He added: "We think we have seen the faintest ones. If we haven't, then we'll have to rethink" the conclusions.
What if some of the white dwarfs have already "gone out"? How would you know?
the Hubble Space Telescope collected light from M4 for eight days over a 67-day period. Only then did the very faintest of the white dwarfs become visible.
If they collected light for an additional 8 days, or 80 days, would additional, "dimmer" white dwarfs become visible?
This was addressed in the press conference today. They took exposures that were adequate to detect fainter white dwarfs if they existed, but they found none; hence they conclude that these ARE the faintest.
White dwarf stars take a gigantically long time to "go out" (which in this case means to cool until they cannot be seen).