# Special & General Relativity Questions and Answers

## Do the different estimates for the age of the universe imply that the speed of light may have changed?

No. At least not directly. The ages for globular cluster stars do not depend on the speed of light except via the propagation speed of light within the stars, and other atomic quantities. The expansion speed of the universe is determined by the redshifts measured for galaxies and their distances.

The 'expansion age' is just 2/3 of the reciprocal of the expansion rate called Hubble's constant. This calculation does involve the speed of light, and also depends on which method was used to determine the distance to the galaxy which might depend in various ways on the value of the speed of light. In converting from a redshift to an expansion speed, you use

```velocity = speed of light x Z
where Z = the change in the wavelength divided by the reference wavelength.
The value for Hubble's Constant, H =  expansion velocity / distance, and the
expansion age = 2/3 x 1/H.  If the speed of light were to have been
different long ago, the expansion speed could have been greater or less than
what we currently measure and so the age would be different due to this
'acceleration or deceleration' effect. A 10 percent change in the speed of
light would produce about a 10 percent change in the expansion speed, but to
get the kind of change needed to solve the current age paradox would
require at least a factor of 2 change in the speed of light over 10 billion
years or so. Such a large change would have serious repercussions for
stellar evolution and other physical phenomena.

There have been cosmological models proposed in the past that assume that
'light gets tired' only as it travels across intergalactic space, however, there
should be other measurable effects that could be detected locally that would
indicate a change in the speed of light over billions of years.
These models are not seriously considered today because they lead to so many
undesirable side effects that are not compatible with observational
evidence. As I mentioned in a
previous question, there is no
evidence that the speed of light changes.