It's almost like an effect that is relational to the amount of space occupied. Almost as if the universe has a built in gravitational constant or acceleration constant across all space. (Maybe those 'zero-point energy' guys are headed in the right direction.)

Why would dark matter form a spherical halo around a galaxy instead of falling into a flattened mass within the galaxy disk?

Is there a different spin vs distance curve for galaxies with low star formation rates vs galaxies with higher star formation rates?  What about elipiticals vs sprials?

I recently read of the discovery of a galaxy with a spin curve that closely matches what one would expect if there were no dark matter.  How broad is the distribution of spin curves?

Finally, I wonder what happens to dark matter when galaxies collide.  I would expect that a lot of information about dark matter could be teased out of collisions between galaxies that are or have been interacting with each other. 

rpowell2u and BULLFOX,

I like both of these posts... to remind me again that "dark matter" is really a catchall phrase for "the solution to the stellar orbital velocity problem".  There are physicists exploring the GR modifier and other physicists "mapping" dark matter locations and yes in both isolated and interacting galaxies.

 The ratio changes for N/D corresponding to galaxy size are a cool piece of information and when someone figures this out I suspect that one of the main confirming pieces will be a prediction that lines up  those ratio correlations.

Thanks

ph