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5.9 Primordial Perturbations of the Cosmological Metric and of the Inflaton

187

Assuming the slow-roll we can estimate the number of e-folds purely in terms of the potential. Indeed we can write:

H dN = −d log = H dt ˙

φ

W(φ)

2 W (φ)

(5.8.48)

 

φinit W (φ)

Ne = 2

 

φend W (φ)

The above integral formula is very useful in order to assess the validity of proposed potentials and as an a priori constraint on their parameters.

5.9Primordial Perturbations of the Cosmological Metric and of the Inflaton

The most significant success of the inflationary paradigm is its ability to interpret the observed anisotropies of the Cosmic Background Radiation in terms of primordial perturbations of the cosmological metric and of the scalar fields.

In this and in the next section we present an introduction to this very challenging field which is presently under very rapid development. For simplicity we focus on the simplest model of one scalar inflation, aiming at illustrating the basic ideas. The reader, however, should be conscious that the most promising scenarios, developed within the framework of supergravity, correspond to many scalar inflation with a lot of extra complicacies.

In order to study the small perturbations of the cosmological metric it turns out to be more convenient to utilize the so called conformal frame described in the following subsection.

5.9.1 The Conformal Frame

Starting from the general isotropic and homogeneous ansatz (5.4.4), for the reasons discussed above, we choose the flat universe case κ = 0 and we introduce the conformal time η by setting:

At the same time we set

 

dt = a(t) dη a(η) dη

 

(5.9.1)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

 

 

+ r2

2 + sin2(θ ) dφ2

=

i 1

 

dr

2

 

dxi2

(5.9.2)

=

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