I believe the current estimated age of the Universe is around 14 billion years. I just read in another post on here that the diameter of the Universe is around 90 billion light years. This doesn't make sense to me, shouldn't the radius of the Universe be equal to it's age? Unless the explosion caused particles to spread out faster than the speed of light? Is that possible? I thought the speed of light was a speed limit that nothing can exceed?

Sorry if there is something I'm not understanding here or if I'm trying to compare apples to oranges. I'm only an astronomy enthusiast.

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    $\begingroup$ Did you read either of these posts by any chance? astronomy.stackexchange.com/questions/1001/… & astronomy.stackexchange.com/questions/3635/… $\endgroup$
    – Dean
    Mar 4, 2016 at 21:33
  • $\begingroup$ Can we start closing duplicates please, not downvoting them? $\endgroup$
    – ProfRob
    Mar 5, 2016 at 11:21
  • $\begingroup$ @RobJeffries -- Yes, this is a duplicate of many others. However, someone who asks this question is almost certainly a lay person who has just entered the field. A lay answer is appropriate. pela's answer to this question accomplishes that. The answers to the duplicates dive too deep into the math for a lay reader, leaving the lay questioner still confused. It might be better to close those earlier questions as duplicates of this one. $\endgroup$ Mar 6, 2016 at 10:36
  • $\begingroup$ @DavidHammen That is not the way it works - the later question is closed. Pela is free to move his excellent answer and post it against the duplicate. $\endgroup$
    – ProfRob
    Mar 6, 2016 at 12:48
  • $\begingroup$ @RobJeffries - That is exactly how it is supposed to work, at least per the founders of the stackoverflow/stackexchange network. Sometimes newer questions are asked better / answered better than older supposed duplicates ones. In such cases, it's better to mark the older questions as duplicates of the newest one. $\endgroup$ Mar 6, 2016 at 13:05

2 Answers 2


The expansion rate of space is not itself the reason that the radius $R_\mathrm{Uni}$ of the observable Universe is larger than 14 billion lightyears (Gly). Just the fact that space expands is the reason. If space did not expand, then $R_\mathrm{Uni}$ would be the expected 14 Gly, as this is the distance that light can travel in the 14 billion years (Gyr) since the Big Bang. But since space has expanded in the meantime, then the distance to some particle that emitted a photon that we observe today, has been continuously increasing during the 14 Gyr, and hence $R_\mathrm{Uni}>14\,\mathrm{Gly}$.

An analogy to this is a worm on a rubber band that is stretched while it crawls along. The distance between its starting point and it's end point is not just a matter of how fast it crawls, but also of how much you stretch the rubber band.

The term "space expands faster than light" is a bit deceptive. Space expands *homogolously*$^1$, meaning that a given point in space recedes from you at a speed depending on its distance from you. If a galaxy is 1 Mpc (= 3.26 Mly) from you, it recedes at $70\,\mathrm{km}\,\mathrm{s}^{-1}$. If it's 2 Mpc from you, it recedes at $140\,\mathrm{km}\,\mathrm{s}^{-1}$. And so on. At some distance the recession velocity becomes larger than $c$, and in fact galaxies at a distance of $R_\mathrm{Uni}$ recede at more than $3c$. This doesn't violate special relativity which says that nothing can travel through space faster than $c$, because the galaxies do not travel through space. They lie approximately still in space, but space itself simply expands, i.e. the distance between everything increases.

Like three worms on a rubber band, with 1 cm between them. If you stretch the band to double length, the distance from worm #1 to #2 is 2 cm, while from #1 to #3 it's 4 cm.

$^1$Homogolous is not the same as homogeneous. Whereas the latter means "the same everywhere", referring to some physical quantity like density, homogolous means "proportional to distance".


The Universe is estimated to be around 14 billion years old, yes. That said, you seem to be talking about the observable universe. The idea is that space is actually capable of expanding faster than the speed of light, as space itself is not matter. This expansion of space is described by something called Hubble's law.


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