# Speed of light just after the Big bang?

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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f95toli
Gold Member
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.

CompuChip
Homework Helper
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).

f95toli
Gold Member
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.

CompuChip
Homework Helper
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.

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*) 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 :-)