Speed of light just after the Big bang?

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Discussion Overview

The discussion revolves around the implications of the speed of light in relation to the expansion of the universe shortly after the Big Bang. Participants explore how the universe could have a diameter of 600 light-years just 10 minutes after the Big Bang, questioning the relationship between the speed of light and the expansion of space.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant questions the feasibility of the universe being 600 light-years in diameter shortly after the Big Bang, suggesting that this would require particles to travel faster than light.
  • Another participant argues that the speed of light limits the travel of particles and light but does not limit the expansion of space itself.
  • Some participants propose that if space is not absolute nothingness, then something must have traveled faster than light during the universe's expansion.
  • One participant introduces the idea of replacing "light" with "information" to resolve paradoxes, stating that while distances can increase faster than light, meaningful signals cannot be transmitted at such speeds.
  • Another participant elaborates on the expansion of space, explaining that while distances can appear to exceed the speed of light due to the expansion, no paradoxes arise since the points cannot be in contact.
  • An analogy involving a balloon is presented, where galaxies are likened to coins on the surface of an inflating balloon, illustrating how distances can increase without violating the speed of light for actual movement.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of the speed of light and the expansion of space. There is no consensus on how to reconcile the apparent contradictions presented by the early universe's expansion.

Contextual Notes

Participants note the limitations of analogies used to explain the expansion of the universe, acknowledging that they may not capture all aspects accurately and could involve mixing reference frames.

Geo212
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I recently saw a documentary on TV in which Stephen Hawking stated that the universe was 600 lightyears in diameter, 10 minutes after the Big Bang. How can this possibly be true? - the particles and matter making up the outer limits will have had to have traveled at many times the speed of light to get there in 10 minutes!
 
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The speed of light only limits how quickly particles (and obviously also light) can travel, it does not limit the rate of expansion of the universe (i.e of space itself).
 
Assuming space is not absolutely nothing, then something must have traveled faster than light.
 
To resolve paradoxes like these, I like to replace "light" with "information".
Even though two neighboring stars, for example, may have moved apart at speeds exceeding c, there is no way you could have used that to transmit any meaningful signal. Quite the opposite in fact: if you have emitted a light signal from one star to the other, it would have taken at least as long as in a "stationary" universe to reach the other one (the main indication of the expansion being the redshift of the photons).
 
Geo212 said:
Assuming space is not absolutely nothing, then something must have traveled faster than light.

Yes, space itself can -as pointed out above- if you will "travel" faster than c. Or, to be more precise, the distance between two points can increase at such a rate that a "naive" calculation of the speed using distance/time will appear to give a result larger than c; but this is just a result of space itself expanding.
Note also that -as Compuchip points out- these two points could never be in contact with each other; so no paradoxes can arise.
 
One well-known analogy* is to consider the universe as a partially inflated balloon. The objects in the universe (like galaxies) can be pictures as coins glued to the surface of the balloon. Now the universe inflating is something like the balloon inflating: the coins are not really moving, but if you are sitting on one of them and you look around, you still see all the other coins receding from you.

To involve the light speed, let's imagine an ant walking on the surface of the balloon from one coin to the next**. If you do the naive measurement of the ant's velocity, you will find that it increases when you start inflating the balloon. After all, the ant covers more distance in the same time, because the surface of the balloon stretches under him while he takes his steps. However, he will clearly not reach the other coin any earlier that he would have in the non-inflating universe.

===*) Disclaimer: One has to be very careful with this analogy, because it doesn't cover all aspects of the universe inflating properly. In other words: note that the universe IS NOT a balloon.

**) Double disclaimer: this is not part of the standard analogy, so I'm not sure how good this extension is. In particular I might be mixing up some reference frames here, and I'm not postulating the the velocity of the ant will be a fixed number for all observers :-)
 

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