How can the universe be 93 billion light years across in size, if it is only 13.7 billion years old?

Question

Physics tells us that the relative speed of two objects cannot exceed the speed of light, but I just read that the universe is estimated to be 93 billion light years in size. If the universe is 13.7 billion years old, how can the universe have expanded to that size without exceeding the speed of light?

Physics tells us that the relative speed of two objects cannot exceed the speed of light, but I just read that the universe is estimated to be 93 billion light years in size. If the universe is 13.7 billion years old, how can the universe have expanded to that size without exceeding the speed of light?

BTW - both of the figures (size and age of the universe) are from this month's Astronomy magazine.

,,and no, I'm not confusing the solar system and the universe - as I said, the current issue of Astronomy magazine states that current estimates place the age of the universe at 13.7 billion years.

Answer 1

Wow what a cool question.
I found an answer to this on Wikipedia:

en.wikipedia.org/wiki/Universe

First it says that very little is known about the size of the universe: )
Then it says 93 billion LY is a guess that has been made (2003).
To understand this I think, we need to see that the light we get from the edge 13.7 billion LY away (LY= light year) was there 13.7 billion years AGO.
That means it has done something since then.
Now all the matter is travelling at nearly the speed of light. So in 13.7 billion years its reasonable to think it has moved on that far. The light has at least, and it is part of our universe.
That should bring us to ~27.4 billion LY for the radius and 54.8 billion LY for the diameter of the universe.
The only problem is that WE are moving away just as fast. That is to say that space in between is expanding, and the space in the opposite direction as well.
This is I think what the article means when it mentions the "comoving distance to the edge of the observable universe". This is saying that the light itself from the beginning is going at, well, the speed of light in both directions.
What it seems to be saying then is that the actual size of everything including light particles can be no more that ~93 billion LY which they seem to present as a definite number based on the time ~13.7 billion years, the observed rate of expansion, and the speed of light itself ~300K km/s.

Anyhow here are the pertinent parts of what Wikipedia says (also is included the section on misconceptions like ~158 billion LY):

Physical structure

[edit] Size
The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field.
The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field.

Main article: Observable universe

Very little is known about the size of the universe. It may be trillions of light years across, or even infinite in size. A 2003 paper[20] claims to establish a lower bound of 24 gigaparsecs (78 billion light years) on the size of the universe, but there is no reason to believe that this bound is anywhere near tight. See shape of the Universe for more information.

The observable (or visible) universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The comoving distance to the edge of the visible universe is about 46.5 billion light years in all directions from the earth; thus the visible universe may be thought of as a perfect sphere with the earth at its center and a diameter of about 93 billion light years.[21] Note that many sources have reported a wide variety of incorrect figures for the size of the visible universe, ranging from 13.7 to 180 billion light years. See Observable universe for a list of incorrect figures published in the popular press with explanations of each.

Misconceptions

Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these are listed below.

* 13.7 billion light-years. The age of the universe is about 13.7 billion years. While it is commonly understood that nothing travels faster than light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.7 billion light-years. This would make sense in the flat spacetime of special relativity. But in the real universe spacetime is highly curved at cosmological scales (general relativity), and light does not move rectilinearly. If a distance is obtained from the product of the speed of light times a cosmological time interval, it has no direct physical significance. [5]

* 15.8 billion light-years. This is obtained in the same way as the 13.7 billion light-year figure, but starting from an incorrect age of the universe which was reported in the popular press in mid-2006[6] [7] [8]. For an analysis of this claim and the paper that prompted it, see [9].

* 27 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.

* 78 billion light-years. This figure, as mentioned above, is a lower bound on the diameter of the whole universe. It yields a lower bound on the radius of 39 billion light-years, which is less than the comoving radius of 46.5 billion light-years. For the 39 billion light-year radius to be correct, light must have circumnavigated the universe, and some regions of space would be visible twice, in opposite directions. This has yet to be proven.

* 156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter (or rather a circumference), the doubled figure is meaningless even in its original context. This figure was very widely reported[10] [11] [12].

* 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources; it was obtained by incorrectly adding 15% to the incorrect figure of 156 billion light-years.

Answer 2

First, the universe did not begin at one point. While the whole observable universe would have fit into a very small region at first, there was more to the universe, even early on, than just that. Those part of the universe that are now 93 billion light years away have always been outside of our light cone.

Second, the universe has expanded faster than the speed of light very early on. The crucial thing here is to realize that it is space itself that expands while the speed of light *through space* is limited. It is possible for space to expand faster than light.

Answer 3

For some reason*, scientists tend to measure the size of the universe linearly, but remember that the universe has been highly curved ever since it's creation, and so has it's expansion.

For all we know, in general relativity, the size of the universe could be of only a few meters(of course it's probably a lot bigger than that, even it's possible that it's as big as it's age in light years. We might never know), but in special relativity(linear to us) still be hundreds of billions of light years in size due to the curvature.

For further reference, read the following article, up until you reach Misconceptions: http://en.wikipedia.org/wiki/Observable_universe#Misconceptions

*I guess the reason is that there is no way for us to know how curved space really is...

Answer 4

That's a good question. Both the age & the size of the universe that you give are generally correct, according to most experts. Also, nothing can move faster than light in a vacuum. So how is this possible?

The answer is that the universe itself expanded during the Big Bang, and by this I mean space and time both where created at the Big Bang & expanded with it. The expansion was faster than the speed of light, but no rules were violated because nothing was moving IN space faster than light. It was space (& time) itself that was expanding (not moving) faster than light. That's the best I can explain it. Okay?

Addendum: The rules of relativity say that nothing can move faster than light thru space (in a vacuum), but there are no rules about what the background (space) can do, & it can expand however it wants, even faster than light.

Answer 5

Dear Sir:

You have confused one major word with another...

Solar System and Universe.

Our Solar System is thought to be some 13 Billion Years old.
Nothing and no body has ventured a guess on the age of the total Universe. The Universe is constantly changing. Objects are in motion and new stars are being born all the time. Old stars are changing into other forms and some stars are just dying.

We can see distant galaxies, but we cannot see the internal makeup of them. Calculation of their age through observation is therefore rather a long shot wouldn't you agree?

Also, just for the record, the Universe has not been sized at 93 Billion Years in width, depth, or height.

What has been stated is that the most distant things we can "observe" with our present optical and radio technology are 40 Billion Light Years away in all directions. That does not in any way suggest that the Universe stops there, we just can't "observe" or "see" any farther than that. That is the technical limitation of our observational equipment, ground or space based.

Answer 6

I need to have a separate list of answers to explain the answers given here. The ANSWER TO THE QUESTION AS ASKED is: your question cites 2 figures. However one figure is representing the "time" or "age of the universe as science accepts it. The other figure is the length in miles of SIZE in the number of miles the universe is. Same wording yet two different explanations. Mankind can only think in theory of space travel as we have not been there ourselves. I am a Creationist, but lets stay scientific. What was there before the big bang? Where is the beginning point of the big bang? Was the big bang just one bang or many? Who detonated the big bang? If no one, that WHAT set off the big bang?

In any event, POOF !!!! nothing then you have Universe in whatever size banged out.

Strangest thing is that the POOF theory works both creation and big bang

Answer 7

space is not linear ie its not straight so its size increases exponentially. also light is subject to the same laws of the universe. as in as time goes on things age and decay as such light has been gradually decaying over the years as in 2000 years ago it was a lot faster than it is now. and 2000 years from now it will be a lot slower than now

Answer 8

Where did you get that size from? I never heard that before.

Surely, the only way we can measure the size of the universe is through the age related distances of the most distant objects detected. So, to my knowledge the distance and age are co-related.

Have you been reading pseudo-science?

By the way, if you have been told it is 93 billion lt years across, then this is not a ridiculous question, as the first respondant so harshly put it. I just think you have got hold of the wrong information.

Answer 9

Space inflation, and the universe is supposedly 158 Billion light years across.

Answer 10

First, you need to divide the 93 billion in half because the Universe likely expanded from a central point.
Second, the Universe has no mass, only the stuff in it does. So it can expand and contract and move as fast as it wants. It's just an empty space.

Obviously a few people don't like my answer (5:24 PM), so let me rephrase:
As the universe expanded from a central point, it expanded, let's say, 46.5 billion light years in each direction (the radius). That totals up to a diameter of 93 billion miles. And according to the smart person whose answer is situated below mine, space can expand as fast as it wants to because it has no mass.