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HST and 'dark matter'
Guest_PhilCo126_*
post May 11 2007, 05:13 PM
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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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Littlebit
post May 11 2007, 05:35 PM
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A dark matter ring? How/why would DM be corraled into a donut? Doesn't the shape imply a distribution of matter with electromagnetic properties?
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Mongo
post May 11 2007, 11:04 PM
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Colour me sceptical on this one -- which should be no surprise, given my stance on 'dark matter'. I do not know much about this latest claim, but I do know that there have been repeated claims of some observation 'proving' the existence of dark matter, which is invariably shown, a little later, to be equally explainable (and in fact, usually more easily explainable) without dark matter (usually under some form of MOND theory). A case in point is the most recent example before this one, the 'bullet cluster', which as you will recall was loudly trumpeted last November as 'proving' the existence of Dark Matter. Here are some results from later, more thorough analysis:

The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter

A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006).

Timing and Lensing of the Colliding Bullet Clusters: barely enough time and gravity to accelerate the bullet

We present semi-analytical constraint on the amount of dark matter in the merging bullet galaxy cluster using the classical Local Group timing arguments. We consider particle orbits in potential models which fit the lensing data. {\it Marginally consistent} CDM models in Newtonian gravity are found with a total mass M_{CDM} = 1 x 10^{15}Msun of Cold DM: the bullet subhalo can move with V_{DM}=3000km/s, and the "bullet" X-ray gas can move with V_{gas}=4200km/s. These are nearly the {\it maximum speeds} that are accelerable by the gravity of two truncated CDM halos in a Hubble time even without the ram pressure. Consistency breaks down if one adopts higher end of the error bars for the bullet gas speed (5000-5400km/s), and the bullet gas would not be bound by the sub-cluster halo for the Hubble time. Models with V_{DM}~ 4500km/s ~ V_{gas} would invoke unrealistic large amount M_{CDM}=7x 10^{15}Msun of CDM for a cluster containing only ~ 10^{14}Msun of gas. Our results are generalisable beyond General Relativity, e.g., a speed of $4500\kms$ is easily obtained in the relativistic MONDian lensing model of Angus et al. (2007). However, MONDian model with little hot dark matter $M_{HDM} \le 0.6\times 10^{15}\msun$ and CDM model with a small halo mass $\le 1\times 10^{15}\msun$ are barely consistent with lensing and velocity data.

The collision velocity of the bullet cluster in conventional and modified dynamics

We consider the orbit of the bullet cluster 1E 0657-56 in both CDM and MOND using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (~ 4700km/s) occur naturally in MOND. CDM can generate collision velocities of at most ~ 3800km/s, and is only consistent with the data provided that the shock velocity has been substantially enhanced by hydrodynamical effects.

In other words, later analysis shows that MOND theories appear to fit the observations better than Dark Matter theories. This has happened again and again. How many times must this cycle repeat before Dark Matter is discarded as an implausible hypothesis?

Bill
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Tman
post May 17 2007, 07:53 AM
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From the Hubble site http://hubblesite.org/newscenter/archive/r...007/17/image/a/
The blue structure shows the possible 'dark matter' and is superimposed on a Hubble image of the cluster. A bit too detailed in this image I guess smile.gif

The PDF data release:
http://www.spacetelescope.org/news/science...70246.proof.pdf

If you like have a look at the big Hubble image of the cluster too... Wow, what a huge linse there! blink.gif

Edited: There is yet another interesting part of the release here. An animation (see Video 2) that zooms in from the naked eye view of the night sky to the small section image of the cluster. At the beginning, on the right is the asterism “Great Square of Pegasus”, and top middle, just out of view, the Andromeda galaxy. To the left is the constellation of Pisces, and leftmost the redish Mira.


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Mongo
post Jun 12 2007, 01:41 AM
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The wedding ring of MOND and non-exotic dark matter

The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND.

No surprise here. As always, the overblown claims that the 'Dark Matter' theory has been proven correct by the latest observation are shown to be mistaken. I was expecting a paper like this to appear (as you can probably tell from my earlier posts on this, and other threads). Not that this refutation will be anywhere near as well-publicized as the original claim.

Bill
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nprev
post Jun 12 2007, 03:46 AM
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I'm with you, man. Dark matter has always been a very hard frog for me to swallow; seems much, much more likely that we just don't fully understand the properties of "light" matter, esp. neutrinos and other exotic little beasts...


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dvandorn
post Jun 12 2007, 04:22 AM
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QUOTE (nprev @ Jun 11 2007, 10:46 PM) *
...Dark matter has always been a very hard frog for me to swallow...

That's because your cosmology is faulty. It has nothing to do with frogs -- it's TURTLES. All the way down.

smile.gif

-the other Doug


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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Mongo
post Jul 18 2007, 01:11 PM
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Using globular clusters to test gravity in the weak acceleration regime
Riccardo Scarpa, Gianni Marconi, Roberto Gilmozzi, Giovanni Carraro

http://xxx.lanl.gov/abs/0707.2459

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.
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nprev
post Jul 18 2007, 02:40 PM
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MOND + neutrino mass is looking more and more plausible all the time.


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Mongo
post Mar 11 2010, 03:36 AM
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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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Explorer1
post Mar 11 2010, 04:46 AM
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So in layman's terms, this is a potential 'theory of everything' if it's proven? Fascinating stuff.
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Greg Hullender
post Mar 11 2010, 05:00 AM
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QUOTE (Explorer1 @ Mar 10 2010, 08:46 PM) *
So in layman's terms, this is a potential 'theory of everything' if it's proven? Fascinating stuff.

No, it's just a new way to look at gravity. But it's a very, very exciting new way to look at gravity. I'm not a physicist myself, but my college friends who became physicists are pretty excited about it, and that's good enough for me.

--Greg
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Mongo
post Mar 11 2010, 05:42 AM
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Just to give a taste of how (some) physicists are viewing this idea, here are some extracts from some of the involved papers:

Part 1 of 2

Notes Concerning "On the Origin of Gravity and the Laws of Newton" by E. Verlinde -- Jarmo Mäkelä

QUOTE
If the idea of gravity as an emergent effect, rather than as a fundamental force turns out to be correct, we may currently be in a somewhat similar position as were the founders of quantum mechanics and atomic physics about 100 years ago. Instead of attempting to understand the microstructure of matter, however, we should, in this time, attempt to understand the microstructure of spacetime itself.


The Coulomb Force as an Entropic Force -- Tower Wang

QUOTE
If one accepts the holographic principle as well as our generalizations and reinterpretations, then the Coulomb's law, the Poisson's equation and the Maxwell's equations can be derived smoothly. Our attempt can be regarded as a new way to unify the electromagnetic force with gravity, from the entropic origin.


Gravity from the Entropy of Light -- Alessandro Pesci:

QUOTE
This suggests a deeper description of what we have discussed. Indeed, holography can be stated to mean that the allowed number of elementary bits of information in layers per unit bit of thickness, at given sum of the energy per bit and pressure, is limited (by an actual value). To the extent that concepts like bit of information, energy associated with a bit and pressure of a collection of bits can be regarded as primeval and, as such, meaningful even in absence of space, holography is pre-existing to space. In this perspective, when space is introduced as the information on ‘where’ information is, the energy in the bit should spread to keep unchanged the elementary amount of information for the bit, and this would be quantum mechanics. When expressed in terms of this space of information, holography would then become the metric theory which describes gravity.


Newtonian Gravity in Loop Quantum Gravity -- Lee Smolin

QUOTE
The idea that the unification of quantum theory with gravity is essentially thermodynamic has been on the table since the discovery of the laws of black hole thermodynamics and Bekenstein’s discovery of black hole entropy. The discoveries of the Unruh temperature and Hawking radiation strengthened the reason for hoping for a deep relationship between gravity, quantum physics and thermodynamics.

Very early in this history, Bekenstein hypothesized that the entropy of any isolated system is bounded by its area. In 1994 ’t Hooft extended this to the bold conjecture that the degrees of freedom needed to describe an isolated system in nature can be considered to live on a two-surface surrounding the region, with the number of degrees of freedom finite and proportional to the area in Planck units. He called this the holographic principle and since then we have come to call any application of the relationship between area and entropy as “holographic.” As developed by Susskind and then Maldacena and many others this led, in the context of string theory and supersymmetric quantum gauge theory, to the AdS/CFT correspondence.

The furthest realization of this idea to date, in the context of gravitational theory, is the discovery by Jacobson that the Einstein equations can be derived from the laws of thermodynamics, assuming only that Bekenstein’s proportionality between area and entropy is universal. This idea has been studied also by Padmanabhan and others, but there has remained the feeling that there was a further discovery, just over the horizon.

In a remarkable paper, Erik Verlinde has provided the next step, which is a non-relativistic analogue of Jacobson’s argument, in which he derives Newton’s law of gravity from thermodynamics plus the relationship between area and entropy. A different argument to the same conclusion has also been provided by Padmanabhan.
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Mongo
post Mar 11 2010, 05:46 AM
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Part 2 of 2

Modified gravity emerging from thermodynamics and holographic principle -- Yi Zhang, Yun-gui Gong, Zong-Hong Zhu

QUOTE
We discuss three different corrections to the area law of entropy. The number of bits N is then modified according to the form of entropy, and the law of gravity is obtained by using the method developed by Verlinde. We also discuss the effect of the entropy correction on the Friedmann equation, and we find that the Friedmann equation got an extra term H^n, which can be used to explain the current accelerating expansion of the universe. The result suggests that dark energy may be an emergent phenomenon based on the holographic principle and thermodynamics.


Towards a Holographic Description of Inflation and Generation of Fluctuations from Thermodynamics -- Yi Wang

QUOTE
Recently, Verlinde conjectured that gravity may be an entropic force, arising from thermodynamics on the holographic screen. We investigate the implications of the entropic force formalism for inflationary cosmology. We find the background dynamics of inflation can be dually described in the holographic language. At the perturbation level, two kinds of novel scale invariant scalar fluctuations arise from thermal fluctuations on the holographic screen. These fluctuations can be responsible for CMB anisotropy and structure formation.


Is Holographic Entropy and Gravity the result of Quantum Mechanics? -- Joakim Munkhammar

QUOTE
In a remarkable paper Verlinde recently proposed a framework for gravity as an entropic force. This theory, while related to Jacobsson's approach, showed that Newtonian gravity easily could be obtained by using entropic and holographic arguments. The assumption was that space is emergent and that the holographic principle holds. Bekenstein entropy was also a key component in his approach. He thus reversed the line of research, assuming that the holographic principle was underlying Newtonian physics. The change of entropy was linked to the change of the Newtonian potential, this led to the conclusion that inertia might be equivalent to the lack of entropy gradients. As Verlinde states, the holographic principle has not been easy to extract from the laws of Newton and Einstein because it is deeply hidden within them. His paper attracted quite some attention and several papers from various fields of theoretical physics, including Loop Quantum Gravity, have been published relating to its topic. A shortcoming of the theory was the unknown origin of the coupling constant h~. This coupling constant was added by Bekenstein in the 1970s mainly for dimensional reasons and has since remained a mystery. We will suggest an origin of this constant in this paper.

In Verlinde's view space is mainly a storage place of information, which is associated with positions, movements and mass of matter. This information is displayed to us on a surface, a screen in front of us. The information is stored in discrete bits on the screen and since the number of bits is limited we get holographic effects. This means that if there is more information on the inside than the amount of information accessible on the screen then information will be hidden from us as we observe the dynamics. This is the holographic principle. Thus the dynamics on the screen is governed by some unknown rules which then only can utilize the information on the screen. Since information is stored on a screen this means that space is emergent in the normal direction of the screen. The microstates may be thought of having all sorts of physical attributes such as energy, temperature etc. This is then related, via entropy, to the information associated with the system.

[...]

The great connections between matter and information made by Bekenstein, Hawking and others in the 1970s has turned out to have very interesting consequences. Verlinde's framework for the origin of the laws of Newton including gravitation based on entropy is perhaps one of the greatest consequences of this. In what way space is emergent and how the holographic principle holds is starting to fall into place. In this paper we have proposed an entropy arising from quantum mechanics and we have investigated its relation to Verlinde's theory. This was then applied to generalize the Newtonian potential arising in Verlinde's theory. There are many open problems remaining as this is a theory in progress. The study of multiple particle situations in the quantum mechanical approach should be interesting. A relativistic approach to quantum entropic gravity also needs to be established and investigated. The construction of various quantum field theories in curved spacetimes based on this approach should be of particular interest. Generally the connection to quantized gravity theories, if there are valid such, is left for further investigation. The nature of space and time as derived concepts, as spoken of by Verlinde, is not addressed in this paper and are features in need of investigation. The relation to AdS/CFT correspondence is also left open for further investigation. In conclusion, this paper, guided by a pure speculation, suggests that the gravitational attraction perhaps could be the result of a particular type of entropy arising in quantum mechanics.

Verlinde proposed a theory of gravity where gravity no longer was a fundamental force, but rather an effect of entropy. His theory, as a reversal of physics research, is a remarkable framework that has many open questions and interesting consequences still to be uncovered. This is why the greatest achievement of this paper is that it provides Verlinde's remarkable theory with a possible physical explanation for the factor h~, which had previously been added by Bekenstein mainly for dimensional reasons.
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nprev
post Mar 11 2010, 05:47 AM
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There's definitely a deterministic flavor to this line of thinking, which seems to be a bit of a flashback to 19th Century physics. I suppose that you could consider Newtonian physics a deterministic sum of random effects @ the quantum level, though.


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