I’m only aware of one “cosmological constant” issue in physics.

When Albert Einstein formulated his new understanding of gravity and acceleration, he found that his description wouldn’t let the universe as a whole be static and stable. So he invented an *ad hoc* principle that permitted the universe to be balanced without collapsing into itself.

When the universal red shift was discovered, suggesting that the universe was expanding, he realized that he could have predicted this was the case, in the same way that the movement of the planets’ not quite matching our predictions were used to justify hypothesizing the existence of undiscovered planets whose gravity perturbed the orbits of the known ones.

Einstein considered his introduction of a ‘fudge factor’ to eliminate the mismatch between his model and observation to be the greatest failure of his career.

The only problem that I know of with the cosmological constant goes something like this: Having a non-zero cosmological constant results in predictions that match fairly well with the best observations we have. But, we have no good idea as to why the cosmological constant has the value that observations seem to be telling us it has. One possible reason for the value of the cosmological constant scientists considered was quantum fluctuations at very small and brief scales, but when this hypothesis was used to estimate the value of the constant, the result was enormously larger than the measured value. The takeaway seems to be that we still don’t know what causes the cosmological constant, and we also don’t have a great idea of what is going on with very small scale quantum fluctuations.

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