During experiments attempting to measure the gravitational constant by doing the experiment inside a deep mine shaft, Australian researchers got a discrepency between the predicted and measured value, with the measured value being two percent too small. They concluded that there was a repulsive fifth force with a range from a few centimetres to a kilometre. Similar experiments have been carried out onboard a submarine (USS Dolphin (AGSS-555)) while deeply submerged.
If this is the fifth force, it would have to have force carriers that somehow vary in mass or some analogous property to explain its strange range.
Critics of this conclusion said that the discrepancy could have been caused by a hidden lode of metal above the shaft. In addition, they pointed out that the gravitational constant had only been measured to its first two digits.
For entirely different reasons, some physicists hold that a repulsive force with infinite range that acts on nucleons and not on a property of them exists. They say that this force would be weaker than gravity, and have either a constant strength or a strength that varies directly with distance. If this is true, at the far distances near the visible edge of the universe, this force would be stronger than gravity and thus contribute to an accelerating universe.
The force mediator for this force would have zero mass.
However, the two above hypotheses are incompatible with each other.
See also: cosmological constant\n