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Is there an event horizon beyond the visible universe?

  1. Aug 11, 2012 #1
    Is there an event horizon beyond the visible universe where the laws of physics kind of get cut off, a limitation to the reach of gravity.

    I ask this because if space is expanding there must be a point that if light left earth it could never come back because the return distance is expanding faster than the light can travel.

    Does it exist? If so what is it called and how far out into space is it.
     
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  3. Aug 11, 2012 #2

    cepheid

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    There are a couple of things in your post that don't make too much sense (to me). In particular, the vague phrase about something being "a limitation to the reach of gravity," and the notion of light leaving Earth and somehow coming back (or failing to). Why do you think it would come back in the first place?

    All that having been said, the answer to your question is yes, there is such a thing as a cosmic event horizon. First of all, a bit about terminology: the radius of the observable universe is the maximum distance within which light has had time to reach Earth in the age of the universe. We can't see anything beyond this distance yet. Cosmologists refer to this boundary (the edge of the observable universe) as the particle horizon, or sometimes just "the horizon." However, this is not an event horizon. An event horizon is defined as a boundary beyond which you can have no knowledge of the events that occur. This is not true of the particle horizon, because although we cannot see anything beyond it right now, eventually, as time goes on, light from more distant things will reach us. In other words, the particle horizon distance will get larger (the portion of the universe that is "observable" will increase).

    A cosmic event horizon would be a distance defining the boundary beyond which you will never see light from objects, no matter how long you wait. Whether it exists or not depends on which cosmological model you use. In the cosmological models that contained only matter, in which the expansion of the universe was slowing, the event horizon distance was at infinity, which is another way of saying that there was no event horizon in these models. If you waited long enough, eventually *everything* would come into view. (You'd have to wait forever to see everything, but that's besides the point -- in principle you could see any object, no matter how distant, as long as you waited long enough for the light from it to reach you). However, the most current cosmological model (which we think most closely describes reality) includes both matter and dark energy, the latter of which is responsible for causing the expansion of the universe to accelerate. Because of this acceleration, there is an event horizon. In other words, there is a finite distance beyond which you will never be able to see anything no matter how long you wait. In other words, light from objects more distant than the event horizon distance will never ever reach you. Therefore, you can have no knowledge of events that occur beyond this horizon. In this sense, the cosmic event horizon is similar to the event horizon of a black hole.

    I don't know off the top of my head what the event horizon distance is in the favoured cosmological model, sorry. EDIT: obviously it would be beyond the particle horizon, just as your thread title suggests.
     
    Last edited: Aug 11, 2012
  4. Aug 12, 2012 #3

    marcus

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    Hi Cepheid, I think you may be mistaken about what is usually called the cosmic event horizon (CEH). A common figure for the particle horizon distance is 46 billion lightyears. A common figure for the CEH distance is around 16 billion lightyears. Lineweaver has a discussion of this in his paper "Inflation and the CMB" and Figure 1 of that paper has proven useful so I keep a link to it in signature. It is the "caltech.edu/level 5" link.
    You may know the Caltech Level 5 stuff, it is an excellent collection of resources for learning cosmology and astrophysics. Lineweaver's article is also available pdf on arxiv.
     
    Last edited: Aug 12, 2012
  5. Aug 12, 2012 #4

    cepheid

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    That doesn't really make any sense given the definition of the event horizon as the distance beyond which you will never be able to see anything, at any time in the future. Clearly this boundary cannot lie within the observable universe. EDIT: I see that what you were trying to say was that the thing called the cosmic event horizon has some other definition than what I just said. I'll respond to that in a moment.
     
    Last edited: Aug 12, 2012
  6. Aug 12, 2012 #5

    marcus

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    If today a galaxy is galaxy is beyond the CEH, that is more than 16 billion ly from us, and if today that galaxy sends us a flash of light, it will never reach us. No matter how long we wait. If today there is a supernova explosion in that galaxy, we will never see it. This is basically how the CEH is defined.

    Another way to think of it is if a galaxy is more than 16 billion ly from us, today, and we leave today traveling at the speed of light, we will never reach it.

    The exact figure for the CEH distance depends of course on what parameters you plug into the standard LCDM model. It is something around that whether you use this or that set of parameters (e.g. 71 km/s per Mpc, 0.73 and 0.27, or 70.4 km/s per Mpc and 0.728 and 0.272). The usual ones all give about the same CEH.

    You can see the CEH and the particle horizon both plotted in Lineweaver's Figure 1.

    The same figure is used in the Davis Lineweaver article "Expanding Confusion" that many people like to quote and recommend. In fact, I'd like to recommend it to QuantumHop! :biggrin: Great article, clears up a lot of confusions!

    http://arXiv.org/abs/astro-ph/0310808
    Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe
    Tamara M. Davis, Charles H. Lineweaver
    (Submitted on 28 Oct 2003)
    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 literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the "observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests...
    26 pages
     
  7. Aug 12, 2012 #6

    cepheid

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    I had a look at your diagram, and to be honest, it seems to support my case. What *I'm* calling the particle horizon, is the radius of the observable universe, which is the spatial extent (at t = 0) of the past light cone of the "now" observer. (For some reason, this is not what is labelled as the particle horizon on this diagram -- meh). From your diagram, it has a radius of somewhere between 40 and 50 Gly. In contrast, what I'm calling the event horizon (and what is also labelled as such on your diagram), is the spatial extent (at t = 0) of the past light cone of an observer at t = ∞. (On the bottom diagram a cosmic time of t = ∞ seems to map to a finite conformal time, which is handy). In other words, what's the farthest distance from which light can have reached you if you wait forever? In this diagram, it appears to be in excess of 60 Gly in radius, and is larger than the particle horizon for all t.

    I will post momentarily with the precise definitions that I'm using for these terms.
     
  8. Aug 12, 2012 #7

    cepheid

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    Okay, here is what I am referring to (maybe somewhat technical for the OP, I don't know). Apologies for the poor quality:
    XTTfx.png
    c9iai.png
     
  9. Aug 12, 2012 #8

    cepheid

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    Yes, I see what you're saying. The 16 Gly is the radius of the past light cone of the t observer at t = now, rather than at t = 0. What I disagree with is whether this is meaningful. Saying that the diameter of the CEH is the diameter of the intersection of this past light cone with the horizontal "now" line makes no sense to me, because light from that galaxy isn't only just leaving to come towards us today. It started leaving 13.7 billion years ago! (Or at least some time in the past approaching that, depending on when that galaxy formed). So, light from that galaxy, which is currently 16 Gly away, will eventually reach us, because it left long before today. In fact, light from any galaxy whose comoving distance (which is also its proper distance *today*) is within 60 Gly will eventually reach us. That is what Lineweaver's diagram clearly shows. For this reason, IMO 60 Gly is the most sensible number to state for the radius of the comoving event horizon (and I think it's also the number that will be returned by integral 12.18 if you exclude the factor of 'a' in front). Light from anything within this comoving distance will reach us in a finite time.
     
  10. Aug 12, 2012 #9

    marcus

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    And I see what you are saying! The Figure actually has 3 layers. I was thinking in terms of proper distance, which is the horizontal scale of the top layer---the top version of the figure.

    You were thinking in terms of *comoving* distance, which is the horizontal scale of the middle and bottom versions.

    As you can see, the CEH distance, in proper distance terms, started out at t=0 as essentially zero and has been constantly increasing all this time, and is gradually approaching a limit of around 16 billion ly (proper distance in the indefinite future.)

    Both are interesting numbers. You point to the 60-some Gly (com. dist) at t=0. I can see that very clearly in the bottom "layer" of the figure. I don't think of it as the CEH, but you do, which is fine.

    For me the 16 Gly CEH is interesting because it is somewhat analogous to a BH event horizon in that as time goes on we will see galaxies drift out to that distance and not pass over but just redshift out of visibility. Analogous to what an outside observer sees when things fall into a BH. He sees them reach the EH and not cross over but just redshift to the limit of perception. He sees their time slow down. this is what we will see happening at the 16 Gly proper distances CEH.
    Or so I think/have read. Tamara Davis (Lineweaver's Phd student) did a thesis on some related matters and a chapter was devoted to this.
     
  11. Aug 12, 2012 #10

    cepheid

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    Nope. I was wrong, and now I have to eat crow. The lower limit on integral 12.18 isn't t = 0. It's t = t1, which is some given time corresponding to "now" for the observer whose event horizon we're talking about. That means that the CEH isn't defined the way I've been saying. Even in Longair (whose book I provided the excerpts from above) it's defined in the sense indicated on Lineweaver's diagram. So you were right all along. What I'm thinking of as the CEH is not the CEH. It's better referred to as just the "maximum (comoving) radius that our observable universe will ever reach (60 Gly)."

    Yes! Brilliant explanation, thank you. That is what the CEH is. Even if we can see objects out to this distance now, objects that are (presently) at or beyond this distance will eventually fade from view (edit: because although light from them has been reaching us in the past, it will eventually stop doing so). You are right. Sorry marcus, I stand corrected.
     
    Last edited: Aug 12, 2012
  12. Aug 12, 2012 #11

    marcus

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    From your scanned pages around equation 12.18:
    ==quote==
    Event horizon. From the discussion of Sect. 12.1 we note the definition (12.13) of the event horizon rE as the greatest proper radial distance an object can have if it is ever to be observable by an observer who observes the Universe at cosmic time t1
    ==endquote==

    I'll have to think about this a little more tomorrow. different people's definitions. Your definer seems to want it to be a PROPER radial distance. And the observer and the object appear to start out at a definite time t1. That is like the "today" I was talking about. the issue is does the light ever reach the observer if he waits long enough.

    Have to think about it later.

    EDIT: Oh wait! I just saw your post. You figured out what Longair was saying and clarified everything. Thanks! The 60 Gly comoving distance however is extremely interesting in its own right, just not what Longair and Lineweaver call CEH. It captures the idea of the matter which we eventually get light from.
     
    Last edited: Aug 12, 2012
  13. Aug 12, 2012 #12
    I don't mean light decides to turn around and come back all of its own accord. I'm asking if there is a distance where it cannot come back (even if it was reflected by a mirror) because its gone so far out that the return distance is expanding faster than speed c.

    I'm just using that as an attempt to describe a distance where the laws of nature become causally disconnected. I.E a distance where gravity would fail to work because the space in-between two objects was expanding faster than gravity waves could travel.

    Maybe an event horizon is not the word I should have used?
     
  14. Aug 12, 2012 #13

    marcus

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    Hi QuantumHop, I realize your question is to Cepheid, and I am barging into the discussion. I should apologize. It's just too interesting.

    C. already answered your question correctly and I did not realize this at first because I have a somewhat rigid understanding of the term "event horizon". Sometimes words get in the way. C. points out that there is this very interesting distance which looks (on Lineweaver's graph) to be about 62 billion ly.

    I'm not sure what to call it. Maybe it is the "ultimate particle horizon". It is the distance measured today of the farthest matter that light from our matter (starting at the very beginning of expansion) could ever reach.

    Conversely, it is the distance measured today of the farthest matter which, if it emitted some light in our direction at the very beginning of expansion, we would eventually see.

    That matter is, today*, about 62 billion ly from us. And the light from it, that it emitted as a hot gas before it formed into stars or anything, is already (today) within 16 billion ly of us, and will arrive here sometime far into the future (when perhaps it's cold and nobody lives here.)

    This distance seems to me very beautiful. It is the ultimate limit of the observable universe, if it were measured today---and since the technical name for the today distance to the edge of the observable is "particle horizon" I guess the correct name for this is "ultimate particle horizon". I suppose one could abbreviate it UPH.

    *to define distance as measured today you have to imagine you could stop the expansion process in order to have time to measure (with radar or string or whatever) without having distances change all the time you were doing it. Freezeframe distance at any particular moment is called the "proper" distance at that moment. And we can imagine labeling batches of material with a permanent label which is its proper distance now at this present moment and calling that its "comoving" distance. The comoving distance of stuff is a convenient handle on it which does not change significantly over time.

    In an effort to sort things out, I've made a short dictionary :biggrin:

    CEH: cosmic event horizon, currently 15 to 16 Gly, today distance of the farthest galaxy you could hit with a flash of light that you send today. Events that occur today beyond that distance cannot have any effect on us. The longterm value of CEH is about 16 Gly, it is still growing but so slowly that practically speaking it is already at 16.

    PH: particle horizon, the radius today of the currently observable region, currently 46 Gly. This is the distance today of any photons of light which our matter emitted at the very start of expansion (if they could have traveled unimpeded).
    Conversely it is the distance today of matter which we could in principle be seeing today if its light hadn't gotten scattered along the way.
    We actually can see ALMOST out to the edge because the universe turned transparent very early (after only a few hundred thousand years)
    The PH keeps extending out because as time goes on there is time for more and more light to reach us. WHAT IS THE ULTIMATE LIMIT of the particle horizon?

    UPH: "ultimate particle horizon"? this is the distance today of the most distant matter whose light, emitted at or near the start of expansion, will eventually reach our matter, in the cold longterm future. Intelligent cockroaches nibbling on the last remaining cosmic crumbs will have a larger observable universe than we do and they will be receiving light from stuff which is now 62 billion ly from us---light from which we haven't seen yet although it is on its way.
    Note that this light, those photons which they are going to receive, must ALREADY be within 16 billion lightyears of us, if it is ever going to make it. (that is the meaning of the CEH, nothing that today is beyond 16 Gly can ever get here.)
     
  15. Aug 12, 2012 #14

    cepheid

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    No, to be honest, I'm glad you did! If only to reinforce that I wasn't completely off the wall when I gave QuantumHop my first answer. So, yes, QuantumHop: no matter how far into the future we wait, we will never ever see light from matter that is presently beyond a distance of 62 Gly (giga-light-years or billion light years).



    I think we are on the same wavelength here, M. "UPH" for the win!. :biggrin: Always fun when you get to coin a new term, although I wouldn't count on it being readily and widely adopted in the literature or anything. :uhh:

    EDIT: an interesting note is that although the present radius of this ultimate limit is 62 Gly, the scale factor goes to infinity. This means that there is no upper limit on the proper size of the observable universe as t → ∞, correct? We will be able to see stuff that will then be arbitrarily far away. But that stuff will not encompass any matter that is presently outside of a 62 Gly radius from us. It's just that the stuff within that radius will eventually be expanded out to ∞.
     
  16. Aug 12, 2012 #15

    marcus

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    I think that's right, Cepheid. Light will be arriving from stuff which is now 62 (or so) Gly and
    1. the light will mostly be too far redshifted to do much with, too long wavelength by then to focus and make images from
    2. the light will mostly have been emitted when the matter was near us [you know the present CMB light was emitted by matter which was only 41 million ly from us and is now 1100 times farther, expect more of same---light from stuff that was even closer at emission time, and even farther now, and even more redshifted.
    3. the light will all consist of photons that, at present, are already within 16 Gly of us. They will just have taken longer to get here because of the struggle with expansion which all light faces.
    In any case this is what I think. Please correct me on anything you find wrong or said badly.

    I'm still enjoying the mindstretching idea of your 60-some Gly, the presentday radius of the ultimately observable.
    =========================
    I'll copy a table from post#358 of the "Same Page" thread because it shows an intriguing seeming contradiction.
    If you go out to stuff that is beyond 5.8 billion ly in today's distance, the farther you go the NEARER the stuff was when it emitted the light.

    The table may need additional explanation for some readers, so I put more explanation over in post#358.
    https://www.physicsforums.com/showthread.php?p=4031639#post4031639
    But this is just the table itself to show how the distance "then" (at emission time) gets less as the distance "now" increases.

    Code (Text):
     Standard model with WMAP parameters 70.4 km/s per Mpc and 0.728.
    Lookback times shown in Gy, distances (Hubble, now, then) are shown in Gly.
    The "now" and "then" distances are shown with their growth speeds (in c)
    time      z     H(conv)   H(d[SUP]-1[/SUP])    Hub      now          back then
       0     0.000     70.4   1/139    13.9      0.0          0.0
       1     0.076     72.7   1/134    13.4      1.0(0.075)   1.0(0.072)
       2     0.161     75.6   1/129    12.9      2.2(0.16)    1.9(0.14)
       3     0.256     79.2   1/123    12.3      3.4(0.24)    2.7(0.22)
       4     0.365     83.9   1/117    11.7      4.7(0.34)    3.4(0.29)          
       5     0.492     89.9   1/109    10.9      6.1(0.44     4.1(0.38
       6     0.642     97.9   1/100    10.0      7.7(0.55)    4.7(0.47)
       7     0.824    108.6   1/90      9.0      9.4(0.68)    5.2(0.57)
       8     1.054    123.7   1/79      7.9     11.3(0.82)    5.5(0.70)
       9     1.355    145.7   1/67      6.7     13.5(0.97)    5.7(0.86)
      10     1.778    180.4   1/54      5.4     16.1(1.16)    5.8(1.07)
      11     2.436    241.5   1/40      4.0     19.2(1.38)    5.6(1.38)
      12     3.659    374.3   1/26      2.6     23.1(1.67)    5.0(1.90)
      13     7.190    863.7   1/11      1.1     29.2(2.10)    3.6(3.15)
     13.6   22.22    4122.8   1/2.37    0.237   36.7(2.64)    1.6(6.66)    
     
    Abbreviations used in the table:
    "time" : Lookback time, how long ago the light was emitted, how long the light has been traveling.
    z : fractional amount distances and wavelengths have increased while light was in transit. Arriving wavelength is 1+z times original.
    H : Hubble expansion rate, at present or at times in past. Distances between observers at rest grow at this fractional rate--a certain fraction or percent of their length per unit time.
    H(conv) : conventional notation in km/s per Megaparsec.
    H(d-1) : fractional increase per convenient unit of time d = 108 years.
    "Hub" : Hubble radius = c/H, distances smaller than this grow slower than the speed of light.
    "now" : distance to object at present moment of universe time (time as measured by observers at CMB rest). Proper distance i.e. as if one could freeze geometric expansion at the given moment.
    "then" : distance to object at the time when it emitted the light.
     
    Last edited: Aug 12, 2012
  17. Aug 13, 2012 #16
    The cosmic event horizon only exists if the universe continues to expand, which, as far as we can tell, it will do.

    Now a philosophical question. Normally, science is only concerned with things which are, in principle, measureable. Yet, there seem to be regions of the universe which are forever hidden from us. Is it meaningful to say that they exist at all? It seems to deny their existence is the same as claiming that we are at the center of the universe, which is a very egocentric view. So, it seems we must accept that there are potentially infinite places that are causally disconnected from us.
     
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