My understanding is that the acceleration rates in most models have different epochs, with plateus and peaks, rather than being a constant or exponential acceleration. There are more than a few distinctly different epoch models, as far as I know.
I'll have to defer to "Physicist" and ThinkPlease" on details such as those....
There are interval estimates and there are point estimates. The two groups' point estimates disagree by about two billion years. Richer said himself his best guess was 13 billion years.
I'm not sure about the error in either of these studies. I've never heard of the 5% standard you cite. For this article, they seem to be using .5 billion ly vs. 5%. But you're definitely right about the contradiction thingy; i.e., I should've said discrepancy instead of contradiction because even if the interval estimates do not overlap we wouldn't necessarily have a contradiction in the strict sense of the word.
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