Modified Theories of Gravity (MoGs)

 Posts: 1
 Joined: January 26 2008
 Affiliation: Princeton University/Astrophysics Dept.
 Contact:
Modified Theories of Gravity (MoGs)
Hi all,
There's been a lot of recent work discussing tests of gravity at large scales– and I'd like to start a thread on this topic (for obvious, selfish reasons).
Topics in theory (analytical calculations, numerical simulations), observations (weak lensing, growth of structure, clustering), and how one can confront the other are all welcome (and encouraged)!
For starters, if anyone can point me to good reviews of "viable" modified theories of gravity, preferably accessible to a nonstring theorist/GR expert, it'd much appreciated!
Cheers,
– Reina
There's been a lot of recent work discussing tests of gravity at large scales– and I'd like to start a thread on this topic (for obvious, selfish reasons).
Topics in theory (analytical calculations, numerical simulations), observations (weak lensing, growth of structure, clustering), and how one can confront the other are all welcome (and encouraged)!
For starters, if anyone can point me to good reviews of "viable" modified theories of gravity, preferably accessible to a nonstring theorist/GR expert, it'd much appreciated!
Cheers,
– Reina

 Posts: 1
 Joined: January 31 2008
 Affiliation: university of florida
Modified Theories of Gravity (MoGs)
Hi,
I would say TeVeS of Bekenstein is reliable, and Skordis has couple of papers that deals with tests of TeVeS. Moffat proposed SVTG and one might look at that as well. There might come up various theories of these kind in the future and the criticism might arise because of the free parameters that they have.
We proposed a modelindependent test of all these types of models by gw observation and I talked about it during the mit (GWDAW−12) meeting on December and posted a paper about that 0801.1984.
emre.
I would say TeVeS of Bekenstein is reliable, and Skordis has couple of papers that deals with tests of TeVeS. Moffat proposed SVTG and one might look at that as well. There might come up various theories of these kind in the future and the criticism might arise because of the free parameters that they have.
We proposed a modelindependent test of all these types of models by gw observation and I talked about it during the mit (GWDAW−12) meeting on December and posted a paper about that 0801.1984.
emre.

 Posts: 7
 Joined: September 25 2004
 Affiliation: CITA
Modified Theories of Gravity (MoGs)
This might be slightly offtopic, but I was wondering – are modified gravity models that have no dark matter still viable? For example, looking at the plots from Skordis' original paper on CMB with TeVeS (astroph/0505519), it looks like these models will have a hard time matching WMAP + ACBAR. In particular, the height of the 3rd peak is supposed to be a problem without dark matter. Does anyone know if you can tune parameters just right to match existing CMB data?
(This is ignoring systems like the Bullet cluster which create a whole different set of problems for MOND, etc...)
(This is ignoring systems like the Bullet cluster which create a whole different set of problems for MOND, etc...)

 Posts: 183
 Joined: September 24 2004
 Affiliation: Brookhaven National Laboratory
 Contact:
Modified Theories of Gravity (MoGs)
I have an embarrassingly trivial paper on this topic, astroph/0508048, and this was some time ago, these days I think it would be sigmas away.
Plus we have bullet cluster, which can be explained in theories with no DM only in a rather contrived way.
I still think that these alternative theories are useful to study, as a reality check and as theoretically interesting, but I don't think they offer a viable alternative these days (this of course does not stop people churning out papers on vanilla MOND as if nothing has happened in the past ten years)
Plus we have bullet cluster, which can be explained in theories with no DM only in a rather contrived way.
I still think that these alternative theories are useful to study, as a reality check and as theoretically interesting, but I don't think they offer a viable alternative these days (this of course does not stop people churning out papers on vanilla MOND as if nothing has happened in the past ten years)

 Posts: 44
 Joined: October 26 2004
 Affiliation: Santa Fe Institute
 Contact:
Modified Theories of Gravity (MoGs)
MoGgies tend to introduce new degrees of freedom; it's why when you try to make the CFT versions of MOND (TeVeS) generally covariant &c., you end up introducing a large number of "dark" fields. On large scales, where 'relativistic' effects are important, this can pretty hard to avoid (this is sometimes seen as a consequence of something called the LovelockGrigore theorem.)
Under a wide set of classical conditions, one can usually sort of separate out the new degrees of freedom into a separate sector, and come up with a new theory that looks pretty much like "regular gravity plus new kinds of matter that also couples to baryons." This is called going from the "Jordan" to the "Einstein" frame. So in many cases an apparent MoG is really a new set of matter fields.
Extensive work on this is done in the context of f(R), which forms a nice class of the simplest (and Ostragradskistable) modifications – definitely Iggy and Wayne's work covers this very clearly. You can also look at the "PPF" ('parametrized postFriedman') and similar efforts. There is really no simple generalization of the "PPN" formalism (for modified gravity in the solar system) to cosmological models.
I think there are ways to get around the "new dof" issue for a consistent phenomenology of modified gravity that doesn't look like 'crazy dark matter' (and I tend to think that these are really cool as toy models of theories that do not have a "classical field theory" limit such as nonlocal, holographic, macroscopic, etc..) Dimitrios, Alan and I have some recent papers on this – http://arxiv.org/abs/0811.3635 & http://arxiv.org/abs/0712.3939 – which also have some references to the other work I've mentioned above. Do drop me an email if you'd like to chat more.
Under a wide set of classical conditions, one can usually sort of separate out the new degrees of freedom into a separate sector, and come up with a new theory that looks pretty much like "regular gravity plus new kinds of matter that also couples to baryons." This is called going from the "Jordan" to the "Einstein" frame. So in many cases an apparent MoG is really a new set of matter fields.
Extensive work on this is done in the context of f(R), which forms a nice class of the simplest (and Ostragradskistable) modifications – definitely Iggy and Wayne's work covers this very clearly. You can also look at the "PPF" ('parametrized postFriedman') and similar efforts. There is really no simple generalization of the "PPN" formalism (for modified gravity in the solar system) to cosmological models.
I think there are ways to get around the "new dof" issue for a consistent phenomenology of modified gravity that doesn't look like 'crazy dark matter' (and I tend to think that these are really cool as toy models of theories that do not have a "classical field theory" limit such as nonlocal, holographic, macroscopic, etc..) Dimitrios, Alan and I have some recent papers on this – http://arxiv.org/abs/0811.3635 & http://arxiv.org/abs/0712.3939 – which also have some references to the other work I've mentioned above. Do drop me an email if you'd like to chat more.

 Posts: 1
 Joined: January 28 2015
 Affiliation: Bu Ali Sina University
Modified Theories of Gravity (MoGs)
I want to constrain my model by COSMOMC but I could not find a manual that can explain me as a very bigginer about the CAMB code , I mean , the variables , subrutines and the structure . my other question is that some patchs are available such as MGCAMB or EFTCAMB do they works for Branse Dicke or I need to change CAMB myself.