- #1

ohwilleke

Gold Member

- 2,385

- 1,374

Date: Sun, 3 Apr 2005 10:51:05 GMT (4kb)

Testing MOND with Ultra-Compact Dwarf Galaxies

Authors: Riccardo Scarpa

Comments: Two pages. Submitted for publication to MNRAS

The properties of the recently discovered Ultra-Compact Dwarf Galaxies (UCDs) show that their internal acceleration of gravity is everywhere above a0, the MOdified Newtonian Dynamics (MOND) constant of gravity. MOND therefore makes the strong prediction that no mass discrepancy should be observed for this class of objects. This is confirmed by the few UCDs for which virial masses were derived. We argue that UCD galaxies represent a suitable test-bench for the theory, in the sense that even a single UCD galaxy showing evidence for dark matter would seriously question the validity of MOND.

But, in the other direction we have:

http://uk.arxiv.org/abs/astro-ph/0503104

Date: Fri, 4 Mar 2005 18:18:46 GMT (161kb)

Date (revised v2): Fri, 4 Mar 2005 21:24:13 GMT (162kb)

Does VIRGOHI21 pose a problem for MOND?

Authors: Scott Funkhouser

Comments: 1 page. 2nd version has minor formatting and wording improvements

If the inferred parameters of the recently discovered dark galaxy VIRGOHI21 are verified then the dynamics of the body may represent a counter-example to the accelerations predicted by MOND.

This involves the so called "dark galaxy" and contains the caveat that the measurements are not yet considered definitive.

And, for all around MOND/TeVeS junkies, we have one of the first really serious critiques of Bekenstein's proposed relativistic modified Newtonian dynamics theory called TeVeS:

http://uk.arxiv.org/abs/gr-qc/0502122

Date (revised v2): Wed, 27 Apr 2005 13:58:24 GMT (15kb)

Spherically symmetric, static spacetimes in a tensor-vector-scalar theory

Authors: Dimitrios Giannios

Comments: 12 pages, accepted for publication in Phys. Rev. D

Recently, a relativistic gravitation theory has been proposed [J. D. Bekenstein, Phys. Rev. D {\bf 70}, 083509 (2004)] that gives the Modified Newtonian Dynamics (or MOND) in the weak acceleration regime. The theory is based on three dynamic gravitational fields and succeeds in explaining a large part of extragalactic and gravitational lensing phenomenology without invoking dark matter. In this work we consider the strong gravity regime of TeVeS. We study spherically symmetric, static and vacuum spacetimes relevant for a non-rotating black hole or the exterior of a star. Two branches of solutions are identified: in the first the vector field is aligned with the time direction while in the second the vector field has a non-vanishing radial component. We show that in the first branch of solutions the \beta and \gamma PPN coefficients in TeVeS are identical to these of general relativity (GR) while in the second the \beta PPN coefficient differs from unity violating observational determinations of it (for the choice of the free function $F$ of the theory made in Bekenstein's paper). For the first branch of solutions, we derive analytic expressions for the physical metric and discuss their implications. Applying these solutions to the case of black holes, it is shown that they violate causality (since they allow for superluminal propagation of metric, vector and scalar waves) in the vicinity of the event horizon and/or that they are characterized by negative energy density carried by the fields.

I'd also welcome comments on this article:

http://uk.arxiv.org/abs/astro-ph/0412415

Date: Thu, 16 Dec 2004 12:51:30 GMT (18kb)

Via Aristotle, Leibniz & Mach to a relativistic relational gravity

Authors: D F Roscoe

Comments: 18 pages, no figures, under review CQG

In previous work we have shown how a worldview that has its origins in the ideas of Aristotle, Leibniz and Mach leads to a quasi-classical (that is, one-clock) metric theory of gravitation (astro-ph/0107397) which, for example, when applied to model low surface brightness spirals (astro-ph/0306228), produces results that have, hitherto, only been matched by Milgrom's MOND algorithm. In this paper we show how the natural generalization of this worldview into a properly relativistic two-clock theory, applied to model a spherically symmetric gravitational source, produces results that cannot be distinguished from the canonical picture for all the standard local tests and which, when interpreted as a radiation model, produces no dipole radiation. Furthermore, although black-holes within this picture have an event horizon at the usual Schwarzschild radius, they do not have an essential singularity at the origin - the solutions are perfectly regular there.

which has something of a cranky alert feeling to it. This probably has something to do with the fact that Roscoe is a co-author of a paper with Arp:

http://uk.arxiv.org/abs/astro-ph/0501090

Date: Thu, 6 Jan 2005 10:24:00 GMT (812kb)

Periodicities of Quasar Redshifts in Large Area Surveys

Authors: H. Arp, C. Fulton, D. Roscoe

Comments: 23 pages, 14 figures

We test the periodicity of quasar redshifts in the 2dF and SDSS surveys. In the overall surveys redshift peaks are already apparent in the brighter quasars. But by analyzing sample areas in detail it is shown that the redshifts fit very closely the long standing Karlssson formula and strongly suggest the existence of preferred values in the distribution of quasar redshifts.

We introduce a powerful new test for groups of quasars of differing redshifts which not only demonstrates the periodicity of the redshifts, but also their physical association with a parent galaxy. Further such analyses of the large area surveys should produce more information on the properties of the periodicity.