Galaxy motions -> hidden superstructure (DM!)

Garth

Understood Ian, but the standard model says DM cannot be the same as ordinary matter, i.e. baryonic, as there is too much of it. Standard GR BB nucleo-synthesis allows only a maximum of 4% critical density as ordinary matter (and that is pushing it) whereas galactic dynamics and lensing observations require about 20%, or more. So what is it?

However if the GR R(t) ~ t^(1/2) then t^(2/3) in the radiation and matter dominated epochs respectively is replaced by R(t) ~ t all the way through in the freely coasting model then nucleo-synthesis allows 20% baryonic density + neutrino etc contributions, so the problem is solved. And you will be right; DM is ordinary matter!

- Garth

Mike2

I've not seen the data. Are the galactic rotation curves and the lensing effects just as strong for Andromeda as for very distant galaxies? Thanks.

Nereid

Good to hear.Indeed! It should be an exciting time, should it not?I'd be interested to hear whether you think the proposals in Cosmic Visions (or the NASA equivalent) will cover this.AFAIK, there are no deep space experiments planned, by anyone, for any purpose (unless you count the Pluto-Kuiper Express, which is all but dead, no?)To what extent would LISA be able to serve as an alternative? What about the resonant mass gravity wave detectors?That's what I'm trying to understand! If none of the missions preclude tests of any alternatives, and if there are but a small number of dedicated missions that the iconclasts would dream of, where's the beef (other than pocket money for tea and biccies)?

Chronos

Speaking of large scale structure and the standard model, this is interesting
http://www.arxiv.org/abs/astro-ph/0404111

Garth

On possible non-GR outcome of GPB experiment:
If such an outcome should occur do you think they will devise another epicycle?
AFAIK it does not - however not having broadband I haven't been able to download all those files on your link - yet!
It has been suggested that a Pioneer Anomaly test be put onboard a low-mass, low-thrust mission to Pluto, the Pluto orbiter Probe or POP. A study of such a mission has been undertaken recently in ESA’s Advanced Concepts Team and detailed results of the system design and trajectory design have been presented in T. Bondo et al, “Preliminary Design of an Advanced Mission to Pluto”. Proceeding of the 24th ISTS, Miyazaki, Japan, June 2004. http://www.esa.int/gsp/ACT/doc/ACT-R...0-ISTS2004.pdf . I have no idea as to whether or when it would get off the ground.
The experiment compares the gravitational attraction of a horizontal light beam against that of a solid apparatus. I do not think LISA will comply, the gravitational bending of light being equal in GR and SCC.
[Such deviation being of two components, one due to the 'free fall' of the photon the other due to the curvature of space - in GR these two contribute 1/2 + 1/2 respectively to the total deviation; in SCC they contribute 3/4 + 1/4 respectively of the same total deviation 1.75".]
- Garth

Nereid

It's pure physics, so lots of $$ will be found to do a bigger and better GPB; ten thousand new 'GR-killer' theories will flow from the word processors of theorists; ten creative ideas to test the new theories will be published, involving little more than what can be found in a leading university lab; three hundred extensive data-mining projects will be commenced, using BOINC (the new grid computing platform on which Seti@Home now runs); ... and key insight for 'the answer' will come from the mind of a bright young PhD student in China (or India).

malthis

Good discussion here.
I used to deride Dark matter as a makeshift idea to plug the emerging holes in GR, or newtonian at least. I guess what turned me around is string theory. For once Ican see how DM could exist.

I don't think dark matter is one long brane covering the universe, though that is possible. But perhaps it has to do with the frequency of the wave nature of particles. For example, photons are considered a string just as baryons are, only at a higher vibration. Light happens to be at the upper vibratory limit of what our expanded dimensions can swallow. But does that mean there's no strings that vibrate faster than light? If there were, wouldn't we not see them at all, nor be able to detect them? Would that be, 'dark matter'?

It's something I've thought about and perhaps Tachyons(sp?) are an offshoot of that dea, I don't know much about them though. Anyone like to comment?

Garth

Lensing effects do not really apply to Andromeda, although searches are made for 'local' (our galactic halo) micro-lensing events that might be caused by mini black holes or Jupiters. [See today's 'Some Consequences of the Baryonic Dark Matter Population' Rudolph E. Schild http://arXiv:astro-ph/0409549 for distant such objects]

However galactic rotation curves certainly do apply not only to Andromeda but also for the Milky Way itself.

Each galaxy seems to have a massive invisible halo and each galaxy cluster seems to have a massive intergalactic medium, also invisible. Visible matter ~ 4%, Dark Matter ~ 23% of total critical density.

Garth