I noticed this news today:
In a paper published today in the science journal Nature, an international team of astronomers predicts that foreground galaxies will affect images of extremely far galaxies.... This means that a significant fraction of far background galaxies will appear on the sky near foreground galaxies. The good news is that the remote galaxies will appear brighter because of a phenomenon called gravitational lensing.
Of course, this kills a lot of old Arp arguments, but I thought further, "Hmm, could such significant lensing explain the data that has, up till now, been the reason we think the expansion is accelerating?"
But no, as Goldsmith says,
"The cosmological constant's claim to a nonzero value fundamentally rests on the finding that distant Type Ia supernovae reach maximum brightnesses approximately 25 percent fainter than the peak brightnesses they would attain in a universe with a cosmological constant equal to zero."
So, going the other way, perhaps the expansion is accelerating more than we thought since some of the Sne IA data may have been lensed, meaning they were actually more than 25% fainter, but the lensing made them appear more luminous.
However, I doubt the supernova search teams overlooked such an effect.