A dynamical dark energy model with a given luminosity distance
It is assumed that the current cosmic acceleration is driven by a scalar field, the Lagrangian of which is a function of the kinetic term only, and that the luminosity distance is a given function of the red-shift. Upon comparison with Baryon Acoustic Oscillations (BAOs) and Cosmic Microwave Background (CMB) data the parameters of the models are determined, and then the time evolution of the scalar field is determined by the dynamics using the cosmological equations. We find that the solution is very different than the corresponding solution when the non-relativistic matter is ignored, and that the universe enters the acceleration era at larger red-shift compared to the standard model.
pacs:95.36.+x; 98.80.Cq; 98.80.Es
According to our current understanding about the cosmos, we live in a flat universe which expands in an accelerating rate and it is dominated by a dark component. Identifying the origin and nature of dark energy is one of the biggest challenges for modern cosmology. Since we can only speculate about what dark energy could be, many cosmological models have been proposed and studied so far. The simplest candidate is a cosmological constant with state parameter , but models with a dynamical component with an evolving state parameter also exist in the literature (for a review on dark energy models see e.g. Copeland:2006wr ()). Scalar field models with a non-canonical kinetic term have been discussed in k-inflation ArmendarizPicon:1999rj (), in which inflation is not due to the potential, but rather to the kinetic term in the lagrangian, and in k-essence models. ArmendarizPicon:2000dh (); ArmendarizPicon:2000ah (), which are designed to address the issue why the cosmic acceleration has recently begun. Very recently, in Bandyopadhyay:2011dh () the authors studied a k-essence model assuming i) a closed form parameterization for the luminosity distance (for distances in cosmology see e.g. Hogg:1999ad ()), proposed previously in Padmanabhan:2002vv (), and ii) that the Lagrangian of the scalar field does not depend on the field itself. However, in that work the matter has been ignored, and in the present article we wish to show that if the presence of matter is also taken into account, the time evolution of the scalar field is very different. We also find that according to this model, the universe enters the acceleration era at a larger red-shift compared to the standard model.
The theoretical framework is basically the same as in Bandyopadhyay:2011dh (), apart from the inclusion of matter in the cosmological equations, and the use of BAOs and CMB data to be mentioned later on. However, we start by giving a summary of all the ingredients we shall be using, so that the present work is self-contained. We focus here on the case of a flat universe, since this is a robust prediction of inflation Guth:1980zm (); Lyth:1998xn (), and it is also supported by current data Komatsu:2010fb (). The framework is based on four-dimensional General Relativity coupled to a single scalar field with a general Lagrangian , where is the scalar field and is the standard kinetic term. Therefore, our model is described by the action
The energy-momentum tensor for the scalar field is given by
where denotes differentiation with respect to . We can recast this energy-momentum tensor into the form of the energy-momentum tensor for a perfect fluid
where are the energy density and the pressure of the fluid respectively. The hydrodynamical quantities are given in terms of as follows
Therefore, the equations of motion for the system gravity+scalar field are just the first Friedmann equation and the equation for energy conservation
where is the Hubble parameter, is the scale factor and the overdot denotes differentiation with respect to cosmic time. Defining the sound speed of the scalar field
the equation for energy conservation takes the form
which generalizes the usual Klein-Gordon equation of a canonical scalar field in a Friedmann-Robertson-Walker background.
If we now include also the non-relativistic matter, the equations of motion for our system are the Friedmann equations and the conservation equation
where , is NewtonÂ´s constant, is the energy density of matter (), are the energy density and pressure of dark energy respectively, is the total energy density and is the total pressure. Assuming no interaction between dark energy and non-relativistic matter, we have two seperate conservation equations
where the second equation for dark energy is equivalent to the equation of motion of the scalar field. If we further assume that the Lagrangian of the scalar field depends on the kinetic term only, , the equation of motion can be integrated once
where is an arbitrary integration constant, which later on will be taken to be , and is the derivative of with respect to the kinetic term. All in all, there are three independent equations, namely the equation of the scalar field, the conservation equation for dust, and the second Friedmann equation. For a given Hubble parameter as a function of the red-shift , the time evolution of the scalar field can be found using the set of the basic cosmological equations. This will be useful later on (see equations (29) and (30), as well as the definitions (27) and (28)).
The comparison of a theoretical model against supernovae data relies on the minimization of
with the distance modulus, , where is the absolute magnitude and is the apparent magnitude. The theoretical apparent magnitude as a function of the red-shift is given by
where the luminosity distance for a flat universe can be expressed as
and is the speed of light, while is the Hubble parameter as a function of the redshift.
We also exploit the CMB shift parameter , since it is the least model dependent quantity extracted from the CMB power spectrum, i.e. it does not depend on the present value of the Hubble parameter . The reduced distance is written as
where is todayÂ´s value of the matter normalized density, and we use the CMB shift parameter value , as derived in Wang:2006ts ().
Independent geometrical probes are Baryon Acoustic Oscillations measurements. Acoustic oscillations in the photon-baryon plasma are imprinted in the matter distribution. These BAOs have been detected in the spatial distribution of galaxies by the SDSS Eisenstein:2005su () at a redshift . The SDSS team reports its BAO measurement in terms of the parameter
where the function is defined to be
and we have used the value . We shall assume that the scalar field Lagrangian, , is chosen so that the solution of the cosmological equations for the Hubble parameter leads to the luminosity distance
Then the CMB shift parameter as well as the BAO parameter, for a given matter density , are certain functions of the two parameters , and if we require that and the two parameters can be determined. Taking we find that the model is viable as long as the two free parameters take the values
and furthermore the distance modulus cannot be distinguished from the corresponding quantity when the matter is ignored, see Figure 1 below.
To find the time evolution of the scalar field, we introduce the dimensionless scalar field and time as follows
where is the age of the universe today, and we use the second Friedmann equation. The dimensionless time as a function of the red-shift is given by
while the dimensionless field as a function of the red-shift is given by
where , and is given in terms of the assumed (dimensionless) luminosity distance by
with having the values determined before, and we have used that . Finally, eliminating the red-shift we obtain as a function of , and the solution can be seen in Figure 2 below. In the same figure we also show the polynomial of second degree that best fits the solution in the same interval, which is the following
We see that the solution exhibits a global minimum at , which corresponds to a red-shift . This is the value for which the two terms in (30) cancel eachother.
Before ending our discussion, let us compare the deceleration parameter of the standard model with that of our model. Using the definitions, the deceleration parameter as a function of the red-shift is given by
and can be seen in Figure 3, where for we have used the values . In our model the universe enters into the acceleration era at , while in the standard cosmological model this takes place at later times.
In summary, in the present work we have assumed that the current cosmic acceleration is driven by a scalar field, the Lagrangian of which is a function of the kinetic term only, and that the luminosity distance is a given function of the red-shift. Upon comparison with Baryon Acoustic Oscillations (BAOs) and Cosmic Microwave Background (CMB) data the parameters of the models are determined, and then the time evolution of the scalar field is obtained by integrating the cosmological equations numerically. We find that the solution is very different than the corresponding solution when the non-relativistic matter is ignored, and that the universe enters the acceleration era at larger red-shift compared to the standard model.
Here we briefly discuss how a given function that vanishes at the origin can be fitted by a polynomial of second degree in a given interval . We require that the deviation of the function from the polynomial is as lower as it can be, or in other words the integral
must be extremized. If we set the first derivatives of with respect to equal to zero
we obtain a two-by-two system
where we have defined
for and . For the given interval and function the above integrals can be computed, and solving the simple algebraic system we find the values of the coefficients given in the text.
The author acknowledges financial support from FPA2008-02878 and Generalitat Valenciana under the grant PROMETEO/2008/004.
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