content_image/297868.jpg content_image/297869.jpg content_image/297867.jpg
We can rearrange Eq. (2.11) slightly so that all four spacetime coordinates have the same scale factor, by defining a Conformal Time variable 

The line element now becomes 

<img src='content_image/297868.jpg'>

<img src='content_image/297869.jpg'>

Let us now assume that the content of the Universe is analogous to a perfect fluid , which is a fluid that has no shear stress, no viscosity and which does not conduct heat. It can be described entirely by its energy density ρ and its isotropic pressure P . In the local rest frame, the Energy- Momentum tensor becomes simply 

<img src='content_image/297867.jpg'>

We have not yet employed GR in the discussion regarding our FLRW Universe. All we have assumed so far is a uniformly expanding/contracting homogeneous and isotropic Universe, which has a content that can be de- scribed as a perfect fluid. We now take the “00” (“ tt ”) component of the 

