# Cosmological constant

LSulayman
Since the discovery of an accelerating expansion in the late 1990s, the cosmological constant has been included in cosmological models for dark energy.

Einstein included the cosmological constant to make the universe static. Dark energy makes the universe expand accelerating. So in the 2 models the cosmological constant explains 2 different things. How is that possible?

The difference between Einstein's cosmological constant and the present one is simply one of magnitude. Einstein chose the cosmological constant to be precisely the right number that he believed would balance out the contracting tendency of a gravitationally dominated universe. When we remove the restriction that the cosmological constant must have that value to make the universe stationary, we can emprically measure what it is and indeed we find a small positive number which is larger than what Einstein had originally postulated (that is, instead of simply balancing the contraction, it actually overpowers the contraction and accelerates the expansion).

cbd1
The difference between Einstein's cosmological constant and the present one is simply one of magnitude. Einstein chose the cosmological constant to be precisely the right number that he believed would balance out the contracting tendency of a gravitationally dominated universe. When we remove the restriction that the cosmological constant must have that value to make the universe stationary, we can emprically measure what it is and indeed we find a small positive number which is larger than what Einstein had originally postulated (that is, instead of simply balancing the contraction, it actually overpowers the contraction and accelerates the expansion).

So, when we refer to a "positive" cosmological constant, this means it is greater than Einstein's original value, where a value smaller than Einstein's would be considered negative?

And, this may seem like a novel question, but when we empirically measure the cosmological constant, what is actually measured?

Is calculating the cosmological constant, on a basic level, taking the current expansion rate and figuring the gravitational attraction of everything and seeing if the expansion will decelerate with gravity or if it will speed up?

And another question, if anyone can answer it. If we assume the current rate of expansion--ignoring the supernova data--what value for the cosmological constant would we need to have the universe eventually stop expanding and contract?

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