# Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be off.

Would this also affect the estimates of the age of the universe, or would the differences of the speed of light would be so minute that it would not cause a drastic change?

• As the article says (in a massive understatement), “Some scientists are a bit skeptical, though.” Jul 12 '19 at 20:51
• There is plenty of evidence that it is, and no evidence that it is not. Jul 12 '19 at 20:53
• Possible duplicates: physics.stackexchange.com/q/2230/2451 and links therein. Jul 12 '19 at 21:11

Even if these theories are correct, it's a tiny effect- $$\sim 5\times 10^{-17}~{\rm s\over \sqrt{m}}$$ of fluctuation according to one of the papers. For the diameter of the observable universe ($$\sim9\times 10^{10}~\rm ly$$), that comes out to about $$400~\rm km$$, or $$5\times10^{-22}$$ times smaller. This is much, much less than the margin of error we already have for the radius of the observable universe, and so would not affect estimates of the universe's size or age at all.