# Slow-Roll Inflation in Scalar-Tensor Models

###### Abstract

The linear and quadratic perturbations for a scalar-tensor model with non-minimal coupling to curvature, coupling to the Gauss-Bonnet invariant and non-minimal kinetic coupling to the Einstein tensor are developed. The quadratic action for the scalar and tensor perturbations is constructed and the power spectra for the primordial scalar and tensor fluctuations are given. A consistency relation that is useful to discriminate the model from the standard inflation with canonical scalar field was found. For some power-law potentials it is shown that the Introduction of additional interactions, given by non-minimal, kinetic and Gauss-Bonnet couplings, can lower the tensor-to-scalar ratio to values that are consistent with latest observational constraints, and the problem of large fields in chaotic inflation can be avoided.

## 1 Introduction

The improvement in the quality of the cosmological observations of the last years [1, 2, 3, 4] has reinforced the theory of cosmic inflation [5, 6, 7]. The inflationary theory gives by now the most likely scenario for the early universe, since it provides the explanation to flatness, horizon and monopole problems, among others, for the standard hot Bing Bang cosmology [8, 9, 10, 11, 12, 13]. In other words, the inflation can set the initial conditions for the subsequent hot Big Bang, by eliminating the fine-tuning condition needed for solving the horizon, flatness and other problems. Besides that, the quantum fluctuations during inflation could provide the seeds for the large scale structure and the observed CMB anisotropies [14, 15, 16, 17, 18, 19, 20]. In particular, inflation allows as to understand how the scale-invariant power spectrum can be generated, though it does not predict an exact scale invariant but nearly scale invariant power spectrum. The deviation from scale invariance is connected with the microphysics description of the inflationary theory which is still incomplete.

The simplest and most studied model of inflation consists of minimally-coupled scalar field with flat enough potential to provide the necessary conditions for slow-roll [6, 7]. But the inflation scenario can be realized in many other models like non-minimally coupled scalar field [21, 22, 23], kinetic inflation [24], vector inflation [25, 26, 27], inflaton potential in supergravity [28, 29, 30], string theory inspired inflation [31, 32, 33], Dirac-Born-Infeld inflation model [34, 35, 36, 37], -attractor models originated in supergravity [38, 39, 40, 41]. Apart from the DBI models of inflation, another class of ghost-free models has been recently considered, named ”Galileon” models [42, 43]. In spite of the higher derivative nature of these models, the gravitational and scalar field equations contain derivatives no higher than two. The effect of these Galileon terms is mostly reflected in the modification of the kinetic term compared to the standard canonical scalar field, which in turn can improve (or relax) the physical constraints on the potential. In the case of the Higgs potential, for instance, one of the effects of the higher derivative terms is the reduction of the self coupling of the Higgs boson, so that the spectra of primordial density perturbations are consistent with the present observational data [44, 45] (which is not possible within the standard canonical scalar field inflation with Higgs potential). Different aspects aspects of Galilean-inflation have been considered in [44, 45, 46, 47, 48, 49]. A particular and important case belonging to the above class of models is the scalar field with kinetic coupling to the Einstein tensor [50, 51, 52, 53, 54] whose application in the context of inflationary cosmology has been analyzed in [55, 56, 57].

In the present paper we consider a scalar-tensor model with non-minimal coupling to scalar curvature, non-minimal kinetic coupling to the Einstein tensor and coupling of the scalar field to the Gauss-Bonnet 4-dimensional invariant, to study the slow-roll inflation and the observable magnitudes, the scalar espectral index and the tensor-to-scalar ratio, derived from it. For studies of inflation with GB coupling see, for instance
[58, 59, 60, 61, 62, 63, 64].
The interaction terms presented in this model have direct correspondence with terms presented in Galileon theories [49] and also give rise to second order gravitational and scalar field equations.
In the appendixes we develop in detail the linear and quadratical perturbations for all the interaction terms of the model and deduce the second order action for the scalar and tensor perturbations.
In appendix A we present the basic formulas for the first order perturbations, needed for the model, in the Newtonian gauge. In appendix B we deduce the gravitational and scalar field equations in a general background. In appendix C and D we give the first order perturbations of the field equations in the Newtonian gauge. In Appendix E we give the details for constructing the second order action using the Xpand tool [syril], and
in appendix F we give a detailed description of the slow-roll mechanism for the minimally coupled scalar field.

The expressions for the primordial density fluctuations in terms of the slow-roll parameters and the corresponding power spectra were found. We have found a consistency relation that is useful to discriminate the model from the standard inflation with canonical scalar field. The latest observational data disfavor monomial-type models with in the minimally coupled scalar field. With the Introduction of additional interactions like the non-minimal coupling, kinetic coupling and Gauss-Bonnet coupling (GB), it is shown that the tensor-to-scalar ratio can be lowered to values that are consistent with latest observational constraints [2, 3]. This is sown in the case of quadratic potential with non-minimal and kinetic coupling, quadratic potential with kinetic and GB coupling and the general power-law potential with GB coupling.

The paper is organized as follows. In the next section we introduce the model, the background field equations and define the slow-roll parameters. In section 3 we use quadratic action for the scalar and tensor perturbations (details are given in the appendix) to evaluate the primordial power spectra. In section 4 we work some explicit models. Some discussion is given in section 5.

## 2 The model and background equations

We consider the scalar-tensor model with non-minimal coupling of the scalar field to curvature, non-minimal kinetic coupling of the scalar field to the Einstein’s tensor and coupling of the scalar field to the Gauss-Bonnet (GB) 4-dimensional invariant

(2.1) |

where is the Einstein’s tensor, is the GB 4-dimensional invariant given by

(2.2) |

(2.3) |

and . One remarkable characteristic of this model is that it yields second-order field equations and can avoid Ostrogradski instabilities. Using the general results of Appendix B, expanded on the flat FRW background

(2.4) |

one finds the following equations

(2.5) |

(2.6) | ||||

(2.7) | ||||

where () denotes derivative with respect to the scalar field. Related to the different terms in the action (2.1) we define the following slow-roll parameters

(2.8) |

(2.9) |

(2.10) |

(2.11) |

The slow-roll conditions in this model are satisfied if all these parameters are much smaller than one, and will be used in the next section. From the cosmological equations (2.5) and (2.6) and using the parameters (2.8)-(2.11) we can write the following expressions for and

(2.12) | ||||

(2.13) | ||||

where we used

(2.14) |

It is also useful to define the variable from Eq. (2.13) as

(2.15) |

where it follows that . Notice that for the simplest case of minimally coupled scalar field (, ), the Eqs. (2.12) and (2.13) give the standard equations

Under the slow-roll conditions and , it follows from (2.5)-(2.7)

(2.16) |

(2.17) |

(2.18) |

showing that the potential gives the dominant contribution to the Hubble parameter, while Eqs. (2.17) and (2.18) determine the dynamics of the scalar field in the slow-roll approximation. The number of -folds can be determined from

(2.19) |

where and are the values of the scalar field at the beginning and end of inflation respectively, and the expression for was taken from (2.18). The criteria for choosing the initial values will be discussed below.

## 3 Quadratic action for the scalar and tensor perturbations

Scalar Perturbations.

After the computation of the second order perturbations we are able to write the second order action for the scalar perturbations as follows

(3.1) |

where

(3.2) |

(3.3) |

with

(3.4) |

(3.5) |

(3.6) |

(3.7) |

And the sound speed of scalar perturbations is given by

(3.8) |

The conditions for avoidance of ghost and Laplacian instabilities as seen from the action (3.1) are

We can rewrite , , and in terms of the slow-roll parameters (2.8)-(2.11) and using Eqs. (2.13) and (2.14), as follows

(3.9) |

(3.10) |

(3.11) |

(3.12) | ||||

The expressions for and in terms of the slow roll parameters can be written as

(3.13) |

(3.14) |

where

(3.15) |

Notice that in general and . Also in absence of the kinetic coupling it follows that . Keeping first order terms in slow-roll parameters, the expressions for y reduce to

(3.16) |

(3.17) |

To normalize the tensor perturbations we perform the change of variables [kobayashi] (see (F.7))

(3.18) |

and the action (3.1) becomes

(3.19) |

where ”prima” indicates derivative with respect to . Working in the Fourier representation, one can write

(3.20) |

and the equation of motion for the action (3.19) takes the form

(3.21) |

From (3.18), and keeping up to first-order terms in slow-roll variables in (LABEL:slr8e) and (LABEL:slr8f), we find the following expression for

(3.22) |

where

Then, under the approximation of slowly varying and up to first-order in slow-roll variables we find the following expression for

(3.23) |

This expression reduces to the one of the canonical scalar field given in Appendix I, Eq. (F.24), in the case where and , with defined in (F.22). In what follows the reasoning is similar to the simplest case, corresponding to minimally-coupled scalar field, which is analyzed in detail in Appendix I. So on sub-horizon scales when the term dominates in Eq. (3.21) we choose the same Bunch-Davies vacuum solution defined for the scalar field, which leads to

(3.24) |

Note that from the expression

(3.25) |

in the approximation of slowly varying and one can integrate the last equation to obtain

(3.26) |

Then in the limit for de Sitter expansion it follows that

(3.27) |

In this last case and neglecting the slow-roll parameters (in this limit ) we can write from (3.23)

(3.28) |

which allows the integration of Eq. (3.21), giving the known solution for the scalar perturbations in a de Sitter background. Taking into account the slow-roll parameters and using (3.26) we can rewrite the Eq. (3.21) in the form

(3.29) |

where

(3.30) |

where we have expanded up to first order in slow-roll parameters. The general solution of Eq. (3.30) for constant (slowly varying slow-roll parameters) is

(3.31) |

and after matching the boundary condition related with the choosing of the Bunch-Davies vacuum (3.24) we find the solution

(3.32) |

using the asymptotic behavior of at , we find at super horizon scales ()

(3.33) |

To evaluate the power spectra we use the relationship

(3.34) |

where we used (3.26) for , and for the last equality we have expanded up to first order in slow-roll parameters, resulting in

(3.35) |

Assuming again the approximation of slowly varying slow-roll parameters we can Integrate this equation to find

(3.36) |

which gives, in the super horizon regime, for the amplitude of the scalar perturbations the following expression

(3.37) |

where the dependence disappears as expected from the solution (3.33) in super horizon scales (). The power spectra for the scalar perturbations takes the following -dependence

(3.38) |

and the scalar spectral index becomes

(3.39) |

It is worth noticing that the slow-roll parameter , related to the kinetic coupling, do not appear in the above expression for the scalar spectral index. This is because appears only in second order terms (or higher) in the expressions for and (see (3.13) and (3.14)).

Tensor perturbations.

The second order action for the tensor perturbations takes the form

(3.40) |

where and are defined in (E.2) and (E.3) (in terms of the slow-roll variables in (3.18) and (3.19)) and the velocity of tensor perturbations is given by

(3.41) |

As in the case of scalar perturbations, in order to canonically normalize the tensor perturbations the following variables are used [49]

(3.42) |

leading to the quadratic action

(3.43) |

which gives the equation

(3.44) |

Or for the corresponding Fourier modes

(3.45) |

which is of the same nature as the equation for the scalar perturbations, and therefore the perturbations on super horizon scales behave exactly as the solutions (F.6). For the evaluation of the primordial power spectrum we follow the same steps as for the scalar perturbations. To this end we write the expression for , up to first order in slow-roll parameters, as follows

(3.46) |

Then, the normalized solution of (3.45) in the approximation of slowly varying slow-roll parameters can be written in terms of the Hankel function of the first kind as

(3.47) |

where the tensor describe the polarization states of the tensor perturbations for the -mode, and

(3.48) |

At super horizon scales () the tensor modes (3.47) have the same functional form for the asymptotic behavior as the scalar modes (3.33), and therefore we can write power spectrum for tensor perturbations as

(3.49) |

where , and the sum over the polarization states must be taken into account. Then, the tensor spectral index will be given by

(3.50) |

An important quantity is the relative contribution to the power spectra of tensor and scalar perturbations, defined as the tensor/scalar ratio

(3.51) |

For the scalar perturbations, using (3.38), we can write the power spectra as

(3.52) |

where

and all magnitudes are evaluated at the moment of horizon exit when (). For we used (3.18) with . In analogous way we can write the power spectra for tensor perturbations as

(3.53) |

where

Noticing that when evaluated at the limit , as follows from (3.35) and (3.48), we can write the tensor/scalar ratio as follows

(3.54) |

taking into account the expressions for given in (3.9), (3.10), (LABEL:slr8e) and (LABEL:slr8f), up to first order, and using the condition , then we can see that (in fact in the limit , independently of the values of and ) and we can make the approximation

(3.55) |

which is a modified consistency relation due to the non-minimal and GB couplings. In the limit it gives the expected consistency relation for the standard inflation

(3.56) |

with . Taking into account the non-minimal and GB couplings we find the deviation from the standard consistency relation in the form

(3.57) |

with given by (3.50). Thus, the consistency relation is still valid in the case of non-minimal coupling, and if there is an observable appreciable deviation from the standard consistency relation, it can reveal the effect of an interaction beyond the the simple canonical scalar field or even non-minimally coupled scalar field models of inflation. It is worth noticing that in the first-order formalism the kinetic-coupling related slow-roll parameter does not appear in the spectral index for the scalar and tensor perturbations and is also absent in the tensor-to-scalar ratio, appearing only starting form the second order expansion in slow-roll parameters. Nevertheless, all the couplings are involved in the definition of the slow-roll parameters trough the field equations. Of special interest are the cases of monomial potentials . These potentials are disfavored by the observational data for in the minimally coupled model. As will be shown for some cases, with the GB and (or) kinetic coupling added, the spectral index and especially the scalar-to tensor ratio can be accommodated within the range of values obtained from the latest observational data.

## 4 Some explicit cases

Model I.

First we consider the particular case of the non-minimal coupling with quadratic potential and kinetic coupling with constant .

(4.1) |

Using the Eqs. (2.16) and (2.18) we can express the slow-roll parameters (2.8)-(2.11) in therms of the potential and the coupling functions, and once we specify the model, we can find the slow-roll parameters in terms of the scalar field and the coupling constants. For the model (4.1) the slow-roll parameters take the form

(4.2) |

where is dimensionless ( has been rescaled as to measure it in units of ) and has dimension of . Additionally, the scalar field at the end of inflation can be evaluated under the condition . Sitting in (4.2) it follows

(4.3) |

From Eq. 2.19 it follows that the number of -foldings can be evaluated as

(4.4) |

This expression allows us to evaluate for a given . We can make some qualitative analysis by assuming that and . In this case from (4.3) it is found that

(4.5) |

and from (4.4) we find for

(4.6) |

giving an approximate relation between the values of the scalar field at the beginning and end of inflation as

So, assuming gives . This will have sense only if the scalar spectral index and the tensor-to-scalar ratio behave properly. In fact from (4.2) and replacing in (3.39) and (3.55), we find (under the condition and )

(4.7) |

and

(4.8) |

where we have used (4.6) for . Additional simplification can be made if we assume that the scalar field at the beginning of inflation is of the order of (). This can be achieved if , as follows from (4.6), which gives

(4.9) |

Thus, for 60 -foldings we find and (). In this case the inflation begins with and ends with . For the numerical analysis with the exact expressions, we assume , . In fact from Eqs. (4.2) follows that the spectral index and the tensor-to-scalar ratio depend on the dimensionless combination . Fig. 1 shows the behavior of and in the interval , for .

Model II

The following example considers a model with kinetic and GB couplings

(4.10) |

where the constant has dimension of and is measured in units of . The slow-roll parameters from , necessary to evaluate and , take the form

(4.11) |

The scalar field at the end of inflation is obtained from the condition , which gives

(4.12) |

And From Eq. (2.19), the number of -foldings can be evaluated as

(4.13) |

which allows to find for a given and from (4.12). From (3.39) and (4.11) we find the scalar spectral index as

(4.14) |

And from (3.55) and (4.11) we find the expression for the tensor-to-scalar ratio as

(4.15) |

For and taking we can find the behavior of and in terms of the dimensionless parameter . In Fig. 2 we show the behavior of the scalar field at the beginning and end of inflation for

In Fig. 3 we show the corresponding behavior of and .

Model III.

The following model considers the general power-law potential and non-minimal power-law functions for the GB and kinetic couplings