How faint would a dwarf have to be for the universe to be 20 billion years old? Really really faint, I guess. :)
They apparently are running at the edge of the sensitivity of the old Hubble scope equipment; this is why they selected M4 -- they just didn't have enough sensitivity to see dwarfs that old any farther away that M4 is.
It's evident that this method is better suited for setting a lower limit on the age, given all the standard physics assumptions, than an upper limit. I'm still skeptical of the "found the faintest one" concept. I'm not even so sure that clusters weren't produced until a fairly long time after the "big bang", or that light isn't reddening due to the expansion of space beyond the effect of recession. Obviously, if the rate of expansion and recession were the same, it wouldn't have been much of a bang.
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