Modeling Quantum Gravity Effects in Inflation
Cosmological models in 1+1 dimensions are an ideal setting for investigating the quantum structure of inflationary dynamics – gravity is renormalizable, while there is room for spatial structure not present in the minisuperspace approximation. We use this fortuitous convergence to investigate the mechanism of slow-roll eternal inflation. A variant of 1+1 Liouville gravity coupled to matter is shown to model precisely the scalar sector of cosmological perturbations in 3+1 dimensions. A particular example of quintessence in 1+1d is argued on the one hand to exhibit slow-roll eternal inflation according to standard criteria; on the other hand, a field redefinition relates the model to pure de Sitter gravity coupled to a free scalar matter field with no potential. This and other examples show that the standard logic leading to slow-roll eternal inflation is not invariant under field redefinitions, thus raising concerns regarding its validity. Aspects of the quantization of Liouville gravity as a model of quantum de Sitter space are also discussed.
1.1 Two-dimensional gravity and cosmological models
The analysis of quantum gravity effects in realistic, four-dimensional cosmological models is hampered by our present inability to quantize gravity in cosmological spacetimes. In two spacetime dimensions, one does have a renormalizable theory of gravity – Liouville field theory Polyakov:1981rd () and variants thereof. As we review below in section 2, Liouville theory describes 2d gravity around a de Sitter or anti-deSitter background; coupling to matter, one finds two-dimensional versions of the Friedmann equations of cosmology DaCunha:2003fm (). Thus we have an ideal setting to investigate the structure of gravitational backreaction at the quantum level – there is enough structure in the single spatial dimension to accommodate the inhomogeneous fluctuations that lead to structure formation, while gravity might be under sufficient control that we can hope to track the quantum back-reaction of the metric on the quantum matter fluctuations.
Of course, the gravity sector does not have independent field-theoretic degrees of freedom in 2d; there are no transverse traceless tensor fluctuations. There is however a sector of scalar metric fluctuations, and as we show in section 3 these behave precisely like their four-dimensional counterparts. We develop a variant of Liouville theory for which there is a one-to-one correspondence between the scalar geometric perturbations in Liouville gravity and those of four-dimensional Einstein gravity. The behavior of these fields under linearized gauge transformations is identified, and invariant combinations are constructed. The quadratic effective action is written in terms of a precise analogue of the Mukhanov-Sasaki variable Sasaki:1986hm (); Mukhanov:1988jd (), which possesses linearized gauge invariance and exhibits a scale invariant fluctuation spectrum at this order. Thus we have an ideal situation in which to model quantum gravity effects in inflationary cosmology – a renormalizable theory of gravity whose field content and perturbative structure matches that of the scalar sector of four-dimensional Einstein gravity.
1.2 The idea of slow-roll eternal inflation
A situation in which quantum effects play a key role is that of inflation. The solutions to the classical equations of motion in an inflation model involve a scalar field, the inflaton , slowly descending its smooth potential . If the descent is slow enough, potential energy dominates over kinetic energy, and the matter equation of state approximates that of a cosmological constant. The quantitative measure of “slow enough” is that the slow-roll parameters which measure the rate of variation of the Hubble scale ,
are much smaller than one over the course of the inflationary epoch.
In the quantum theory of a scalar field rolling down its potential in curved spacetime, there are fluctuations about the classical field value. These fluctuations back-react on the geometry to make curvature perturbations – indeed, these curvature perturbations are thought to seed the fluctuations in the Cosmic Microwave Background that we observe today.
The idea of slow-roll eternal inflation is that large coherent fluctuations over Hubble-size volumes, seemingly far out on the tails of the scalar field probability distribution, nevertheless have an extraordinary effect on the structure of the wavefunctionSteinhardt:1982 (); Vilenkin:1983xq (); Linde:1986fd (); Goncharov:1987ir (); Linde:1993xx (). Consider what happens in a single Hubble volume in spatial dimensions, over a Hubble time . The classical motion shifts the inflaton from its initial value by . The wavefunction of the inflaton is centered on the classical value, with a width Linde:1982uu (); Vilenkin:1982wt (); Starobinsky:1982ee (). The net shift of the inflaton
determines an effective cosmological constant . The amount of inflation in instances where shifts the field to a higher value of the potential is greater than in instances where it shifts the field to a lower value of the potential. If is sufficiently larger than , then there is a substantial probability to make large volumes where the inflaton has fluctuated up, relative to the spatial volume generated by the classical motion. Eventually, when one looks at large volume, the probability density is concentrated on field configurations where the inflaton has never rolled down its potential. A quantitative analysis Creminelli:2008es (); Dubovsky:2011uy () indicates that a phase transition to this eternally inflating state occurs when
1.3 Quantum dS spacetime, and slow-roll eternal inflation in 2d
To investigate the mechanism of slow-roll eternal inflation in the full quantum theory, in section 4 we focus on the particular choice of potential for a scalar matter field in a modified two-dimensional Liouville gravity. For small , this potential satisfies the criterion (3) for slow-roll eternal inflation. On the other hand, a simple field redefinition relates this theory to pure de Sitter Liouville gravity and a decoupled free matter field . Tracked through the field redefinition, the prediction of slow-roll eternal inflation translates into a force pushing the field in a preferred direction, which on the face of it seems absurd. In particular, in conformal gauge the field is completely decoupled. The generalization from an exponential quintessence potential to a cosh potential exhibits similar inconsistencies. Thus, to the extent that these models capture the structure of inflationary dynamics, it appears that the paradigm of slow-roll eternal inflation is logically inconsistent, since it is not invariant under field redefinitions. Possible objections to the analysis are raised and discussed.
This result leads us to examine the quantization of de Sitter Liouville theory in section 5, using both WKB analysis as well as a relation to free field theory via the (canonical) Bäcklund transformation. We undertake this analysis in part to address some of the concerns raised in section 4, but also because this theory is a model of two-dimensional quantum de Sitter spacetime and therefore of interest in its own right. Classically, the Bäcklund transformation is a canonical change of variables; at the quantum level, it is the functional equivalent of an integral transform Braaten:1982fr (); Braaten:1982yn (); Moore:1991ag (); Teschner:2001rv (), and allows one to quantize the dual free field, then construct the wavefunctional of the Liouville theory as an integral transform of the free field wavefunctional; the kernel of the integral transform is given by the generating functional of the canonical transformation. We explore some properties of the resulting Liouville wavefunctional.
2 2d gravity and 2d inflationary cosmology
2.1 Timelike Liouville field theory
Einstein gravity is trivial in two dimensions – the Einstein-Hilbert action is a topological invariant (the Euler character of the two-dimensional spacetime); hence the Einstein tensor vanishes identically. Liouville gravity Polyakov:1981rd () provides a useful substitute, however – especially when it comes to cosmology Polchinski:1989fn (); DaCunha:2003fm (). The Liouville action (for a review and further references, see Ginsparg:1993is (); our conventions differ slightly so as to conform more closely to Einstein gravity in higher dimensions)
describes the dynamics of the scale factor of the (Lorentz signature) 2d metric in conformal gauge
Here is a fixed background metric, whose scalar curvature is . If one sets , the classical equation of motion
is solved by constant curvature ‘dynamical’ metrics , i.e. two-dimensional (anti)de Sitter spacetimes. The parameter plays the role of Newton’s constant here; weak coupling is small . In contrast to Einstein gravity in higher dimensions, the coupling is dimensionless.
Quantum consistency of 2d gravity coupled to conformally invariant matter requires the vanishing of the total stress tensor
which includes contributions from the quantum Liouville field theory, conformal matter fields, and Faddeev-Popov ghosts for the coordinate gauge choice (for the moment, we work in conformal gauge where is fixed). In particular, the contributions to the conformal anomaly
must cancel; this leads to a condition relating the various contributions to the conformal central charge
where the coefficient appearing in the Liouville central charge receives a modification from its classical value due to the quantization of the Liouville field itself,
This modification is determined by the condition of scale invariance of the exponential potential for the Liouville field at the semiclassical level.
Conformally invariant matter only couples to the Liouville field through the conformal anomaly and requirements of residual symmetry under conformal transformations, i.e. only through the requirement (9) of vanishing of the total central charge, and through the stress tensor constraints (7). In this case, the classical solution to the Liouville dynamics is a constant curvature dynamical metric . More generally – either when the matter is non-conformal, or in gauges other than conformal gauge – the gravitational and matter sectors interact non-trivially.
In Einstein gravity, the natural (de Witt) metric on deformations in the metric configuration space has negative signature for deformations of the conformal factor . This feature allows a nontrivial solution to the Hamiltonian constraint of the theory, for generic initial conditions in the classical theory; in cosmology, the timelike signature of the scale factor allows one to think of the scale factor as a measure of time in eras of uniform expansion or contraction. To mimic this property in the 2d model, one wants the kinetic term of the Liouville field to similarly have a sign opposite to that of the matter fields. This criterion is met for , which leads to pure imaginary , and the desired opposite sign kinetic term for the metric conformal factor. In order to restore reality to the gravitational coupling, one maps . The action for this timelike Liouville theory reads
where now . The requirement can be satisfied by having a large number of matter fields; free scalars have , free fermions . In the language of inflationary cosmology, there is a large number of isocurvature modes. Gravity is semiclassical in the limit of large .
2.2 More general models – quintessence
Models of inflationary cosmology arise if we allow a nontrivial potential for some of the matter fields, dressed by the dynamical metric DaCunha:2003fm (). A generic parity-symmetric action for scalar matter coupled to Liouville gravity is
where we account for the timelike Liouville dynamics via a Lorentz signature field space metric, together with the identification among the fields . At the semiclassical level, the gravitational equations of motion imply the vanishing of the beta functions for the quantum field theory with this action; these are conditions on the coupling functions , , and Friedan:1980jf (); Friedan:1980jm (); Fradkin:1984pq (); Callan:1985ia (); Sen:1985eb () At leading semiclassical order, one solution to these conditions is timelike Liouville theory coupled to free scalar matter fields; another solution which we will consider is the quintessence model
together with additional, decoupled free matter fields. The conditions of conformal invariance are
We will be interested in weakly varying matter potentials for which , and thus , so that the gravitational dressing of the matter potential is close to the classical one.
Let us consider the equations of motion in the proper time coordinate instead of conformal time ,
and specialize to spatially homogeneous field configurations, and canonically scaling matter potential . In terms of the ‘Hubble expansion rate’
(here overdots denote proper time derivatives), and the matter potential , the equations of motion and the Hamiltonian constraint then become
Here is the number of spatial dimensions, and , are the spatially homogeneous energy density and pressure of the additional matter fields. For spatial dimensions, these are precisely the Friedmann equations of Einstein gravity, if we identify with the Newton constant . The terms in the last two equations are Casimir energy corrections to the stress tensor in a compact spatial geometry, and are absent if space is non-compact. Thus the timelike Liouville/quintessence dynamics is very much the version of standard cosmological dynamics.
The conditions for slow-roll inflation are then that the dimensionless parameters
are much smaller than one. Slow-roll in the quintessence potential (13) thus holds provided , which is simply the condition that the metric and matter potential have approximately their canonical scaling dimensions.
2.3 Gauge-invariant formulation
We will be interested in formulating cosmological perturbation theory in the above models, in a framework that allows comparison to results in four dimensions. In particular we wish to begin from a gauge invariant starting point, rather than selecting conformal gauge at a very early stage of the analysis. In the classical limit , a covariant generalization of the conformal anomaly term in the action (4) is
where the spacetime curvature and scalar Laplacian are written in terms of a general metric . Upon choosing conformal gauge , this action yields the Liouville kinetic term and background curvature coupling, up to a total derivative. Its shortcoming is that it is nonlocal. Therefore we introduce an auxiliary field and write
which yields (19) upon eliminating via its equation of motion. Thus (20) is a fully gauge invariant local action which is classically equivalent to Liouville theory. At the semiclassical level, this action receives quantum corrections and in conformal gauge becomes
Because is null direction in the field space, the term generates no net contribution to the central charge, and the condition (9) on the total central charge is , i.e. as a string theory one has a light-like linear dilaton in the critical dimension. This property is readily seen by diagonalizing the kinetic term; introducing , one has
i.e. timelike Liouville theory together with a free matter field having a conformally improved stress tensor. The conformal improvement terms of the spacelike and timelike contribute equal and opposite amounts to the central charge.
The scale dimension of the cosmological term determines again . This value only arises after resumming self-contractions of the exponential, and one might expect the quantum corrections of the effective action to look different in other gauges. In performing the covariant analysis of cosmological perturbations in the next section, we will consider the classical action and set , though in principle one could determine systematically the quantum corrections in a general gauge.
While on the subject of the covariance of 2d gravity, it is worth noting that an analysis of Liouville theory as a constrained Hamiltonian system was carried out by Teitelboim Teitelboim:1983ux (). The action takes the form
where the Hamiltonian and momentum constraints are
Naively one might think that this leads to a Lorentz covariant, gauge invariant theory when one allows the lapse and shift to be arbitrary, however upon passing to the Lagrangian formalism there are non-covariant terms involving the lapse and shift. Upon eliminating the field momentum from the action, one finds
This can be organized into the form (11) with the background metric
apart from a term . This residual term has no Lorentz covariant expression. Thus, while the Hamiltonian dynamics (23), when coupled to matter, seems to have local reparametrization symmetry, it is not Lorentz invariant in a general gauge. In particular, when performing the cosmological perturbation theory described in the next section around a slow-roll background, we have found that the action for small fluctuations is generically not Lorentz covariant. Therefore, in what follows we will work with the covariant action (21), as well as its specialization to conformal gauge.
3 Cosmological perturbation theory
A standard parametrization of the 2d metric is
We expand around a spatially homogeneous background
3.1 Background solutions
For the purposes of this section, we will analyze the classical () covariant 2d gravity theory (21) coupled to classical matter, since that is the procedure followed in analyzing cosmological perturbations in higher dimensional Einstein gravity. Working in conformal gauge (and in particular, conformal time) for the background, , , the background equations of motion are
(here prime denotes derivative with respect to conformal time); one also has the Hamiltonian constraint
For instance, when one has deSitter solutions. There are three general classes of such solutions for homogenous backgrounds:
where is conformal time. Evaluating the stress-energy tensor of these solutions
(in light-cone coordinates , with compact spatial sections ), one finds the Liouville field energy for the first, ‘hyperbolic’ solution; for the second, ‘parabolic’ solution; and for the third, ‘elliptic’ solution. These various solutions can be used to satisfy the stress tensor constraints (7) for homogeneous states of the matter fields, depending on their energy. The matter vacuum is paired with the global de Sitter solution, i.e. the elliptic case (32) with ; increasing the matter energy decreases the Liouville momentum , pinching the neck of the de Sitter “bounce”, until at the critical value and the neck pinches off. Increasing the matter energy further leads to the hyperbolic solutions, which have a Milne-type cosmological singularity as . The additive contribution in the stress tensor can be thought of as the Casimir energy of the matter fields on a spatial circle. If we choose to work in a geometry with non-compact spatial sections (which is allowed for the parabolic and hyperbolic solutions), this Casimir term is absent, and the geometry is that of the parabolic solution.
3.2 The quadratic fluctuation action
We now wish to derive the effective action for perturbations to quadratic order Mukhanov:1988jd (); Mukhanov:1990me (). Substituting the expansion (3) into the action (21), expanding to second order in , and using the background equations of motion, one finds
up to total derivatives. The equations of motion of the lapse and shift and yield constraint equations that can be solved for the lapse and shift, with the result
Substituting into (34) (which is allowed because these variables are non-dynamical), one finds
(again up to total derivatives). Note that the effective action is independent of .
Having solved the constraints, the resulting action should be expressible in terms of gauge invariant quantities. Under linearized gauge transformations
the fields transform as
in terms of which one can rewrite (36) as
In fact, the map between perturbations of our 2d theory and the scalar sector of 4d cosmological perturbations can be made quite precise. A standard parametrization of the scalar metric perturbations in 4d is Baumann:2009ds (); Mukhanov:1990me ()
Under linearized gauge transformations
the 4d scalar modes transform as
One can arrange combinations of the fields such that the transformations (43) cancel, leading to the gauge invariant combinations (the so-called Bardeen potentials)
Comparison to the linearly perturbed 2d metric (28)
together with its gauge transformation properties (37), suggests the classical identifications
Including the auxiliary field , the gravitational sector in 2d has as many scalar modes as that of 4d; provides the necessary fourth scalar mode in this correspondence. The 2d analogue of the Mukhanov-Sasaki variable (39) is thus in fact precisely the same as its 4d analogue
apart from a canonical normalization factor of ; while similarly the quantity again differs by a canonical scaling by
This different factor of results in a time-dependent tachyonic mass for the perturbation in four-dimensional slow-roll, whereas in two dimensions and the variable is an ordinary massless free field. This difference in scaling is just what is needed to have a scale-invariant spectrum in both cases.
Since is a standard, conformally invariant free scalar field in the slow-roll approximation, the physical fluctuation spectrum in 2d is scale invariant, as in higher dimensions. However the normalization of the 2d fluctuations is of order
rather than of order as in 4d (as one might have predicted on the basis of dimensional analysis). In the quintessence model (13), the condition that is simply the slow-roll condition , and so slow-roll eternal inflation predicts that the inflaton is always trying to climb its potential.
For completeness, it is worth displaying the full set of equations of motion in the standard 4d parametrization of the metric. This exercise extends to the fluctuations the remarkable parallel between Liouville cosmology in 2d and Einstein cosmology in 4d, observed at the level of homogeneous backgrounds in DaCunha:2003fm (), see equations (2.2). Using the identifications (3.2), the perturbed Hamiltonian and momentum constraint equations for the quadratic effective action (34) read
These two can be combined into a constraint equation for the Bardeen potential which has precisely the same form as its counterpart in Einstein gravity
The dynamical equations of motion can also be written in forms closely resembling their 4d counterparts; under the map (3.2), the equation of motion becomes, after using the Hamiltonian and momentum constraints,
which is the counterpart of the trace component of the spatial Einstein equations ; the traceless part of these equations also has an analogue in the equation of motion
and finally, one has the inflaton equation of motion
The equations of motion (53)-(55) and constraints (51) are direct analogues of the equations of linearized cosmological perturbation theory in four dimensions Baumann:2009ds (); Mukhanov:1990me (). The expressions above differ slightly because the coefficients of various terms are dimension dependent.
The correspondence between two- and four-dimensional scalar perturbations means that one has available in two dimensions all the standard gauge choices used in cosmological perturbation theory. Some standard gauges are
Uniform density gauge. One chooses a time slicing such that the inflaton is a global clock, ; and spatial reparametrizations allow one to set (in 2d, one could alternatively set ). This gauge is singular if the classical inflaton velocity vanishes, because one cannot then adjust the time slicing forward or backward to eliminate (i.e. the gauge slice fails to be transverse). In this gauge, the constraints can be solved for and ; the physical, fluctuating degree of freedom is , which in four dimensions is the scalar curvature perturbation
The scalar determines the scalar curvature perturbation via . Of course, in 2d there is no spatial curvature, nevertheless is a gauge invariant observable equal to in this gauge.
Spatially flat gauge. Here one sets (i.e. in 2d), so that the spatial scale factor is a global clock, and solves the constraints for the lapse and shift perturbations and . This gauge is singular when the expansion rate vanishes, such as at the de Sitter bounce. The dynamical variable is the inflaton fluctuation , and the observable in (56) is determined by the matter density fluctuations rather than directly in terms of the metric. Discussions of slow-roll eternal inflation usually take place in this gauge.
Synchronous and conformal gauges. Gauge choices where one fixes the Lagrange multipliers for the gauge constraints, such as synchronous gauge
or conformal gauge
leave additional fluctuating fields to be quantized, and the constraints that eliminate the unphysical ones are only imposed weakly on the space of states, after quantization. This leads to a potential difficulty – there are negative metric fluctuations, and so instabilities are a concern. In string theory, one typically deals with this issue through analytic continuation of the fields to/from some regime where the path integral is convergent.
This last point is underscored by the fact that in 2d, and in 4d, are absent from the gauge-invariant effective action (40). We will see below that Liouville perturbation theory suffers large infrared divergences; this does not mean that de Sitter timelike Liouville theory is doomed, but it does mean that a more sophisticated approach is called for.
Physical gauges such as spatially flat or uniform density gauge are convenient, in that the only fluctuating quantities are physical degrees of freedom. The quadratic effective action (40) is written directly in terms of the gauge invariant quantity for which we can choose as representative either the spatial metric perturbation or the density perturbation (which is basically ). These gauges have a sensible perturbation theory, since part of the gauge choice is in 4d, or in 2d. However, the expansion of the effective action in these gauges is cumbersome; it may be worth braving the subtleties and pitfalls of conformal gauge, if one can make sense of de Sitter timelike Liouville theory – the action is quite simple, even if its quantization is subtle. We will explore some aspects of conformal gauge quantization after we have related our two basic models (11) and (13).
4 Relating quintessence to pure de Sitter gravity
It turns out that the two models discussed above – the de Sitter timelike Liouville theory (11) and the Liouville-quintessence model (13) – are related by a field redefinition. In this section, we exhibit the field redefinition and point out that the Liouville quintessence appears to provide a counterexample to the phenomenon of slow-roll eternal inflation. A generalization to a cosh potential for the inflaton provides a further counterexample. We then proceed to discuss a variety of potential objections to this line of reasoning. We end with a brief exploration of four-dimensional analogues of the Liouville-quintessence model.
4.1 Field redefinitions
The quintessence model (13) with fields , is related to Liouville theory (11) with fields , by a ‘boost’ field redefinition111Our conventions for which fields carry a tilde are henceforth reversed relative to the introduction.
which leaves the kinetic term and the path integral measure invariant, and relates the parameters in the respective actions via
(in the modified Liouville, theory, one must also shift the auxiliary field to maintain the form of the action). Thus the potentials in the two frames are related by
with , . For , the slow-roll conditions (2.2) are satisfied. With this choice, we can immediately write down the solution of the model at linearized order using the results of the previous section.
The field redefinition (59) relates the background solution for quintessence to that of the de Sitter solution of Liouville theory (32), plus an additional free field. Let us choose non-compact spatial sections; then the quintessence solution is
This classical solution has the property that , i.e. small potential, as one evolves to large volume , as one would expect. If, on the other hand, slow-roll eternal inflation is in operation, one expects the dominant measure for at large volume to be concentrated on configurations with . The field redefinition (59) then implies the behavior
of the free matter field in Liouville gravity. For to climb its potential via slow-roll eternal inflation implies, in the other frame, the presence of a mysterious drift force that pushes the free field to large negative values . It is not clear how such a drift force could arise, since in conformal gauge the free field is completely decoupled from the gravitational sector.
Note that nowhere in this argument does one need to invoke the small fluctuation expansion elaborated above in section 3. Thus, on the one hand, the class of two-dimensional models being considered has exactly the same perturbative content and structure of scalar fluctuations as inflationary models in higher dimensions, and for an appropriate parameter regime satisfies the criteria for slow-roll eternal inflation; on the other hand, it is related by an exact field redefinition to free field theory in de Sitter space. In the latter description, the sorts of fluctuations predicted by slow-roll eternal inflation do not seem to occur. It is hard to see how slow-roll eternal inflation could arise in this model.
4.2 More general models
The quintessence model is free field theory in disguise, therefore it has many hidden symmetries. One might object that these symmetries secretly suppress eternal inflation in the 2d quintessence model, but since they are not generic, the mechanism of slow-roll eternal inflation might still be valid in general. However, already this argument should give one pause, since it suggests that the standard argument involving gravitational back-reaction of gravity on decohered matter fluctuations would need to be refined in light of this subtlety, since it would seem to apply as well to the quintessence model.
We believe that such hidden symmetries are not the reason for the apparent absence of slow-roll eternal inflation. Instead of a quintessence potential as in (13), consider a potential with several exponentials. There will then be no field redefinition that exhibits a large class of hidden symmetries, and yet field redefinitions lead to apparent contradictions with the slow-roll eternal inflation paradigm.
For example, let the potential for the inflaton be
with so that both exponentials in the cosh have the same scale dimension. Again, for sufficiently small, according to the standard criteria this potential exhibits slow-roll eternal inflation where the inflaton is driven to large positive values, or large negative values, depending on whether the initial condition is placed at positive or negative ; on the other hand, classical evolution always predicts that the inflaton is driven to zero at large scale factor .
We can now apply boost field redefinitions that convert either of the exponentials into a pure cosmological constant term; the transformation (59) that leads to (61) for instance relates the original potential (65) to
In the redefined frame, slow-roll eternal inflation predicts that is always driven to at large scale factor , independent of the initial value of , whereas classical evolution predicts that is driven to . If we had boosted in the opposite direction, we would have predicted that there is only quantum jumping in the direction of minus infinity, whereas classical evolution drives the redefined field to plus infinity.
Translating back to the evolution in the cosh frame , , slow-roll eternal inflation in predicts the dynamics for
independent of the initial value of . If we had performed the boost in the opposite direction, we would have concluded that the field is always driven to minus infinity, independent of its initial value.
Thus, the issue is not whether one has a model that is secretly free field theory; rather it’s that the logic of slow-roll eternal inflation is internally inconsistent – when one applies that logic to the same theory viewed in different coordinate frames in field space, one predicts different outcomes, not remotely compatible with one another.
4.3 Possible objections
The preceding result flies in the face of conventional wisdom. Therefore, it behooves us to consider whether there are any subtleties that might prevent the above quintessence model from being a counterexample to the logic of slow-roll eternal inflation. Let us list here a few:
The theory might not exist.
It is not clear that the action (11) describes a conformal field theory. The exponential potential solves the conformal invariance condition at semiclassical order, but it is not clear that this remains true at large where the exponential becomes large; yet this is precisely this late-time de Sitter regime of large scale factor that we wish to consider. In the semiclassical spacelike Liouville theory ( large and negative, and the AdS sign of the cosmological constant), the exponential is self-consistent for small Liouville energy since the dynamics never explores the regime of large values of the potential. The weak-coupling perturbation expansion is fully self-consistent. The status of timelike Liouville theory, with the de Sitter sign of the cosmological constant, is much less clear.
There have been a few investigations of the structure of the beta functions in the presence of potential interactions (a ‘tachyon background’ in the language of string theory). In two dimensions, the generalized beta functions that determine the conditions for scale invariance are thought to be derivable from a variational principle on the space of couplings (this is the target space effective action in the string theory interpretation of the 2d dynamics). There are claims Banks:1991sg () that, due to field redefinition ambiguities, one could take the form of the field space effective potential for the 2d coupling 222Note that there are two uses of the notion of ‘potential’ here which need to be distinguished. The 2d potential is a coupling in the 2d field theory describing dynamics in 2d ‘spacetime’. There is also an effective potential in the space of couplings , , etc, which governs which forms of lead to consistent quantum theories of 2d gravity; it is the properties of this effective potential in the space of couplings that is the subject of the present discussion. in this field space effective action to be given exactly by , which would seem to indicate that the growth of never subsides; on the other hand, Tseytlin:2000mt () argued for a modification to , which would seem to lead to an endpoint to ‘tachyon condensation’, since the potential for has a minimum. A more recent analysis Headrick:2008sa () showed that there is no target space effective action involving the couplings , , and of (12) satisfying certain expected properties – that when the 2d action is a sum of decoupled field theories, the beta functions factorize in an appropriate way. Tachyon dynamics was also studied in Freedman:2005wx (); Suyama:2005wd (); Hellerman:2006nx (); Swanson:2008dt (); Adams:2009yb (). The situation seems at the moment rather murky.
For our purposes, what is needed is that there is no endpoint to ‘tachyon condensation’ in the coupling space – that the exponential growth of the potential persists for an arbitrarily long time, so that the 2d dynamics is some approximation of de Sitter geometry all the way out to arbitrarily large , and thus arbitrarily large spatial volume.333Of course, if there were an endpoint to tachyon condensation, that would also be interesting, as it would be an example of a mechanism for dynamical relaxation of the cosmological constant through quantum effects. Actually, this property may not be completely necessary – it should be sufficient that there is some sufficiently long epoch in which the 2d classical dynamics looks like de Sitter expansion, and there are many e-foldings of that expansion during which the criteria for slow-roll eternal inflation are met, while at the same time we can find a field redefinition that orients the gradient of along the direction (this would be the approximately decoupled ‘Liouville plus free field’ frame).
Note that the late time properties of local (sub-Hubble scale) observables in this class of models is intimately bound up with the ultraviolet behavior of the Liouville field theory at large positive , since fixed proper distance becomes ever smaller coordinate separation as the scale factor continues to grow. Thus it would be helpful to understand the nature of closed string tachyon condensation in string theory well above the critical dimension (or in the modified Liouville theory, which is critical string theory with a large light-like dilaton).
Timelike Liouville theory might exist, but not have the requisite properties.
The timelike Liouville theory that appears in conformal gauge has been the subject of a number of investigations Schomerus:2003vv (); Zamolodchikov:2005fy (); Kostov:2005kk (); Kostov:2005av (); McElgin:2007ak (); Harlow:2011ny (), which have sought to adapt to the timelike regime () the conformal bootstrap technology that solves the spacelike () Liouville theory. The bootstrap considers the properties of correlation functions involving insertions of a class of degenerate operators (operators having a null vector in its tower of descendants under the action of the conformal algebra). Conformal Ward identities then lead to constraints on the correlators. Additional analytic properties of the correlators, such as crossing symmetry and factorization, together with the conformal Ward identities and the assumption of a unique operator of each conformal highest weight, lead to a set of discrete functional identities on correlators. For , these relations are sufficient to uniquely specify the correlation functions Dorn:1994xn (); Zamolodchikov:1995aa (). For , the corresponding exercise leads to two candidate solutions for the correlation functions Zamolodchikov:2005fy (); Schomerus:2003vv (); Kostov:2005kk (); Kostov:2005av (); McElgin:2007ak (); Harlow:2011ny (), neither of which satisfies all the expected properties of a conformal field theory such as vanishing of the two-point function for two conformal fields of different scale dimension. Thus, at the moment there are unresolved issues with the conformal bootstrap. Even if these are resolved, one will then need to understand whether these largely Euclidean techniques are applicable to the intrinsically Lorentz signature issues being addressed here.
Nevertheless, the semiclassical expansion around the de Sitter background seems no less consistent than in higher dimensions, and has the additional advantage of perturbative renormalizability. We have seen that there is a precise map between the auxiliary field Liouville-quintessence model in two dimensions, and the scalar sector of cosmological perturbation theory in four dimensions. In both cases there is a scale-invariant spectrum of linearized perturbations, and the small fluctuation expansion is quite similar in structure. At the perturbative level, the theory seems fully consistent, provided that there is a scheme to renormalize UV divergences, and a method to regulate and resum IR singularities.
There might not be an interpretation in terms of a single universe.
Since 2d gravity can also be interpreted as string theory, there is the issue of topology change in the 2d geometry – it would be difficult to give a single-universe interpretation to the dynamics if there is a significant probability of a catastrophic topology-changing event occurring in a given spatial domain. The coupling in the 2d action can be thought of as governing the amplitude for topology-changing processes for string worldsheets, which here represent an ensemble of 2d cosmologies. In timelike Liouville theory, the coupling enhances the likelihood of topology change at large negative where the exponential Liouville potential is vanishingly small, and exponentially suppresses topology change in the regime of large positive which governs the late-time de Sitter dynamics. Thus, while the early history of the 2d cosmology at small spatial volume may be rife with topology-changing processes, at late time and large spatial volume, these processes are highly suppressed. If one adopts a Hartle-Hawking ansatz for the generation of the expanding universe through a tunneling process, the region of strong topological fluctuation is in an exponentially suppressed region of field space.
On the other hand, consider the rate of string pair production in timelike Liouville theory. This rate was estimated in Strominger:2003fn (), where it was shown that string pair production is exponentially suppressed as for a string (i.e. 2d universe) with energy (Liouville momentum) . However it is exponentially enhanced by the density of states ,444This is the density of states when the spacelike fields have no (or very small) conformal improvement terms in their stress tensor; large conformal improvement terms of these fields will reduce the density of states. For instance, in the modified Liouville theory with light-like dilaton, the density of states only grows as and for small enough the pair production rate is finite. and so the total rate of production diverges. If we are forced to think about an ensemble of universes, according to this analysis that ensemble is dominated by the proliferation of highly excited states in the Hilbert space of a single universe.555The back-reaction of this proliferation of excited strings was argued in Aharony:2006ra (); Frey:2008ke () not to cause a significant perturbation of the background at late times.
Conformal gauge might be pathological.
The disparity between the predictions of slow-roll eternal inflation and conventional semiclassical dynamics is particularly stark in conformal gauge for the quintessence model (13), where in the redefined frame one has a decoupled Liouville field and free field. The conclusion might be on somewhat shakier ground if conformal gauge were somehow pathological; and indeed, we will see below that the small fluctuation expansion is problematic. Outside of conformal gauge, there is a coupling between the gravitational sector and the matter sector through the curvature term , which now contains dynamical fields. One could then worry that fluctuations of provide some sort of drift force that indeed pushes the inflaton in a preferred direction through an effective potential . However, one can choose to start with a conformal improvement in the quintessence frame which is cancelled by the field redefinition, leaving as an ordinary free field with no curvature coupling, or one can choose either sign of this coupling through an appropriate value in the quintessence frame. It seems unlikely that the effects of the improvement term alter the conclusion. Furthermore, the more general example of the cosh potential doesn’t really need conformal gauge to exhibit an internal inconsistency in the logic of eternal inflation.
4.4 Four-dimensional models
The two-dimensional models parallel the structure of four-dimensional cosmology quite closely. However, one might wish to study directly a four-dimensional cosmological model exhibiting the same structure as the 2d quintessence model explored above. Are there four-dimensional quintessence models similarly related to free field theory?
Consider the class of 4d metrics
where spatial indices are raised and lowered with the unit determinant metric . The 4d Einstein action coupled to a scalar inflaton is
where to simplify further developments we have chosen a non-canonical normalization for . The analogue of (modified) timelike Liouville theory is Einstein gravity with a cosmological constant , and we similarly couple it to a free scalar field.
The gradient terms in square brackets again are invariant under a boost transformation
under which the action transforms into
with , . This action is the analogue of the Liouville-quintessence model (13). Unfortunately, it cannot be written in Einstein frame – that would get us back to (68). However, this canonical normalization is not necessary for investigating the logic of slow-roll eternal inflation. In the slow-roll approximation in a Jordan frame such as (70), both the effective gravitational coupling and the Hubble scale are evolving slowly. The standard argument, that quantum fluctuations trump classical displacement of the inflaton, is not affected by this slow evolution of the Newton constant, any more than it is affected by the slow classical evolution of the Hubble scale; both are slow-roll suppressed effects, and the claim is that quantum fluctuations of the inflaton far outweigh them in the eternal inflation regime. We have
where , constitute the slowly evolving background values, locally constant over a Hubble volume. To leading order in the slow-roll approximation, the equation of motion of is
and thus the slow-roll eternal inflation criterion (3) is then met when
Again, the standard paradigm predicts slow-roll eternal inflation where clearly it does not happen.
5 Semiclassical quantization of timelike Liouville
In this section, we collect some results on the quantization of timelike Liouville theory in conformal gauge. A sensible quantization would help allay some of the potential concerns raised above; and provide us with a theory of quantum two-dimensional de Sitter spacetime.
5.1 Perturbation theory is insufficient
At small coupling , the semiclassical approximation to the Liouville dynamics should be accurate, however perturbation theory is not. Expanding the action (11) around a classical solution
we choose the background de Sitter solution
The linearized equation of motion for fluctuations about this background is
The solutions (in momentum space for the spatial coordinate ) are the same spherical Bessel functions that appear in the analysis of inflaton dynamics in four dimensions: