# How big is the known universe? **URL:** \ **Category:** Factual Questions\ **Created:** [November 14, 2011, 6:32pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920 "2011-11-14T18:32:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [November 14, 2011, 6:32pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/1 "2011-11-14T18:32:07Z")
If we see light from a distant galaxy, and we determine that such a galaxy is, say, 2 billion light years away, then the light we’re seeing has traveled for 2 billion years, correct? But in that 2 billion years, the galaxy has traveled at least twice that distance, right? So, is the estimated size of the known universe calculated with that additional motion taken into account, or is the “known universe” based on where those farthest objects appear to be now? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 14, 2011, 6:51pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/2 "2011-11-14T18:51:49Z")
> [@](#): > > But in that 2 billion years, the galaxy has traveled at least twice that distance, right? I’m not sure how you get “twice that distance”, but yes, the galaxy has continued to recede while its light has been en route. To get back to your original question, there are actually multiple different definitions of “distance”. They’re all calibrated to be the same at short distances, but on cosmological scales, they disagree. Conversions between them will depend on the exact cosmological model you’re using. Any figure you find for the “size of the Universe” or “size of the known Universe” will be implicitly assuming one of those definitions, but popular presentations don’t usually specify which. In technical contexts where it does matter, the type of distance will be specified. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [November 14, 2011, 7:02pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/3 "2011-11-14T19:02:05Z")
Yes, I should have said it this way: … But in that 2 billion years, the galaxy has traveled _ **TO** _ at least twice that distance, right? But, even if different definitions are used, couldn’t one definition be used to answer my question? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [November 14, 2011, 7:27pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/4 "2011-11-14T19:27:39Z")
I’m only gonna throw these out there, because I’m not a cosmologist, but any galaxy is moving at good clip relative to any other galaxy. On top of this, we have the expansion of space-time, the “canvas” upon which matter and energy is “painted”. So, getting a strait answer is a bit hairy, I imagine. Also, correct me if I’m wrong, but we can’t assume the universe “ends” where matter and energy finally dissipate. How much void of space-time might lay beyond the finite matter and energy in our universe? Do have even a moderate consensus among the experts, or even an inkling at all? ---
**Author:** ![fiddlesticks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiddlesticks/32/18129_2.png) [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)\ **Post date:** [November 14, 2011, 7:31pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/5 "2011-11-14T19:31:36Z")
The Wikipedia entry for [Observable universe](http://en.wikipedia.org/wiki/Observable_universe) pegs it at 93 billion light years. Or 28 billion parsecs if you prefer that scale. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [November 14, 2011, 7:56pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/6 "2011-11-14T19:56:23Z")
Whooo, not easy reading. But I think that indicates that the known or observable universe is the size it would figure to be if the expanding “edge” were to recede while light from the farthest observable points were moving toward us, which is sort of what I wanted to know. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 14, 2011, 8:27pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/7 "2011-11-14T20:27:17Z")
> [@fiddlesticks](#): > > The Wikipedia entry for [Observable universe](http://en.wikipedia.org/wiki/Observable_universe) pegs it at 93 billion light years. Or 28 billion parsecs if you prefer that scale. How does that work if the universe is only 15 billion years old? Surely the observable universe by definition could only have a 15 billion light year radius (and thus a 30 billion light year diameter)? Is the expansion of space really that much? I knew it was accelerating, but I had no idea it was that fast! But, even so, there’s no way we can see something 90 billion light years away, so in what sense is it “observable”? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 14, 2011, 9:01pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/8 "2011-11-14T21:01:07Z")
The things we see now, whose light left 13.7 billion years ago, have since receded further away, and are now 46.5 billion lightyears away. **cmyk** , there is no point at which the matter and energy dissipate away. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [November 14, 2011, 9:03pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/9 "2011-11-14T21:03:24Z")
> [@fiddlesticks](#): > > The Wikipedia entry for [Observable universe](http://en.wikipedia.org/wiki/Observable_universe) pegs it at 93 billion light years. Like Archimedes said, “Give me a lever long enough, and I shall peg the universe”. ---
**Author:** ![tellyworth](https://avatars.discourse-cdn.com/v4/letter/t/977dab/32.png) [@tellyworth](https://boards.straightdope.com/u/tellyworth)\ **Post date:** [November 14, 2011, 9:08pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/10 "2011-11-14T21:08:56Z")
> [@Candyman74](#): > > How does that work if the universe is only 15 billion years old? Surely the observable universe by definition could only have a 15 billion light year radius (and thus a 30 billion light year diameter)? Because the light from distant objects _started_ traveling towards us billions of years ago. The universe has continued expanding during that time, so those objects are now much further away. In The Hidden Reality, Brian Greene says the observable universe has a diameter of about 82 billion light-years (page 28, footnote 12 in my copy). ---
**Author:** ![fiddlesticks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiddlesticks/32/18129_2.png) [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)\ **Post date:** [November 14, 2011, 9:12pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/11 "2011-11-14T21:12:23Z")
Here’s another related Wikipedia article “[Comoving distance](http://en.wikipedia.org/wiki/Comoving_distance)”, which is all about the difficulties in measuring something that has moved since you’ve “seen” it. ---
**Author:** ![Randy\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/r/ee7513/32.png) [@Randy\_Seltzer](https://boards.straightdope.com/u/Randy_Seltzer)\ **Post date:** [November 14, 2011, 10:47pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/12 "2011-11-14T22:47:19Z")
> [@Chronos](#): > > **cmyk** , there is no point at which the matter and energy dissipate away. Sure there is, just not in the way **cmyk** is thinking. > **[Heat death of the universe](https://en.wikipedia.org/wiki/Heat_death_of_the_universe)** > > The heat death of the universe (also known as the Big Chill or Big Freeze) is a scientific hypothesis regarding the ultimate fate of the universe which posits the universe will evolve to a state of no thermodynamic free energy and, having reached maximum entropy, will therefore be unable to sustain any further thermodynamic processes. The hypothesized heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer If th... ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 15, 2011, 12:50am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/13 "2011-11-15T00:50:17Z")
> [@tellyworth](#): > > Because the light from distant objects _started_ traveling towards us billions of years ago. The universe has continued expanding during that time, so those objects are now much further away. But we’re not observing them at 90 billion light years away, we’re observing them at the location they were when the light started travelling towards us; 15 billion or less light years away. So surely we can’t see more than 15 billion light years away, even though we _know_ that the object 15 billion light years away has since moved much further away? Hell, for all we know it may have disappeared. We can only observe what it was like 15 billion years ago. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 15, 2011, 1:05am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/14 "2011-11-15T01:05:35Z")
Well, yes, but we don’t see _anything_ right now. Your computer screen? You’re not seeing it as it is now; you’re seeing it as it was a few nanoseconds ago. Does this mean that the size of the Universe is zero? ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 15, 2011, 2:54am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/15 "2011-11-15T02:54:40Z")
> [@Chronos](#): > > Well, yes, but we don’t see _anything_ right now. Your computer screen? You’re not seeing it as it is now; you’re seeing it as it was a few nanoseconds ago. Does this mean that the size of the Universe is zero? No. I didn’t say that at all. The light from my computer screen has had time to reach me within the age of the universe. I’m talking about objects from which the light has not had time to reach me in the 15 billion years the universe has existed - objects further than 15 billion light years away. I’ll try to explain what I mean more clearly. We can see 15 billion light years because the universe is 15 billion years old, and light moves at the speed of light. Thus light that originated 15 billion light years away from us has had 15 billion years to reach us, and can thus be seen. Light that originated 16 billion light years away from us has also only had 15 billion light years away, and is therefore still 1 billion light years away from us. In a billion years, it’ll reach us, and we’ll see that object. So, to my mind, that’s the observable universe - anything up to the age of the universe in light years away from us. Now, what was said above (and I’m not saying I disagree - I’m not a physicist; I’m merely asking for an explanation) is that something that was 15 billion light years away from us at one point in time and is now 20 billion light years away still “counts” because we can see where it was when it was 15 billion light years away. While it is true that we can see where it was when it was 15 billion light years away, we can’t see where it is\* now\*, because it would take light from it longer than the age of the universe to reach us. So it’s not observable in its present location, only in its past location. So to my mind, how does that make the “observable” universe any greater than 15 billion light years? Surely it’s still 15 billion light years, but we know that the object is further out than that because we know where it was headed; we just can’t observe that. Like I said, I’m trying to grok it. If the physicists and articles say it’s so, I guess it must be so; but I’m struggling a little with the meaning of “observable”. ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [November 15, 2011, 4:37am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/16 "2011-11-15T04:37:25Z")
The universe has existed 13.7 billion years, not 15 billion years. ---
**Author:** ![Jamicat](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@Jamicat](https://boards.straightdope.com/u/Jamicat)\ **Post date:** [November 15, 2011, 4:59am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/17 "2011-11-15T04:59:34Z")
The universe could very well exist, as infinite… But, we can only see within our viewing bubble. That bubble would be the time that light has, to reach back to us from billions of years ago, traveling at light speed. We will most likely see further eventually, with better equipment, but we will forever still be within the light bubble. regardless. But of course, there’s always WARP DRIVE…🆒 ---
**Author:** ![Jamicat](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@Jamicat](https://boards.straightdope.com/u/Jamicat)\ **Post date:** [November 15, 2011, 5:03am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/18 "2011-11-15T05:03:25Z")
if i had a plane at the edge of the observable universe…and a treadmill…would… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 15, 2011, 6:14am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/19 "2011-11-15T06:14:17Z")
> [@](#): > > While it is true that we can see where it was when it was 15 billion light years away, we can’t see where it is now, because it would take light from it longer than the age of the universe to reach us. So it’s not observable in its present location, only in its past location. The age of the Universe is how long the Universe _has_ existed, not how long it _will_ exist. If a galaxy is 20 billion lightyears away right now, then we’ll see it at that location in 20 billion years. If it’s 10 billion lightyears away right now, we’ll see it in 10 billion years. 10 billion years from now is not qualitatively different from 20 billion years from now, and in either case, we can’t see it in its current location right now. ---
**Author:** ![brocks](https://avatars.discourse-cdn.com/v4/letter/b/7feea3/32.png) [@brocks](https://boards.straightdope.com/u/brocks)\ **Post date:** [November 15, 2011, 8:51am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/20 "2011-11-15T08:51:54Z")
> [@Chronos](#): > > The things we see now, whose light left 13.7 billion years ago, have since receded further away, and are now 46.5 billion lightyears away. So doesn’t that mean that they were 13.7 billion light years away from us, 13.7 billion years ago? And doesn’t _that_ mean that the universe was 27 billion light years across within a very short time after the big bang? And doesn’t **that** mean that it expanded much, much faster than the speed of light? And if the expansion is accelerating, as I read somewhere, and it has always been much faster than the speed of light, doesn’t _ **that** _ mean that we’ll never see anything we can’t already see? [Next page](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920.md?page=2)