Abstract

We study the effects of the nonminimal derivative coupling on the dissipative dynamics of the warm inflation where the scalar field is nonminimally coupled to gravity via its kinetic term. We present a detailed calculation of the cosmological perturbations in this setup. We use the recent observational data from the joint data set of WMAP9 + BAO + and also the Planck satellite data to constrain our model parameters for natural and chaotic inflation potentials. We study also the levels of non-Gaussianity in this warm inflation model and we confront the result with recent observational data from the Planck satellite.

1. Introduction

Although the hot big bang scenario has considerable successes in explaining the general properties of our universe, this model cannot address properly some issues such as flatness, horizon, and relics problems. The latest observations show that a period of accelerating expansion (inflation) during the early history of the universe with () can solve these problems. Inflation also provides the primordial perturbations which seed the formation of structure in the universe [1]. Despite the great successes of the inflation paradigm, there is a problem for realization of this scenario that we do not know how to integrate it with ideas of particle physics. Also problems such as unexpected low power spectrum at large scales and egregious running of the spectral index [2] are some yet unsolved problems in this paradigm. From a thermodynamic viewpoint, there are two different dynamical realizations of inflation which are called cold and warm inflation. In the standard inflationary model, called supercooled or isentropic inflation, the universe rapidly supercools during the inflation and its temperature decreases, which is a result of the exponential expansion. When the inflation ends, a reheating phase occurs, which explains the subsequent processes and evolution. The fluctuations created during inflation are zero-point ground state fluctuations and evolution of the inflaton field is governed by the ground state evolution equations. The other picture of inflation dynamics is the warm or nonisentropic inflation [3–5]. In this picture, the dissipative effects are significant. So, that radiation production and inflationary expansion occur concurrently. Several mechanisms for implementing such a dissipation during inflation have been proposed [6, 7]. In warm inflationary models, the sources of density fluctuations are the thermal fluctuations. Matter fields interact with particles that are in a thermal bath with mean temperature smaller than the grand unified theories (GUT) critical temperature [8]. The most important candidate for the inflaton is the Standard Model’s Higgs field. Nevertheless, if Higgs field is minimally coupled to gravity, it cannot realize a good inflation stage [9]. To preserve the Higgs field as an inflaton candidate, a model has been postulated in which the Higgs field is nonminimally coupled to gravity [10]. It has been argued in [11, 12] that this nonminimal coupling breaks the unitary bound by giving rise to nonrenormalizabe operators. However, the authors in [13] have shown that this is not actually the case and the unitary bound is preserved in this setup. On the other hand, if gravity is nonminimally coupled to derivatives of the scalar field, the unitary bound is preserved during inflation. So, this model can explain the inflationary phase properly [14, 15]. In recent years, the issue of inflation and also the late time accelerated expansion with nonminimal derivative coupling are studied in detail [16–20]. Nevertheless, the warm inflation dynamics in the context of nonminimal derivative coupling and perturbations in this setup have not been studied yet. Therefore, the present study is devoted to filling this gap.

With these preliminaries, the goal of the present study is to investigate the effects of the nonminimal derivative coupling on the dissipative dynamics of the warm inflation. We investigate the effects of the nonminimal derivative coupling and dissipation on the inflationary dynamics of the model. Then, we present a detailed calculation of the perturbations in this framework. By using the recent observational data from WMAP9 + BAO + and also the Planck satellite data, we constrain our model parameters for natural and chaotic inflation potentials. Also we study the levels of non-Gaussianity in this scenario and we confront the obtained results with the recent observational data from the Planck satellite.

2. The Setup

The action of a 4-dimensional model in the presence of the nonminimal derivative coupling between the scalar field and gravity is given by where is the Einstein tensor and is a constant having the dimension of mass and  GeV is the reduced Planck mass. is the Ricci scalar and is the potential of the scalar field . Variation of action (1) with respect to the scalar field gives the following equation of motion: If we consider phenomenologically a dissipation coefficient that is responsible for the decay of the scalar field into the radiation during the inflationary regime, the scalar field equation of motion will be as follows: can be assumed to be a constant or a function of the scalar field or the temperature or both of them. In the slow-roll approximation where and , the scalar field equation of motion takes the following form: Note that the sufficient condition for canonical normalization of the scalar field is given by [14, 15]. The energy density and the pressure of the scalar field are given by Also, the energy conservation equation of the model is given by During the inflaton evolution, the dissipation effect leads to the production of entropy. The entropy density of the radiation in this nonminimal setup is defined by . Now the total energy density of the system is given by and using the relation (6), the rate of entropy production can be deduced. In this setup, entropy production is affected by the nonminimal derivative coupling between the scalar field and gravity. This can be seen more explicitly in the following relation: where by (4), is directly related to the nonminimal derivative coupling. The basic idea of the warm inflation is that radiation production is occurring concurrently with inflationary expansion due to dissipation from the inflaton field system. The equation of state for radiation is given by . Therefore, the continuity equation for the radiation yields the following result: We assume a quasi-stable radiation production during the warm inflation phase [21, 22]; that is, and . So we can write Substituting (4) into this relation, we get Now we define the dissipation factor as follows: which is a dimensionless parameter. With this definition, we can write During the slow-roll stage, the scalar field evolution is damped. For the high (or weak) dissipation regime we have (or ), respectively.

Now, in the spatially flat FRW background, the Friedmann equation can be written as follows:

The slow-roll regime can be parametrized by a set of slow-roll parameters , , and , defined by

Inflation can be realized only if ; once one of these parameters reaches the unity, the inflation phase terminates. We need to calculate these parameters in our nonminimal derivative coupling model. For we get and the second slow-roll parameter becomes And finally, the third slow-roll parameter takes the following form: where, as usual, a prime marks the differentiation with respect to . The number of e-foldings during inflation is defined as which in the slow-roll approximation can be written as where is the value of the scalar field at the end (beginning) of the inflationary phase.

3. Perturbations

In this section we study scalar and tensor perturbations in our model. The scalar perturbations can be decomposed to entropy or isocurvature perturbations and adiabatic or curvature perturbations. In the framework of warm inflation, the scalar and radiation fields are interacting. Therefore, isocurvature perturbations are generated besides the adiabatic ones. This occurs because warm inflation can be considered as an inflationary model with two basic fields. Dissipative effects can produce a variety of spectral indexes, ranging between red and blue and thus producing the running blue to red spectral index suggested by WMAP data [23–25]. In the longitudinal gauge, the scalar metric perturbations of the FRW background are given by [26, 27] where is the scale factor and  and are the metric perturbations. The spatial dependence of perturbed quantities is in the form of plane waves , where is the wave number. Therefore, the perturbation of the metric leads to the perturbation in the energy-momentum tensor through Einstein’s field equations. The perturbed Einstein’s field equations can be obtained as follows: where appears from the decomposition of the velocity field as , , [16] and we have omitted the subscript . We perturb the scalar field equation of motion and the continuity equation for the radiation field as

We consider these equations in the slow-roll approximation and on large scales where [28, 29]. In this situation, (26) become, respectively, and (24) takes the following form: where we have used the relation of the velocity field as .

We can solve the above set of equations by substituting (29) into (27), where we get Now, by defining a function as (30) can be rewritten as

A solution of this equation is given as where is an integration constant. So, from (31), we find For simplicity we define the following quantity:

With this definition, we can rewrite (34) as and, therefore, the density perturbation is given by

The fluctuations of the scalar field are generated by thermal interaction with the radiation field, instead of quantum fluctuations where the wave number is defined by and corresponds to the freeze-out scale at which dissipation damps out the thermally excited fluctuations. So, the density perturbation amplitude becomes

The scalar spectral index is given by The interval in wave number is related to the number of e-foldings by the relation So we obtain

The running of the spectral index which is defined as in our model takes the following form:

Now we pay attention to the tensorial perturbations. As has been mentioned in [30], the generation of tensor perturbations during inflation period produces stimulated emission in the thermal background of gravitational waves. This process changes the power spectrum of the tensor modes by an extra, temperature-dependent factor given by . Therefore, the spectrum of tensor perturbations is given by The ratio between the amplitudes of tensor and scalar perturbations (tensor-to-scalar ratio) is given by where .

This is the complete treatment of perturbations in this setup. In what follows, we consider two specific inflaton potentials to study the inflationary aspects of this model and confrontation with recent observational data from Planck + WMAP9 + BAO + [23–25].

4. Specific Examples

4.1. Chaotic Inflation

With , the number of e-foldings in the slow-roll approximation at the end of the inflation is given by where , , and are defined as follows: Note that if we set for the end of the inflation, the dimensionless field at the end of the inflation attains the following form: Substituting (49) into (47), we find as follows:

Using (42) and (46), the scalar spectral index and the tensor-to-scalar ratio in the warm chaotic inflation are given by respectively, where is given by the relation (50). The left panel of Figure 1 shows the behavior of the tensor-to-scalar ratio versus the scalar spectral index with . For , the model with , for , the model with , and for , the model with lie inside the CL of the Planck + WMAP9 + BAO + joint data. We have also plotted the evolution of the running of the scalar spectral index versus the scalar spectral index in the right panel of Figure 1. For this case, the running is negative and in some range of compatible with recent observational data. From another perspective, we can fix at, for instance, and let vary in order to see the effect of nonminimal derivative coupling. In Figure 2, we survey the effects of the nonminimal derivative decoupling. The left panel of this figure shows the behavior of the tensor-to-scalar ratio versus the scalar spectral index with . For , the model with , for , the model with , and for , the model with are compatible with observational data. Also, evolution of the running of the scalar spectral index versus the scalar spectral index in the background of Planck + WMAP9 + BAO + joint data is shown in the right panel of Figure 2. For this case, the running is negative in some range of and lies inside the CL of the Planck + WMAP9 + BAO + .