HST and 'dark matter' |
HST and 'dark matter' |
Guest_PhilCo126_* |
May 11 2007, 05:13 PM
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Guests |
ASA Updates Plans for Hubble 'Ring Of Dark Matter' Briefing
GREENBELT, Md. - NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal. |
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Mar 11 2010, 03:36 AM
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#2
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Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
Sorry to resurrect this long-dead thread, but a paper just appeared on ARXIV that bears on the possible physics behind MOND.
As background, in recent months a new theory of gravity has received a lot of attention. A Dutch physicist named Erik Verlinde appears to have made the crucial connections that allow for a physical explanation of gravity (both Newtonian and relativistic) as well as related concepts like Newtonian mechanics, inertia and indeed space and time itself. This is a bold claim, but top physicists like Lee Smolin, George Smoot and Gerard 't Hooft have jumped on board in recent weeks. This is no crackpot idea. On the Origin of Gravity and the Laws of Newton -- Erik Verlinde QUOTE Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic. What Verlinde found is that gravity can be explained as an entropic force -- a pseudo-force that results from the universal tendency to maximize entropy (the Second Law of Thermodynamics). Other examples of entropic forces include osmosis, and the 'force' causing large polypeptide molecules suspended in a fluid to curl up into a roughly spherical shape, instead of stretching out in a linear fashion. (This is NOT due to intra-molecular bonding forces, but instead is due to the fact that there are many more internal degrees of freedom in a spherical configuration than in a linear one, and hence a 'random walk' between states, driven by random collisions with the fluid molecules, will almost inevitably result in a spherical configuration.) In Verlinde's theory, the most fundamental quality is information, which is stored in what are called 'holographic screens' that separate and define every point in space-time. These holographic screens are not new, they have become an indispensable concept in black hole physics over the past decade or so. They have the effect of 'smearing' information over a large area, rather than having it concentrated at a single point -- resulting in quantum mechanics. The amount of information associated with matter and its location is measured in terms of entropy. Changes in entropy associated with changes in location result in an entropic force, which when the math is done exactly matches gravity, both Newtonian and relativistic. The theory also automatically generates an additional force that exactly matches the effects of so-called 'dark energy' which had been recently postulated in order to explain the observed changes in the Hubble constant at extreme distances. In this theory it is a consequence of information being stored at the 'universal horizon', covering every point in the universe. Additionally, it provides an easy and natural explanation of inertia, which had always been a big puzzle. Over the past two months a great many additional people have written papers, filling in and extending the theory. So far, it has been found to accurately describe both Newtonian and relativistic gravity, Newtonian mechanics including inertia, the early period of cosmological inflation, the current cosmological acceleration commonly ascribed to 'dark energy', and much of electrodynamics (!) including the Coulomb law, the Poisson equation and indeed all the Maxwell equations -- raising the possibility that electromagnetism is also a non-fundamental entropic force, with the photon perhaps being akin to the phonon (soliton) as a derived 'particle'. The newest such result concerns MOND: Entropic corrections to Newton's law -- Leonardo Modesto, Andrew Randono QUOTE It has been known for some time that there is a deep connection between thermodynamics and gravity, with perhaps the most dramatic implication that the Einstein equations can be viewed as a thermodynamic equation of state. Recently Verlinde has proposed a model for gravity with a simple statistical mechanical interpretation that is applicable in the non-relativistic regime. After critically analyzing the construction, we present a strong consistency check of the model. Specifically, we consider two well-motivated corrections to the area-entropy relation, the log correction and the volume correction, and follow Verlinde's construction to derive corrections to Newton's law of gravitation. We show that the deviations from Newton's law stemming from the log correction have the same form as the lowest order quantum effects of perturbative quantum gravity, and the deviations stemming from the volume correction have the same form as some modified Newtonian gravity models designed to explain the anomalous galactic rotation curves. [...] Let us now focus on the corrections to Newton's laws stemming from the volume correction to the area-entropy relation. We notice here that the form of the potential is of the same functional form as the correction to the Newtonian potential posited by Modified Newtonian Dynamics (MOND) in order to model the anomalously flat galactic rotation curves. Of course this does not prove that some form of MOND is correct, but it does raise the likelyhood a good deal, in my opinion. p.s. -- if anybody wants to see links to other papers regarding aspects of this theory of gravity, please let me know on this thread and I will post links to the more important papers. |
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