Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is our entire universe observable?

  1. Nov 18, 2011 #1
    In an earlier thread--What is space?--a few people indicated that only a part of our universe is observable: Because space is expanding, photons from distant parts of the of the universe haven't had time to get here yet.

    So that's my next question. Is our entire universe observable?

    My thinking--based on what I've read, of course--is that we can observe all of our universe. When we look out into space, we're also looking back in time, so when we see the very earliest galaxies--which we do--we're looking back about 10 or 12 billion years, and (I think) when we observe the cosmic microwave background radiation, we're looking back even further in time; in fact, we're listening to the "noise" from the Big Bang.

    Put another way, our universe began with the Big Bang about 13.7 billion years ago. Since then, it has expanded--and the expansion is getting even more rapid--but it can't have expanded to infinite size because that would require that the rate of expansion also be infinite. We've measured the rate of expansion, and it is increasing, but it's not infinite.

    At some time in the distant future, our universe will have expanded enough so that we won't be able to observe distant galaxies; in fact, we'll be able to see only our local galactic cluster.

    But we're not there yet. Since we can look back almost as far as the Big Bang, we can observe everything in our universe.

    Am I even close on this one?
  2. jcsd
  3. Nov 18, 2011 #2
    Philip, The Universe is believed to be far bigger than the observable universe. Penrose put a number on it, somewhere around 10 to power 30 times bigger. Therefore if this number is correct, only 10 to the power -30 of the whole Universe is observable!
  4. Nov 18, 2011 #3
    Scratching my head.

    Taking a shower (where I do some of my best thinking) after I posted my question, I realized that part of the problem is the word "observable."

    When we look (at light or radio waves or X-rays) out into space, we're looking back in time. I forget how far away the Andromeda galaxy is--let's say 100 million light years--so if I want to know what's happening there at 00:00:00 GMT on 11/18/2011, I'm gonna have a long wait.

    So we can't observe what is. We can only observe what was. Even as I continue to scratch my head and look out the window, it takes a small fraction of a second for the light hitting my retinas to be be converted into an electro-chemical neural impulses sent through my optic nerves to my brain where they're "converted" into a sensory impression.

    We're all living a few hundred nanoseconds in the past. In that sense, nothing is "observable" in the present.

    But back to your comment and Penrose: Does that 1030 number come from the expansion of the universe since the Big Bang? And is "The Universe" the same as "our universe"?
  5. Nov 18, 2011 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Philip you don't seem to me to be listening to what people are telling you here. We try to base our information (at least some of it) on the professional research literature, not on commercial popularizations. Maybe we sometimes could do a better job, but it is a different domain of discourse. There is a subtle divide.

    Where you seem to have gone wrong is by starting with some pop-sci account (Hawking/Greene, I forget what sources you mentioned) and then on top of that apparently misunderstanding what the popularization said.

    I don't know how we can help you if you don't throw out the fundamental misconceptions and misinterpretations and listen very carefully.

    Standard mainstream cosmology does not assume that the big bang had a finite volume and involved only a finite amount of matter.

    If you think some commercial author told you otherwise then either they lied or you misinterpreted.

    So, after a finite amount of expansion you can have an infinite universe. It is not ruled out.

    Also a FINITE volume big bang is not ruled out! It is not favored by the observation data, but it is not ruled out either. And that would lead to a FINITE spatial volume universe.

    But what YOU say IS RULED OUT by observation with 95% certainty, namely that the U IF it is finite coincides with the currently observable portion. There was a conservative 95% lower bound published on the ratio back in 2007 as I recall. IF finite then the whole's circumference has to be at least 600 billion lightyears---an order of magnitude (roughly a factor of ten) bigger than the diameter of the currently observed portion.

    That was the Komatsu et al WMAP5 report from a while back. Haven't seen anything more recent. If anyone wants the link, please ask and I will go find it.

    Also I think you said you have seen one or more of those pretty pictures the largescale structure----wispy cobbwebby pictures with their voids and crisscross strands.

    That is assumed to be coextensive with space. Filling all space whether finite or infinite. It has resulted (apparently) from an approximately uniform distribution of matter gradually falling together into strands and clumps. This has been confirmed by computer simulations! They start with an approximately uniform distribution coextensive with space and simulate the action of gravity, gradually condensing stuff, and they get results which are visually quite similar.

    Google "TED Smoot" for a 15 minute talk with computer simulations of this process. Watch the structure forming. It's neat.

    I have to go, no time to say more.
    Last edited: Nov 18, 2011
  6. Nov 18, 2011 #5
    There are three aspects of the word universe, and it's important to be very clear about which aspect you are speaking. Any question prefaced with "our" universe becomes automatically ambiguous.

    The first and most popularly thought of aspect is "the" universe. That comprises all of the infinite matter. "The" universe is popularly mistaken for the second notion.

    The second notion of the word is the "observable" universe (OU). We cannot see beyond the sphere that is the observable universe because, as you say, the light has not reached us yet. We see back in time because there are still galaxies and other matter from closer to the BB time that is within the OU. The radius of the OU sphere is slowly increasing as light from further distant galaxies finally reach us. Light that began its journey many billions of years ago and is only reaching us now, is therefor within the OU and logically gives us information from many billions of years ago.

    The third notion of the word is in discussing "other" universes, which is rarely referred to because it is entirely theoretical. The notion of "our" universe is sometimes spoken of in this third context.

    When reading popular material or seeing documentaries, it's important to keep the first 2 notions of the universe in mind as they are often not clearly delineated in such works.

    Edit: but to answer the original question, slightly reworded: Is "the" entire universe observable? No it is not.
    Last edited: Nov 18, 2011
  7. Nov 18, 2011 #6
    I am not sure whether this would be off-topic or what , I want to know something in relation of past and light(observance),

    Lets take example of Andromeda Galaxy, AS we all know whatever would have happened on andromeda galaxy few millions of years ago, we will see that now as light takes time to travel,,

    Is there anything else like light that can travel in space/universe and takes more time than light to travel, and we can see that...like some sort of signals or radiations , or anything like that ??? ........If possible then is there any possibility that these signals may show us something of early begining.......


    also why does universe appears black/colorless to us ??
  8. Nov 19, 2011 #7
    The well-known term observable universe refers precisely to what we can observe. observable universe and universe are not synonymous
  9. Nov 19, 2011 #8
    Marcus, et al:

    A few, some, many, most (take your pick) readers of this blog are non-scientists. A deep understanding of many of the questions non-professional people ask requires knowledge of the mathematics behind them. What you dismiss as "pop-sci" is, by necessity, written without any math. So, of course, we don't get it.

    Another part of the problem is that laypeople like me need to be able to visualize things, and sometimes that's simply impossible. Writers who popularize science provide lots of visual representations, but these images have to be imprecise, as are the metaphors used to "explain" the real science.

    So, please, lighten up just a little bit. It's in your best interest: Popularizers like Green and his NOVA program on public television spark public interest--and even enthusiasm. I'm someone who may be technically ignorant, but I'll always support (with my vote) billion-dollar research programs because I'm fascinated by what we're learning even though I may be dumb about exactly what it is.
  10. Nov 19, 2011 #9
    Philip, I believe that much of what we call the observable universe will no longer be observable for light leaving stars in our current time frame. What we are calling the most distant parts of the observable universe is actually light that was emitted from early galaxies as little as 700M years after the Big Bang. Soon even these most distant early stage galaxies will receed, never to be seen again, and those same galaxies, as they are now, have long since become unobservable.

    (I dislike the term BB!)
    Last edited: Nov 19, 2011
  11. Nov 19, 2011 #10
    By "recede, never to be seen again", do you mean that light from them will redshift until we can no longer detect it, or that light they are emitting today will somehow never reach us? (Just checking, because I currently understand the former to be the case, and not the latter).
  12. Nov 19, 2011 #11
    Cephron, my interpretation is that galaxies would become further and further redshifted until the frequency of light drops all the way through the radio spectrum to zero, and then no more signals from them at all. The light can no longer reach us because the distance between us and the light is increasing so fast that the light can not reach us.
  13. Nov 19, 2011 #12


    User Avatar
    Gold Member
    2016 Award

    Yep, that's how I understand it too. Things will, over time, move out of our observable universe until there's nothing left in it but the Milky Way and probably the Local Group.
  14. Nov 19, 2011 #13


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You might want to consider essentially the same scenario but with this refinement. Say the cosmic event horizon stabilizes at 16 billion LY. Galaxies are gradually receding out to that horizon and, in actual fact, crossing it and leaving our observable universe. But we don't see them cross!

    It is somewhat analogous to things falling into a BH horizon. We see them going slower and slower and getting more and more redshifted---they sort of get hung up on the horizon and redshift into undetectability. But we never see them actually pass thru.

    I recall that Lawrence Krauss has an article about the distant future where he describes things like this. He speculated that the Local Group would eventually all merge. Our galaxy would be merged into a larger combined galaxy.

    To get that 16 billion LY, take your figure for the Hubble rate, say 71 km/s per Mpc
    and take your estimate of the "dark energy" fraction*, say 73%
    and type this into google:
    1/(71 km/s per Mpc)/sqrt(.73)

    You should get 16.1 billion years. So thats what "hubble time" will be far in the future. So
    hubble radius will be 16.1 billion LY.

    But in the far distant future the Hubble radius will have stopped growing and will approach an asymptotic value. So coincides with cosmic event horizon. Light from beyond it cannot get to us.

    Dark energy fraction is a conventional handle on the cosmological constant. I think energy is probably a bad way to think of the cosmological constant. The most natural way is a "zero point curvature" a very slight irreducible warping
    Last edited: Nov 19, 2011
  15. Nov 19, 2011 #14
    Hi phinds, I dug the following up from one of the sig links on Marcus' posts, the Charley link.

    Charles H. Lineweaver and Tamara M. Davis in 'Misconceptions about the Big Bang' say that the light from receding galaxies will reach us because the Hubble distance changes with time.
    I'm not sure if I agree with this either because we would expect to see light from beyond our observable universe under the same principles. For this to be true the observable universe must be equivalent to the entire universe.

    The following was the wrong answer.
    Last edited: Nov 19, 2011
  16. Nov 19, 2011 #15


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi Laurie, I think you said you were in Australia. Lineweaver is a fellow countryman. I'm a fan of both authors (Lineweaver and Tamara Davis). Glad you had a look at the "charley" link.

    I don't think they say that light from ALL receding galaxies will eventually reach us, no matter how far and how rapidly receding.

    The interesting thing is to figure out how far and how rapidly receding a galaxy can be and still have its light eventually reach us.

    The misconception is that if ANY galaxy is outside our Hubble radius (which means that it is receding > c) then its light can automatically never get here.

    that's simply not true, because the Hubble radius, which is now about 13.8 billion LY, is still expanding and is due to grow out to around 16 billion LY before its growth essentially ends and it levels off.

    Laurie, if you (or anyone you know) likes numbers, try this. Type this into google.
    1/(71 km/s per Mpc)
    You should get 13.8 billion years.

    Now divide that by sqrt(.73) or type this into google:
    1/(71 km/s per Mpc)/sqrt(.73)
    You should get around 16 billion years.

    convert to distances:
    Present Hubble radius = 13.8 billion LY
    Future Hubble radius far in future when it stops growing = 16.1 billion LY.

    These numbers depend on an estimate for the Hubble constant which is 71 km/s per Mpc.
    Some people prefer 70 or 72 and that would change the estimates of distance, but the idea is clear enough. The 0.73 comes from assuming the cosmo constant or "dark energy" fraction is 73%. If you like 75% then the numbers also change accordingly.
    Last edited: Nov 19, 2011
  17. Nov 19, 2011 #16


    User Avatar
    Gold Member
    2016 Award

    Good points as always, Marcus. Thanks.
  18. Nov 19, 2011 #17
    Hi Marcus, I think the yes answer is also specific to certain receding galaxies but these galaxies are identified in the wrong answer as being past the Hubble radius.
    So if the light from a receding galaxy is currently in a specific range beyond the current Hubble radius we may be able to observe its light in the future. This implies that our observable universe will increase in mass over time.

    Marcus, do you know if anybody is looking into this problem because there should be a certain measurable and consistent rate of change of universal mass associated with it that would need to be factored in. If not the scenario is irrelevant and the observable universe is equal to the entire universe (BB).
    Last edited: Nov 19, 2011
  19. Nov 20, 2011 #18
    The key words in what you quoted are "can grow to encompass the photon", so can, not will. The way I understand this is that receding galaxies are not all on a linear trajectory directly away from us. There would be many vectors out there, some towards us, some away. And depending on a combination of their trajectory and how near or how close to our Hubble sphere, some will come into view while others will not.
  20. Nov 20, 2011 #19
    Sorry if someone else already pointed this out, haven't had time to read the responses.

    I think one part of your misunderstanding stems from the fact that you seem to think we're at the center of the universe or something. The fact that we see a sphere surrounding us (the CMB) the furthest we can see in every direction doesn't mean that if we were an entire observable universe's distance away in another direction we wouldn't still see the CMB from just as long a time ago in all directions around us. Where we are right now was once the location of the origin of the universe, just as everywhere else is. If you could be located the exact right distance from the planet earth and looked at the planet earth, you would this part of our universe from that 300,000 or w/e yr age. Our location is not special or unique.

    The universe could be much larger in extent or even infinite and until more time has passed you'd still be able to see just as far back, that doesn't mean there is nothing else further away.
  21. Nov 20, 2011 #20

    Marcus, I like this solution. Being an optimist I have a preference for solutions which avoid the big rip or the big crunch. This solution seems to be the one which is most stable for the longest period of time. In this scenario the entire observable universe at that time would ultimately become a single stable galaxy with all the rest of the whole universe similarly divided.


    I find it amazing that something as complex as the whole Universe could unfold with such stability, without adjustment along the way and just from the initial starting conditions, if that is actually the case.
    In engineeering we cant make even a simple process control system which doesnt need some kind of adjustment along the way.

    Marcus I was trying to understand what you meant here. Is this something that is plugged into the Cosmos calculator?
    Last edited: Nov 20, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook