How can white dwarf supernovae tell us that the universe is expanding at an accelerating rate?
White dwarf supernovae are used because they are good standard candles which can be used to out to very far distances. From the redshift of a distant supernova, you can tell the average distance between the galaxies at the time the light was emitted: this is because the light has been stretched by cosmological redshift the same amount the universe has stretched since the light was emitted. By measuring the brightness of the supernovae, and comparing that to what you expect for the different scenarios, you can tell whether the universe has been accelerating or not. We see very distant supernovae as dimmer than they would be for a constant expansion rate universe. This means that they are fact farther away than they would be for constant expansion- so the expansion must have been speeding up.
