2002 research: speed of light slowing down?

That result has been controversial since the beginning. A comparable survey looking at a different part of the sky saw no effect, but the original authors and some new collaborators combined data from a most-of-the-sky survey and found hints that the fine-structure constant might be large in one direction of space and small in another.

One of the strengths of the quasar observation was that was based on spectroscopic observations of atomic transitions. Since a slight change to the fine-structure constant pushes some energy levels up and others down, there were transitions from the same sources which were both redder and bluer than predicted. This was the main argument against the effect being some sort of redshift miscalibration.

If the fine-structure constant is changing over time, or if Earth is moving through regions of space where the fine-structure constant has different values, those same sorts of energy-level shifts would occur on Earth. A long-running experiment has compared the atomic-clock transition in cesium, which should be relatively insensitive to changes in α, to a particular transition in dysprosium which should have enhanced sensitivity to changes in α. So far, no earthbound effect has been seen.

Conclusion: still an open question. Stay tuned.