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Natural Science Forum / Physics / Research / August 2007Radial Velocity Profile in Globular Clusters
See this paper by Scarpa et. al. http://arxiv.org/abs/0707.2459 Quoting abstract: "We report on the results from an ongoing program aimed at testing Newton's law of gravity in the low acceleration regime using globular clusters. It is shown that all clusters studied so far do behave like galaxies, that is, their velocity dispersion profile flattens out at large radii where the acceleration of gravity goes below 1e-8 cm/s/s, instead of following the expected Keplerian fall off. In galaxies this behavior is ascribed to the existence of a dark matter halo. Globular clusters, however, do not contain dark matter, hence this result might indicate that our present understanding of gravity in the weak regime of accelerations is incomplete and somehow incorrect." Cooperstock and Tieu had previously understood the "anomalous" rotation curves of galaxies as a misapplication of GR. This work of theirs is on-going - here is another fertile field for modeling, using their ideas. -drl > See this paper by Scarpa et. al. > [quoted text clipped - 8 lines] > behavior is ascribed to the existence of a dark matter halo. Globular > clusters, however, do not contain dark matter**" How is it concluded that Globular clusters "do not contain dark matter" if they have the "dark matter" characteristic related to "velocity dispersion profile flattens out at large radii"? Richard Saam > > See this paper by Scarpa et. al. > [quoted text clipped - 15 lines] > > Richard Saam Tidal streams from old clusters. Even small amounts of DM would cause them to be gravitationally bound. Instead, they shed stars owing to tidal interactions with the Milky Way. http://www.sdss.org/news/releases/20020603.pal5.html Also remember we are talking about EDM halos, not unaccounted-for dwarfs etc. -drl Thus spake Richard Saam <rdsaam@att.net> >> See this paper by Scarpa et. al. >> http://arxiv.org/abs/0707.2459 [quoted text clipped - 10 lines] >if they have the "dark matter" characteristic related to >"velocity dispersion profile flattens out at large radii"? Dark matter is generally assumed diffuse in galactic haloes - the size of the galaxy being enough to provide the required mass. Globular clusters are orders of magnitude smaller than galaxies and would require large concentrations of dark matter to generate the velocity dispersion profile. I would think that this could be eliminated by looking at the gravitational effect of the cluster on surrounding galactic stars. Regards  SignatureCharles Francis moderator sci.physics.foundations. substitute charles for NotI to email > Thus spake Richard Saam <rdsaam@att.net> >>> See this paper by Scarpa et. al. [quoted text clipped - 17 lines] > profile. I would think that this could be eliminated by looking at the > gravitational effect of the cluster on surrounding galactic stars. So what's the state of play? It seems to me that everything is being thrown into question in cosmology. We need increasingly bizarre DM, plus supposedly unrelated Dark Energy and then we have *partial* explanations of data using MOND etc. So my question is this - is there likely to be a single explanation for all of this or have the experimentalists discovered evidence for a slew of fairly unconnected phenomena? SignatureDirk http://www.onetribe.me.uk - The UK's only occult talk show Presented by Dirk Bruere and Marc Power In article <5jgppaF3u4ib2U1@mid.individual.net>, Dirk Bruere at NeoPax <dirk.bruere@gmail.com> writes: > So what's the state of play? It seems to me that everything is being > thrown into question in cosmology. We need increasingly bizarre DM, plus > supposedly unrelated Dark Energy and then we have *partial* explanations > of data using MOND etc. So my question is this - is there likely to be > a single explanation for all of this or have the experimentalists > discovered evidence for a slew of fairly unconnected phenomena? Does it matter? The universe is what it is; it is the job of science to figure it out. Some of the ancient Greeks wanted to explain everything with 4 or 5 or even just 1 element. It turns out that there are roughly a hundred elements. Aesthetic perceptions based on the ancient Greek ideal have to go, but that's a small price to pay for understanding chemistry. Science proceeds by coming up with the simplest explanation which fits the data---not because there is any guarantee that it is correct, but just because it is a good working hypothesis. The process is known as Occam's razor. If more data come a long which require one to make the working hypothesis more complicated, then there is no need for a crisis of faith---that's just the way science works. Of course, if the explanation becomes so contrived that it is difficult to believe it, then it might be worth looking for a radically different explanation via some paradigm shift, but the burden of proof is on those calling for the shift (i.e. they have to demonstrate that the new hypothesis explains all the data but is simpler). Also a new hypothesis should make a testable prediction by which it could in principle be falsified. Cosmology as a data-driven science is relatively new. There seems to be a bias against "bizarre" dark matter. However, why should most matter glow for our convenience, or even be composed of baryons? Isn't this just chauvinism? Most people probably expect that, by weight, if not humans then at least large multicellular creatures (including plants) make up the bulk of the mass of life on Earth, but actually bacteria have that honour. So, in a sense, bacteria are the dark matter of biology (and for a long time, they were invisible, but due to their small size). |
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