To what accuracy do we know the value of the cosmological constant? The Wikipedia article just says that the cosmological constant 
"is measured to be on the order of $10^{−52} \text{ m}^{−2}$," but the only citation goes all the way back to 2004.  What is $\Lambda$'s current measured value and error bars?
 A: From the Planck satellite collaboration the 2015 results from the CMB measurements and other data then available are best depicted in terms of the percent of mass/energy (called the dark energy) that is due to the cosmological constant, and the total mass/energy density. From the Wikipedia article at https://en.m.wikipedia.org/wiki/Lambda-CDM_model, they are:
1) Fraction of mass/energy that is dark energy $\Omega_\Lambda = 0.6911 \pm 0.0061$
2) the critical density $\rho_\text{crit} = (8.62 \pm 0.12) \times 10^{−27} \text{ kg}/\text{m}^3$
3) The actual measured total mass/energy density parameter $\Omega_{tot} = 1.0023^{+0.0056}_{−0.0054}$
Note that to get the actual mass/energy density you need to multiply item 2 by item 3. It's just that the above are the ways each is stated in measurements and derived parameters. The fact that $\Omega_{total}$ is 1 within the error uncertianties says that the universe is flat, within that uncertainty. The fact that the number is not exact, but has an error uncertainty means we are still not certain it is flat, but feel pretty good about it. 
Note also that there are various other measurements that are accounted for in those numbers above. The Wikipedia article shows what they are. It is the standard procedure for what is called the concordance model. If you mistrust some of those there are other, very close but not identical, values in the table in Wikipedia.
