Is Modified Gravity Theory a Viable Alternative to Dark Matter?

[MonstersFromTheId]

Dark matter seems to have the upper hand at the moment, at least with what I read.

But now I keep hearing about Modified Gravity Theory as a alternative explanation for the large structure of the Universe, (often paired with comparisons between Dark Matter, and what was once called the "luminiferous ether").

How much support has MGT garnered at this point? Is this a serious point of view, or something that belongs more in the "debunking" section of the forum?

What has been "modified" with respect to gravitational theory?

 

[Wallace]

There are a number of modified gravity schemes that have been proposed. Early suggestions altered Newtonian gravity (known as Modified Newtonian Dynamics or MOND) and more recently this has been extended to General Relativity in a theory known as Tensor Vector Scalar gravity (TVS). By analogy with GR going to Newtonian in the low mass limit, TVS reduces to MOND.

The basics of MOND are not so much a change in gravity as such but a change in the way gravity accelerates mass. Instead of the usual Newtons Second Laws, F=ma, MOND suggests that we have F=m µ(a/a0) a. The function µ(x) describes the modification and depends on a new parameter a0 that is suggested to be a fundamental constant. There are various forms of µ(x) that have been proposed. Try having a look at the Wikipedia article on MOND for some more info.

There are also a large number of modified gravity theories that attempt to deal with dark energy, i.e. they explain the observations that lead us to think dark energy exists as a modified gravity in a similar fashion to how MOND is supposed to do away with the need for dark matter.

As far as I'm aware no one has proposed a modified gravity theory that consistently accounts for all observational data. So some theory might work really well for galaxies but would imply something for Cosmology that we don't observer or vice versa.

That being said, the current dark energy + dark matter model for the Universe is not completely without its own warts so modified gravity remains a possibility, albeit less likely on current evidence than the current model.

 

[cristo]

and more recently this has been extended to General Relativity in a theory known as Tensor Vector Scalar gravity (TVS). By analogy with GR going to Newtonian in the low mass limit, TVS reduces to MOND.

Are these tensor vector scalar theories the same as "f(R)" theories of which I have heard. That is, theories in which the Einstein Hilbert action of GR is changed from R to some function f(R)?

 

[MonstersFromTheId]

Tx Wallace! That wiki article is great, but MAN have *I* got some reading to do!

 

[Wallace]

Are these tensor vector scalar theories the same as "f(R)" theories of which I have heard. That is, theories in which the Einstein Hilbert action of GR is changed from R to some function f(R)?

I think they are different, but I'm very far from certain about that.

 

[nrqed]

Are these tensor vector scalar theories the same as "f(R)" theories of which I have heard. That is, theories in which the Einstein Hilbert action of GR is changed from R to some function f(R)?

I think that there is some differences in the way those things are defined depending on your source. I am used to see the following definition:


Scalar tensor theories have a scalar field introduced via
<br /> \int d^4x \sqrt{-g} ~\bigl( \frac{f(\phi)}{2} R - \frac{\omega(\phi)}{2} \nabla^c \phi \nabla_c \phi - V(\phi) \bigr)

As special case of th eabove is the Brans-Dicke theory.

I am used to seeing "generalized scalar-tensor theories" defined as

<br /> \int d^4x \frac{\sqrt{-g}}{16 \pi} ~\bigl( f(\phi,R) - \frac{\eta}{2} \nabla^c \phi \nabla_c \phi \bigr)

and modified gravities as simply
<br /> \int d^4x \sqrt{-g} \, f(R)

But these are not the most general models. One can introduce functions of the Riemann tensor (as opposed to only the Ricci scalar) or the Weyl tensor and so on.

The possibilities are endless

 

[Wallace]

The possibilities are endless

Aye there's the rub. With an endless range of possible modifications it's no surprise that modified gravity is capable of solving just about any problem in cosmology. If modified gravity successfully mimics some other result how are we supposed to tell the theories apart? Maybe the LHC will save us!

 

[nrqed]

Aye there's the rub. With an endless range of possible modifications it's no surprise that modified gravity is capable of solving just about any problem in cosmology. If modified gravity successfully mimics some other result how are we supposed to tell the theories apart? Maybe the LHC will save us!

I agree completely. Actually, it has been proven that if one simply adds a scalar (like in the scalar-tensor model I mentioned above) with an arbitrary potential, one can reproduce *any* time evolution for the scale factor!

So just adding a scalar is not interesting. But corrections in powers of R are to be expected if one thinks of GR as an effective field theory. So if one would reproduce observations (including effects such as acceleration of the expansion or drak matter effects) from higher order terms in R or the Riemann tensor, I personally think that would be interesting. well, a paper by Carroll et al on exactly that led to a large number of citations (I think) so it's not devoid of interest.

 

[cristo]

Thanks for highlighting the differences, nrqed.

 

[Garth]

I agree completely. Actually, it has been proven that if one simply adds a scalar (like in the scalar-tensor model I mentioned above) with an arbitrary potential, one can reproduce *any* time evolution for the scale factor!

And that might be significant if there is indeed an age problem in the early universe. Critique of Mainstream Cosmology

Garth

 

[emc2cracker]

I very much like the most recent theory by John Moffat. People have the misconception that this is somehow degrading to Einstiens theory, in fact I believe Einstien would be excited about this theory, he went on trying to make a similar theory.

I have a question for you math experts here. Modified gravity makes no attempt to couple the repulsive gravitational field near massive objects with electromagnetism as Einstien did. However does this mean under MOG that is not possible? I am intrigued at what MOG means for magnetar stars. I hope this theory is correct, it seems to me dark matter and energy are simply made up in order to make our current accepted gravity theory work. Seems to me this is much like the planet Vulcan prediction, just shoveling in more matter to make the math work.

I am no physicist, but I am a diligent enthusiast trying to grasp all this and study it. Professor John Moffat is kind of my role model, coming from limited educational background he is basically a self made physicist so maybe there is some hope for enthusiasts like me.

Strange this topic has been silent so long, according to the professor nothing has come in observational data within the past two years to debunk his theory. If that is so it seems it should be gathering support, but it seems it has been forgotten and disregarded.

 

[twofish-quant]

I have a question for you math experts here. Modified gravity makes no attempt to couple the repulsive gravitational field near massive objects with electromagnetism as Einstien did.

Quite true. In fact most modified gravity theories go out of their way to avoid making any changes to gravity near massive objects. The reason for this is that there is no evidence of any odd gravitational behavior near massive objects, so all of the modified gravity theories are designed match standard gravity at short non-galactic distances.

I am intrigued at what MOG means for magnetar stars.

You try to make it mean nothing. The trouble is that magnetars and anything involving magnetism are incredibly hard to model even with standard gravity. If you add strange gravitational effects, then you make it impossible to get any sort of prediction out.

Pretty much all simulations involving magentars and accretion discs use Newtonian gravity. You want to keep the gravity model dead simple since the magnetic fields and hydrodynamics end up so complicated.

I hope this theory is correct, it seems to me dark matter and energy are simply made up in order to make our current accepted gravity theory work. Seems to me this is much like the planet Vulcan prediction, just shoveling in more matter to make the math work.

It is. However it's not clear whether this is the prediction of Neptune, Vulcan, or Pluto.

Strange this topic has been silent so long, according to the professor nothing has come in observational data within the past two years to debunk his theory. If that is so it seems it should be gathering support, but it seems it has been forgotten and disregarded.

That's because its one of about several dozen different theories involving modified gravity that people are looking at. There's nothing about Moffat's theory of gravity that says "pick me!" over the several dozen other modified gravitational theories that are in play right now. There are probably several hundred different papers on modified gravity models.

Also a lot of these observations take a lot of time and effort. There is an interaction between theorists and observationalists, because a lot of what theorists do is to come up with smoking guns for observationalists to look for.

One thing that is pretty cool to see plotted is what I call an "exclusion chart". What you do is to take a diagram, plot predictions that different models have, and as data comes in, you knock out models that don't work. The really cool part of these charts is that when it comes time to write grant proposals, you have this chart that says "look here."

 

[Jonathan Scott]

MOND has a very nasty feature which probably creeps into most other modified gravity theories as well. It claims that the MOND-type gravitational effect on objects depends on whether the acceleration of that object (in some absolute or relative sense) is less than some threshold value. However, a macroscopic object such as a star is made up of smaller objects (atoms and molecules) which are undergoing much higher accelerations (due to the local gravitational field), which means that by the same logic they should be immune to such MOND-type accelerations. MOND supporters seem to think that this doesn't matter and assert vaguely that the motion of the star as a whole doesn't need to be determined by the motion of its constituent particles, but I don't buy that.

The reason for this postulated cut-off is because it is thought that MOND-type deviations from Newtonian or GR gravity would have been noticed by now within solar system dynamics or the laboratory.

Certainly such forces should be just about observable at a laboratory level, but because they are proportional to sqrt(m)/r instead of m/r^2, it seems likely that any standard Cavendish-type measurements would normally try to minimize them in comparison with the Newtonian forces (by maximizing m and minimizing r), and although there have been experiments to look for non-Newtonian gravitational forces, I think they have always been concentrating on even higher powers of 1/r, not lower ones. (It's well known that different experiments have given unexpectedly different values of G, so perhaps this could be a factor).

Also, such effects should now be observable from space probe orbits in the solar system, as they are of a similar order to the well-known "Pioneer anomaly". There could of course be a connection with that anomaly, but I don't personally think that the figures match at the moment.

I have therefore not yet ruled out the possibility that a MOND-like modification to gravity might still work WITHOUT any cut-off threshold, and I'd be very interested to see any experimental evidence for or against that hypothesis.

 

[Chronos]

MOND is very unpalatable to me. With MOND you must rewrite the laws of the universe in ways consistent with observation. It might be right, but, I want to see all the rewritten laws and their consistency with observation.

 

[Jonathan Scott]

I think that a theory which predicts the MOND effect can be derived from the assumption that a volume of space containing a fraction m/M of the mass of the universe has a surface solid angle deficit of 8 \pi m/M, in an analogous way to angular deficits on an ordinary sphere. This has only one parameter, M, which effectively represents the mass of the universe. It effectively matches MOND when M = 2c⁴/Ga₀ where a₀ is the MOND acceleration parameter. This gives M approximately equal to 2 x 10⁵⁴kg which is high but still plausible.

Edit: Couldn't get typos in Latex to go away, so converted them to text.

 

[Wallace]

The theoretical basis for MOND aside, recent evidence has gone against the phenomenology being a correct description of gravity. Some of the problems include MOND not being able to explain galaxy-galaxy strong lenses as well as not getting even close to describing galaxy merger rates and timescales observed. MOND's problems make CDM's warts look like beauty spots!

Even most of those working in MOND now accept that some amount of dark matter is needed for MOND to work. You can't yet rule out MOND + less dark matter than the CDM model needs, but you can rule out MOND with no dark mattter.

More broadly, the comparison between some kind of modified gravity on large scales and dark energy is far less settled. I think it would be a brave call to say that modified gravity in that context is ruled out, but this is a different regime to that which MOND deals with.

 

[twofish-quant]

More broadly, the comparison between some kind of modified gravity on large scales and dark energy is far less settled. I think it would be a brave call to say that modified gravity in that context is ruled out, but this is a different regime to that which MOND deals with.

That is one big theoretical reason that people like LCDM. Theorists like elegant models that seem to explain completely unrelated things. If you need two theories of modified gravity to explain things at different scales, then this fails the "beauty and elegance" test.

 

[Wallace]

There are elegant and simple modified gravity models with no more free parameters than LCDM. They still have dark matter, that's looking incresinly certain, but they address dark energy. One example is the DGP braneworlds model. It has only one free parameter, a length scale, and a simple theoretical basis (as long as you can accept higher spatial dimension...).

I believe the most recent data has shown some tension with the DGP model, so it may not stand, but there are 'elegant' alternatives to LCDM. Fundamentally though, I think even alternatives would suffer some kind of fine tuning problem analgous to LCDM, so I don't think all of LCDM's theoretical unpleasantness can be solved by invoking a different theory.

 

[Chronos]

I'm a big LCDM fan because it was cobbled from so many different and unrelated pieces. MOND, like other unorthodox theories, attempts to beautify physics. I think physics is ugly and our universe has warts.

 

[emc2cracker]

Thank you for the enlightening comments all,

Please allow me to expand on my notion about magnetars I was being a little too vague. I believe if we ever get to witness the actual supernova explosion that forms a magnetar we will discover Einstien was indeed correct. I believe the only reason magnetars do not collapse into black holes is because of the link of electromagnetism to the repulsive gravitational force, or the fifth force in John Moffats equations. I also believe we will discover this same force is responsible for quasar jets and probably was crucial in the formation of the universe. Now some may think that is unlikely or impossible, but the logic behind the notion is pretty strong if the math is lacking at least I have yet to see an equation to show such a notion is impossible.

And I do not see how invoking dark matter makes any long term sense. Now I am certain that dark matter exists, but I do not think it is completely undetectable mass, and I do not think it is an adequate explanation to uphold relativity as is. To me it is obvious that gravity is stronger farther away than relativity allows. Let's be realistic dark matter was never discovered, it was thought up to make our galactic simulations work without reprogramming it with new math. That was when dark matter was come up with, imagine programing relativity into a simulated universe only to discover your galaxies all flew apart. I believe I would have invented dark matter too so I can actually run some simulations, but I wouldn't go on tv preaching like science knows all when clearly that was an uh-oh moment in theoretical physics. Dark matter is a patch not a fix, I'm content with it for now until a final equation explains all the movements, and I am further convinced the final equation will have to invoke some matter that we cannot see or detect.

 

[inflector]

The theoretical basis for MOND aside, recent evidence has gone against the phenomenology being a correct description of gravity. Some of the problems include MOND not being able to explain galaxy-galaxy strong lenses as well as not getting even close to describing galaxy merger rates and timescales observed. MOND's problems make CDM's warts look like beauty spots!

Can you point me to papers or publications that discuss these problems in more detail? I'm especially interested in how the galaxy merger rates point to dark matter over MOND.

 

[Wallace]

I did a quick search and found http://adsabs.harvard.edu/abs/2007MNRAS.381L.104N" to find papers. Note that almost all the papers found on ADS will have links to the free ArXiv version, so you can read them without paying a journal subscription.

 

[inflector]

Thanks, that search site was great. I found a bunch of papers that are relevant.

 

[emc2cracker]

Personally I find MOND far too simple, I agree the dark matter model is more accurate. HOwever I still believe MOG is a better explanation than both. Sure its uglier, but it allows for dark matter models still however not the 60% makeup that we currently adhere to.

The new observational data on magnetars also points to the possibility that there is far less dark matter than we had thought. It is proposed that possibly 50% of all empty supernovas could house a dead magnetar. That could add up to 100 million in our galaxy, that is possibly an additional 2.8 billion solar masses in the galaxy. Ok so it doesn't dent 2 X 10^12 much, but what good is it to have a theory with a variable that you can just change to anything to come up with the right answer.

MOG has to be the best mathematical explanation of gravity in my opinion. ONly the very most distand clusters and objects defy explanation in MOG, and I believe that has to do more with the problems of seeing that far than the mathematics. MOG goes all the way down to the quantum level and the equations are mind blowing.

I admit I may be biased because of the postulation of the fifth force that my book depends on. And yes maybe MOG's physics are my model for my favorite fictional space vehicles, but MOG came first. I did not hunt out the best solution to fit my view of the universe (or the physics of my science fiction novel), it was the physics that spawned the dream in my case.

But I am very inquisitive, those of you who do not accept MOG would you care to point me to the conflicts you can hang your hat on?

Also I am having a difficult time finding raw data on magnetar observations, are there any observational researchers here that can point me in the right direction? I have a colleague waiting on me to give him numbers to crunch and I am very new to this kind of research.

Thanks again you all, this is a great forum filled with all kinds of food for thought. I must be getting old lol, physics is becoming exciting I think I missed my calling in career choice!

 

[Wallace]

If dark matter was in the form of many many magnetars they would have been seen through many more microlensing events than have been found in experiments like MACHO. Essentially 'compact' dark matter is all but ruled out by these kind of observations, so if it exists, dark matter has to be a diffuse substance, many small particles (dubbed WIMPs for Weakly Interacting Massive Particles).

When you say MOG, what are you referring to? There are a large number of modified gravity theories (other than MOND) and some are more promising than others, but none yet have been able to show a better explanation for the available data than the LCDM model. The key part of that sentence is 'yet' and hopefully within a decade or so the question will be reasonably sorted one way or the other given the new data that will be coming.

 

[emc2cracker]

Now come on surely you didn't just misconstrued my entire statement just now? I didn't say I could point to every single object in the universe, and I didn't say dead magnetars made up all. However Magnetars ARE more common than was thought when they told us our galaxy has 2 X 10 ^ 12 solar masses invested in dark matter. So actually the total dark matter mass of our galaxy is dropping, when the findings are complete it could be 2.8 billion solar masses less dark matter.

So my point is of course dark matter explains everything, its a changing number you just say ok we have X missing mass, so X = dark matter. That's not real science that's a cop out. Come on guys it seems you all are still reading 2007 observational data, I mean MOND? MOND can't explain gravitational lensing period, MOND is just not quite there. And instead of fixing gravity MOND made it worse by weakening it, so sure no dark matter but now we need more dark energy.

MOG doesn't need either, matter of fact dark energy is completely void and gone under mod. And we cannot detect lensing for every single object out there, there is dark matter out there for sure. But its not invisible or undetectable its just space gravels far away from a source of light. The newest version of MOG has the good ideas of MOND like vector fields, and has the right gravity strength at far distances.

here guys look at these please:

http://arxiv.org/abs/astro-ph/0702146 bullet cluster data and mog

http://adsabs.harvard.edu/abs/2009arXiv0911.3552M

http://physicsworld.com/cws/article/news/39102

http://physicsworld.com/cws/article/news/39102 searching for dark matter in small galaxies

http://arxiv.org/PS_cache/gr-qc/pdf/9910/9910112v2.pdf (might have to register)

Scalar Vector Tensor or Modified Gravity has explained the background radiation, has accounted for the expanding universe, and the matter power spectrum. The flaws most here have picked out are corrected in the theories latest form. And really it have much the same fundamentals as MOND did, the big difference was the strength of the vector fields.

All this from a failed painter.

 

[Wallace]

Now come on surely you didn't just misconstrued my entire statement just now? I didn't say I could point to every single object in the universe, and I didn't say dead magnetars made up all. However Magnetars ARE more common than was thought when they told us our galaxy has 2 X 10 ^ 12 solar masses invested in dark matter. So actually the total dark matter mass of our galaxy is dropping, when the findings are complete it could be 2.8 billion solar masses less dark matter.

No no, I'm not misconstruing anything, let's not get defensive. You've got to keep the numbers in perspective, so yes we do think there are more magnetars now, compared to when we knew less about magnetars, but the abundance is still far short of anything that effects calculations to do with dark matter, it's orders and orders of magnitude away. I know what you're saying, not that magnetars could replace DM, but just reduce the need for as much, but the difference that extra magnetars would make is negligible. My comment about the MACHO project is the key, if magnetars were relevant to the mass census then we'd have seen them in this project.

From many independant measurements (i.e. not only from galaxy rotation curves etc) we know that there is roughly 10 times the mass of DM compared to baryons, and that of the baryon mass, there is around 10 times more gas than stars/planets/neutron stars/black holes. So magnetars are a subset of that last group, so changes in the number we observe of them makes no difference to the total amount of dark matter.

Magnetars require supernovae to form, and the supernovae rate is given by the star formation rate, which measure as a function of redshift by observing galaxies. To have made enough magnetars to have a mass fraction relevant to the amount of DM required you would need many orders of magnitude more star formation to have occurred in the past compared to what is observed, which would have required a much greater baryon abundance to have provided the fuel than is observed to be there.

Magnetars are a very interesting class of objects, but there study is not relevant to questions around the amount of DM required.

So my point is of course dark matter explains everything, its a changing number you just say ok we have X missing mass, so X = dark matter. That's not real science that's a cop out. Come on guys it seems you all are still reading 2007 observational data,

Fortunately this bears no resemblance to how cosmology is done. We do not simply add it whatever matter is needed wherever it is required. The abundance and internal structure of dark matter structure is carefully modeled with simulations and semi-analytic means and these are compared with observations such as that from lensing. We don't just add whatever dark matter we need to balance the sums, instead you take the predictions from simulations which have modeled the gravitational evolution of structure from the early universe to today (or to whatever redshift you are looking at) and compared that to observations. There is not the freedom to invent whatever mass is needed, you're constrained by the physics, which gives results which agree with observations.

MOG doesn't need either, matter of fact dark energy is completely void and gone under mod. And we cannot detect lensing for every single object out there, there is dark matter out there for sure. But its not invisible or undetectable its just space gravels far away from a source of light. The newest version of MOG has the good ideas of MOND like vector fields, and has the right gravity strength at far distances.

Scalar Vector Tensor or Modified Gravity has explained the background radiation, has accounted for the expanding universe, and the matter power spectrum. The flaws most here have picked out are corrected in the theories latest form. And really it have much the same fundamentals as MOND did, the big difference was the strength of the vector fields.

TeVeS is a well known theory that has had a lot of discussion. It still hasn't outperformed LCDM compared to data when all the data are compared together, using model selection. To be fair, the current data is possibly not good enough, the extra model complexity in something like TeVeS means you prefer a simpler model like LCDM unless the data was clear enough. The biggest difference between LCDM and modified gravity models tends to come from the time evolution of structure, observed through observations like the halo mass function vs z (from for instance strong lensing counts, or weak lensing surveys or cluster counts). If the ESO mission Euclid gets funded and built, we should also get some excellent information from redshift space distortions, which are and excellent way of discriminating between dark energy and modified gravity.

 

[twofish-quant]

The new observational data on magnetars also points to the possibility that there is far less dark matter than we had thought. It is proposed that possibly 50% of all empty supernovas could house a dead magnetar. That could add up to 100 million in our galaxy, that is possibly an additional 2.8 billion solar masses in the galaxy. Ok so it doesn't dent 2 X 10^12 much, but what good is it to have a theory with a variable that you can just change to anything to come up with the right answer.

Ummm... No. Magnetars are baryonic matter which means that they don't change the amount of dark matter that is required. Also if they numbers that you give are accurate, then the existence or non-existence of magnetars is almost completely and totally irrelevant to dark matter, since those those numbers are too small to make a difference.

MOG goes all the way down to the quantum level and the equations are mind blowing.

Except that no one has come up with an MOG that seems to fit observation that well.

 

[twofish-quant]

However Magnetars ARE more common than was thought when they told us our galaxy has 2 X 10 ^ 12 solar masses invested in dark matter. So actually the total dark matter mass of our galaxy is dropping, when the findings are complete it could be 2.8 billion solar masses less dark matter.

One important thing is that 2 x 10^12 solar masses is different from 2.000000000 x 10^12 solar masses. If it turns out that there is 2.8 x 10^9 solar masses of magnetars, that doesn't change at all the need to get 2 x 10^12 solar masses of dark matter, since compared to 2 x 10^12, 2.8 x 10^9 is a rounding error.

When someone says 2 x 10^12, it's usually implied that they 1 is OK and 3 is OK. If you want to exclude 1x10^12 and 3x10^12 then you say 2.000 x 10^12. Also the mass of magnetars also works the same way. If you can get a magenet researcher to say, well maybe there are 10^10 solar masses of magnetars then things start becoming interesting.

Also one thing I try to teach in my intro astronomy classes is how to think about large numbers. Most people that take intro astronomy aren't going to be astronomers, but a large fraction are going to be business managers, and it's really important to realize that if you start talking about $2 trillion, then $2.8 billion is an insignificant rounding error that can basically be ignored.

So my point is of course dark matter explains everything, its a changing number you just say ok we have X missing mass, so X = dark matter.

Dark matter doesn't explain everything, but it explains a lot. One thing that dark matter does explain nicely is the first acoustic peak. If you assume that dark matter exists, then you have sound waves going through that dark matter and those sound waves determine the distribution of galaxies.

Also you have to backtrack and ask the question "so why do we think we have missing mass?". If it's *only* galaxy rotation curves, then yes it's probably simplier to assume modified gravity. But it isn't. You also have galaxy distribution and deuterium abundances.

Scalar Vector Tensor or Modified Gravity has explained the background radiation, has accounted for the expanding universe, and the matter power spectrum. The flaws most here have picked out are corrected in the theories latest form. And really it have much the same fundamentals as MOND did, the big difference was the strength of the vector fields.

Which is great and that means that you have a viable modified gravity theory that we can then through observations at and see if it works. There's a cottage industry of modified gravity theories out there, and whether dark matter is winning or modified gravity is winning changes from month to month.

But even if it turns out that LCDM is *wrong* there is still a good reason that people use it for cosmological models. The problem is that if you try to run a simulation of galaxy formation on a specific theory of modified gravity, and that specific theory turns out to be wrong, then your results are pretty much useless. If you use LCDM as the basis of your calculations, and it turns out that there isn't any dark matter, then there is going to be a lot of work in "translating" your results to vector gravity or whatever. You can think of LCDM as something like Newtonian physics. Even if it turns out to be wrong, it's "good enough" for some things.

"Standard models" are "standards" for the same reasons that Microsoft Windows or C++ or Fortran are standards. It's not that I think that Microsoft Windows is better than OpenOffice, but that because everyone uses "doc" files, it's natural that I use ",doc" files because the world decides that something else should be the "new standard" then I can translate things over into the new format.

 

[Haelfix]

Aye there's the rub. With an endless range of possible modifications it's no surprise that modified gravity is capable of solving just about any problem in cosmology. If modified gravity successfully mimics some other result how are we supposed to tell the theories apart? Maybe the LHC will save us!

Arguably the biggest knock against many (not all) of the modifications to GR is that they are inconsistent at the quantum level. They have ghost modes that do not cancel.

So while the quantization of gravity is a big mystery in general, it of course has to happen at some point and any theory that fails the most basic principles (loss of unitarity, gauge anomalies, etc) should be ruled out.

Another constraint on modifications to GR, involves having a well posed Cauchy problem as well as satisfying correct cosmological perturbation spectrums (this is suprisingly difficult to get right).

FYI: f(R) gravity is equivalent to scalar tensor theories when (and only when) f''(R) is nonzero

 

[twofish-quant]

We don't just add whatever dark matter we need to balance the sums, instead you take the predictions from simulations which have modeled the gravitational evolution of structure from the early universe to today (or to whatever redshift you are looking at) and compared that to observations. There is not the freedom to invent whatever mass is needed, you're constrained by the physics, which gives results which agree with observations.

And the other thing is that there are *tons* of observations. One other thing is that a lot of the observations are culmulative, which means that my just taking more of the same type of observation, you beat down the errors, and you have situations in which knowing that the number is 2, doesn't make a difference but knowing that the numbers if 2.3 and not 2.6 makes a huge difference.

To be fair, the current data is possibly not good enough, the extra model complexity in something like TeVeS means you prefer a simpler model like LCDM unless the data was clear enough.

One reason that people also tend to dark matter is that it's easier to intuitively think about dark matter than general relativity. General relativity (and generalizations of it like TEVES) is a beautiful elegant theory. It's also a pain in the rear end to get any sort of calculation from it. So if you do a calculation, you *assume* that weird gravity isn't that important, because if it's not important, you get a result. If your results don't make sense then you bite the bullet and go back and add in the complex stuff later.

This is the sort of thing that leads to scientific revolutions. If there is something from measurements that suggests that TEVES explains some weird thing about the early universe, then what I'll do is to spend about three months and put TEVES physics into my supernova code. Now it might be that when I do that, suddenly I get realistic explosions, in which case I publish something, which causes people to apply TEVES to their stuff, and you could get a snowball effect in which within a year or two, modified gravity becomes the new "standard model"

Or maybe not. The reason I'm not putting strange gravity into my supernova code right now is because it's going to take three to six months to get it to work, and *right now* there are other pieces of physics that I could spend my time looking at. (And yes, people have tried to put dark matter candidates into supernova code, and that doesn't do much.)

 

[twofish-quant]

Arguably the biggest knock against many (not all) of the modifications to GR is that they are inconsistent at the quantum level. They have ghost modes that do not cancel.

On the other hand straight GR is not renormalizable at the quantum level. I've heard it said that it's a good thing that Einstein proposed GR when quantum field theory was less will understood, because people would have used the non-renormaliziability of GR as evidence against it.

So while the quantization of gravity is a big mystery in general, it of course has to happen at some point and any theory that fails the most basic principles (loss of unitarity, gauge anomalies, etc) should be ruled out.

Observation beats theory. If it turns out that we need to modify GR to get cosmology to work, this is just more funding for theorests to think about the problem. Also pretty much all of the major cosmological theories are phenomenonlogical. We have this fudge factor parameter that we adjust to make everything fit. What's fun about this is that it turns out to be really, really hard to get that to work.

 

[twofish-quant]

This is also an example of how "real science" doesn't match "science in the movies." There is this idea of this lone genius that fights the "establishment" and then wins from shear brilliance. It doesn't really work that way.

 

[atyy]

On the other hand straight GR is not renormalizable at the quantum level. I've heard it said that it's a good thing that Einstein proposed GR when quantum field theory was less will understood, because people would have used the non-renormaliziability of GR as evidence against it.

But renormalizability isn't a requirement for a useful quantum field theory, so GR is a useful QFT.

 

[Haelfix]

GR may or may not be a useful quantum theory (it might have to be modified at very high energies), otoh it is not a manifestly inconsistent quantum theory, at least at this level of discussion.

A theory with residual gauge or conformal anomalies, or those with ghosts by contrast is always inconsistent, and is thus ruled out automatically from the beginning. For instance, Palatini f(R) gravity is an example, as well as many of the higher derivative theories, bimetric ones and so forth. You would have to insist upon a modification of quantum mechanics to get them to work.

Its a very nice constraint, b/c some of those theories can be tweaked to pass purely cosmological and relativity tests (eg the absense of curvature singularities, no ctcs, etc).

 

[atyy]

GR may or may not be a useful quantum theory (it might have to be modified at very high energies), otoh it is not a manifestly inconsistent quantum theory, at least at this level of discussion.

I was just thinking along the lines of Donoghue's GR as an effective quantum field theory at low energies, needing a still unknown UV completion.

 

[twofish-quant]

For instance, Palatini f(R) gravity is an example, as well as many of the higher derivative theories, bimetric ones and so forth. You would have to insist upon a modification of quantum mechanics to get them to work.

So modify quantum mechanics. I don't have much of a problem with that.