# Positive energy density of the universe

There is a Baez essay about the vaccum energy density, where he says:
The reason they get a positive energy density is very interesting. Thanks to the redshifts of distant galaxies and quasars, we've known for a long time that the universe is expanding. The new data shows something surprising: this expansion is speeding up. Ordinary matter can only make the expansion slow down, since gravity attracts - at least for ordinary matter.

So did they do what I might naively think of doing, namely just plotting the redshift data over a few years? Or is it a more subtle method that directly measures the time derivative of the expansion?

And if I understand correctly the vacuum energy density is the same thing as whatever is driving the acceleration in the expansion. Does that imply that, at least in principle, the Casimir force is changing over time?

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