# 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:** 2
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 9:34am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/21 "2011-11-15T09:34:21Z")
deleted ---
**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, 11:17am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/22 "2011-11-15T11:17:10Z")
> [@Chronos](#): > > 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. Yes; that is exactly what I said. An excellent summation! Although I’m not sure where you got the impression that I was under the impression that the age of the universe is anything other than how long it has existed thus far. ---
**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, 11:21am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/23 "2011-11-15T11:21:37Z")
> [@brocks](#): > > 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? That’s what I’ve always understood. And as everything expands away from us, more and more of it will be doing so faster than light, and will therefore actually disappear from our view forever as it moves outside our bubble of observability (which is expanding _at_ the speed of light). Eventually, in billions of years, future astronomers will see a big empty universe and if historical records don’t survive there will be no way of ever knowing about all that stuff we can see now. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 15, 2011, 12:05pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/24 "2011-11-15T12:05:52Z")
> [@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. [No you are not](http://boards.straightdope.com/sdmb/showthread.php?p=14391151#post14391151). Sheesh! > [@](#): > > The paradoxical (and thus, for some, kinda thrilling) notion that our experience is always experience of the past arises only when we apply inappropriate time-scales to our thinking about cognition and subjective experience, and when we baselessly privilege certain events (such as the first arrival of certain photons at the retina) over others that are in fact, far more significant to the formation and timing of our experiences. With astronomical distances, different considerations come into play, but for ordinary distances, no you are not seeing your monitor as it was in the past. Indeed, one can make just as good a case (but also ultimately wrong) that you are seeing it as it will be fractionally in the future. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 12:23pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/25 "2011-11-15T12:23:07Z")
I’ll just point out that if a star emits light and it is received on Earth 13.7 billion years later, then, in an expanding universe, it must’ve been closer than 13.7 billion light years when the light was emitted. The actual maths is a bit of a pain, especially as the Hubble parameter is a function of time, but it will be signifcantly less than 13.7 billion light years. ---
**Author:** ![Icerigger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/icerigger/32/432_2.png) [@Icerigger](https://boards.straightdope.com/u/Icerigger)\ **Post date:** [November 15, 2011, 12:26pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/26 "2011-11-15T12:26:01Z")
Eventually everything but our own local group of galaxies bound together will be the entire observable universe as all the other galaxies pass over the light horizon. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 12:42pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/27 "2011-11-15T12:42:31Z")
> [@Candyman74](#): > > That’s what I’ve always understood. And as everything expands away from us, more and more of it will be doing so faster than light, and will therefore actually disappear from our view forever as it moves outside our bubble of observability (which is expanding _at_ the speed of light). Eventually, in billions of years, future astronomers will see a big empty universe and if historical records don’t survive there will be no way of ever knowing about all that stuff we can see now. I assume you’re talking about the cosmological event horizon, which is not (necessarily) the same as the Hubble sphhere (the surface receding from us with a recession velcoity of c). In all of the best current models the cosmological event horizon is receding from us faster than c. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [November 15, 2011, 1:29pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/28 "2011-11-15T13:29:18Z")
> [@brocks](#): > > 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? First of all, you have to understand that the interviening space is expanding at a rate which is, as far as we can tell, roughly proportional to the distance between points, such that the further twompoints are away from each other the faster they are moving apart. At the boundary of the observable universe, sometimes called the cosmic event horizon (which is, as previously mentioned, currently estimated to be about 46.5 Bly) the speed at which space is retreating from the centroid is almost _c_. (Note that this is true for any position in space, such that the boundary vhanges with the observer’s position and velocity.) Things outside of this boundary, like anything within the event horizon of a black hole, are forever beyond the accessible area of space-time (called the “light cone”) for the observer, and no direct observation of the amount or organization of mass and energy can be made. The extent of the non-observable universe is unknown and may well be infinite. Stranger ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [November 15, 2011, 1:55pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/29 "2011-11-15T13:55:43Z")
> [@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. Or in other words: [![](https://i.ytimg.com/vi/JcAp1ogn79g/hqdefault.jpg?sqp=-oaymwEmCOADEOgC8quKqQMa8AEB-AH-BIAC4AOKAgwIABABGE8gZSheMA8=&rs=AOn4CLDxvazeTDsmvJQDCMVK9LSfUN2doQ "BIG BIG BIG Smartcar small Commercial 2011") ](https://www.youtube.com/watch?v=JcAp1ogn79g) Link safe for work. Has audio. Of course there is one roque cosmologist in there thats just plain wrong but there is always one of those. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 2:08pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/30 "2011-11-15T14:08:34Z")
I think we need to clear this up, there are 3 surfaces associated with expansion: **Hubble sphere** Radius: ~13.9B ly (note that this is a little larger than the estimated age of the universe multiplied by c) Recession velocity: c (exact) The Hubble sphere is the surface receding from us with recession velocity c. anything beyond has a recession velocity greater than c. **Cosmolgical event horzion** Radius: ~16B ly Recession velocity: ~1.2c (current) The cosmological event horizon is the surface dividing events which will be able to affect us in the future. Events happening now (i.e. now in cosmological time) beyond the cosmological event horizon will not be able to affect us at any point in the future. Is not universal feature of expanding universes (though it is a feature of our universe). **Particle horizon** Radius: ~45.7B light years Recession velocity: ~3.3c (current) The particle horizon is the limit of our expanding universe and is the sphere containing all stars,galaxies, etc that we can see now (though obviously if they lie outisde our cosmological event horizon, but inside the particle horizon we will only ever be able to observe their past). Not all speeds and distances are coordinate speeds and coordinate distances for the standard cooridinates. This paper below is quite accessible and it clears a lot of the confusion that surrounds these issues. > **[Expanding Confusion: common misconceptions of cosmological horizons and the...](https://arxiv.org/abs/astro-ph/0310808)** > > We use standard general relativity to illustrate and clarify several common > misconceptions about the expansion of the Universe. To show the abundance of > these misconceptions we cite numerous misleading, or easily misinterpreted, > statements in the... ---
**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, 3:45pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/31 "2011-11-15T15:45:22Z")
> [@These are my own pants](#): > > I assume you’re talking about the cosmological event horizon, which is not (necessarily) the same as the Hubble sphhere (the surface receding from us with a recession velcoity of c). I’m talking about the term used earlier, which was “observable universe”; and trying to understand what that term is being used in reference to. I understood it to mean the Hubble sphere (being that which is observable), but it seems that it actually refers to the cosmological event horizon. > [@](#): > > In all of the best current models the cosmological event horizon is receding from us faster than c. Yes, I know. I said that. 🙂 ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 3:52pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/32 "2011-11-15T15:52:09Z")
> [@These are my own pants](#): > > I’ll just point out that if a star emits light and it is received on Earth 13.7 billion years later, then, in an expanding universe, it must’ve been closer than 13.7 billion light years when the light was emitted. The actual maths is a bit of a pain, especially as the Hubble parameter is a function of time, but it will be signifcantly less than 13.7 billion light years. Actually I should clarify, if 13.7 billion years is taken to be the age of the universe then the proper distance between us and a star emitting light 13.7 billion years (not that a star could exist that early in the universe) away would vanish (i.e. go to zero) as all proper distances vanish at the big bang singularity. If we assume the star existed a litle after the big bang, say 13.2 billion years ago (the age of the oldest known star), light emitted 13.2 billion years ago would’ve been emitted when we (or at least the spot that has the same comoving coorindates as us) had a proper distance of about 2 or 3 billion light years away from it. That figures taken from the graph on page 3 of the paper I’ve posted above. Though due to the scale used, it’s quite difficult to read accurately for 13.2 billion years ago, so take that as a very rough figure! ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 3:58pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/33 "2011-11-15T15:58:04Z")
> [@Candyman74](#): > > I’m talking about the term used earlier, which was “observable universe”; and trying to understand what that term is being used in reference to. I understood it to mean the Hubble sphere (being that which is observable), but it seems that it actually refers to the cosmological event horizon. > > Yes, I know. I said that. 🙂 That’s fair enough, though the Hubble sphere doesn’t directly relate to what you can and can’t see or what you will or what you won’t see. In fact it’s quite an arbitary surface, just dividing recession velocities into sub- and superluminal ---
**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, 9:59pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/34 "2011-11-15T21:59:50Z")
> [@These are my own pants](#): > > This paper below is quite accessible and it clears a lot of the confusion that surrounds these issues. > > [[astro-ph/0310808] Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe](http://arxiv.org/abs/astro-ph/0310808) Thanks very much for that link. ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [November 15, 2011, 10:05pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/35 "2011-11-15T22:05:14Z")
> [@cmyk](#): > > 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? AFAIU, yes, due to the inflation theory, our light-cone bubble is a tiny spot in the surrounding super-universe. I don’t think there is a consensus among the experts on just how fast the inflation was and thus no consensus on how big this super-universe is. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [November 15, 2011, 10:05pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/36 "2011-11-15T22:05:14Z")
> [@These are my own pants](#): > > Not all speeds and distances are coordinate speeds and coordinate distances for the standard cooridinates. Note, not not i.e. Note: all speeds and distances are coordinate speeds and coordinate distances for the standard cooridinates. ---
**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, 10:42pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/37 "2011-11-15T22:42:24Z")
**njtt** , I’m a physicist, not a physiologist, neurologist, nor psychologist, so I treat the eye as just an idealized detector, without worrying about all the complicated brain processing that goes on behind it. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [November 16, 2011, 7:17am UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/38 "2011-11-16T07:17:14Z")
Isn’t the horizon of last scattering (i.e., the CMB) the oldest thing we can see? How far away is that right now? ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [November 16, 2011, 6:44pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/40 "2011-11-16T18:44:24Z")
Toward the edge of the Hubble Sphere…where expansion equals lightspeed… frequencies/wavelengths are extremely redshifted. Processes which generate cosmic and gamma rays redshift to visual or even radio waves, right? The microwave cosmic background noise, when un-redshifted, reveals what processes at the dawn of time? ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [November 16, 2011, 11:24pm UTC](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920/41 "2011-11-16T23:24:59Z")
> [@Enola\_Straight](#): > > Toward the edge of the Hubble Sphere…where expansion equals lightspeed… frequencies/wavelengths are extremely redshifted. > > Processes which generate cosmic and gamma rays redshift to visual or even radio waves, right? > > The microwave cosmic background noise, when un-redshifted, reveals what processes at the dawn of time? I believe it records the moment, about 300,000 years after the big bang, that matter and energy decoupled, and the universe became transparent. Others are better informed than I, though. [Previous page](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920.md?page=1) [Next page](https://boards.straightdope.com/t/how-big-is-the-known-universe/602920.md?page=3)