#### Abstract

We review the status of the fourth-order (quartic in the spacetime curvature) terms induced by superstrings/M-theory (compactified on a warped torus) in the leading order with respect to the Regge slope parameter, and study their (nonperturbative) impact on the evolution of the Hubble scale in the context of the four-dimensional FRW cosmology. After taking into account the quantum ambiguities in the definition of the off-shell superstring effective action, we propose the generalized Friedmann equations, find the existence of their (de Sitter) exact inflationary solutions without a spacetime singularity, and constrain the ambiguities by demanding stability and the scale factor duality invariance of our solutions. The most naive (Bel-Robinson tensor squared) quartic terms are ruled out, thus giving the evidence for the necessity of extra quartic (Ricci tensor-dependent) terms in the off-shell gravitational effective action for superstrings. Our methods are generalizable to the higher orders in the spacetime curvature.

#### 1. Introduction

The homogeneity and isotropy of our Universe, as well as the observed spectrum of density perturbations, are explained by inflationary cosmology [1, 2]. Inflation is usually realized by introducing a scalar field (inflaton) and choosing an appropriate scalar potential. When using Einstein equations, it gives rise to the massive violation of the strong energy condition and the exotic matter with large negative pressure. Despite the apparent simplicity of such inflationary scenarios, the origin of their key ingredients, such as the inflaton and its scalar potential, remains obscure.

Theory of superstrings is the leading candidate for a unified theory of Nature, and it is also the only known consistent theory of quantum gravity. It is therefore natural to use superstrings or M-theory for the construction of specific mechanisms of inflation. Recently, many brane inflation scenarios were proposed (see, e.g., [3] for a review), together with their embeddings into the (warped) compactified superstring models, in a good package with the phenomenological constraints coming from particle physics (see, e.g., [4]). However, it did not contribute to revealing the origin of the key ingredients of inflation. It also greatly increased the number of possibilities up to (known as the string landscape), hampering-specific theoretical predictions in the search for the signatures of strings and branes in the Universe.

The inflaton driven by a scalar potential and its engineering by strings and branes are by no means required. Another possible approach can be based on a modification of the gravitational part of Einstein equations by terms of the higher order in the spacetime curvature [5]. It does not require an inflaton or an exotic matter, while the specific higher-curvature terms are well known to be present in the effective action of superstrings [6, 7].

The perturbative strings are defined on-shell (in the
form of quantum amplitudes), while they give rise to the infinitely many
higher-curvature corrections to the Einstein equations, to all orders in the
Regge slope parameter and the string coupling .
The finite form of all those corrections is unknown and beyond our control. However,
it still makes sense to consider the *leading* corrections to the Einstein
equations, coming from strings and branes. Of course, any result to be obtained
from the merely leading quantum corrections cannot be conclusive. Nevertheless,
they may offer both qualitative and technical insights into the early Universe
cosmology, within the well-defined and highly restrictive framework. In this paper,
we adopt the approach based on the Einstein equations modified by the leading
superstring-generated gravitational terms which are *quartic* in the spacetime curvature. We
treat the quartic curvature terms *on
equal footing* with the Einstein term, that is, nonperturbatively.

We consider only geometrical (i.e., pure gravity) terms in the low-energy M-theory effective action in four space-time dimensions. We assume that the quantum -corrections can be suppressed against the leading -corrections, whereas all the moduli, including a dilaton and an axion, are somehow stabilized (e.g., by fluxes, after the warped compactification to four dimensions and spontaneous supersymmetry breaking).

Our paper is organized as follows. In Section 2, we review our starting point: M-theory in 11 spacetime dimensions with the leading quantum corrections, and the dimensional reduction to four spacetime dimensions. In Section 3, we discuss the problem of the off-shell extension of the gravitational part of the four-dimensional effective action for superstrings. In Section 4, we review the physical significance of the on-shell quartic curvature terms. In Section 5, we prove that it is impossible to eliminate the fourth-order time derivatives in the four-dimensional equations of motion with a generic metric. The structure of equations of motions for the special (FRW) metrics is revealed in Section 6, which contains our main new results. The exact (de Sitter) solutions, stability, and duality constraints are also discussed in Section 6. Our conclusion is Section 7. In Appendix A, we give our notation and compute some relevant identities. The two-component spinor formalism (for completeness) is summarized in Appendix B.

#### 2. M-Theory and Modified Einstein Equations

There are five perturbatively consistent superstring models in ten spacetime dimensions (see, e.g., the book [6, 7]). All those models are related by duality transformations. In this paper, we are going to consider only the gravitational sector of the heterotic and type-II strings. In addition, there exists a parent theory behind all those superstring models, it is called M-theory, and it is eleven-dimensional [6, 7]. Not so much is known about the nonperturbative M-theory. Nevertheless, there are the well-established facts that (i) the M-theory low-energy effective action is given by the 11-dimensional supergravity [8], and (ii) the leading quantum gravitational corrections to the 11-dimensional supergravity from M-theory in the bosonic sector are quartic in the curvature [9–12] (see, e.g., [13] for some recent progress). Our purpose in this section is to emphasize what is not known.

All the bosonic terms of the M-theory corrected
11-dimensional action read as follows [9–12]:where is the 11-dimensional gravitational constant, is the M2-brane tension given bywhere is a 3-form gauge field
of the 11-dimensional supergravity [8], and is its four-form field strength, is the gravitational scalar curvature, stands for the 11-dimensional Levi-Civita
symbol in the Chern-Simons-like coupling, while are certain *quartic* polynomials in the 11-dimensional
curvature. The is given bywhere the is the 11-dimensional extension of the
eight-dimensional Euler densityand the is the eight-formwhere the traces are taken with
respect to (implicit) Lorentz indices in eleven space-time dimensions. The
(world) vector indices are also suppressed in (2.1).

The -contribution (2.3) is defined *modulo* Ricci-dependent terms by its derivation
[9–12]. The basic reason is the *on-shell* nature of the perturbative
superstrings [6, 7],
whose quantum on-shell amplitudes determine the gravitational effective action
modulo field redefinitions. Via the Einstein-Hilbert term, the metric field
redefinitions contribute to the next (quartic) curvature terms with at least
one factor of Ricci curvature. Therefore, some additional physical requirements
are needed in order to fix those Ricci-dependent terms in the off-shell
M-theory effective action.

To match the constraints imposed by particle physics, M-theory is supposed to be compactified to one of the superstring models in ten dimensions, and then down to four spacetime dimensions, for example, on a Calabi-Yau complex three-fold [6, 7]. Alternatively, M-theory may be directly compactified down to four real dimensions on a 7-dimensional special holonomy manifold [14]. The bosonic fields of the action (2.1) are just an eleven-dimensional metric and a 3-form (there is no dilaton in eleven dimensions). In other words, the 11-dimensional action (2.1) is the most general starting point to discuss the M-theory/superstrings compactification.

In the presence of fluxes, we should consider the *warped* compactification, whose metric is
of the form [15]where is the FRW metric in (uncompactified)
four-dimensional spacetime (see (5.1) below), is a metric in compactified seven dimensions
with the coordinates , ,
and is called a warp factor.

Since we are interested in the gravitational sector of the four-dimensional type-II superstrings, an explicit form of the 7-metric is not needed. In the case of heterotic strings, one has to include the “anomalous” term quadratic in the curvature (see below). We put all the four-dimensional scalars (like a dilaton, an axion, and moduli) into the matter stress-energy tensor (in Einstein frame), and assume that they are somehow stabilized to certain fixed values. In addition, we do not consider any M-theory/superstrings solitons such as M- or D-branes. After dimensional reduction, the only gravitational terms coming from type-II superstrings in four dimensions are given bywhere we have introduced the Einstein coupling in four dimensions, and the four-dimensional counterpart of in (2.3), ,

The relation between the coupling constants and is given bywhere we have introduced the *Kaluza-Klein* (KK) compactification scale and the *average* warp factor (with an integer weight )We also findof mass dimension .
For instance, when substituting the Planck scale and , and ignoring the warp factor, ,
we get the incredibly small (and, in fact, unacceptable—see Section 6) value

As regards the four-dimensional heterotic strings, the action (2.7) is to be supplemented by the term [16, 17]whereagain modulo Ricci-dependent terms.

The gravitational action is to be added to a matter
action, which lead to the *modified* Einstein equations of motion (in the type-II case, for definiteness)where stands for the energy-momentum tensor of all
the matter fields (including dilaton and axion).

Due to the ambiguities in the definition of the -polynomial, it is also possible to replace it bywhere we have introduced the Weyl tensor in four dimensions [18], which is the traceless part of the curvature tensor—see Appendices A and B.

#### 3. Going Off-Shell with the Curvature Terms

There are about Ricci-dependent terms in the most general
off-shell gravitational effective action that is *quartic* in the curvature. It also means
about new coefficients, which makes the fixing of
the off-shell action to be extremely difficult. The quartic curvature terms are
thus different from the *quadratic* curvature terms, present in the on-shell heterotic string effective action
(2.13), whose off-shell extension is very simple (see below). It is, therefore,
desirable to formulate some necessary conditions that any off-shell extension
has to satisfy.

(i) The first condition is, of course, the vanishing of all extra terms (i.e., beyond those in (2.8)) in the Ricci-flat case [19, 20]. The perturbative superstring effective action is usually deducted from the superstring amplitudes, whose on-shell condition is just the Ricci-flatness. In the alternative method, known as the nonlinear sigma-model beta-function approach, the Ricci-dependent ambiguities in the effective equations of motion (associated with the vanishing sigma-model beta-functions) arise via the dependence of the renormalization group beta-functions of the nonlinear sigma-model upon the renormalization prescription, starting from two loops (see, e.g., [21] for details).

(ii) Supersymmetry requires all quantum bosonic
corrections to be extendable to locally supersymmetric invariants. It can be
made manifest in four spacetime dimensions, where the off-shell superspace
formalism of supergravity is available [22]. The Weyl tensor, Ricci
tensor and scalar curvature belong to three different superfields called , and ,
respectively, while the first superfield is
chiral. (We use the two-component spinor notation [22], —see Appendix
B.) * * In
particular, the Weyl tensor appears in the first order of the superspace chiral anticommuting coordinates asso that the terms (with all curvatures being replaced by
Weyl tensors) are easily supersymmetrizable in superspace asThe terms in (2.16) are also extendable to the
manifest superinvariantwhere we have introduced the
supervielbein densities, and ,
in the chiral and central superspaces, respectively (see [22] for details).

Those invariants were extensively studied in the past
because they naturally appear as the possible counterterms (with divergent
coefficients) in quantum four-dimensional supergravity (see, e.g., [23, 24]). In superstring theory,
one gets the *same* structures,
though with *finite* coefficients
(see, e.g., [25, 26]). Thus, in four dimensions, the structure of the *on-shell superstrings* quartic curvature
terms is fixed by local supersymmetry alone, up to normalization.

(iii) The absence of the higher-order time derivatives
is usually desirable to prevent possible unphysical solutions to the equations
of motion, as well as to preserve the perturbative unitarity, but it is by no
means necessary. As is well known, the standard Friedmann equation of general
relativity is an evolution equation, that is, it contains only the first-order
time derivatives of the scale factor [1, 2, 27].
It happens due to the cancellation of terms with the second-order time
derivatives in the mixed -component of Einstein tensor—see, for
example, the appendix of [28] for details. It can also be seen as the consequence
of the fact that the second-order dynamical (Raychaudhuri) equation for the
scale factor in general relativity can be integrated once, by the use of the
continuity equation (3.5), thus leading to the evolution (Friedmann) equation
[1, 2]. As regards the
quadratic curvature terms present in the heterotic case, their unique off-shell
extension is given by the Gauss-Bonnet-type combination [29, 30]In the expansion around
Minkowski space, ,
the fourth-order derivatives (at the leading order in ) coming from the first term in (3.4) cancel
against those in the second and third terms [31]. As a result, the off-shell extension (3.4) appears
to be ghost-free in all dimensions. As regards *four* space-time dimensions, the terms
(3.4) can be rewritten as the four-dimensional Euler density
(A.7). Therefore,
being a total derivative, (3.4) does not contribute to the four-dimensional
effective action. (Of course, adding Euler
densities to the Einstein-Hilbert term matters in higher (than four) dimensions [32, 33], or with the dynamical dilaton and axion fields
[34–36].) The higher-time derivatives are apparent in the
gravitational equations of motion with the quartic curvature terms (see also
[37]). It is natural
to exploit the freedom of the metric field redefinitions in order to get rid of
those terms. However, in Section 5 we prove that it is impossible to eliminate
the fourth-order time derivatives in the quartic curvature terms via a metric
field redefinition. It may still be possible for some special (like FRW)
metrics, after imposing the string duality requirement (Section 6).

(iv) The matter equations of motion in general
relativity imply the covariant conservation law of the matter energy-momentum
tensorBy the well-known identity ,
(2.15) and (3.5) implyFor instance, when as in (3.4), (3.6) readsBy the use of Bianchi identities
for the curvature tensor, we find by an explicit calculation that the left-hand
side of (3.7) identically vanishes. We believe that
(3.6) should be *identically* satisfied by *any* off-shell gravitational correction because, otherwise, the consistency of the
gravitational equations of motion may be violated.

Given the quartic curvature terms (2.8), the modified Einstein equations of motion (2.15) are

(v) We may also add the *causality* constraint as our next
condition: the group velocity of ultraviolet perturbations on a gravitational
background with the higher-curvature terms included must not exceed the speed
of light. As was demonstrated in [38, 39], the causality condition merely affects the sign
factors of the full curvature terms in the action, namely, the signs in front
of and should be positive. It must be automatically
satisfied by the perturbative superstring quartic corrections (2.8) due to the
known unitarity of superstring theory, and it is the case indeed—see the
identity (A.20—just because .

Of course, our list is not complete, and it could be easily extended by more conditions, for example, by requiring the consistency with black hole physics, gravitational waves, nucleosynthesis, and so forth. For example, in Section 6, we impose the scale factor duality as yet another constraint.

#### 4. On-Shell Structure and Physical Meaning of the Quartic Curvature Terms

The detailed structure and physical meaning of the
quartic curvature terms in (2.8) and (2.16) are easily revealed via their
connection to the four-dimensional *Bel-Robinson* (BR) tensor [40–43]. The latter is well known
in general relativity [44–47]. We review here the main properties of the BR tensor,
and calculate the coefficients in the important identities—see
(4.4) and
(4.5) in the section below. (Those coefficients
were left undetermined in [45–47]).

The BR tensor is defined bywhose structure is quite similar to that of the Maxwell stress-energy tensor (see also Appendix A for more details).

The Weyl cousin of the BR tensor is obtained by replacing all curvatures by Weyl tensors in (4.1)—see (A.10). The Weyl BR tensor can be factorized in the two-component formalism (see Appendix B)

In this section, we consider all the quartic terms
on-shell, that is, *modulo* Ricci
tensor-dependent terms. Therefore, we are not going to distinguish between and here. The Ricci tensor-dependent additions will
be discussed in Sections 5 and 6.

The significance of the BR tensor to the quartic curvature terms is already obvious from superspace (see Section 3), where the locally supersymmetric extension of the quartic Weyl terms (2.16) is given by (3.3) whose bosonic part is the BR tensor squared due to (4.3). As regards a straightforward proof, see Appendix A and our derivation of (A.20) there, which implyIn addition, when using another identity (A.17), (4.4) yieldswhere we have introduced the Euler and Pontryagin topological densities in four dimensions—see (A.7) and (A.8), respectively.

In addition [40–43, 45–47], the on-shell BR tensor is *fully symmetric* with respect to its vector
indices, it is *traceless,
*(ii) It is covariantly *conserved* (though the BR tensor is not a
physical current),and it has *positive* “energy”
densityEquation (4.6) is most easily
seen in the two-component formalism (see Appendix B),
(4.7) is the consequence
of Bianchi identities [48], whereas
(4.8) just follows from the definition
(4.1).

The BR tensor is related to the gravitational
energy-momentum *pseudo* tensors
[45–47]. It can be most
clearly seen in *Riemann normal
coordinates* (RNCs) at any *given* point in spacetime. The RNCs are defined by the relationsso that the derivatives of
Christoffel symbols read as follows:Raising and lowering of vector
indices in RNCs are performed with Minkowski metric and its inverse ,
whereas all traces in the last two equations (4.9) and
(4.10) vanish

Moreover, there exists the remarkable noncovariant
relation (valid only in RNC) [45–47]where the symmetric *Landau-Lifshitz* (LL) gravitational
pseudotensor [27]and the nonsymmetric *Einstein* (E) gravitational pseudotensor
[49, 50]have been introduced in RNC, in
terms of Christoffel symbols.

#### 5. Off-Shell Quartic Curvatures in Cosmology

The main cosmological principle of a *spatially* homogeneous and isotropic -dimensional universe (at large scales) gives
rise to the standard *Friedman-Robertson-Walker* (FRW) metrics of
the form [49, 50]where the function is known as the scale factor in “cosmic”
coordinates ;
we use and ,
while is the FRW topology index taking values .
Accordingly, the FRW metric (5.1) admits a 6-dimensional isometry group that is either , or ,
acting on the orbits ,
with the spatial 3-dimensional sections , ,
or ,
respectively. By the coordinate change, ,
the FRW metric (5.1) can be rewritten to the formwhich is manifestly
(4-dimensional) conformally flat in the case of .
Therefore, the 4-dimensional Weyl tensor of the FRW metric obviously vanishes
in the “flat” case of .
It is well known that the FRW Weyl tensor vanishes in the other two cases, and ,
too [28, 51]. Thus we have

Inflation in an early universe is defined as the epoch during which the scale factor is accelerating [1, 2]where the dots denote time derivatives, and is Hubble “constant.” The amount of inflation is given by a number of e-foldings [1, 2]:which should be around [1, 2].

Though the leading purely geometrical (perturbative)
correction in the heterotic string case is given by the Gauss-Bonnet
combination (3.4), and thus it does not contribute to the equations of motion
in four space-time dimensions, the situation changes when the *dynamical* moduli (axion and dilaton) are
included. The effective string theory couplings are moduli dependent, which
gives rise to a nontrivial coupling with the moduli in front of the
Gauss-Bonnet term, so that the latter is not a total derivative any more. At
the level of the one-loop corrected heterotic superstring effective action in
four dimensions, the cosmological solutions were studied in [34–36]. As regards the realization
of inflation in M-theory, see, for example, [52].

In the case of type-II superstrings (after stabilizing the moduli) we are left with the quartic curvature terms in the four-dimensional effective action (Section 2). Let us address the issue of the higher-time derivatives in the general setting. It is quite natural to use the freedom of the metric field redefinitions in string theory in order to try to get rid of the higher-time derivatives in the effective action. The successful example is provided by the Gauss-Bonnet gravity (Section 3) that we are now going to follow. Let us consider a weak gravitational fieldin the harmonic gaugeThe linearized curvatures are given bywhereas the Ricci tensor and the scalar curvature in the gauge (5.7) read

(We assign the lower case *latin* letters to spacetime indices, ,
and the lower case *middle greek* letters to spatial indices, .)

As is clear from the structure of those equations, it is possible to form the Ricci terms after integration by parts in the quadratic curvature action. As a result, there is a cancellation of all terms with the fourth-order time derivatives in the leading order of the Gauss-Bonnet action (3.4) in all spacetime dimensions, as was first observed in [31].

Unfortunately, we find that it does not work for the quartic curvature terms, even in four spacetime dimensions, as we now going to argue.

When using the linearized curvature (5.8), the quartic terms (2.8) in four spacetime dimensions have the structurewhere we have introduced the notation andwhile all the index contractions above are performed with Minkowski metric.

Equation (5.10) is not very illuminating, but it is enough to observe that the dangerous terms and do contribute, and thus lead to the terms with the fourth- and third-order time derivatives in the equations of motion, when all are supposed to be independent. The last possibility is to convert those terms into some Ricci tensor-dependent contributions. However, in the harmonic gauge (5.7), getting the Ricci tensor requires the two spacetime derivatives to be contracted into the wave operator, as in (5.9), in each dangerous term, which is impossible for the quartic curvature terms, unlike their quadratic counterpart because any integration by parts in the quartic terms does not end up with a wave operator in each term. The equations of motion in the case of -gravity with the FRW metric are explicitly computed in Section 6, as an example.

Having failed to remove the higher-time derivatives
for a generic metric, one can try to get rid of them for a special class of
metrics, namely, the FRW metrics of our interest. The simplest example arises
when all the Riemann curvatures in the quartic curvature terms are replaced by
the Weyl tensors, as in (2.16). It also amounts to adding certain quartic
curvature terms with at least one Ricci factor to the effective action (2.7).
This proposal is based on the reasonable assumption [53] coming from the AdS/CFT
correspondence that the and spaces seem to be the exact solutions to the
(eleven-dimensional) M-theory equations of motion. Of course, such assumption
is just the sufficient condition, not the necessary one, because there may be
many more solutions. The substitution leads to the contributions with three Weyl
tensors (from the quartic terms) in the equations of motion, which implies *no* perturbative superstring corrections to
the FRW metrics at all because of (5.3).

In Section 6, we find that the scale factor duality requirements allow a family of the generalized Friedmann equations coming from the most general quartic curvature terms, with just a few real parameters.

#### 6. Exact Solutions, Stability, and Duality

Our motivation in this paper is based on the observation that the standard model (SM) of elementary particles does not have an inflaton. (The proposal [54] to identify the inflaton with the SM Higgs boson requires its nonminimal coupling to gravity, which does not fit to string theory.) In addition, M-theory/superstrings have plenty of inflaton candidates but any inflationary mechanism based on a scalar field is highly model dependent. When one wants the universal geometrical mechanism of inflation based on gravity only, it should occur due to some Planck scale physics to be described by the higher-curvature terms (cf. [5]).

On the experimental side, it is known that the vacuum energy density during inflation is bounded from above by a (non)observation of tensor fluctuations of the cosmic microwave background (CMB) radiation [55]:It severely constrains but does not exclude the possibility of the geometrical inflation originating from the purely gravitational sector of string theory because the factor of above may be just due to some numerical coefficients (cf. Section 2).

In this section, we consider the structure of our
generalized Friedmann equation with *generic* quartic curvature terms. We get
the conditions of stability of our inflationary solutions, and solve the
duality invariance constraints coming from string theory [56, 57].

Due to a single-arbitrary function in the FRW Ansatz (5.1), it is enough to take
only one gravitational equation of motion in (2.15) without matter, namely, its
mixed -component. As is well known [1, 2], the spatial
(3-dimensional)
curvature can be ignored in a very early universe, so we choose the manifestly
conformally-flat FRW metric (5.1) with in our Ansatz. It leads to a purely
gravitational equation of motion having the formwhere is a *polynomial* with respect to its arguments:Here, the sum goes over the *integer* partitions of ,
the dots stand for the derivatives with respect to time ,
and are some real coefficients. The highest
derivative enters linearly at most, .

The FRW Ansatz with gives the following nonvanishing curvatures:where . For example, in the case of the gravity (3.8), after a straightforward (though quite tedious) calculation of the mixed -equation without matter and with the curvatures (6.4), we findIt is remarkable that the fourth-order time derivatives (present in various terms of (3.8)) cancel, whereas the square of the third-order time derivative of the scale factor, , does not appear at all in this equation. (Taking Weyl tensors instead of Riemann curvatures leads to the vanishing coefficients.)

Our generalized Friedmann equation (6.2) applies to *any* combination of the quartic curvature
terms in the action, including the Ricci-dependent terms. The coefficients in (6.3) can be thought of as linear
combinations of the coefficients in the most general quartic curvature action.
The polynomial (6.3) merely has undetermined coefficients, that is
considerably less than a of the coefficients in the most general
quartic curvature action.

The structure of (6.2) and (6.3) admits the existence of rather generic exact inflationary solutions without a spacetime singularity. Indeed, when using the most naive (de Sitter) Ansatz for the scale factor,with some real positive constants and , and substituting (6.6) into (6.2), we get , whose coefficient is just a sum of all -coefficients in (6.3). Assuming the to be positive, we find an exact solution:This solution in nonperturbative in , that is, it is impossible to get it when considering the quartic curvature terms as a perturbation. Of course, the assumption that we are dealing with the leading correction implies . Because of (2.11) and (6.7), it leads to the natural hierarchywhere we have introduced the four-dimensional Planck scale and the compactification scale .

The *effective* Hubble scale of (6.7) should be lower than the *effective* (with warping) KK scale in order to validate our four-dimensional
description of gravity, that is, the ignorance of all KK modes:It rules out the naive KK
reduction (with ) but still allows the warped compactification
(2.6), when the average warp factor is tunedwhere we have used (2.11) and
have estimated by order .

The exact solution (6.6) is nonsingular, while it describes an inflationary isotropic and homogeneous early universe. (The exact de Sitter solutions in the special case (2.8) were also found in [58–61].) Given the expanding universe, the curvatures decrease, so that the higher-curvature terms cease to be the dominant contributions against the matter terms we ignored in the equations of motion. The matter terms may provide a mechanism for ending the geometrical inflation and reheating (i.e., a graceful exit to the standard cosmology).

To be truly inflationary solutions, (6.6) and (6.7) should correspond to the stable fixed points (or attractors) [1, 2]. The stability conditions are easily derived along the standard lines (see, e.g., [62, 63]). When using the parameterizationwe easily find

Equation (6.3) now takes the formwhere the -coefficients are linear combinations of the -coefficients (easy to find). Equations (6.6)
and (6.7) are also simplifiedThe solution (6.14) can be
considered as the fixed point of the equations of motion (6.2) in a generic
casewhere we have introduced the
notationEquation (6.15) can be brought
into an autonomous formthat is quite suitable for the
stability analysis against small perturbations about the fixed points, ,
where .
We findwhere all the partial
derivatives are taken at the fixed point (denoted by ). The fixed points are stable when all the eigenvalues
of the matrixin (6.18) are *negative* or have negative real parts
[62, 63]. Then the fixed point is a
stable attractor.

To the end of this section, we would like to investigate how the symmetries of string theory are going to affect the coefficients of our generalized Friedmann equation. Here, we apply the scale factor duality [56, 57] by requiring our equation (6.2) to be invariant under the duality transformationThis duality is a cosmological version of the genuine stringy T-duality (which is the symmetry of the nonperturbative string spectrum), in the case of time-dependent backgrounds. The scale factor duality is merely the symmetry of the (perturbative) equations of motion of the background fields. It is used, for example, in the so-called prebig bang scenario [64] in order to avoid the cosmological singularity.

In the -parameterization (6.11), the duality transformation (6.20) takes the very simple formThe equations of motion in the form (6.15) are manifestly invariant under , where is an arbitrary constant.

It is straightforward to calculate how the right-hand side of (6.2) transforms under the duality (6.20) by differentiating (6.20). We find

To see how the duality affects the polynomial ,
we consider the case with the third-order time derivatives, motivated by
(6.5).
We introduce the notationso that the duality invariance
condition readsThe structure of the polynomial in
(6.3), as the sum over partitions of ,
restricts a solution to (6.24) to be most quadratic in :whose coefficients are
polynomials in ,
of the order being given by their subscripts, that is,After a substitution of (6.25)
and (6.14) into
(6.24), we get an *over* determined system of linear equations
on the coefficients. Nevertheless, we find that there is a consistent general
solutionparameterized by merely five real
coefficients .
Requiring the existence of the exact solution (6.6), that is, the positivity of in (6.7) yields

As regards the gravity representing the “minimal”
candidate for the off-shell superstring effective action, we checked that
neither the duality invariant structure (6.15) nor the inequality
(6.28) are
satisfied by the coefficients present in
(6.5). We interpret it as the clear
indications that some additional Ricci-dependent terms *have to be added* to the terms or, equivalently, the gravity is ruled out as the off-shell
effective action for superstrings.

Finally, we would like to mention about some possible simplifications and generalizations.

The last equation (6.4) apparently implies that the Ricci-dependent terms in should have the factor of . Hence it may be possible to completely eliminate both the fourth- and third-order time derivatives in our generalized Friedmann equations, though we are not sure that this choice is fully consistent. However, if so, instead of (6.24), we would get another duality conditionwhose most general solution is simpler:with merely three, yet to be determined coefficients .

We would like to emphasize that our results above can
be generalized to any finite order with respect to the spacetime curvatures in
the off-shell superstring effective action because it amounts to increasing the
order of the polynomial .
The list (6.10) can be continued to any higher order in the derivatives. We can
now speculate about the form of the generalized Friedmann equation to *all* orders in the curvature. It depends
upon whether (i) there will be some finite maximal order of the time
derivatives there, or (ii) the time derivatives of arbitrarily high order
appear (we do not know about it). Given the case (i), we just drop the
requirement that the right-hand side of our cosmological equation
(6.2) is a
polynomial, and take a duality-invariant *function* instead. In the case (ii), we should replace
the function by a *functional*,
thus getting a nonlocal equation having the formwhose functional is subject to the nontrivial duality
constraint

Imposing *simultaneously* both conditions of
stability and duality invariance leads to severe constraints on the -coefficients. Hence it also severely restricts
the quantum ambiguities in the superstring-generated quartic curvature gravity.
Finding their solutions seems to be a nontrivial mathematical problem, we would
like to investigate it elsewhere [65].

#### 7. Conclusion

The higher-curvature terms in the gravitational action defy the famous Hawking-Penrose theorem [66–68] about the existence of a spacetime singularity in any exact solution to the Einstein equations. As we demonstrated in this paper, the initial cosmological singularity can be easily avoided by considering the superstring-motivated higher-curvature terms on equal footing (i.e., nonperturbatively) with the Einstein-Hilbert term.

Our results predict the possible existence of the very short de Sitter phase driven by the quartic curvature terms, in the early inflationary epoch.

Though we showed the natural existence of inflationary (de Sitter) exact solutions without a spacetime singularity under rather generic conditions on the coefficients in the higher-derivative terms, it is not enough for robust physical applications. As a matter of fact, we assumed the dominance of the higher-curvature gravitational terms over all matter contributions in the very early Universe at the Planck scale. However, given the expansion of the Universe under the geometrical inflation, the spacetime curvatures should decrease, so that the matter terms can no longer be ignored. The latter may effectively replace the geometrical inflation by another matter-dominated mechanism, thus allowing the inflation to continue substantially below the Planck scale.

In addition, the number of e-foldings (5.5) is just about one in our scenario based on the quartic curvature terms, which makes it difficult to compete with the conventional inflation mechanisms [1, 2]. An investigation of the possible “graceful exit” strategies, toward a matter-driven inflation is, however, beyond the scope of the given paper.

The quartic curvature terms are also relevant to the Brandenberger-Vafa cosmological scenario of string gas cosmology [69]—see, for example, [63] for a recent investigation of the higher-curvature corrections there. (The higher-curvature terms were considered only perturbatively in [63].)

The higher-time derivatives in the equations of motion may be unavoidable when using the higher-curvature terms, but we do not see that they constitute a trouble.

Gravity with the quartic curvature terms is a good playground for going beyond the Einstein equations. Our analysis may be part of a more general approach based on superstrings, including dynamical moduli and extra dimensions.

#### Appendices

#### A. Our Notation and Identities

We use the basic notation of [27] with the signature . The (Riemann-Christoffel) curvature tensor is given byin terms of the Christoffel symbolsIt followsThe traceless part of the curvature tensor is given by a Weyl tensor:where we have introduced the Ricci tensor and the scalar curvature

The dual curvature is defined bywhere is Levi-Civita tensor.

The *Euler* (E) and *Pontryagin* (P)
topological densities in four dimensions arerespectively.

The *Bel-Robinson* (BR) tensor is defined by
[40–43]

Its Weyl cousin is given by

The Riemann-Christoffel curvature (modulo Ricci-dependent terms) is most easily described in the Petrov formalism [70] by imposing the Ricci-flatness condition . A metric at a given point in spacetime can always be brought into Minkowski form , whereas the curvature tensor components can be represented bywhere the 3d tensors and are symmetric by definition, , and is 3d Levi-Civita symbol normalized by .

The Ricci-flatness condition implies that is traceless, is symmetric, and . It is now natural to introduce a symmetric (traceless) complex 3d tensorand bring it into one of its canonical (Petrov) forms, called I, II, or III, depending upon a number (3, 2, or 1, resp.) of eigenvectors of . For our purposes, it is most convenient to use the form I with three independent (complex) eigenvectors, so that the real matrices and can be simultaneously diagonalized as in terms of their real eigenvalues.

(We use the lowercase greek letters to represent vector indices in three (flat) spatial dimensions.)

It is straightforward to write down any Riemann-Christoffel curvature invariants as the polynomials of their eigenvalues (A.13) in the Petrov I form. It is especially useful for establishing various identities modulo Ricci-dependent terms. We findso thatSimilarly, one findsFor example, when being compared to (A.14), it yields the identity

As regards the superstring correction (2.8) in four dimensions, we findThe BR tensor (A.9) squared in the Petrov I form readsAs a result, we find the identitieswhich are valid on-shell, that is, modulo Ricci tensor-dependent terms.

#### 9. Two-Component Formalism

To complete our notation, we summarize basic definitions and main features of the two-component spinor formalism in gravitation (cf. [18, 22]). The main point is the use of an algebra isomorphic to the Lorentz algebra .

We use lowercase (middle) latin indices for the curved spacetime vector indices, capital (early) latin letters for the tangent (flat spacetime) vector indices, and lowercase (early) greek letters for the (tangent spacetime) spinor indices, and , whereas and .

A four-component Dirac spinor can be decomposed into its chiral and antichiral parts, and , by using the chiral projectors , where . The simplest form of chiral decomposition is obtained in the basis for Dirac gamma matrices with a diagonal matrixHere, is a unit matrix, and are three Pauli matrices.

Given a vector field in a curved spacetime, it can always be represented by a bispinor field where we have introduced the vierbein , together with its inverse , obeying the relations

For instance, one easily finds that the metric in the two-component formalism can be represented by a product of two Levei-Civita symbols:

As regards the curvature tensor, it can be naturally
decomposed in the two-component formalism as follows:The four-spinor (or ) is *totally symmetric* with respect to its
chiral (or antichiral) spinor indices, while is is also traceless, thus
representing the self-dual (or antiself-dual) part of the Weyl tensor (A.4)The four-spinor is symmetric with respect to its first two
indices, as well as with respect to the last two indices, while it is also
traceless, thus representing the traceless part of the Ricci tensor:The scalar represents the scalar curvature .
One easily finds

The Bianchi II identities in the two-component formalism read as follows:

#### Acknowledgments

One of the authors (S. V. Ketov) would like to thank the Institute for Theoretical Physics, Leibniz University of Hannover, Germany, for kind hospitality extended to him during part of this investigation. This work is partially supported by the Japanese Society for Promotion of Science (JSPS) under the Grant-in-Aid programme for scientific research, and the bilateral German-Japanese exchange programme under the auspices of JSPS and DFG (Deutsche Forschungsgemeinschaft). The authors are grateful to D. Berenstein, E. Elizalde, A. Hebecker, L. Kofman, O. Lechtenfeld, U. Lindstrom, K. Maeda, N. Ohta, N. Sakai, I. Shapiro, and M. Vasquez-Mozo for discussions and correspondence. They also would like to thank the referees for their constructive remarks. (The authors are grateful to A. Morishita for his help with calculations.)