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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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  • Well, as I feared, this got closed as a dupe of a different, unrelated question. I was asking about whether causality has a speed limit, not how space can expand faster than light-speed. Oh well. – Doradus Aug 14 '21 at 16:03

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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  • Interesting point about the dimensionality of the expansion rate. Is there some cosmological frequency that is comparable to the expansion rate instead? – Daddy Kropotkin Aug 13 '21 at 14:03
  • Let me explain what I meant by "expands faster than light-speed" then: during the inflationary epoch, the universe grew by a factor of 10^26 in the space of 10^-32 seconds. Two events separated by one Planck length at the start of inflation, if my math is right, would recede from each other at an average of 10^23 m/s during this period. – Doradus Aug 14 '21 at 14:28
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    @Doradus yes, I understand exactly that. It doesn’t change the answer. The first patagraph is a specific and direct answer regarding your causality concern, and the second paragraph is a more general answer to this whole category of questions about the universe expanding faster than c. – Dale Aug 14 '21 at 16:41
  • @Dale ok thanks, just wanted to be clear why I felt a speed is appropriate here. I don't think it's meaningless just because other people (not me) use that phrase to describe something else that is measured in different units. – Doradus Aug 14 '21 at 19:16
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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.

Marco Ocram
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  • The cosmological redshift is "caused" by the Universe's expansion. – Daddy Kropotkin Aug 13 '21 at 13:23
  • Indeed. The expansion also 'causes' galaxies to move further apart. They are both qualities of the expansion itself. – Marco Ocram Aug 13 '21 at 13:30
  • ... so you're statement that "Nothing is 'caused' by the expansion." is incorrect. Unless you're using the word "cause" in some other, atypical meaning. – Daddy Kropotkin Aug 13 '21 at 13:48
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    Hi Daddy, the point I intended to convey is that what we usually think of as causality- namely event A triggers event B which triggers event C and so on- is different from describing facets of event A in itself. – Marco Ocram Aug 13 '21 at 14:09
  • I do not think that was my mistake: I assumed the expansion is the effect, not the cause. See the last sentence of my question: "Or, alternatively, that the expansion of the universe was not caused by the big bang?" – Doradus Aug 14 '21 at 14:15