The Speed of Light and the Size of the Universe


Is the speed of light slowing? Read on!

The purpose of this page is to explore the significance of the speed of light, in calculating the size of the universe. Also, there is evidence that the speed of light was much higher, and is falling. That concept will be examined with data from the experts.


Scientists generally believe that all matter emanated from a common point. (The Big Bang theory) Further, the assumption is made that objects in the universe have always moved apart, and are continuing to move apart, at the same velocity. The rate of expansion is uncertain, but many accept the rate of expansion as one-fifth the speed of light.

The speed of light is believed to be constant at 299,792.4586 +/- 0.00003 km/ sec Or, approximately:

300,000 km/sec

If we use an expansion rate of one fifth the speed of light, (Some have suggested considerably higher, but this number facilitates our calculations.) the most distant object is moving away from the earth at a velocity of:

60,000 km/sec

With this background, it is possible to look at a textbook calculation of the age of the universe.

An example is used of the Hydra cluster, which is estimated to be at a distance from the earth of:

1,9 x 10 to the 9th power (in light years)

While the light was coming back to us, the cluster must have receded 20% father, so that it is now at a distance from the earth of:

2.28 x 10 to the ninth power (in light years)

To get to it's present location at a velocity of one-fifth the speed of light, it would have taken the above distance divided by 20,000 km/sec (1/5 the velocity of light). With proper accounting for units, we arrive at:

11,400,000,000 years

This gives us the presumed age of the universe


But an interesting thing has been discovered. Apparently, the speed of light is slowing.

Originally, it was assumed that the speed of light was infinite. (Johannes Kepler in the 17th century)

However, others attempted to measure the speed of light. In fact, the speed of light has been measured employing 16 different methods, during the past 300 years. The first was by Olaf Roemer in 1677. He used the eclipses of Jupiter with its moons to make the first known crude estimate. It is significant that he calculated error bounds on his estimates.

Later, in 1729, James Bradley confirmed Roemer's work. He used Io in his measurements of elapsed time between eclipses by Jupiter.

In 1875, Harvard used the same method as Roemer, but refined the measurements to obtain a new estimate of the speed of light.

The National Bureau of Standards obtained a highly refined measurement in 1983.

The interesting thing is that each of the measurements resulted in a lower value for the speed of light. This might be explained by accepting more recent measurements as better. However, another interesting fact is that the each measurement fell below the lower error bound of the prior measurement!

The data are summarized in the following table:


c min


c max













Measurements of the Speed of Light, c, in kilometers per second

What can one conclude? Physicists are debating the issue, but articles are beginning to appear in respected journals. (e.g. J. Magueijo and A. Albrecht, Physical Review, 1999)

The possibility that light is not a constant, but is decreasing in value is fascinating.


If the measurements above are accepted, then the speed of light may have diminished dramatically. If observations over a relatively small period of time, 300 years, show this much of a drop, then it is possible to expect that the speed of light was much higher.

Discussions on this are showing up in the literature:

The Speed of Light Is Not Constant.

Was the Speed of Light Faster in the Early Universe?

An interesting example would be to assume that it has exponentially dropped from infinity (instantaneous transmission) to the value measured today. The implication is that the prior calculation of the age of the universe is incorrect. Higher early values for the speed of light will yield correspondingly lower values for the age of the universe.

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