List of Footnotes

1		In the case of ‘discretization’ or ‘deconstruction’ the higher dimensional approach has been useful only ‘after the fact’. Unlike for DGP, in massive gravity the extra dimension is purely used as a mathematical tool and the formulation of the theory was first performed in four dimensions. In this case massive gravity is not derived per se from the higher-dimensional picture but rather one can see how the structure of general relativity in higher dimensions is tied to that of the mass term.
2		The equation of motion with respect to $χ$ gives $□&tidle;χ = 0$ , however this should be viewed as a dynamical relation for $&tidle;χ$ , which should not be plugged back into the action. On the other hand, when deriving the equation of motion with respect to $&tidle;χ$ , we obtain a constraint equation for $&tidle;χ$ : $χ&tidle;= 2□χ$ which can be plugged back into the action (and $χ$ is then treated as the dynamical field).
3		This is already a problem at the classical level, well before the notion of particle needs to be defined, since classical configurations with arbitrarily large $ϕ 1$ can always be constructed by compensating with a large configuration for $ϕ 2$ at no cost of energy (or classical Hamiltonian).
4		In this review, the notion of fully non-linear coordinate transformation invariance is equivalent to that of full diffeomorphism invariance or covariance.
5		Up to other Lovelock invariants. Note however that $f(R )$ theories are not exceptions, as the kinetic term for the spin-2 field is still given by $√ --- − gR$ . See Section 5.6 for more a more detailed discussion in the case of massive gravity.
6		Strictly speaking, the notion of spin is only meaningful as a representation of the Lorentz group, thus the theory of massive spin-2 field is only meaningful when Lorentz invariance is preserved, i.e., when the reference metric is Minkowski. While the notion of spin can be extended to other maximally symmetric spacetimes such as AdS and dS, it loses its meaning for non-maximally symmetric reference metrics $fμν$ .
7		This procedure can of course be used for any reference metric, but it fails in identifying the proper physical degrees of freedom when dealing with a general reference metric. See Refs. [145 , 154 ] as well as Section 8.3.5 for further discussions on that point.
8		In the normal branch of DGP, this brane-bending mode turns out not to be normalizable. The normalizable brane-bending mode which is instead present in the normal branch fully decouples and plays no role.
9		Note that in DGP, one could also consider a smooth brane first and the results would remain unchanged.
10		The local gauge invariance associated with covariance leads to $d$ first class constraints which remove $2d$ degrees of freedom, albeit in phase space. For global symmetries such as Lorentz invariance, there is no first-class constraints associated with them, and that global symmetry only removes $d(d− 1)∕2$ degrees of freedom. Technically, the counting should be performed in phase space, but the results remains the same. See Section 7.1 for a more detailed review on the counting of degrees of freedom.
11		Discretizing at the level of the metric leads to a mass term similar to (2.83 *) which as we have seen contains a BD ghost.
12		This special fully non-linear and Lorentz invariant theory of massive gravity, which has been proven in all generality to be free of the BD ghost in [295 , 296 ] has since then be dubbed ‘dRGT’ theory. To avoid any confusion, we thus also call this ghost-free theory of massive gravity, the dRGT theory.
13		The analysis performed in Ref. [95 *] was unfortunately erroneous, and the conclusions of that paper are thus incorrect.
14		In the previous section we obtained directly a theory of massive gravity, this should be seen as a trick to obtain a consistent theory of massive gravity. However, we shall see that we can take a decoupling limit of bi- (or even multi-)gravity so as to recover massive gravity and a decoupled massless spin-2 field. In this sense massive gravity is a perfectly consistent limit of bi-gravity.
15		The field redefinition is local so no new degrees of freedom or other surprises hide in that field redefinition.
16		See Refs. [169 , 434 , 242 , 340 ] for additional work on deconstruction in five-dimensional AdS, and how this tackles the strong coupling issue.
17		Some dofs may ‘accidentally’ disappear about some special backgrounds, but dofs cannot disappear non-linearly if they were present at the linearized level.
18		More recently, Alexandrov impressively performed the full analysis for bi-gravity and massive gravity in the vielbein language [15 *] determining the full set of primary and secondary constraints, confirming again the absence of BD ghost. This resolves the potential sources of subtleties raised in Refs. [96 , 351 , 349 , 348 ].
19		We stress that multiplying with the matrix $λ$ is not a projection, the equation (7.52 *) contains as much information as the equation of motion with respect to $λ$ , multiplying the with the matrix $λ$ on both sides simply make the rank of the equation more explicit.
20		If only $M$ vielbein of the $N$ vielbein are interacting there will be $(N − M + 1)$ copies of diffeomorphism invariance and $M − 1$ additional Hamiltonian constraints, leading to the correct number of dofs for $(N − M +1)$ massless spin-2 fields and $M − 1$ massive spin-2 fields.
21		Technically, only one of them generates a first class constraint, while the $N − 1$ others generate a second-class constraint. There are, therefore, $(N − 1)$ additional secondary constraints to be found by commuting the primary constraint with the Hamiltonian, but the presence of these constraints at the linear level ensures that they must exist at the non-linear level. There is also another subtlety in obtaining the secondary constraints associated with the fact that the Hamiltonian is pure constraint, see the discussion in Section 7.1.3 for more details.
22		This is actually precisely the way ghost-free massive gravity was originally constructed in [137 , 144 ].
23		The non-renormalization theorem protects the parameters $α$ ’s and the mass from acquiring large quantum corrections [140 , 146 ] and it would be interesting to understand their implications in the case of a mass-varying gravity.
24		Note that the Vainshtein mechanism does not occur for all parameters of the theory. In that case the massless limit does not reproduce GR.
25		This result has been checked explicitly in Ref. [146 *] using dimensional regularization or following the log divergences. Taking power law divergences seriously would also allow for a scalings of the form $4 n n (ΛCutoff∕M Pl)h$ , which are no longer suppressed by the mass scale $m$ (although the mass scale $m$ would never enter with negative powers at one loop.) However, it is well known that power law divergences cannot be trusted as they depend on the measure of the path integral and can lead to erroneous results in cases where the higher energy theory is known. See Ref. [84 ] and references therein for known examples and an instructive discussion on the use and abuses of power law divergences.
26		If $c3 = 0$ , we can easily generalize the background solution to find other configurations that admit a superluminal propagation.
27		We thank the authors of [177 , 178 ] for pointing this out.
28		This is not to say that perturbations and/or perturbativity do not break down earlier in the quartic Galileon, see for instance Section 11.4 below as well as [88 , 58 ], which is another sign that the Vainshtein mechanism works better in that case.
29		The minimal model does not have a Vainshtein mechanism [435 ] in the static and spherically symmetric configuration so in the limit $α3 → − 1∕3$ , or equivalently $α → 0$ , we indeed expect an order one correction.
30		In taking this limit, it is crucial that the second metric $fμν$ be written in a locally inertial coordinate system, i.e., a system which is locally Minkowski. Failure to do this will lead to the erroneous conclusion that massive gravity on Minkowski is not a limit of bi-gravity.
31		In the context of DGP, the Friedmann equation was derived in Section 4.3.1 from the full five-dimensional picture, but one would have obtained the correct result if derived instead from the decoupling limit. The reason is the main modification of the Friedmann equation arises from the presence of the helicity-0 mode which is already captured in the decoupling limit.
32		See [478 *] for a recent review and more details. The convention on the parameters $bn$ there is related to our $βn$ ’s here via $bn = − 1(4 − n )!βn 4$ .
33		Notice that this is not an issue in massive gravity with a flat reference metric since the analogue Friedmann equation does not even exist.
34		Notice that even if massive gravity is formulated without the need of a reference metric, this does not change the fact that one copy of diffeomorphism invariance in broken leading to additional degrees of freedom as is the case in new massive gravity.
35		See also [335 , 195 , 49 , 50 , 51 ] for other ghost-free non-local modifications of gravity, but where the graviton is massless.

	Abstract
1	Introduction
2	Massive and Interacting Fields
	2.1	Proca field
	2.2	Spin-2 field
	2.3	From linearized diffeomorphism to full diffeomorphism invariance
	2.4	Non-linear Stückelberg decomposition
	2.5	Boulware–Deser ghost
I	Massive Gravity from Extra Dimensions
3	Higher-Dimensional Scenarios
4	The Dvali–Gabadadze–Porrati Model
	4.1	Gravity induced on a brane
	4.2	Brane-bending mode
	4.3	Phenomenology of DGP
	4.4	Self-acceleration branch
	4.5	Degravitation
5	Deconstruction
	5.1	Formalism
	5.2	Ghost-free massive gravity
	5.3	Multi-gravity
	5.4	Bi-gravity
	5.5	Coupling to matter
	5.6	No new kinetic interactions
II	Ghost-free Massive Gravity
6	Massive, Bi- and Multi-Gravity Formulation: A Summary
7	Evading the BD Ghost in Massive Gravity
	7.1	ADM formulation
	7.2	Absence of ghost in the Stückelberg language
	7.3	Absence of ghost in the vielbein formulation
	7.4	Absence of ghosts in multi-gravity
8	Decoupling Limits
	8.1	Scaling versus decoupling
	8.2	Massive gravity as a decoupling limit of bi-gravity
	8.3	Decoupling limit of massive gravity
	8.4	$Λ3$ -decoupling limit of bi-gravity
9	Extensions of Ghost-free Massive Gravity
	9.1	Mass-varying
	9.2	Quasi-dilaton
	9.3	Partially massless
10	Massive Gravity Field Theory
	10.1	Vainshtein mechanism
	10.2	Validity of the EFT
	10.3	Non-renormalization
	10.4	Quantum corrections beyond the decoupling limit
	10.5	Strong coupling scale vs cutoff
	10.6	Superluminalities and (a)causality
	10.7	Galileon duality
III	Phenomenological Aspects of Ghost-free Massive Gravity
11	Phenomenology
	11.1	Gravitational waves
	11.2	Solar system
	11.3	Lensing
	11.4	Pulsars
	11.5	Black holes
12	Cosmology
	12.1	Cosmology in the decoupling limit
	12.2	FLRW solutions in the full theory
	12.3	Inhomogenous/anisotropic cosmological solutions
	12.4	Massive gravity on FLRW and bi-gravity
	12.5	Other proposals for cosmological solutions
IV	Other Theories of Massive Gravity
13	New Massive Gravity
	13.1	Formulation
	13.2	Absence of Boulware–Deser ghost
	13.3	Decoupling limit of new massive gravity
	13.4	Connection with bi-gravity
	13.5	3D massive gravity extensions
	13.6	Other 3D theories
	13.7	Black holes and other exact solutions
	13.8	New massive gravity holography
	13.9	Zwei-dreibein gravity
14	Lorentz-Violating Massive Gravity
	14.1	SO(3)-invariant mass terms
	14.2	Phase $m1 = 0$
	14.3	General massive gravity ( $m0 = 0$ )
15	Non-local massive gravity
16	Outlook
	Acknowledgments
	References
	Footnotes
	Figures