 Conformal gravity

Conformal gravity is a generic name for gravity theories which are invariant under conformal transformations in the Riemannian geometry sense; more accurately, they are invariant under Weyl transformations where g_{ab} is the metric tensor and Ω(x) is a function on spacetime.
Contents
Weylsquared theories
The simplest theory in this category has the square of the Weyl tensor as the Lagrangian
where C_{abcd} is the Weyl tensor. This is to be contrasted with the usual Einstein–Hilbert action where the Lagrangian is just the Ricci scalar. The equation of motion upon varying the metric is called the Bach equation,
where R_{ab} is the Ricci tensor. Conformally flat metrics are solutions of this equation.
Since these theories lead to fourth order equations for the fluctuations around a fixed background, they are not manifestly unitary. It has therefore been generally believed that they could not be consistently quantized. This is now disputed ^{[1]}.
Four derivative theories
Conformal gravity is an example of a 4derivative theory. This means that each term in the wave equation can contain up to 4 derivatives. There are pros and cons of 4derivative theories. The pros are that the quantized version of the theory is more convergent and renormalisable. The cons are that there may be issues with causality. A simpler example of a 4derivative wave equation is the scalar 4derivative wave equation:
The solution for this is in a central field of force is:
 Φ(r) = 1 − 2m / r + ar + br^{2}
The first two terms are the same as a normal wave equation. Since this equation is a simpler approximation to Conformal gravity then m corresponds to mass of the central source. The last two terms are unique to 4derivative wave equations. It has been suggested to assign small values to them to account for the galactic acceleration constant (also known as dark matter) and the dark energy constant ^{[2]}.
The main issue with conformal gravity theories, as well as any theory with higher derivatives, is the typical presence of ghosts, which point to instabilities of the quantum version of the theory, although there might be a solution to the ghost problem ^{[3]}.
Conformal Unification to the Standard Model
By adding a suitable gravitational term to the standard model action with gravitational coupling, the theory develops a local conformal (Weyl) invariance in the unitary gauge for the local SU(2). The gauge is fixed by requiring the Higgs scalar to be a constant. This mechanism generates the masses for the vector bosons and matter fields with no physical degrees of freedom for the Higgs. The gravity sector can be decoupled in the weak field limit and the resulting quantum field theory can be argued renormalizable in the unbroken SU(2) phase.^{[4]}^{[5]}
See also
References
 E.S. Fradkin and A.A. Tseytlin (1985). "Conformal Supergravity". Phys. Rept. 119 (4–5): 233–362. Bibcode 1985PhR...119..233F. doi:10.1016/03701573(85)901383.
 Conformal gravity on arxiv.org
 ^ Mannheim, Philip D. (20070716). "Conformal Gravity Challenges String Theory". PASCOS07, Imperial College London, July 2007. 0707. pp. 2283. arXiv:0707.2283. Bibcode 2007arXiv0707.2283M
 ^ Mannheim, Philip D. (20050801). "Alternatives to Dark Matter and Dark Energy". Prog.Part.Nucl.Phys. 56 (2): 340. arXiv:astroph/0505266. Bibcode 2006PrPNP..56..340M. doi:10.1016/j.ppnp.2005.08.001.
 ^ Mannheim, Philip D. (20060906). "Solution to the ghost problem in fourth order derivative theories". Found.Phys. 37 (4–5): 532. arXiv:hepth/0608154. Bibcode 2007FoPh...37..532M. doi:10.1007/s1070100791197.
 ^ Montag, J. Lee (1992), "Spontaneously Broken Conformal Symmetry and the Standard Model", Unpublished Research. http://mysite.verizon.net/lmontag/sitebuildercontent/sitebuilderfiles/conformal_standard_model.pdf
 ^ Pawlowski, M.; Raczka, R. (1994), "A Unified Conformal Model for Fundamental Interactions without Dynamical Higgs Field", Foundations of Physics 24 (9): 1305–1327, arXiv:hepth/9407137, Bibcode 1994FoPh...24.1305P, doi:10.1007/BF02148570
Theories of gravitation  Classical theories of gravitation
 Conformal gravity
 f(R) gravity
 Scalar theories
 Scalartensor theories
 Bimetric theories
 Einstein–Cartan
 Whitehead's theory
 Nonsymmetric gravitation
 Scalartensorvector
 Tensorvectorscalar
 Superfluid vacuum theory
 Logarithmic BEC vacuum
 Teleparallelism
 Geometrodynamics
 Quantum gravity
 Semiclassical gravity
 Twistor theory
 Causal dynamical triangulation
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 Induced gravity
 Causal sets
 Noncommutative geometry
 Canonical GR
 Wheeler–DeWitt eqn
 Loop quantum gravity
 Superfluid vacuum theory
 Logarithmic BEC vacuum
Other
 Higherdimensional GR
 Kaluza–Klein
 DGP model
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 Lovelock theory of gravity
 Lower dimensional gravity
 2+1D topological gravity
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 Jackiw–Teitelboim gravity
 Liouville gravity
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