In the 1920s-1930s Friedmann, Lemaître, Robertson, and Walker independently modeled a universe with the premise that the matter in it would be observed to be uniform from anywhere in it. Since they did so independently and expressed their results with different choices of coordinates there are different coordinate expressions for the resulting line element and different sources will refer to the line element with different subsets of these names depending on who the source was relying upon as its reference matterial. RW and FRW are very common. The line element expressed in a very common choice of coordinates can be written
where k = -1 is the case of a negatively curved universe, k=0 is the case of a flat universe, and k = 1 is the case of a positively curved universe.
Where and this latter G is the gravitational constant of the universe.
A positive cosmological constant yields accelerating universe solutions and is sometimes refered to as "dark energy". Such an acceleration term has been observed in the expansion of the universe and so there is evidence that there exists a positive , dark energy. So lets look at the flat universe FLRW line element for the case of a positive cosmological constant.
First let us define a function of time as
Then the Einstein tensor for this line element can be written as
And yields a Ricci-scalar of
The solution from this equation for the Ricci-scalar for a positive cosmological constant is
Putting the big bang at yields , and choosing A appropriately results in the flat FLRW line element for a positive cosmological constant as
where is the age of the universe in this choice of time coordinate.
The resulting Einstein tensor is
which for very small times near the big bang becomes
The behavior of was derived for a Ricci-scalar equation above modeling the universe to contain only dark energy and electromagnetic radiation. In the early history of the real universe, much of the electromagnetic radiation made a phase transition to ordinary matter and dark matter. As such the amount of electromagnetic radiation actually observed left over in the cosmic microwave background radiation is only about 10-5 of the electromagnetic radiation energy density represented in this model.