Question of the Week

Does the expansion of space slow the speed of light?

Tue, 14th Jan 2014

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"Gorman, Ray" asked:

Greetings, Naked Folks!


I sort of get the explanation that, not only is the Universe expanding but that the actual SPACE in the Universe in expanding. Does this not affect the speed of light? So, for example, if you are looking at the universe from OUTSIDE the universe, would the distance of ˜a mile' a few billion years ago be LESS than what ˜a mile' is today? This would make the speed of light "slower" back then compared to now, wouldn't it? As I said, if ONLY viewed from outside the universe. IF this is the case, how is the speed of light a "constant"? Love the podcast- makes the commute shorter- "relatively" speaking!




Raymond Gorman





Hannah - So, does the speed of light change? Dr. Andrew Pontzen from University The first stars in the Universe turn on at about 400 million years after the Big Bang.College London provides the answer.

Andrew - So far as anyone can tell, the speed of light in our universe is absolutely fixed. It does not change.

Hannah - The speed of light travelling in the near vacuum of space clocks in at just shy of 300 million meters per second or 186,000 miles per second. So, if space is expanding, doesn’t that mean that objects in space are also expanding including atoms in our meter ruler that we use to measure distance by? Well, space expansion isn’t actually uniform. Our Milky Way and the galaxy next to us, Andromeda are moving closer to each other but in general, galaxies are getting further apart as space expands. In this case, does this non-uniform expansion affect the meter rule somehow and therefore, affects speed of light? Well, back to cosmologist Andrew Pontzen.

Andrew - As you can imagine, the fact that the universe is expanding and distances change over time, does make this a bit more complicated. You might think it even makes what we call speed of light ambiguous because distances mean something different at different times. But actually, it doesn’t. You can still define what we mean by the speed of light pretty well because it has an effect for instance on the structure of atoms themselves. This is through something called the fine structure constant which is just a number that the speed of light appears in. So, if the speed of light were different, the structure of the atoms would also be different. Because of that, by looking through telescopes, we can actually tell that the structure of atoms hasn’t changed over time and that's what makes us pretty confident that the speed of light must be fixed.

Hannah - Okidok, rest assured, our speed of light is not decreasing and time is not speeding up either, or is it?

Andrew - There is a possible complication which is something called string theory which is a possible theory of everything, sort of a better version of physics. It does allow for this fine structure constant to change. So, it is possible that in our universe, the fine structure constant and so the speed of light is effectively changing over very long timescales. But whenever anybody has looked for any evidence of that, they have found no evidence. So, as far as we can tell, the speed of light is fixed.


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No. Pmb, Sat, 5th Oct 2013


(the universe) all existing matter and space considered as a whole; the cosmos. The universe is believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; it has been expanding since its creation in the Big Bang about 13 billion years ago.

There ain't no outside

Source The Oxford English Dictionary and Oxford Dictionaries Online
syhprum, Sat, 5th Oct 2013

I recommend posting the source of a definition when one is quoted (or stating its yours otherwise).

That's merely by one definition. The one most cosmologists use differs than that. They consider the universe to be the entirety of space where each two points in the space can be connected by a continuity of points in the same space. That's how we can say that the universe is expanding.

Take a look at the diagram at

The child universe is considered to be a seperate universe. This is the definition of universe that physicists use, especiaylly when speaking of multiple universes and the mutpliple universe interpretation of QM. The cosmos would then be the summation of all universes, i.e. what the syhprum's definition calls "universe.
" Pmb, Sat, 5th Oct 2013

The speed of light used to be something to be measured in relation to meters and seconds, which were defined in relation to Earth's size and rotational speed. Having refined those measurements, we discovered that the speed of light makes a much more useful constant, so we now define meters and seconds in relation to the defined speed of light. This redefinition allows the size and rotational speed of Earth to vary in terms of meters and seconds.

4D space-time is defined in terms of the constant speed of light. The warp of space-time in a gravitational field preserves that constant speed. The only way the speed of light, in meters per second, can change is by changing the definitions of meters and seconds, again.

Alternatively, you could create new units of distance and time and define the speed of light in terms of those new units. For example, you might set the Hubble limit (radius of the observable universe) as the fundamental unit of distance, and the inverse of the Hubble constant (rate of expansion of space) as the fundamental unit of time. If the rate of expansion of space (in sec-1) changes, then the speed of light in those new units would vary. Phractality, Tue, 8th Oct 2013

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