How Does Modified Gravity Theory Address Black Holes and the Universe's Origins?

[emc2cracker]

Friends:

I have been studying John Moffats Modified Gravity theory. I find it very interesting and I agree with him in that I think Dark Matter and Dark Energy are merely mediums to make our current equations work so we can predict the movements of the cosmos. However I am unclear on what modified gravity means to black holes, gravitational waves, and especially confused about how modified gravity explains the beginning point in time and space and the age of the universe.

I am hoping someone here read his book and made more sense of it than I did. Or can it be that some of these things are yet to be explained under that model?

 

[ohwilleke]

A summation of the state of quantum gravity theory by the proponent of a conceptually similar theory, by a disciple of one of the founders of the theory, describes his theory, how he got there, and how it is fared in light of empirical research (see the conclusion).

http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.3876v1.pdf

 

[emc2cracker]

Yes I have read about MOND, TeVes, and quantum gravity. I don't see anything in this link you posted that answers my question above either. So if anyone has any info on relativities singularities in alternative models let me know I'd love to study that.

I think MOG makes black holes kind of like gray holes... just really dense objects without a source of its own light.

 

[marcus]

A summation of the state of quantum gravity theory by the proponent of a conceptually similar theory, by a disciple of one of the founders of the theory, describes his theory, how he got there, and how it is fared in light of empirical research (see the conclusion).

http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.3876v1.pdf

Ohwilleke you point to Bekenstein's recent article and say "see the conclusion".

I didn't find a conclusion section in Bekenstein's article. And I didn't find a summing-up at the end. Maybe I'm overlooking some key passage. How about quoting for us some conclusions from Bekenstein? Make it easier, spell it out so to speak.

I have to say that if I had to choose I would probably lean towards Bekenstein rather than Moffatt. Bekenstein impresses me as having more on the ball. But I could easily be wrong.

Interest in MOND and improved MOND-like theories has greatly declined in the past 2 or 3 years, wouldn't you say? The weak-lens mapping of clouds of DM, and the bullet cluster pictures, took a lot of the wind out of MOND sails.

If there still is reason to pursue MOND, then offhand I would say that Bekenstein (not Moffatt) is the most credible person to explain why and to assess the field's status. Just my two cents.

 

[ohwilleke]

From Part 5, starting at page 16 (some citations omitted, emphasis added, paragraph breaks adjusted for ease of reading):

How does TeVe S measure up to the task of describing gravitational lensing?

n TeVeS an extragalactic system lenses light, or radio waves, just as would GR, were the latter supplemented by DM in the amount and with the distribution necessary to reproduce the observed galactic dynamics. . . the TeVe S scheme is falsifiable—by comparison of the calculated potential with that inferred from the lensing —to a larger extent than is the DM paradigm for which any discrepancy can be tucked away into the invisible component. . . .

Zhao et al. (2006) . . . compare TeVeS predictions with a large sample of quasars doubly imaged by intervening galaxies. . . . The corresponding mass-to-light ratios are found to be in the normal range for stellar populations, with some exceptions. This result clashes with the claim by Ferreras et al. (2008) that lensing by galaxies from the very same sample can only be explained in MOND by including a lot of DM apart from neutrinos. But the last authors use a mixture of MOND and GR instead of TeVeS.

What should the probability distribution by angular separation of the two images in a sample of lensed quasars?

This important question has proved troublesome for the DM paradigm. In TeVeS it has been investigated by Chen and Zhao (2006) and lately by Chen (2008). . . . these workers compare predictions of both TeVeS for a purely baryonic universe with cosmological constant and of GR with DM and baryons with the CLASS/JVAS quasar survey. After the preliminary work the later paper reports that TeVeS comes out on top. All the above is accomplished with spherical mass models of the galaxies; a step towards the modeling of asymmetric lenses within TeVeS has been taken by Shan, Feix et al. (2008).

When it comes to weak lensing (distorted but unsplit images) by clusters of galaxies, a pure MOND account is less than satisfactory. The case of spherically symmetric clusters is fairly summarized by Takahashi and Chiba (2007). . . . These authors . . fail to get a fit with observations unless they add a neutrino component a la Sanders (2003, 2007); the required neutrino mass is unrealistically large, so it seems that a DM component is needed to buttress the MOND effect. . . .

Nonspherical cluster systems are also problematic. In the massive colliding clusters systems MACSJ0025.4–1222 (Bradaˇc et al. 2008) and 1E0657–56 (Clowe et al. 2004) the galaxy components have been rudely separated from the hot gas concentrations. Weak lensing mapping using background galaxies shows the gravitating mass to be preponderately located in the regions containing the galaxies, rather than in the gas which accounts for the bulk of the visible baryonic mass (Clowe et al 2006, Bradaˇc et al 2008). Collisionless DM would indeed be expected to move together with the galaxies and get separated from the collisionless gas; hence the widespread inference that much DM exists in these systems. However, this view conflicts with the finding (Mahdavi et al. 2007) that in the merging clusters A520 the lensing center is in the hot gas which is separate from the galaxy concentration. Angus, Famaey et al. (2006) considered it possible to explain the lensing seen in 1E0657–56 by TeVeS with a reasonable purely baryonic matter distribution, but later concluded (Angus, Shan et al. 2007) that a collisionless component is needed after all, with neutrinos just barely supplying a resolution. This conclusion is confirmed by a careful study of Feix, Fedeli et al. (2008) who . . . conclude that the source of gravity in 1E0657–56 must include an invisible component.

The weak lensing by cluster Cl0024+17 provides another relevant case study. Jee et al. (2007) find its deduced mass surface density to exhibit a ring which does not coincide either with the galaxy distribution, or the hot gas. Again this has been hailed as graphic proof of DM. But Milgrom and Sanders (2008) argue that such feature is actually expected in MOND, lying as it does at the transition between the Newtonian and the MOND regime. Famaey et al. (2007) conclude that the lensing in Cl0024+17 can be modeled in MOND by including 2 eV neutrinos. A truly TeVe S model of Cl0024+17 is still outstanding.

Turn now to cosmology.

Critics of MOND used to argue that the complex power spectrum of cosmological perturbations of the background radiation, which is said to be well fit by the “concordance” DM model of the universe, proves that DM is essential to any rational picture of the cosmos. . . . Skordis et al. (2006) have shown that, without invoking DM, TeVeS can largely be made consistent with the observed spectrum of the spatial distribution of galaxies, and of the cosmic microwave radiation, if one allows for contributions to the energy density of massive neutrinos, and of a cosmological constant. . . . Thus although the elimination of galaxy bound DM was the original motivation for MOND, and thus for TeVe S, the later may potentially provide a way to eliminate cosmological (homogeneously distributed) DM!

Apart from the DM mystery, cosmology furnishes us with a “dark energy” mystery. Dark energy is the agent responsible for the observed acceleration of the Hubble expansion in the context of GR cosmological models. . . . Diaz-Rivera et al. (2006) find an exact deSitter solution of TeVeS cosmology which can represent either early time inflation epochs or the late time acceleration era. Hao and Akhoury (2005) conclude that with a suitable choice of the TeVeS function F, the scalar field can play the role of dark energy. According to Zhao (2006) the choice of F implicit in the work of Zhao and Famaey (2006) leads to cosmological models that evolve at early times like those of standard cold DM cosmology, and display late time acceleration with the correct present Hubble scale, all this without needing DM or dark energy. . . .

The advent of large lensing surveys may open a way to distinguish between these two theories, as well as between GR and other modified gravities, by exposing correlations between galaxy number density and weak lensing shear (Schmidt 2008, Zhang et al. 2007). The effect of the dark energy on the expansion can be separated out by comparing cosmological models with the same expansion history in two theories. And the ultimate confrontation between GR and TeVeS cosmology may be accomplished by cross-correlating galaxy number density with cosmic microwave background antenna temperature (Schmidt et al. 2007).

Moffat, if I recall correctly, has claimed that he can overcome the bullet cluster issue (no, I don't have a cite and haven't seen it since shortly after the bullet cluster implications for MOND were first discussed).

Bekenstein has the benefit of greater established respectability in the field and a small cadre of people working with his TeVeS theory that Moffat lacks. There are a fair number of TeVeS articles in the last two or three years (and the lack of 2009 articles in linked chapter may be a function of the gap between actually writing date and arvix publication).

Bekenstein acknowledges the failures of TeVeS while pointing out the successes. He also points out that DM can serve as a post-hoc justification for just about anything. Notably, in Bekenstein's view the bullet cluster, which can conceivably be explained through simple neutrinos acting as DM, actually poses less of a threat to TeVeS than the overall difficulty that TeVeS has explaining weak lensing by galactic clusters generally.

Of course, the other virtue of TeVeS is that it provides an analytic framework which very closely replicates lambda cold dark matter theories in a compact and constrained manner that can be used to make models that work. Even if modified gravity is not the correct mechanism, the success of MOND/TeVeS in so many areas suggests that it is useful (in much the way that the Tully-Fischer relation that inspired it is useful) in making predictions to test, and in understanding the deeper relationships that are driving lambda CDM in all but a few very extreme circumstances.

Even if DM is right, there is still work to be done to explain why we have the quantities of DM in places that we do. Why, for example, is there a so much larger DM effect in galactic clusters than there is in mere galaxies? Are there, for example, multiple kinds of DM, with "type one" DM explaining most DM effects, and "type two" DM explaining what TeVeS fails to get right?

 

[czes]

If I good understand a difference between Beckenstein and Moffat propositions it is that Beckenstein proposes a modification of the Newton's law due a modification of the metric tensor and Moffat found a possible new massive field there.
Here is an interesting link of Janssen and Prokopec
http://arxiv.org/PS_cache/gr-qc/pdf/0611/0611005v1.pdf
There is shown a problem with an instability close to Black Hole.

I prefer Moffat idea and I would like to find a connection in vacuum fluctuation where the virtual particles do carry a relativistic mass. The virtual particle-antiparticle is not a stable real particle but it exists for a not defined time. If there is a fild with a set of the great amount virtual particles the mass could be defined then. So though the particle is instable the properties of the set of the instable particles may be stable.