# Speed of Universe Expansion [duplicate]

This question already has an answer here:

How much is the universe expanding every second (Earth second)?

If you could be a bit simple in the answer that would be good.

## marked as duplicate by Michael Brown, Waffle's Crazy Peanut, ManishearthMar 19 '13 at 10:57

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## 1 Answer

As Stefan has mentioned in his comment, there are already several questions on this issue. However I'd guess from the way you have phrased your question that the existing answers might be a bit technical so I'll attempt a simpler explanation. Suppose you draw a line that measures out some distance, $d$, then as shown above the Hubble expansion means that the end of the line is moving away from you at a speed $v$ given by:

$$v = H \space d$$

where $H$ is Hubble's constant. So your question "how much is the universe expanding every second" doesn't have a simple answer because the speed of expansion depends on the length of your line, $d$.

You may have heard that the size of the observable universe is 13.7 billion light years. What we could do is take this distance and find out what speed it's travelling at. If we feed in $d$ as 13.7 billion light years into our equation above (and assuming that $H$ has remained constant during that time) we get the speed, $v$, about equal to the speed of light. So we calculate that the edge of the observable universe is travelling away from us at about the speed of light. See this question for a more technical explanation of why the answer is about the speed of light but not exactly equal to it.

You could try a different calculation and ask how fast your drive to work is expanding. Actually it isn't expanding, or at least not due to Hubble expansion, since the expansion of the universe only overrides other forces at huge distances but let's ignore this for now. Suppose you drive 10km to work, then your office is receding from you at a speed of:

\begin{align} v &= H \times 10,000 \\ &= 2.3 \times 10^{-14} m/s \end{align}

• Hi John Rennie, in your answer you indicate the static distance (13.7 billion light years) thus neglecting the expansion of space itself (according to all sources including wikipedia, the size of the observable universe is 92-94 billion light-years, that means that the distance from earth to the edge of the observable unverse should be around 46-47 billion light years). Is that a mistake or did you put that value on purpose? (or am I missing something?) In any case, even according to that value, d should be 13.7/2. Correct? – Aldo Cernuto Mar 19 '13 at 13:18
• There is an ambiguity about the phrase "size of the observable universe". The furthest we can see with our telescopes is 13.7 Glyrs, but of course we're seeing it 13.7Glyrs in the past because the light has taken that long to get to us, and in the intervening 13.7 billion years that point has moved farther away from us. The figure of 47Glyrs is where the furthest point we can see is now. NB both figures are the radii of spheres centred on the observer. – John Rennie Mar 19 '13 at 15:37
• But light that has travelled 13.8 Gyrs to reach us definitely was not emitted from a distance of 13.8 Glyrs, it was much closer - I seem to remember something like 40 Mlyrs but might be wrong. Point is, the figure 13.8 Glyrs makes very little sense as a measure of the size of the observable universe, as it is averaged over an enormous range of sizes. I think it would make more sense to pick one time - emission or absorption - and state the according size. – Thriveth Dec 14 '13 at 13:16