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Lorenzo Sebastiani, Sunny Vagnozzi, Ratbay Myrzakulov, "Mimetic Gravity: A Review of Recent Developments and Applications to Cosmology and Astrophysics", Advances in High Energy Physics, vol. 2017, Article ID 3156915, 43 pages, 2017. https://doi.org/10.1155/2017/3156915
Mimetic Gravity: A Review of Recent Developments and Applications to Cosmology and Astrophysics
Mimetic gravity is a Weyl-symmetric extension of General Relativity, related to the latter by a singular disformal transformation, wherein the appearance of a dust-like perfect fluid can mimic cold dark matter at a cosmological level. Within this framework, it is possible to provide a unified geometrical explanation for dark matter, the late-time acceleration, and inflation, making it a very attractive theory. In this review, we summarize the main aspects of mimetic gravity, as well as extensions of the minimal formulation of the model. We devote particular focus to the reconstruction technique, which allows the realization of any desired expansionary history of the universe by an accurate choice of potential or other functions defined within the theory (as in the case of mimetic gravity). We briefly discuss cosmological perturbation theory within mimetic gravity. As a case study within which we apply the concepts previously discussed, we study a mimetic Hořava-like theory, of which we explore solutions and cosmological perturbations in detail. Finally, we conclude the review by discussing static spherically symmetric solutions within mimetic gravity and apply our findings to the problem of galactic rotation curves. Our review provides an introduction to mimetic gravity, as well as a concise but self-contained summary of recent findings, progress, open questions, and outlooks on future research directions.
The past decade has seen the astounding confirmation of the “dark universe” picture, wherein the energy budget of our universe is dominated by two dark components: dark matter and dark energy [1–50]. The race to determine the nature and origin of these components is in progress on both the observational and theoretical fronts. Theories of modified gravity appear quite promising in this respect, particularly given that gravity remains the least understood of the four fundamental forces. For an incomplete list of comprehensive reviews, as well as seminal works on the subject, we refer the reader to [51–68] and references therein.
A particularly interesting theory of modified gravity which has emerged in the past few years is mimetic gravity . In mimetic gravity, as well as minimal modifications thereof, it is possible to describe the dark components of the universe as a purely geometrical effect, without the need of introducing additional matter fields. In the past three years, interest in this theory has grown rapidly, with over 90 papers following up on the original idea or at least touching on it in some way [70–161]. For this reason, we believe it is timely to present a review on the progress achieved thus far in the field of mimetic gravity. This review is not intended to provide a detailed pedagogical introduction to mimetic gravity but rather summarize the main findings and directions in current research on the theory, as well as providing useful directions to the reader, should she/he wish to deepen a given topic in mimetic gravity. For this reason, this review should not be seen as a complete introduction to mimetic gravity, nor should it substitute consultation of the original papers. No prior knowledge on the subject is assumed.
This review is structured as follows: in this section we will provide a historical and technical introduction to modified gravity, which shall justify our subsequent endeavour in mimetic gravity. In Section 2, we will introduce mimetic gravity. Given that understanding the reason behind the equations of motion of mimetic gravity differing from those of General Relativity is not obvious, a major goal of the section will be to provide a clear and concise explanation for this fact. Subsequently, in Section 3 we will explore a few solutions of mimetic gravity and correspondingly some extensions of the theory, as well as correspondences with related theories of modified gravity. Section 4 will provide a brief interlude focusing on perturbations in mimetic gravity. In Section 5 we will present a case study of a mimetic-like model, namely, mimetic covariant Hořava-like gravity, with a focus on its solutions and cosmological perturbations, and the need to extend the model beyond its basic formulation. Section 6 will be devoted to studying spherically symmetric solutions in mimetic gravity. In Section 7 we will touch upon the issue of rotation curves in mimetic gravity and how this issue is addressed. Finally, we will conclude in Section 8.
1.1. Why Modify General Relativity?
General Relativity (GR henceforth), first formulated by Einstein in 1915 [162–166] (for a pedagogical review, see, e.g., ), is an extremely successful and predictive theory, and together with Quantum Field Theory forms one of the pillars of modern physics. The traditional picture of GR is a geometrical one, with the theory being one of space-time and its metric. A more modern view is free of geometrical concepts and sees GR as the unique theory of massless spin-2 particles.
Confirmations of GR abound (see, e.g.,  for a complete review), ranging from gravitational lensing  to the precession of Mercury’s orbit . Shortly after its centennial in 2015 one of the pillars of GR, the existence of gravitational waves, was grandiosely confirmed by the detection of GW150914  and GW151226  by LIGO (see also ). Before we even embark on a review of mimetic gravity, then, it is worth reminding the reader why one should even consider questioning a theory as successful as GR. Aside from the philosophical perspective that questioning theories and exploring other approaches are a sensible route in science, provided of course that there is agreement with observations, hints persist in the literature that complicating the gravitational action may indeed have its merits. In fact, the reader should be reminded that as early as 1919 (four years after GR had been formulated), proposals started to be put forward as to how to extend this theory, notably, in the form of Weyl’s scale independent theory  and Eddington’s theory of connections . These early attempts to modify GR were driven solely by scientific curiosity with no formal theoretical, or let alone experimental, motivation.
Nonetheless, theoretical motivation for modifying the gravitational action came quite soon. The underlying reason is that GR is nonrenormalizable and thus not quantizable in the way conventional Quantum Field Theories are quantized. However, it was proven that 1-loop renormalization requires the addition of higher order curvature terms to the Einstein-Hilbert action. In fact, it was later demonstrated that, while actions constructed from invariants quadratic in curvature are renormalizable , the addition of higher order time derivatives which follows from the addition of terms higher order in the curvature leads to the appearance of ghost degrees of freedom, which entail a loss of unitarity. More recent results show that when quantum corrections or string theory are taken into account, the effective low-energy gravitational action admits higher order curvature invariants (see, e.g.,  for a general review). However, all these early attempts to modify GR had a common denominator in the fact that terms which modified the gravitational action were only considered to be relevant in proximity of the Planck scale, thus not affecting the late universe.
With the emergence of the “dark universe” picture in recent years, the limits of GR have been fully exposed, and further motivations to modify this theory have emerged. A series of experiments and surveys, including but not limited to CMB experiments, galaxy redshift surveys, cluster surveys, supernovae surveys, lensing experiments, and quasar surveys, have depicted a peculiar picture of our universe [1–50]. This scenario suggests that our näive picture of the world we live in being described by the Standard Model of Particle Physics (SM) supplemented by General Relativity is, at best, incomplete. The concordance cosmology model suggests a scenario where only ~4% of the energy budget of the universe consists of baryonic matter, whereas ~24% consists of nonbaryonic dark matter (DM), and the remaining ~76% consists of dark energy (DE). Of the last two extra components, dark matter is (presumably) the one with properties most similar to ordinary matter. It shares the clustering properties of ordinary matter, but not its couplings to SM gauge bosons (e.g., electromagnetic ones), and is believed to be responsible for structure formation during the matter domination era of the cosmological history. As ordinary matter, DM satisfies the strong energy condition. Dark energy, instead, is more peculiar still, given that it does not share the clustering properties of ordinary matter or DM, as it violates the strong energy condition. It is believed to be responsible for the late-time speed-up of the universe, which has been inferred from a variety of cosmological and astrophysical observations, ranging from type Ia-supernovae to the CMB. Whereas evidence for DE is somewhat indirect and exclusively of cosmological origin, clues as to the existence of DM are instead present on a wide variety of scales, from cosmological to astrophysical (galactic and subgalactic) ones. For technical reviews on DM, see, for instance, [177–181]; for similar reviews on DE, see, for instance, [182–186].
The late-time acceleration of our universe, however, is most likely not the only period of accelerated expansion that our universe has experienced. A period of accelerated (exponential) expansion during the very early universe, prior to the conventional radiation and matter domination epochs, is required to solve the horizon, flatness, and monopole problems. This period of accelerated expansion is known as inflation (see, e.g., [187–196] for pioneering work), and a vast class of models attempting to reproduce such period exists in the literature (for an incomplete list, see, e.g., [197–206] and references therein, see also, e.g., [207–231] for more recent inflationary model-building which is extremely closely relevant to mimetic gravity and variations of it). For reviews on inflation, see, for instance, [232–236]. Inflation also purports to be the mechanism generating primordial inhomogeneities which are quantum in origin [237, 238], providing the seeds which grow under gravitational instability to form the large-scale structure of the universe. The fact that the universe presumably undergoes acceleration at both early and late times or, equivalently, at high and low curvature is very puzzling and might be hinting to a more profound structure.
It thus appears that concordance cosmology requires at least three extra (possibly dark) cosmological components: one or more dark matter components, some form of dark energy, and one or more inflaton fields. There is no shortage of ideas as to what might be the nature of each of these components. Nonetheless, adding these three or more components opens another set of questions, which include but are not limited to the compatibility with the current SM and the consistency of formulation. On the other hand, gravity is the least understood of the four fundamental interactions and the most relevant one on cosmological and astrophysical scales. If so, it could be that our understanding of gravity on these scales is inadequate or incomplete, and modifying our theory of gravitation could indeed be the answer to the dark components of the universe. One could argue that this solution is indeed more economical and possibly the one to pursue in the spirit of Occam’s razor. In other words, modifications to Einstein’s theory of General Relativity might provide a consistent description of early and late-time acceleration and of the dark matter which appears to pervade the universe. Modified theories of gravity not only can provide a solution to the “dark universe riddle” but also possess a number of alluring features such as unification of the various epochs of acceleration and deceleration (matter domination) of the universe’s evolution, transition from nonphantom to phantom phase being transient (and thus without Big Rip), solution to the coincidence problem, and also interesting connections to string theory.
Having presented some motivation to modify our theory of gravitation, we now proceed to briefly discuss systematic ways by means of which this purpose can be achieved.
1.2. How to Modify General Relativity?
Essentially all attempts to modify General Relativity are guided by Lovelock’s theorem . Lovelock’s theorem states that the only possible second-order Euler-Lagrange expression obtainable in 4D space from a scalar Lagrangian density of the form , where is the metric tensor, is given by the following:where and are constants and and are the Ricci tensor and scalar, respectively. It follows that constructing metric theories of gravity whose equations differ from those of GR requires at least one of the following to be satisfied:(i)Presence of other fields apart from or in lieu of the metric tensor(ii)Work in a number of dimensions different from 4(iii)Accept metric derivatives of degree higher than 2 in the field equations(iv)Giving up locality or Lorentz invariance
Therefore, we can imagine broadly classifying the plethora of modified gravity theories according to which of the above assumptions is broken.
1.2.1. New Degrees of Freedom
Relaxing the first assumption leads to what is probably the largest class of modified gravity theories. Theories corresponding to the addition of scalar degrees of freedom include quintessence (e.g., [240–242]) and coupled quintessence (e.g., ) theories, the Chern-Simons theory (e.g., ), Cuscuton cosmology (e.g., [245–247]), Chaplygin gases (e.g., [248, 249]), torsion theories such as theories (e.g., [250–253], see also, e.g., [254–264] for recent work) or the Einstein-Cartan-Sciama-Kibble theory (e.g., [265–268]), scalar-tensor theories (e.g., ) such as the Brans-Dicke theory , ghost condensates (e.g., ), galileons (e.g., [272, 273]), KGB , Horndeski’s theory , and many others.
One can instead choose to add vector degrees of freedom, as in the case of the Einstein-aether theory (e.g., [276–280]). The addition of a vector field leads to the introduction of a preferred direction in space-time, which entails breaking Lorentz invariance.
Theories where tensor degrees of freedom are added include Eddington-Born-Infeld gravity (e.g., [281–284]) and bimetric MOND (e.g., [285, 286]) among others. TeVeS (tensor vector scalar gravity, ) is instead a theory which features the addition of all three types of degrees of freedom together.
Broadly speaking, mimetic gravity belongs to the class of theories of modified gravity where an additional scalar degree of freedom is added. Caution is needed with this identification though because, as we shall see later, mimetic gravity does not possess a proper scalar degree of freedom, but rather a constrained one.
1.2.2. Extra Dimensions
Relaxing the second assumption instead brings us to consider models with extra dimensions, the prototype of which is constituted by Kaluza-Klein models (e.g., [288–290]). Models of modified gravity with extra dimensions abound when considering string theory, including Randall-Sundrum I  and II  models, Einstein-Dilaton-Gauss-Bonnet gravity (e.g., [293, 294]), cascading gravity (e.g., [295–298]), and Dvali-Gabadadze-Porrati gravity (e.g., [299, 300]). Another interesting theory that falls within the extra dimension category is represented by 2T gravity .
1.2.3. Higher Order
The most famous and studied example of a theory falling within this category is undoubtedly represented by gravity (, see also, e.g., [60, 65–67, 303–309] or  for black holes phenomenology). In fact, unification of inflation and late-time acceleration was proposed in the context of gravity in [303, 305, 311–313]. Belonging to the same family are also variations of the former such as , , , gravity (see, e.g., [132, 314–323]), but also Gauss-Bonnet (see, e.g., [324–334]) and conformal gravity (see, e.g., [335–337]). Another well-known theory which lies within this family is represented by Hořava-Lifshitz gravity (, see also [339–363], which also violates Lorentz invariance explicitly), and correspondingly the vast category of Hořava-like theories, including those which break Lorentz invariance dynamically (e.g., [356, 361, 362, 364–372]).
If we choose to relax the assumption of locality (we have already seen cases where the assumption of Lorentz invariance is relaxed above), we can consider nonlocal gravity models whose action contains the inverse of differential operators of curvature invariants, such as and gravity (e.g., [373–377]). Some degravitation scenarios belong to this family as well (e.g., [378, 379]).
Broadly speaking, mimetic gravity belongs to the class of theories of modified gravity where an additional scalar degree of freedom is added. Caution is needed with this identification though because mimetic gravity does not possess a proper scalar degree of freedom. Instead, the would-be scalar degree of freedom is constrained by a Lagrange multiplier, which kills all higher derivatives. As such, the mimetic field cannot have oscillating solutions and the sound speed satisfies , confirming that there is no propagation of scalar degrees of freedom (however, this is true only in the original mimetic model but does not necessarily hold in extensions thereof). Furthermore, the same Lagrange multiplier term introduced a preferred foliation of space-time, which breaks Lorentz invariance (although this is preserved at the level of the action). These aspects of mimetic gravity will be discussed in more detail in the following section.
2. Mimetic Gravity
The expression “mimetic dark matter” was first coined in a 2013 paper by Chamseddine and Mukhanov  although, as we shall see later, the foundation for mimetic theories had actually been developed a few years earlier in three independent papers [380–382]. In , the proposed idea is to isolate the conformal degree of freedom of gravity by introducing a parametrization of the physical metric in terms of an auxiliary metric and a scalar field , dubbed mimetic field, as follows:From (2) it is clear that, in such a way, the physical metric is invariant under conformal transformations of the auxiliary metric of the type , being a function of the space-time coordinates. It is also clear that, as a consistency condition, the mimetic field must satisfy the following constraint:Thus, the gravitational action, taking into account the reparametrization given by (2) now takes the form:where is the space-time manifold, is the Ricci scalar, is the matter Lagrangian, and is the determinant of the physical metric.
By varying the action with respect to the physical metric one obtains the equations for the gravitational field. However, this process must be done with care, for the (variation of the) physical metric can be written in terms of the (variation of the) auxiliary metric and the (variation of the) mimetic field. Taking this dependency into account, variation of the action with respect to the physical metric yields where is the Einstein tensor, with being the Ricci tensor, while and are the trace of Einstein’s tensor and the stress-energy tensor of matter, respectively. Notice that the auxiliary metric does not enter the gravitational field equation explicitly, but only through the physical metric, whereas the mimetic field enters explicitly. In fact, the mimetic field contributes to the right-hand side of Einstein’s equation through an additional stress-energy tensor component:We note that both energy-momentum tensors, and , are covariantly conserved, that is, (with the covariant derivative), whereas the continuity equation for with the mimetic constraint (3) leads toFinally, the trace of (5) is found to beIt is clear that the above is automatically satisfied if one takes into account (3) even if . Thus, the theory admits nontrivial solutions and the conformal degree of freedom becomes dynamical even in the absence of matter ( but ) .
Let us examine the structure of the mimetic stress-energy tensor. Recall that the stress-energy tensor of a perfect fluid whose energy density is and pressure is given byNotice that the mimetic stress-energy tensor in (6) assumes the same form of the one of a perfect fluid with pressure and energy density , while the gradient of the mimetic field, , plays the role of 4-velocity. Thus, the mimetic fluid is pressureless, suggesting it could play the role of dust in a cosmological setting. To confirm whether the mimetic fluid can indeed play the role of dust, it is necessary to investigate cosmological solutions. In fact, this is easy to do on a Friedmann-Lemaître-Robertson-Walker (FLRW) setting, with a metric of the form:where is the scale factor. If we take the hypersurfaces of constant time to be equal to those of constant , we immediately see that the constraint equation (3) is automatically satisfied if the mimetic field is identified with time up to an integration constant (which we arbitrarily set to 0). Thus, the mimetic field plays the role of “clock” on an FLRW background. It is then easy to show that (7) implies that , which corresponds to the energy density of the mimetic stress-energy tensor, decays with the scale factor of the FLRW universe as . Recall that the energy density of a component with equation of constant state parameter evolves as in an FLRW universe, and hence the evolution in the energy density of the mimetic field corresponds to , namely, the equation of state for dust. In other words, the conformal degree of freedom of gravity can mimic the behaviour of dark matter at a cosmological level, hence the name “mimetic dark matter” .
Lagrange Multiplier Formulation. Before further discussing some fundamental aspects of mimetic dark matter (or mimetic gravity henceforth), such as the reason behind the different solutions from GR despite the seemingly innocuous parametrization given by (2), let us discuss an alternative but equivalent formulation of mimetic gravity. The equations of motion obtained from the action written in terms of the auxiliary metric are equivalent to those one would conventionally obtain from the action expressed in terms of the physical metric with the imposition of an additional constraint on the mimetic field. This suggests that the mimetic constraint given by (3) can actually be implemented at the level of the action by using a Lagrange multiplier. That is, the action for mimetic gravity (4) can be written asVariation of the Lagrangian with respect to the Lagrange multiplier field leads to (3), while variation with respect to the physical metric yieldswhose trace, when one takes into account (3), is given byThus, one recovers (5) again. In this review, we will always make use of the action given by (11) to introduce the mimetic field.
A remark is in order here. Actions such as (11) had actually been introduced three years before the term “mimetic dark matter” was first coined. Three independent papers in 2010, by Lim et al. , Gao et al. , and Capozziello et al. , respectively, presented models with two additional scalar fields, one of which playing the role of Lagrange multiplier enforcing a constraint on the derivative of the other (see also  for recent work on the role of Lagrange multiplier constrained terms in cosmology). In fact, it was shown that these types of models can produce a unified theory describing dark energy and dark matter, because the term inside the Lagrange multiplier can always be arranged in such a way to reproduce the conventional expansion history of CDM. Thus, it is fair to state that mimetic theories had actually seen birth prior to the 2013 paper by Chamseddine and Mukhanov.
2.1. Understanding Mimetic Gravity
Before we can make further progress in exploring solutions in mimetic gravity, generalizing the theory, or studying connections to other theories, we need to touch on two very important points: first, why the seemingly innocuous parametrization given by (5) has led to a completely new class of solutions not contemplated by GR, and second, whether the theory is stable or not. As we shall see, the first point can ultimately be explained in terms of singular disformal transformations.
It might appear puzzling at first that, only by rearranging parts of the metric, one is faced with a different model altogether. A first explanation appeared in , which explained this property in terms of variation of the action taking place over a restricted class of functions. This is the case in mimetic gravity, precisely because the consistency equation (3) enforces an additional condition for any admissible variation of the action, in particular demanding thatThus, for a spatially homogeneous mimetic field , . Varying over a restricted class of functions now provides less conditions for the stationarity of the action and hence more freedom in the dynamics. This is a well-known property relevant when one makes derivative substitutions into an action : the class of functions over which the variation is admissible does not only comprise those for which the variation of is vanishing at the boundary but also those where the integral of the variation of is zero. This extra restriction leads to less conditions for stationarity of the action and the appearance of additional dynamics with respect to the original case.
Another explanation was presented in , which identified mimetic gravity as a conformal (Weyl-symmetric) extension of GR. The first important point to notice is that the parametrization is noninvertible even for fixed , owing to the fact that the map is a map from ten variables to eleven. With this parametrization, the theory is manifestly conformally invariant, that is, invariant with respect to transformations of the auxiliary metric (and correspondingly the action ) of the form:where is a function of the space-time coordinates. Two immediate corollaries of the theory’s conformal invariance are its yielding of identically traceless equations of motion for the gravitational field and requiring conformal gauge fixing. In fact, recall that the equation of motion for the gravitational field (5) is automatically traceless if the consistency condition given by (3) is satisfied. Therefore, we can identify (3) with the conformal gauge condition in the locally gauge-invariant theory with action . Mimetic gravity can thus be seen as a conformal extension of GR which is Weyl-invariant in terms of the auxiliary metric . However, this theory is quite different from the off-shell conformal extensions of GR proposed in  (which preserves GR on-shell but modifies its effective action off-shell); here the gravitational action is already modified at a classical level, by adding an extra degree of freedom provided by a collisionless perfect fluid. This additional degree of freedom, which can mimic collisionless cold dark matter for cosmological purposes, arises from gauging out local Weyl invariance.
2.1.1. Singular Disformal Transformations
As we anticipated above, mimetic gravity and the appearance of the extra degree of freedom which can mimic cosmological dark matter are rooted into the role played by singular disformal transformations. As was shown by Bekenstein , because GR enjoys invariance under diffeomorphisms, one is free to parametrize a metric in terms of a fiducial metric and a scalar field . The map between the two is defined as a “disformal transformation,” or a “disformation,” and takes the following form:where . The functions and are referred to as conformal factor and disformal factor, respectively. In general the functions , are arbitrary, with . It is easy to show that, provided the transformation is invertible, the equations of motion for the theory (obtained by variation of the action with respect to and ) reduce to those of obtained by varying with respect to the metric .
To make progress, it is useful to contract the two equations of motion with and . Although we will not perform the steps explicitly, it is easy to show that this leads to the following two equations of motion:where the two quantities and are defined asThe determinant of system (17) is given byIf the above is nonzero, it is trivial to obtain that the resulting set of equations consists of Einstein’s equation () and a second empty equation. Therefore, the theory does not feature new solutions with respect to GR .
The situation is quite different if the determinant in (19) is zero, which corresponds to the physical case when the disformal transformation given by (16) is noninvertible or singular. In this case, being the function , this immediately determines , which is of the form:where is an arbitrary function. We will not show the steps explicitly, which are instead discussed in detail in Section of , but it is not hard to obtain the equations of motion and notice that they differ from those of GR, due to the presence of an extra term on the right-hand side of Einstein’s equation (i.e., an additional contribution to the stress-energy tensor). Therefore, when the disformal transformation is singular, one is faced with the appearance of extra degrees of freedom which result in equations of motion differing from those of GR .
The parametrization (5) defining mimetic gravity can be identified with a singular disformal transformation, with and in (16), and correspondingly in (20). In general, when the relation defined by (20) exists between the conformal factor and the disformal factor , the resulting disformal transformation is singular and, as a result, the system possesses additional degrees of freedom, explaining the origin of the extra degree of freedom in mimetic gravity which mimics a dust component. This aspect has been at the center of a number of studies recently; see, for instance, [79, 90, 96, 102, 104, 107, 128, 140, 142, 386–389]. Moreover,  has shown that the two approaches towards mimetic gravity (and further extensions to be discussed later, such as mimetic Horndeski theories), namely, Lagrange multiplier (11) and singular disformal transformation (16), are in fact equivalent.
2.1.2. Mimetic Gravity from the Brans-Dicke Theory
There actually exists a third route to mimetic gravity, apart from disformal transformations and Lagrange multiplier, whose starting point is the singular Brans-Dicke theory. Namely, by starting from the action (4) (neglecting matter terms) and by performing the conformal transformation given by (5), we end up with the action :where we have defined . One immediately sees that (21) corresponds to the singular/conformal Brans-Dicke action  with density parameter . Thus, we conclude that a third way of obtaining mimetic gravity is by substituting the kinetic term in lieu of the scalar field in the singular Brans-Dicke action . In case matter fields are included, the substitution has to be performed on the matter part of the Lagrangian as well, which means that matter will not be coupled to but to .
Is mimetic gravity stable? In other words, does its spectrum contemplate the presence of states with negative norm, or fields whose kinetic term has the wrong sign (corresponding to negative energy states), which could possibly destabilize the theory? This is an important question which has yet to find a definitive answer. Recall that the original mimetic theories formulated in 2010 were found to suffer from a tachyonic instability ; therefore, the question of whether mimetic gravity is stable is totally pertinent.
If we formulate the theory of mimetic gravity using the physical metric , we inevitably incur into the risk of the appearance of higher derivatives of the mimetic field, which could entail the emergence of ghosts. Addressing this question requires performing a Hamiltonian analysis of the theory, identifying all constraints and counting the local degrees of freedom. A preliminary analysis of this problem was conducted in , which concluded that the theory is stable if the energy density of the mimetic field is positive. This condition is, of course, easy to understand physically. Moreover, it indicates a preference for de Sitter-type backgrounds with a positive cosmological constant, since in that case both contributions to the energy density, given by curvature and trace of the matter stress-energy tensor, are positive. Therefore, it is presumed that mimetic is gravity is stable provided the time evolution of the system preserves the positivity of the energy density stored in the mimetic degree of freedom. The work of  also identified another possible instability issue, namely, caustic singularities (which are not dangerous at the quantum level, unlike ghost instabilities). These are presumably due to the pressureless nature of the mimetic field and can possibly be circumvented if one modifies the theory with higher derivative terms, as we will discuss later.
The analysis of  imposed the conformal gauge condition (3) prior to proceeding to the canonical formulation. A proper analysis should instead take place in full generality and has been conducted in . This analysis finds that the Hamiltonian constraint depends linearly on the momentum, which in most cases signals that the Hamiltonian density of the theory is unbounded from below, a classical sign of instability. As anticipated, this occurs frequently for higher derivative theories, which are prone to the Ostrogradski instability. The work in  concluded, as , that mimetic gravity is stable as long as the energy density in the dust degree of freedom in the theory remains positive. However, this is not always consistent with the dynamics of the theory, given that for some initial configurations, the energy density could start its evolution with a positive value but then end up with a negative value, which would cause instability. In fact,  finds that the requirement that the theory be stable correspond to the requirement that initial configurations do not cross the surface for which the momentum conjugate to the mimetic field, , satisfies .
A possible solution to these instability issues was presented in  and studied in . The idea is to modify the parametrization (5) by making use, instead of the gradient of a scalar field, of a dynamical vector (Proca) field:The Proca field is made dynamical by adding a Maxwell kinetic term to the action, where is the field-strength tensor of the vector field. It is beyond the scope of our review to provide details of the analysis, conducted in  which finds that the Hamiltonian in the Proca mimetic model shows no sign of instability. Furthermore,  proposes an interesting extension of mimetic gravity to a mimetic tensor vector scalar model, by generalizing (2) towhere now both the scalar and vector degrees of freedom contribute to mimetic matter. It is furthermore demonstrated that the theory is free of ghosts .
Another recent work confirmed in all generality that the original mimetic gravity theory suffers from ghost instability , in the following way. It is immediate to show that mimetic gravity is invariant under the local symmetry defined bywhere as usual and correspond to the conformal and disformal factors. Being and for mimetic gravity, (24) corresponds as expected to invariance of the physical metric under conformal transformations of the auxiliary metric. In the Hamiltonian description, this symmetry is associated with a first class constraint. In fact, one can show that the primary constraint, which corresponds to the generator of infinitesimal conformal transformations, is first class, with its Poisson commuting with the Hamiltonian and momentum constraints. This leaves no place for a secondary constraint which could eliminate the Ostrogradski ghost. Thus, this result confirms indeed that the original mimetic gravity proposal suffers from a ghost instability.
3. Solutions and Extensions of Mimetic Gravity
Having discussed the underlying physical foundation of mimetic gravity, and its stability, we can now proceed to study solutions and extensions of this theory.
3.1. Potential for Mimetic Gravity
Recall that, in a cosmological setting, the mimetic field plays the role of “clock.” Therefore, one can imagine making the mimetic field dynamical by adding a potential for such field to the action. A field-dependent potential corresponds to a time-dependent potential which, by virtue of the Friedmann equation, corresponds to a time-varying Hubble parameter (and correspondingly scale factor). Therefore, by adding an appropriate potential for the mimetic field, one can in principle reconstruct any desired expansion history of the universe. This is the idea behind the minimal extension of mimetic gravity first proposed in . The action of mimetic gravity (in the Lagrange multiplier formulation) is thus extended to include a potential for the mimetic field:The equations of motion of the theory are then given bywhich, by taking the trace, can be used to determine the Lagrange multiplier:When plugged into (26), (27) yieldsOf course, variation with respect to the Lagrange multiplier as usual yields the constraint (3). Thus, when a potential is added to the action, the mimetic field contributes a pressure and energy density of and , respectively . One further equation of motion can be derived by varying the action with respect to the mimetic field, which giveswhen taking into account the expression for the Lagrange multiplier (27).
To study cosmological solutions, it is useful to consider a flat FLRW background (10), since therein the mimetic field can be identified with time. In this case, it is not hard to show that (29) reduces to which can be integrated to givewhereas the pressure remains . The Friedmann equation can instead be manipulated to the form:where as usual the Hubble parameter is defined as . Further progress can be made by performing the substitution , which yields the following equation :It should be noticed that the equations of motion simplify greatly because of the identification of the mimetic field with time on an FLRW background. In this way, the pressure becomes a known function of time and satisfies a linear differential equation. We now proceed to study a few interesting potentials and the corresponding solutions.
Let us consider the following potential :Solving the corresponding (33) and substituting for the scale factor, , yield the following solution for :where is an integration constant. For the solution describes an oscillating flat universe with amplitude of oscillations which grows with time; however, the solution presents singularities and for this reason we will not write it down explicitly . We can furthermore determine the equation of state parameter (EoS) if we recall that the energy density is given by (31) whereas the pressure reads . Explicit calculation gives the following:It is interesting to note that, for , the EoS approaches , that is, a cosmological constant, at late times.
We can consider the case where mimetic matter is a subdominant energy component in the universe, which is instead dominated by another form of matter with EoS . The scale factor in this situation evolves asand hence (31) can be used to deduce the evolution of the energy density of mimetic matter, which decays asGiven that the pressure of mimetic matter reads , we immediately see that the EoS for mimetic matter is , demonstrating that mimetic matter, when subdominant, can imitate the EoS of the dominant energy component . A comment is in order here. Mimetic matter can only be subdominant if . If this condition is not satisfied, mimetic matter will only start imitating the dominant matter component at late times, while acting as a cosmological constant at earlier times.
3.1.2. Power-Law Potential
We can consider an arbitrary power-law potential:for which the solution of (33) can be written in terms of the Bessel functions :For (with corresponding to the case we have studied previously) the limiting behaviour of the scale factor is that of a dust-dominated universe, with EoS . For and (which corresponds to a positive pressure), the corresponding solution is a singular oscillating universe. For and instead, the pressure is negative and hence we expect accelerating solutions . In fact, corresponds to a cosmological constant as expected (the potential is simply a constant), whereas gives an inflationary expansion solution with scale factor:which resembles that of chaotic inflation sourced by a quadratic potential .
3.1.3. Inflation in Mimetic Gravity
One can always reconstruct the appropriate potential for the mimetic field which can provide an inflationary solution. The method is very simple: choose a desired expansion history of the universe [encoded in the Hubble parameter or equivalently in the scale factor ], find the corresponding -parameter through , and then invert (33) to find the corresponding potential which can provide the desired expansion:As an example, we can consider the following potential :whose corresponding solution for the scale factor is exponential () for large negative times and corresponding to EoS (i.e., dust, ) at late times. Thus, we see that with this potential mimetic gravity can provide us with an inflationary solution with graceful exit to the matter dominated era .
Another interesting possibility is given by an exponential potential :In this case, the scale factor can be expressed in terms of Bessel functions of the first and second kinds :where and are integration constants and and are the modified Bessel functions of order zero, of the first and second kinds, respectively. At late times, the behaviour of the scale factor is that of a matter dominated universe, that is, with EoS (). At early times the behaviour of the scale factor depends on the sign of , in particular providing us with an inflationary solution for , whereas the solution is a bouncing nonsingular universe for . A similar behaviour can be obtained if one chooses the potential:that is, a solution with inflation at early times and matter domination at late times. More complicated potentials which can reproduce qualitatively similar behaviours were studied in .
3.1.4. Bouncing Universes in Mimetic Gravity
As we have already seen in previous cases, one can easily construct bouncing solutions in mimetic gravity. Let us work through one further example here. Consider a potential of the form:As usual, the scale factor can be determined by solving (33), which yields If we set the integration constant to 0, the corresponding energy density and pressure (one again, refer to (31) and recall that ) are given byAt very early times (large negative ) the EoS approaches ; the universe is dominated by dust and contracts. At a certain time corresponding to , the energy density drops suddenly to zero, after which the universe begins expanding. During the first instants of the expansion (within one Planckian time), the energy density of the universe is Planckian but subsequently drops as the expansion proceeds as a conventional expansion in a dust-dominated universe. The interesting feature of this potential is that the EoS crosses the phantom divide without singularity. This remains true even in the general case where the integration constant is nonzero, provided .
In the case we have just examined, the bounce occurs at the Planck scale, and hence the classical analysis we provided might not be valid as quantum gravity effects would be playing an important role. However, a minimal modification allows lowering the scale of the bounce and correspondingly increases the duration of the bounce (which now lasts more than a Planckian time). The corresponding potential which can provide this behaviour is given by Although we will not show the solution explicitly, in this case the scale of the bounce is reduced to and the duration of the bounce is now .
3.2. Mimetic Gravity
The next step which was performed by Nojiri and Odintsov is to generalize mimetic gravity to mimetic gravity . In this theory we expect to have two additional degrees of freedom compared to GR: the constrained (nonpropagating) scalar degree of freedom of mimetic gravity and the additional scalar degree of freedom arising from the term. The action of the theory is given by where as usual the relation between the physical and auxiliary metric and the mimetic field is given by (2), and the constraint equation (3) has to be satisfied. Because of this, the action of mimetic gravity can be equally written employing a Lagrange multiplier field analogously to (11) :where the Lagrange multiplier enforces the constraint on the gradient of the mimetic field and in addition we have added a potential for the mimetic field.
The equations of motion of the theory are slightly more complicated than that of conventional mimetic gravity. Varying with respect to the metric gives the gravitational field equations :Variation with respect to the mimetic field instead yields the following equation :As usual, by construction, variation with respect to the Lagrange multiplier gives the mimetic constraint:
As we have mentioned previously, in mimetic gravity one has two additional degrees of freedom. Therefore, by appropriately tuning either or both the potential for the mimetic field, or the form of the function, one can reconstruct basically any desired expansion history of the universe. On the other hand, the interpretation of the cosmological role played by mimetic dark matter remains the same as in mimetic gravity .
Let us proceed to study some of the properties of mimetic gravity in a cosmological setting. As usual, we consider a flat FLRW universe, and we model the matter contribution as that of a perfect fluid with energy density and pressure . Assuming that the mimetic field depends only on time, (53), (54), and (55) can be expressed as follows :The last equation shows that up to an integration constant, which we can set to zero, the mimetic field can be identified with time just as in ordinary mimetic gravity. Thus, (58) can be expressed as follows:which, if we assume that the contribution of ordinary matter is negligible (), reduces toOn the other hand, (57) can be solved for as follows:which shows that (59) is automatically satisfied.
The above equations put on a quantitative footing the statement we previously made: namely, that by tuning the behaviour of either or both the two additional scalar degrees of freedom, we can reconstruct any possible expansion history of the universe . For instance, one can imagine fixing the form of the scalar potential and then reconstruct the form of which gives the wanted evolution [encoded in or, equivalently, ]. Alternatively, one can start from a given form of which might not admit the wanted evolution (e.g., matter dominated-like expansion followed by accelerated expansion) and reconstruct the form of the scalar potential which can allow for such expansion. It should also be remarked that any solution in conventional gravity is also a solution in mimetic gravity, but the converse is not true. In  specific solutions which allow unification of early-time inflation and late-time acceleration with intermediate matter domination era, as well as bouncing Universes, are studied and it is shown that they can be implemented in mimetic gravity. Of course, the exact forms of the mimetic potential or the function in these cases are quite complicated; nonetheless, the study serves as a proof of principle that, in such theories, one can realize any given expansion history of the universe without the need for dark components, which remains the main goal of modified theories of gravity.
Three further recent studies by Odintsov and Oikonomou [112, 120, 130] have demonstrated how one can, in mimetic gravity, realize inflationary cosmologies which are compatible with Planck and BICEP2/Keck Array constraints on the scalar spectral index and on the tensor-to-scalar ratio, and , respectively. Both the reconstruction (determination of the potential once the form of and the evolutionary history of the universe are given) and the inverse reconstruction (determination of the form of once the potential and the evolutionary history of the universe are given) are studied in detail and it is demonstrated that several viable options for realizing inflation compatibly with observational constraints are possible. However, the studies also point out a possible weakness of mimetic gravity in this respect: namely, the forms of both the mimetic potential and the function can become extremely complicated. The forms of both the mimetic potential and the Lagrange multiplier increase in complexity as the complexity of the form increases. Therefore, these and other studies on mimetic gravity and extensions thereof should not be viewed as the ultimate cosmological theory of everything, but rather as a proof of principle that within these theories one can reproduce basically any cosmological scenario and thus solve the “dark universe” problems, although this might come at the cost of sacrificing simplicity.
In addition, [112, 120, 130] also remarked that, although in principle the forms of the mimetic potential and the function can be arbitrary, it must be kept in mind that, in order to realize viable inflation, a mechanism for graceful exit to the conventional radiation dominated era must be achieved. This entails ensuring that the theory contains unstable de Sitter vacua, which eventually becomes the cosmological attractor of the dynamical system. It is precisely the functional form of which has to ensure that graceful exit takes place. Therefore, in mimetic gravity, although in principle the form of the potential is arbitrary, the same cannot be said about the functional form of , which has to be such as to ensure graceful exit from inflation. Therefore, in the interest of simplicity, a practical approach to constructing a minimal model of mimetic gravity with the desired inflationary properties would be to choose the simplest possible functional form of which ensures graceful exit from inflation, then performing the reconstruction technique to determine the form of the mimetic potential which allows the desired expansion history following inflation to be realized. Another possible solution, which we will discuss shortly, is to consider inflation [95, 116], where a dynamical scalar field is coupled to gravity.
3.2.1. Late-Time Evolution in Mimetic Gravity
So far we have discussed mimetic gravity and variants thereof at early times, that is, at the epoch when primordial curvature perturbations were generated. However, it is also interesting to consider late-time evolution in mimetic gravity. The equations of motion are incredibly complex and in principle do not allow for analytical solutions. However, this complexity can be bypassed by means of the method of dynamical analysis (see, e.g., [391–394]), which gives information about the global behaviour of solutions. In particular, one proceeds by transforming the equations of motion into their autonomous form and extract the critical points. Subsequently perturbations are linearized around these critical points and expressed in terms of the perturbation matrix, the eigenvalues of which determine the type and stability of the critical points.
A detailed dynamical analysis of mimetic gravity was presented in . This type of analysis allows us to bypass the complexity of the equations of motion by extracting critical points and studying the corresponding observables, such as the energy densities of the various energy components, the corresponding EoS, and the deceleration parameter. In particular, the analysis finds that the only stable critical points, that is, those that can play the role of attractors at late times, are those that exist in gravity as well. In other words, stable solutions in mimetic gravity can only affect the expansion history of the universe at early and intermediate times, whereas at late times the expansion history has to coincide with that driven by conventional gravity. An immediate implication of this finding is that, although mimetic gravity could drive inflation differently from gravity, the late-time acceleration of the universe in these theories has to coincide with the usual gravity one . However, these conclusions have been reached only by studying the theory at the level of the background. It is expected that different conclusions would be reached if the same study would be performed at the level of perturbations. This is true because the new terms present in the equations of motion of mimetic gravity compared to conventional gravity can contribute to the perturbation equations, although they do not contribute at the background level . Finally, the energy conditions required to avoid the Dolgov-Kawasaki instability in mimetic gravity were studied in , which found that these are the same as in conventional gravity.
As we mentioned above, the conclusions reached about the late-time evolution in mimetic gravity hold only at the background level. However, in conventional gravity, there exists a serious problem during the late-time evolution at the perturbation level, namely, that of dark energy oscillations  (see also, e.g., [396–399]). The degree of freedom associated with the modification of GR (that is, ) leads to high frequency oscillations of the dark energy around the line of the phantom divide during matter era. As a consequence, some derivatives of the Hubble parameter may diverge and become singular, and the solution is unphysical. Usually in conventional gravity the problem is solved by adding power-law modifications by hand.
In , it was instead argued that in mimetic gravity it is possible to overcome the problem by a suitable choice of the potential. By appropriately choosing the potential and the Lagrange multiplier, it is possible to damp the oscillations within a mimetic model whose corresponding conventional model suffered from the oscillations problems. The oscillations die out for redshifts , so there is no issue with dark energy oscillations at our current epoch. Moreover, the values of the dark energy equation of state and the total equation of state are very close to the observed values. The model can in principle be discerned from gravity in that the predicted growth factor is lower in magnitude, a very testable prediction in view of future experiments, further supporting the viability of mimetic gravity as a cosmological framework.
3.2.2. Mimetic Gravity
The question of constructing a theoretically motivated but at the same time simple model for mimetic gravity which provides an early-time inflation epoch, but at the same time graceful exit from the latter, was addressed in . Here, a model of mimetic gravity was considered, where is a scalar field coupled to gravity. We will not go into the details of the work, for which we refer the reader to the original paper . The basic idea is to use the sector to reproduce a variety of cosmological scenarios: among the ones considered in the paper were accelerated cosmologies at high and low curvatures (thus unifying inflation and late-time acceleration), with Einstein gravity at intermediate curvatures. In particular, the accelerated cosmologies are realized by making use of a “switching-on” cosmological constant. The dynamical field evolves in such a way to allow for graceful exit from the inflationary period, thus making the vacua of the first de Sitter period (corresponding to inflation) unstable. Entry into the late-time accelerated epoch, represented by a stable de Sitter attractor, is also made possible by the dynamical field, which thus links all epochs of the expansion history of the universe in a unified way. In the minimal case studied in , the mimetic component ensures the presence of cosmological nonbaryonic dark matter although, as we have extensively discussed, it is possible by adding a suitable potential for the mimetic field to obtain similar solutions, but with a different form of .
3.2.3. Nonlocal Mimetic Gravity
A further extension of mimetic gravity was presented in , which embeds the theory into the framework of nonlocal theories of gravity. Recall that these theories were first presented in , inspired by quantum loop corrections. These theories feature nonlocal operators (i.e., inverse of differential operators) of the curvature invariants. The prototype of nonlocal mimetic gravity is given by the action :where as usual is the mimetic field and the Lagrange multiplier term enforces the constraint on its gradient. It is actually more useful to introduce an additional scalar field , which allows us to translate the action given by (63) to a local scalar-tensor form, as follows :where is an additional Lagrange multiplier which enforces the constraint on the scalar field :Aside from the two constraint equations obtained by varying the action with respect to the Lagrange multipliers, variation of the action with respect to the yields the gravitational field equations :Instead, variation with respect to the two scalar fields leads to the following equations of motion :In , this model was studied in detail making use of the reconstruction technique. In particular, two forms for the function have been studied: exponential and power-law. It was shown that appropriate choices for the mimetic potential, as expected, can give the desired expansion history of the universe, which in the cases studied included viable inflation unified with late-time acceleration with intermediate epoch compatible with Einstein’s gravity and cosmological dark matter provided by the mimetic field, as well as solutions with cosmological bounces.
3.3. Unimodular Mimetic Gravity
As we have seen, mimetic gravity provides a geometric explanation for dark matter in the universe, with dark matter emerging as an integration constant as a result of gauging local Weyl invariance, without the need for additional fluids. An older theory, known as unimodular gravity  (see also [98, 401–419]), had instead been proposed much earlier to solve, in a geometrical fashion as well, one more of the conundrums of modern cosmology: the dark energy problem. In this framework, dark energy emerges in the form of a cosmological constant from the trace-free part of Einstein’s field equations, with the trace-free part which results in turn by enforcing the condition that the square root of (minus) the determinant of the metric is equal to 1, or in general a constant. It would therefore be interesting to combine the two different approaches of mimetic gravity and unimodular gravity into a single framework which could geometrically explain both dark matter and dark energy by a vacuum theory, without need for additional fluids. This is the proposal of Nojiri et al. in .
In order to combine mimetic gravity and unimodular gravity it is necessary to enforce two constraints. The first is the constraint on the gradient of the mimetic field (3), whereas the second is the unimodular constraint:In order to enforce these two constraints, it is conceptually simple to make use of two Lagrange multipliers. This approach has two advantages. First, it keeps the two concepts of mimetic and unimodular gravity separate and facilitates the extraction of physical information. Second, it is as usual more convenient to have the two constraints emerge from the equations of motion. The action for unimodular mimetic gravity thus reads where variation with respect to the Lagrange multiplier enforces the mimetic constraint (3), whereas variation with respect to the Lagrange multiplier enforces the unimodular constraint (68).
The equations of motion for the gravitational field are obtained by varying the action with respect to the metric and are given by :whereas variation with respect to the mimetic field yields the usual equation of motion:Although we do not show the steps explicitly, for which we refer the reader to the original paper , in the usual FLRW setting it is possible to manipulate the Einstein equations in order to get the following reconstruction equation for the mimetic potential (as usual, the mimetic field can be identified with time):The content of the above equation is clear: as in all extensions so far discussed of mimetic gravity, one can always reconstruct the potential for the mimetic field which can provide the desired expansion encoded in or . The reconstruction technique is very powerful although, as we have seen, the corresponding potentials are complicated and somewhat hard to justify from first principles, although the reconstruction technique serves in this case as a proof of principle tool.
Having made this consideration, let us consider a few examples where the reconstruction technique is applied. Let us consider the following simple potential :It can be easily shown that it leads to the following solution for the scale factor and the Hubble parameter :that is, a de Sitter cosmology. The functional forms of the Lagrange multiplier are also quite simple and are given by the following:Another choice for the potential which leads to a physically interesting solution is the following :for which the scale factor and the Hubble parameter readThe above solution is that corresponding to an universe dominated by a fluid with EoS . The two Lagrange multipliers are given byThus, we see that with two relatively simple choices of potential it is possible to reconstruct two important expansion histories of the universe: the late-time de Sitter phase and the expansion dominated by a perfect fluid with arbitrary EoS. Although we will not discuss this case explicitly here, for which instead we redirect the reader to , it is possible by a choice of a more complicated potential, to realize a viable inflationary model within unimodular mimetic gravity, which is compatible with bounds from Planck and BICEP2/Keck Array. Moreover, it has been shown that graceful exit from these types of inflationary periods can be achieved by ensuring that the corresponding de Sitter vacua which drives the period of accelerated expansion is unstable.
Two further comments are in order here. First, it is possible to provide an effective fluid description of unimodular mimetic gravity . Namely, manipulation of the Einstein equations shows that the contribution of the unimodular and mimetic parts of the action can be interpreted as that of a perfect fluid carrying energy density and pressure as follows:where is defined asFurthermore, the effective energy density and pressure defined as per above satisfy the continuity equation.
The second comment is related to the fact that the unimodular constraint is enforced in a noncovariant way (cf. the action given by (69), where one of the terms in is not multiplied by ). It is nonetheless possible to present a covariant formulation of unimodular mimetic gravity via the following action:where is a three-form field, variation of which gives the constraint , implying that the Lagrange multiplier is constant. On the other hand, the covariant version of the unimodular constraint is obtained by variation with respect to , from which one is left withFurther manipulation, for which we refer the reader to , shows that the Friedmann equations one obtains from the covariant version of unimodular mimetic gravity are equivalent to those of the noncovariant version, and thus one can reproduce precisely the same cosmological scenarios in both theories.
3.3.1. Unimodular Mimetic Gravity
A minimal extension of the unimodular mimetic gravity framework we have discussed so far is to consider unimodular mimetic gravity, which is described by the action :As expected, the equations of motion are slightly more complicated than in the unimodular mimetic case, but no conceptual difficulty is added. Specifically, variation with respect to the two Lagrange multipliers enforces the usual unimodular and mimetic constraints, whereas variation with respect to the metric gives rise to the equations for the gravitational field :Finally, variation with respect to the mimetic field yields the following equation:The more complex structure of the equations of motion complicates the reconstruction procedure, that is, the equivalent of (72), which now reads where the function is given byDespite the increased complexity of the equations of motion, the same considerations apply as for unimodular mimetic gravity, as well as all extensions of mimetic gravity hereto considered. Namely, it is always possible to reconstruct any viable cosmological expansion scenario, including unification of inflation and late-time acceleration with intermediate radiation and matter domination, with graceful exit from inflation triggered by unstable de Sitter vacua. This can be achieved by appropriately choosing either or both the form of the mimetic potential or the function . The price to pay would eventually be a considerable complexity in the functional form of both, which of course does not represent a first principle obstacle .
3.4. Mimetic Horndeski Gravity
One can further consider more general scalar-tensor theories, which can be “mimetized” according to the procedures we have described so far, namely, through a singular disformal transformation or through a Lagrange multiplier term in the action enforcing the mimetic constraint. In fact, analogously to GR, one can show that the most general scalar-tensor model is invariant under disformal transformations, provided the latter is invertible. This has been shown in all generality in . One can then “mimetize” such theories by considering the following action :where are integers and is the Lagrangian density which is a function of the metric and the mimetic field. In general, the constraint enforced by the Lagrange multiplier can be generalized to , but for the sake of simplicity we will set here and redirect the reader to the work of  for more general discussions, and for the explicit form of the equations of motion. In fact, setting is basically equivalent to assigning a potential to the mimetic field. In  it was shown that the two approaches to mimetic Horndeski gravity, namely, singular disformal transformation and Lagrange multiplier, are equivalent.
Of course, the considerations made above can be applied in the case of a specific scalar-tensor model, namely, Horndeski gravity  (see, e.g., [420–438] for further recent work on the topic of Horndeski gravity and theories beyond Horndeski). Recall that Horndeski gravity is the most general 4D local scalar-tensor theory with equations of motion no higher than second order. The Horndeski action can be written as a sum of four terms:where s readwhere , , , the functions , , , are free, and denotes differentiation with respect to .
The mimetic version of the above Horndeski model has been studied in a variety of papers recently (e.g., [104, 114, 128]). We report some particular cases taken into consideration. We remark that the freedom in the free functions , , , and , as well as in the function (which, when , is equivalent to providing a potential for the mimetic field), results in the possibility of reproducing basically any given expansion scenario of the universe. A specific model studied in  is one where the functions take the form:In this case, on a flat FLRW background the solution is given bywith and integration constant and . Thus, we see that the scenario under consideration has reproduced the expansion history of a universe filled with a perfect fluid with EoS . Another case considered in  is the mimetic cubic Galileon model, where the functions take the following form:where the cut-off scale is subsequently set to . It is then found that the model can reproduce the expansion history of a universe filled with nonrelativistic matter, followed by a cosmological constant dominated expansion analogous to the late-time acceleration we are experiencing . The case of a nonminimal coupling to the auxiliary metric, was examined as well, which we will not report on here and for which we will redirect the reader to the original paper .
To conclude, we report on the following specific case of mimetic Horndeski model which was studied in . The Horndeski part of the action of the theory is given bywhich corresponds to the following choice for the functions discussed previously:A number of solutions, including cosmological bounces, inflation unified with late-time acceleration, and future singularities, have been discussed. As a specific example, on a flat FLRW background, the following choice of potential for the mimetic field ,gives rise to the following solution:which represents a regular bounce solution. Another bounce solution can be obtained by considering the following potential :for which the scale factor and correspondingly the Hubble parameter read:In Section 5 we will present a detailed discussion on a specific mimetic Horndeski model, constructed as an extension of a covariant Hořava-like theory of gravity.
3.5. Einstein-Aether Theories, Hořava-Lifshitz Gravity, and Covariant Renormalizable Gravity
In closing, we comment on some connections between mimetic gravity and other theories of modified gravity, such connections having been identified recently: namely, the scalar Einstein-aether theory, Hořava-Lifshitz gravity, and a covariant realization of the latter, that is, covariant renormalizable gravity.
3.5.1. Einstein-Aether Theories
An interesting connection which can be identified is that between mimetic gravity and Einstein-aether theories [276, 277] (see also [278–280, 439–444]). These are a class of Lorentz-violating generally covariant extensions of GR. By Lorentz-violating generally covariant we mean that Lorentz invariance is preserved at the level of the action, only to be broken dynamically. In fact, the theory contains a unit time-like vector (which is called the aether) whose norm is fixed by a Lagrange multiplier term in the action. This entails the fixing of a preferred rest frame at each space-time point. In fact, mimetic gravity itself dynamically violates Lorentz symmetry because the gradient of the mimetic field fixes a preferred direction in space-time.
To be precise, mimetic gravity is in correspondence with a particular version of the Einstein-aether theory, namely, the scalar Einstein-aether theory [77, 145]. In this theory, the role of aether is played by the gradient of a scalar field, precisely as occurs in mimetic gravity: the role of aether is played by the four-velocity vector which in turn is the gradient of the mimetic field. It should be noted that the scalar Einstein-aether theory is quite different from the original vector theory. Moreover, in the case where the potential for the potential for the scalar field in such theories (corresponding to the potential for the mimetic field in mimetic gravity) is constant, the model corresponds to the IR limit of projectable Hořava-Lifshitz gravity, which we will comment on further in Section 3.5.2.
3.5.2. Hořava-Lifshitz Gravity
Recall that Hořava-Lifshitz gravity  (HLG hereafter) is a framework and candidate theory of quantum gravity, wherein gravity is made power-counting renormalizable by altering the graviton propagator in the UV. This is achieved by abandoning Lorentz symmetry as a fundamental symmetry of nature, in favour of a Lifshitz anisotropic scaling in the UV. For an incomplete list of references concerning further work in Hořava-Lifshitz gravity, see, for instance, [339–363] and references therein. If the lapse function in HLG is only a function of time, that is, , the theory takes the name of projectable Hořava-Lifshitz gravity. Recall that in the Arnowitt-Deser-Misner decomposition of space-time :the function takes the name of lapse.
Previous work has shown that the IR limit of the nonprojectable version of Hořava-Lifshitz gravity can be obtained from the Einstein-aether theory (the vector version) by requiring that the aether be hypersurface orthogonal: that is, , where is a scalar field and is chosen in such a way to ensure has unit norm. In this case is responsible for the preferred foliation in Hořava gravity, and is the lapse function. If is set equal to coordinate time, the resulting action is that of nonprojectable Hořava gravity [357, 443].
Further requiring that , where is a scalar field, reduces the vector Einstein-aether theory to the scalar Einstein-aether theory . This condition implies that , which upon identification of with coordinate time corresponds exactly to the defining condition for projectable Hořava gravity. In this case, the unit norm constraint cannot be solved for a generic but has to be imposed at the level of the action, for instance, via a Lagrange multiplier term . Therefore, the condition under which is invariant is that be invariant under a shift symmetry: , which shows why the equivalence between scalar Einstein-aether theory and mimetic gravity fails if a nonzero potential for the mimetic field is included . Notice also that, as is known, dark matter emerges as an integration constant in the IR limit of projectable Hořava-Lifshitz gravity . Given the correspondence between this theory and mimetic gravity, then, it should not come as a surprise that dark matter emerges in a similar fashion within the framework of mimetic gravity, as a purely geometrical effect.
A complete proof of the equivalence between mimetic gravity and the IR limit of projectable Hořava-Lifshitz gravity was presented in . In particular, it was shown that the action for the IR limit of projectable Hořava-Lifshitz gravity can be written aswhich we immediately recognize as the action for mimetic gravity without potential, with the addition of a higher derivative term which, as we shall see shortly, was actually added shortly after the original formulation of the theory on purely phenomenological grounds, to cure some undesirable properties at the perturbation level. In Hořava-Lifshitz gravity, is a Lagrange multiplier which had been introduced to enforce the projectability condition. This equivalence was demonstrated rigorously in Appendix A of . Therefore, mimetic gravity appears in the IR limit of a candidate theory of quantum gravity.
3.5.3. Covariant Renormalizable Gravity
Recall that HLG achieves power-counting renormalizability by breaking diffeomorphism invariance. However, this breaking appears explicitly at the level of the action. This has been at the center of criticism, which has related this explicit breaking to the appearance of unphysical modes in the theory which is coupled strongly in the IR [350, 351, 357–359]. Therefore, in order to circumvent these issues it would be desirable to have a theory which preserves the renormalizability properties of Hořava gravity in the UV but retains diffeomorphism invariance at the level of the action. Therefore, diffeomorphism invariance should be broken dynamically in the UV.
An example of such theory has been presented by Nojiri and Odintsov [356, 361, 362] (see also [364–372]) and comes under the name of covariant renormalizable gravity (CRG henceforth). The theory features a nonstandard coupling of a perfect fluid to gravity. When considering perturbations around a flat background, this nonstandard coupling dynamically breaks diffeomorphism invariance. The price to pay is the presence of this exotic fluid, which could have a stringy origin. In particular, one formulation of CRG introduces the fluid via a Lagrange multiplier term, precisely as done in mimetic gravity. For this reason, in the limit where the nonstandard coupling of the fluid to gravity disappears, one recovers the action of mimetic gravity. In one of its equivalent formulations, the action of CRG is given by [356, 361, 362]where and are constants and to recover the mimetic constraint. In particular, the above case corresponds to the version of CRG, where is the dynamical critical exponent which quantifies the degree of anisotropy between space and time in the UV regime of the theory.
Following the initial proposal by Nojiri and Odintsov, other CRG-like models were studied in recent years. For instance, one particular CRG-like model was studied by Cognola et al. in , where black hole and de Sitter solutions were also studied. The action of such CRG-like theory takes the following form :In the above action, the Horndeski-like coupling of the scalar field to curvature (for the case ) has been considered copiously in the literature; for an incomplete list see, for instance, [420, 426, 433, 447–455] and references therein.
In the continuation of our review, after a brief interlude on perturbations in mimetic gravity, we shall consider a case study of a mimetic-like theory. Our choice of case study will fall upon the CRG-like model of Cognola et al. as defined by the action in (103). We shall study in detail its cosmological solutions and perturbations around a flat background. A pathological behaviour of the model when considering perturbations around a flat FLRW background will be cured by appropriately modifying the model.
4. A Brief Interlude: Cosmological Perturbations in Mimetic Gravity
Before proceeding to the case study of a specific mimetic-like model, it is mandatory to discuss the issue of perturbations in mimetic gravity. Recall in Section 2 that we remarked that mimetic gravity does not properly belong to the class of modified theories of gravity with a full extra scalar degree of freedom. Instead, the additional scalar is constrained by the Lagrange multiplier, a situation quite different from, for instance, that of Horndeski models which have a proper scalar degree of freedom.
However, it is clear that the Lagrange multiplier kills the wave-like parts of the scalar degree of freedom: in other words, given that the constraint takes out any higher derivative, it is not possible to have oscillating (wave-like solutions). As a consequence, we can already envisage that the sound speed in the minimal mimetic gravity model will satisfy . This implies, as we have anticipated, that there are no propagating scalar degrees of freedom in the theory.
The property of vanishing sound speed in mimetic gravity can be rigorously demonstrated, by considering small longitudinal perturbations around a flat background . The line element is then given by and are functions of the space-time coordinates such that , and , . Here, we used the conformal Newtonian gauge. Moreover, from field equations we also haveCorrespondingly, given that around a flat FLRW background the mimetic field plays the role of “clock,” we perturb it aswhere , which together with (3) implies thatThen, by perturbing the components of the Einstein equations, a relatively straightforward calculation  shows that the evolution equation for the perturbation to the mimetic field, , satisfies the following equation:The aforementioned pathological property of perturbations in mimetic gravity can be noticed from (108). The evolution equation for perturbations to the mimetic field does not depend on the Laplacian (or in general on spatial derivatives) of the latter. In other words, there is no term of the form in (108), which implies that the sound speed is identically zero. This means that, even when pressure is nonvanishing, the dust degree of freedom induced in mimetic gravity behaves as dust with zero sound speed, and as such quantum perturbations to the mimetic field cannot be defined in the usual fashion. Else, they would fail in providing the seeds for the observed large-scale structure of the universe which grow via gravitational instability. We remark once more that this behaviour is not unexpected, given that the condition enforced by the Lagrange multiplier eliminates wave degrees of freedom. Moreover, this fact has been shown in all generality for mimetic Horndeski models in .
In order to have a theory whose quantum perturbations can be defined in a sensible way, the minimal action for mimetic gravity has to be modified, for instance, by introducing higher derivative (HD) terms. As an example, consider the following action :The corresponding equations of motion readwhere . Thus, identification of the mimetic field with time on a flat FLRW background implies that . There are two main effects of the introduction of the specific HD term on the theory: one at the level of the background and one at the level of perturbations. At the level of the background, it can easily be shown that the Friedmann equation is modified from (32) to which corresponds to a renormalization of the amplitude of the potential. At the level of perturbations, it can be shown that (108) is modified to the following :whereTherefore, the addition of the higher derivative term results in a small but nonvanishing sound speed, which implies that the behaviour of mimetic matter deviates from the usual perfect fluid dust. The nonvanishing sound speed also results in the possibility of defining quantum perturbations in a sensible way. It is beyond the scope of our review to provide further technical details on the matter, but it can be shown that the simple model we have just described is capable of producing red-tilted (i.e., ) scalar perturbations which are enhanced over gravity waves (implying a small value of ), which is consistent with observations from Planck and BICEP2/Keck Array .
In concluding this brief interlude, let us also spare a few words on further modifications of mimetic gravity involving higher derivative terms. These have been studied in [84, 85], in particular by adding the previously discussed term, as well as a term proportional to . It was noticed that these terms affect the growth of perturbations below the sound horizon, in particular suppressing the growth of those with large momenta. The result is the presence of a cut-off in the matter power spectrum for perturbations below a certain wavelength. On larger scales, instead, the predictions for the matter power spectrum match those of collisionless cold dark matter.
The suppression of power on small scales is particularly intriguing in the light of the observation that the collisionless cold dark matter paradigm appears to suffer from a number of shortcomings on subgalactic scales. The core-cusp problem refers to the discrepancy between -body simulations of collisionless cold dark matter, which predict a cuspy profile for the dark matter halo in galaxies, and observations which instead suggest a cored profile towards the center. The discrepancy is particularly large for dwarf galaxies, but indications that a cored profile is favoured for larger galaxies as well persist (see, e.g., [456–460]). Moreover, these same simulations predict an abundance of substructure which is approximately 10 times larger than what we actually observe, an issue which is referred to as the “missing satellites problem” (see, e.g., [461–463]). Although this problem is most acute for satellite galaxies, it exists for field galaxies as well (e.g., ). To make matters worse, the most massive subhalos in these simulations find no observed counterpart, despite one would expect star formation to be more efficient within them: this is known as the “too big to fail problem” (see, e.g., [465–468]). For an incomplete list of comprehensive reviews on these issues, refer, for instance, to [469, 470] and references therein.
Several approaches to solving these problems exist in the literature. If one insists that dark matter is cold and collisionless, then an important role must be played by the baryonic content of the universe. In fact, it has been argued in several works that baryonic feedback processes (see, e.g., [471–480]) due to supernovae or to the stellar and gaseous content of galaxies, or dynamical friction between dark matter and baryon clumps (see, e.g., [481, 482]), can in principle solve or at least alleviate these problems. Alternatively, the small-scale discrepancies might be taken as an indication that something is lacking in the collisionless cold dark matter picture, with DM possibly having sizeable self-interactions. This approach has been undertaken in a number of works; see, for instance, [483–492] and references therein. In particular, a possibility which has received a lot of attention recently is that where the paucity of structure on small scales is explained by modifying the properties of dark matter in such a way that the resulting matter power spectrum is suppressed at large wavenumbers, by coupling the dark matter to a bath of dark radiation (e.g., a massless or light dark photon) or to a light scalar, which delays kinetic decoupling; refer, for instance, to [493–507] and references therein.
A different mechanism, but with similar outcomes, occurs in mimetic gravity. Namely, the suppression of small-scale power, operated by the higher derivative terms, has the potential to solve the missing satellite problem and the too big to fail problem, as shown in . Moreover, the same higher derivative terms could potentially also cure the caustic singularities from which mimetic gravity suffers. The reason for this is that the higher derivative terms effectively correspond to terms parametrizing dissipation, or viscosity, which emerges due to the fact that the velocity dispersion of the dust is now nonzero. Actually, in  it was also argued that these same dissipative terms could alleviate the core-cusp problem. Although these discussions remain preliminary, it is very interesting to note that modifying the action for mimetic gravity by the addition of higher derivative terms could provide a solution to the small-scale structure puzzles of collisionless cold dark matter.
5. A Case Study: Mimetic Horndeski Covariant Hořava-Like Gravity
Having discussed in detail the physics behind mimetic gravity, and many of its extensions, we now provide a detailed case study of a specific mimetic model. Our choice falls on the covariant Hořava-like theory of gravity first discussed by Cognola et al. , with action defined by (103). We will argue that this model can be viewed as a mimetic Horndeski model. We will study the model, its background solutions, and scalar perturbations in detail. Not unexpectedly, we will find that the sound speed of the minimal model is vanishing, and thus quantum perturbations cannot be defined in the usual way. To circumvent this problem, we modify the model by the addition of higher order terms, repeating the analysis of background solutions and scalar perturbations, and show that it will be necessary to go beyond the Horndeski framework in order to have a nonvanishing sound speed. The discussion in this section is largely based on the work of .
5.1. Mimetic Covariant Hořava-Like Gravity
Let us start from the action of the CRG-like model first discussed by Cognola et al. , which is given by the following:where , are arbitrary constants, is the cosmological constant, is a natural number, is a Lagrange multiplier, and determines the constraint imposed on the gradient of the cosmological field . Recall that the Horndeski-like coupling in the action above, for , has been considered in several works (e.g., [420, 433, 447–455]). If we set , we see that the constraint on the gradient of the scalar field corresponds precisely to that of mimetic gravity (3). Thus, we can extend the model to include the mimetic field by adding a potential for the scalar field, which is now made dynamical. The action now readswhere we have set the dimensionless parameter and added a potential for the mimetic field, . Variation with respect to immediately leads to (3), whereas variation with respect to the field modifies (7) towhere we define the following quantities:Finally, variation of the action with respect to the metric leads to the gravitational field equations, which for this theory read