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(I hope this question is a little more profound than the standard "if nothing can move faster than light..." question that pops up here regularly.)

It has been said that the speed of light could actually be called the "speed of causality", since causal influences cannot propagate through space faster than this speed.

But the universe expands faster than light-speed, and this expansion was caused by the big bang.

Doesn't this demonstrate that the speed of light cannot be the speed of causality? That this speed is a much less fundamental one, describing merely causal relations between events in spacetime, and not causality itself? Or, alternatively, that the expansion of the universe was not caused by the big bang?

Dale
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Doradus
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2 Answers2

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But the universe expands faster than light-speed, and this expansion was caused by the big bang. Doesn't this demonstrate that the speed of light cannot be the speed of causality?

No, because the big bang happened everywhere. So there is no spatial distance between any point in the universe and the big bang. More broadly, for any event in the universe and any timelike geodesic through that event, if you maximally extend that geodesic to the past you will reach the big bang.

In any case, the phrase "the universe expands faster than light-speed" is actually a meaningless phrase. The expansion of the universe is usually measured in km/s/Mpc which in SI base units is 1/s. But light speed is measured in m/s. They are incompatible units. It is a speed compared to an inverse time. You cannot compare such quantities; it literally is meaningless to state that an inverse time is greater than a speed.

Dale
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Your mistake is to have assumed that the expansion of the Universe is in itself a cause of something else. Nothing is 'caused' by the expansion.

Professor Sushing
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