Wednesday, May 06, 2015

Testing modified gravity with black hole shadows

Black hole shadow in the movie “Interstellar.” Image credit: Double Negative artists/DNGR/TM © Warner Bros. Entertainment Inc./Creative Commons (CC BY-NC-ND 3.0) license.

On my visit to Perimeter Institute last week, I talked to John Moffat, whose recent book “Cracking the Particle Code of the Universe” I much enjoyed reading. Talking to John is always insightful. He knows the ins and outs of both particle physics and cosmology, has an opinion on everything, and gives you a complete historical account with this. I have learned a lot from John, especially to put today’s community squabbles into a larger perspective.

John has dedicated much of his research to alternatives to the Standard Model and the cosmological Concordance Model. You might mistake him for being radical or having a chronical want of being controversial, but I assure you neither is the case. The interesting thing about his models is that they are, on the very contrary, deeply conservative. He’s fighting the standard with the standard weapons. Much of his work goes largely ignored by the community for no particular reason other than that the question what counts as an elegant model is arguably subjective. John is presently maybe best known for being one of the few defenders for modified gravity as an alternative to dark matter made of particles.

His modified gravity (MOG) that he has been working on since 2005 is a covariant version of the more widely known MOdified Newtonian Dynamics (or MOND for short). It differs from Bekenstein’s Tensor-Vector-Scalar (TeVeS) model in the field composition; it also adds a vector field to general relativity but then there are additional scalar fields and potentials for the fields. John and his collaborators claim they can fit all the evidence for dark matter with that model, including rotation curves, the acoustic peaks in the cosmic microwave background and the bullet cluster.

I can understand that nobody really liked MOND which didn’t really fit together with general relativity and was based on little more than the peculiar observation that galaxy rotation curves seem to deviate from the Newtonian prediction at a certain acceleration rather than at a certain radius. And TeVeS eventually necessitated the introduction of other types of dark matter, which made it somewhat pointless. I like dark matter because it’s a simple solution and also because I don’t really see any good reason why all matter should couple to photons. But I do have some sympathy for modifying general relativity, though, having tried and failed to do it consistently has made me vary of the many pitfalls. For what MOG is concerned, I don’t see a priori why it’s worse adding a vector field and some scalar fields than adding a bunch of other fields for which we have no direct evidence and then giving them names like WIMPS or axions.

Quite possibly the main reason MOG isn’t getting all that much attention is that it’s arguably unexciting because, if correct, it just means that none of the currently running dark matter experiments will detect anything. What you really want is a prediction for something that can be seen rather than a prediction that nothing can be seen.

That’s why I find John’s recent paper about MOG very interesting, because he points out an observable consequence of his model that could soon be tested:
Modified Gravity Black Holes and their Observable Shadows
J. W. Moffat
European Physics Journal C (2015) 75:130
arXiv:1502.01677 [gr-qc]
In this paper, he has studied how black holes in this modification of gravity differ from ordinary general relativity, and in particular calculated the size of the black hole shadow. As you might have learned from the movie “Interstellar,” black holes appear like dark disks surrounded by rings that are basically extreme lensing effects. The size of the disk in MOG depends on a parameter in the model that can be determined from fitting the galaxy rotation curves. Using this parameter, it turns out the black hole shadow should appear larger by a factor of about ten in MOG as compared to general relativity.

So far nobody has seen a black hole shadow other than in the movies, but the Event Horizon Telescope will soon be looking for exactly that. It isn’t so much a telescope but a collaboration of many telescopes all over the globe, which allows for a very long baseline interferometry with unprecedented precision. In principle they should be able to see the shadow.

What I don’t know though is whether the precision of both radius of the shadow and the mass will be sufficient to make a distinction between normal and modified general relativity in such an experiment. I am also not really sure that the black hole solution in the paper is really the most general solution one can obtain in this type of model, or if not there is some way to backpedal to another solution if the data doesn’t fulfill hopes. And then the paper contains the somewhat ominous remark that the used value for the deviation parameter might not be applicable for the black holes the Event Horizon Telescope has set its eyes on. So there are some good reasons to be skeptic of this and as the scientists always say “more work is needed.” Be that as it may, if the event horizon telescope does see a shadow larger than expected, then this would clearly be a very strong case for modified gravity.

27 comments:

kashyap vasavada said...

Hi Bee:
Are there particles associated with these fields? If there are, then, will they have zero mass like gravitons or can they have non zero mass? In either case HEP will have something to say about the model.

Uncle Al said...

"Much of his work goes largely ignored by the community" Management is rewarded for enforcing process, counting things, and avoiding risk. Discovery is blackest insubordination toward established process, prior art, and safe conduct. Four pages! Be afraid, be very afraid.

"The size of the disk in MOG depends on a parameter in the model that can be determined from fitting the galaxy rotation curves." Quantum gravitation, MoND, or MOG are argued, experiment is observed sourcing: Physics and chemistry use existing bench top apparatus (png). Gravitation ignores composition. Quickly, sensitively test spacetime geometry with geometry.

Chris K said...

If you were to take a random sample of 20 galaxies in the universe, you would get a very wide range of sizes along with the fact that no two galaxies would have the exact same baryonic mass distributions. Given those facts, endorsers of Dark Matter theory would have to convince the rest of us that for each of those galaxies, there happens to be just the right amount of dark matter situated in just the right location to produce near-flat rotation curves for all. In the end, I think that rejecting dark matter has less to do with cosmological principles than it does with understanding probability and statistics.

Sabine Hossenfelder said...

Kashyap,

I'm not sure what you mean by that. Do you mean are the fields quantized? I don't think so. The fields don't couple to standard model particles, so I don't know what you think hep will have to add to this, unless you mean Planck scale scattering. Best,

B.

Sabine Hossenfelder said...

Chris,

The rotation curves differ from case to case too. But yes, while the dark matter people need to get the statistics right to find some universalities, the modifiers of gravity need to find a way to get the statistics right so it's not too universal. That sword cuts both ways. Best,

B.

kashyap vasavada said...

Bee: OK.What you are saying is that if the fields are not quantized there is no need for associating particles with them.But I think,so far,we do not know any field with which there are not associated particles(?).That may be also a criticism of the model. Also sooner or later, one has to quantize fields to make connection with QM. That may be why Moffat's model has not received much attention.

Plato Hagel said...

I wonder what Kip Thorne would of thought of John's proposal?

b.e. hydomako said...

Hi.

I merely wanted to say that this is the best science related article I have read online in quite awhile. That you are willing to recognize and explicate that the Standard Model is not without difficulties and also acknowledge that, let's say, "orthodoxy" within the scientific community sometimes gets in the way of scientific progress, well, it's simply delightful to see some honesty for a change instead of either pop-science reporting sensationalism or dogmatic pronouncement from some scientists who wish to convince others that their favoured theories are the true ones.

Quite possibly the main reason MOG isn’t getting all that much attention is that it’s arguably unexciting because, if correct, it just means that none of the currently running dark matter experiments will detect anything.

Heh. I can't say I understand all the more technical details involved in either MOG or the current accepted framing of the issue, but I do understand the issues outside of the equations, let's say, and my intuition tells me quite strongly that the "standard" modes of investigations into dark matter are simply not going to find it because the it's a problem of the model and not of the phenomena. So, again, it is refreshing to see an article the not only acknowledges that as a possibility, but that also presents the fact that at least some scientists feel this way as well and are also working on alternate or modified models as opposed to merely sticking to guns that seem to be shooting blanks (to turn and twist a phrase).

So, thanks very much for writing and sharing this article: more science related articles ought to have the same balance and temperament.

Robert L. Oldershaw said...

It is possible that the ultracompact objects at the centers of galaxies do not have event horizons. This would make them scantily clad singularities.

In fact this has been predicted to be the case. If this prediction is vindicated it could be an important step forward after decades of sos.

Sabine Hossenfelder said...

Robert:

It is correct that it has been demonstrated that in some collapse situations horizons might not be formed. However, this is most likely not the case for the black holes in question here. You are right in that one should look at it anyway. I found it somewhat surprising, but according to several studies it isn't all that easy to tell the difference. I wrote about this here. Best,

B.

Sabine Hossenfelder said...

kashyap,

No, this isn't what I am saying. I said I don't understand your question. If you quantize the fields, you can associate particles with them, so I don't know whether you are really asking if the fields are quantized or what else did you mean? For the solution in question here, no, because the fields only interact gravitationally, there's no need to quantize them, you wouldn't see the effects anyway. I don't think this is why this model has been largely ignored because you could definitely quantize it if you wanted to. Probably one should do it to figure out whether it's nicely stable and so on, but then I would think of this as an effective theory anyway, so not sure it makes much sense. Best,

B.

Unknown said...

The crucial bit about the claim of viability of MOG seems to be the test against the bullet cluster. Is the result here indeed convincing? (I am genuinely asking.)

Phillip Helbig said...

"John has dedicated much of his research to alternatives to the Standard Model and the cosmological Concordance Model. You might mistake him for being radical or having a chronical want of being controversial, but I assure you neither is the case."

Unfortunately, there are a few other people who suffer from the same wrong classification. There are those who sincerely believe in some alternative theory, which might be crackpot or might not be. Then there are those who are just careful: they want to make sure that all assumptions are justified and so on. Then there are the devil's advocates, who don't really believe in their alternative theories but use them to test orthodoxy. And, of course, there are those who question orthodoxy without providing any alternative.

Sabine Hossenfelder said...

Unknown,

I don't know. It isn't really my area. But I don't have much doubts. See, all you really need to fit the bullet cluster is some way to dislocate the center of gravity from the visible 'stuff'. You can't do this with vanilla GR, you need something else that has its own mind, ie additional degrees of freedom. And if you have a vector field and some scalar fields, I don't see what prevents you from fitting the data any less good than you can do with dark matter. Best,

B.

David Brown said...

What is the meaning of MOG in terms of string theory? CONJECTURE: Empirical proof that the Brans-Dicke coupling constant ω > 400000 is equivalent to convincing the majority of physicists that Milgrom is the Kepler of contemporary cosmology.

nicolas poupart said...

"what counts as an elegant model is arguably subjective"

Sorry but the simplicity of the axiomatic has always been a standard criterion in particular the insulation and removal of non-independent axioms (which are in fact theorems). For example GR -> NEWTON + SR is not at all equivalent to NEWTON + SR -> GR and thus NEWTON + SR = GR. In this situation, it would be ridiculous to even use a mathematical trick with a unnecessary complexity. Here's a question that deserves further development and it should start with the affine version of the GR. This unfortunately requires time and expertise. I would like to mention here the work of Stéphane Le Corre (http://arxiv.org/abs/1503.07440) that unifies GR and Gravitomagnetism with an amazing experimental adequacy on rotational curves.

"I can understand that nobody really liked MOND which didn’t really fit together with general relativity"

If it was only that. First, MOND normalizes the acceleration, which is arbitrary ; with F = ma and F = mM / r ^ 2 we might as well normalize the mass which would make more sense in a Maxwellian / Einsteinian thinking where the only real existence is the field. In QT it is the mass that is successfully normalized (http://arxiv.org/abs/1406.4088).

Moreover, experimentally MOND fails on the lower limit of dark matter of 2M for galaxies at almost zero of rotation.

Sabine Hossenfelder said...

Nicolas,

I fail to see what is the point of your comment. I agree that A => B is not equivalent to B => A, but why is that relevant to the perception of elegance? Besides this, it doesn't follow from Newton's law and Special Relativity that you get General Relativity, there are several more assumptions that go into this, to begin with there is the equivalence principle and, maybe most importantly to the topic, an implicit assumption of minimalism that discards for example the possibility of massive gravitons or Brans-Dicke type theories or bi-metric theories and so on and so forth. GR is, basically, the simplest thing you can do. That may be right or it may not be right. Best,

B.

Patrick Johnson said...

The best way to go is to observe both sides,using the standard model and looking into what the MOG has to offer. Keeping silent to explore beyond the standard model is not going to help astrophysicists and cosmologists in this regard. I hope something is done to look into what other people views are in relation to real world phenomena.

nicolas poupart said...

Sabine,

I grant you the strong equivalence principle, conservation of energy, momentum, etc. I join the Bauhaus school, beauty is the fusion of form and function; with an equivalent complete theory, beauty is the simplicity of production of theorems. "GR is, Basically, the Simplest thing you can do" effectively when using tensor calculus to first seek to preserve the general covariance but nothing says that the covariance is not a consequence of another set of more simple axioms... no ?!?

Sabine Hossenfelder said...

Nicolas,

'Simplicity' is not a good criterion unless you define what you mean because it depends on how familiar you are with the mathematical structures used. I would argue for example that a Lagrangian approach to GR is 'simpler' than trying to work out the equations, but of course this is only so if you have already done the work to derive the Euler-Lagrange equations from the action principle to begin with. You see what I mean? In any case, I still don't understand what your comment had to do with the paper in question. Best,

B.

Uncle Al said...

@nicolas poupart "I grant you the strong equivalence principle, conservation of energy, momentum, etc."

The EP can be measurably violated on a bench top without contradicting any prior observation. Noetherian symmetries (homogeneity of time for mass-energy conservation, homogeneity and isotropy of space for linear and angular momentum conservation) do not exactly obtain at different heights in a gravitational potential. Newton is wrong (GR, QM) because postulated c = infinity h = 0 are not empirical. Observation says postulating mirror-symmetric vacuum (boson photons) is not exact for fermionic (quarks) matter.

45 years' hectares of rigorously derived theory may have empirically inexact founding postulates. Execute falsification by exploiting anomalies as diagnostics: look.

outerhoard said...

Well, I suppose anyone can *CLAIM* that their version of modified gravity can explain the Bullet Cluster without the need for dark matter, but until someone can explain in terms comprehensible to the layperson how that's possible even in principle (given that either the apparent distribution of dark matter is dependent on the distribution of observable matter or it isn't), I see no reason to pay attention. The paper abstract is sheer gobbledegook from my perspective.

nemo said...

The need of Dark Matter is a question of phenomenology, not of theory.
The point is, not to make an hypothesis that can be fitted this time for an aspect or another, but building a larger understanding of Nature rules. That's my own dream and the paper is going in this direction, even if I can't say whether or not it is correct.
Just experiments or, observations, may solve the subject.

Jay Poynting said...

For any theory to be regarded seriously, it needs to make predictions that differentiate it from existing theory. Theories that explain existing data in a different way are almost too numerous to count. MOG and other theories like it need to provide verifiable predictions that can't be continuously tweaked if the observations do not agree with the theory's current version.

Phillip Helbig said...

"What is the meaning of MOG in terms of string theory? CONJECTURE: Empirical proof that the Brans-Dicke coupling constant ω > 400000 is equivalent to convincing the majority of physicists that Milgrom is the Kepler of contemporary cosmology."

I am not unsympathetic to MOND. I think it should get a fair shake. But it is difficult (like it or not) with pundits claiming that Milgrom is, if not the modern Einstein, then the modern Kepler. Blog comments will not convince those set in their ways, no matter which side of the fence they are on. At most, you can hope to spark interest in those not yet committed. But doing so in a manner very similar to that which most crackpots use to tout their own claims is really not constructive.

tobychev said...

As I recall the EHT has published some preliminary (one dimensional) plots of the shadow and they where not super surprised by the size (http://www.eventhorizontelescope.org/science/eh_structure.html) which somewhat speaks against there being a 10x difference.

This is a 2008 result so I'm surprised to see it missing getting some mention here, do you know anything about this absence?

(To be honest I didn't know about it either until hearing about it last year despite it appearing in nature.)

andrew said...

Another important test for MOG will be to see if it accurately predicts the behavior of RAVE stars in the Milky Way, which are outside of the plane of the main disk of the galaxy.

This is an area where other modified gravity theories have failed, vis-a-vis CDM theories, but where the theories tested are arguably straw men that MOG can be expected to outperform.

The Chris K comment on halo scatter relative to luminous mass as a test of modified gravity v. dark matter theories is a solid one, however. The papers that have looked at this issue have found that halo scatter relative to luminous mass is indeed far lower than any kind of collisionless dark matter theory would predict and also produces the wrong frequency of "bulge-less" spiral galaxies. This is one of the strongest arguments against dark matter theories generally.