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

Big Bang inflation/expansion vs centre of universe vs Speed of light query

  1. Oct 8, 2012 #1

    Firstly, I want to thank all those knowledgeable people here that offer their input so freely. I have found it very enlightening and educational.
    I apologise in advance if the following is quite long but there are a couple of cosmic rules that I can't quite grasp so I will try to articulate it in a structured manner :)

    I was watching a program on Discovery channel (UK) last night called Stephan Hawkings Universe (http://www.discoveryuk.com/web/stephen-hawkings-universe/videos/).
    A 2 hour documentary that cover the universe from start to theorised end.

    This program seemed to counter some of what I have read here and my understanding of the cosmos and the rules that governs it... yet, I suspect that it is my misunderstanding of the rules and/or the concepts.

    Center of the Universe:
    Now having read several of the post here that say that there is no center to the universe and that the big bang happened everywhere all at once.
    Whilst I have a little bit of difficulty comprehending this, I can accept that is entirely possible. However, in the program last night SH said that if we could freeze time and run it in reverse, all matter in the universe would end up at a single point, being infinitely small (slight paraphrasing - sorry). I understood this to say that the Expansion/inflation of the Universe started from a single point.
    He further went on to say that after a trillionith (spelling?) of a second, our universe was the size of an orange and that after 10 minutes, it was the size of our solar system or maybe the milky way (cant remember which for sure), being trillions of miles across.

    My question on this point is, on the assumption that SH knows his stuff, how could it be that we could not calculate the center?
    (I am positive I have misunderstood something here - being purely an interested layman).

    Cosmic Speed limit:
    Following on from the above, again on the assumption that the statement that the universe was the size of our solar system (or galaxy) in 10 minutes, this would mean that the matter expanded / inflated faster than light? I base this on my understanding that it takes light 8 minutes to reach the Earth from the Sun, which is 93 million miles away.

    My question is, at what point did the laws of physics start (in cosmic terms) that prevents Faster Than Light (FTL) travel?
    This seems to mean to me (incorrectly most likely) that FTL speed was possible and hypothetically, at some distant future time in our scientific development - it would be able to harness those principles to travel beyond the speed of light - therefore potentially allowing travel in much short time frames to distant places in our Galaxy.

    Whilst I appreciate that the above is mainly conjecture on my part, I would like to understand where or what it is I misunderstand.

    Thank you,
  2. jcsd
  3. Oct 8, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    It is true that if we could rewind time we would view everything getting closer together. However, once we get to the point that the density is very very high, we start to get infinities in our math. It is at this point that we have a "singularity". The key thing to understand is that just because we get infinities doesn't mean that a "real singularity" is formed. It is very likely that we simply don't know how the rules of the universe work at the high energy, high density scales of the early universe.

    It is purely a miscommunication on their part. If you could rewind time back to 1 second after the start of the universe, we would still see the universe as being infinite in size. Take it back to 0.01 seconds, and we STILL see an infinite universe. The only thing is that the density and energy of everything continues to increase as we go further back in time.

    The speed of light, commonly known as c, is the speed limit for things that are moving "through" space. Expansion doesn't obey this rule, as it is an effect of spacetime itself dragging stuff with it. (This is actually not an accurate statement, but without an understanding of the math of General Relativity it is extremely difficult to understand properly. But the simply, easy way to view it is that space is expanding and things are moving with it.)

    Being that the huge recession velocity of faraway objects is purely due to the expansion of spacetime, which we have no control over, it is not possible to harness this.
  4. Oct 8, 2012 #3
    I liked the other parts of your answer but I think "the size of an orange" is too much of a "miscommunication" to be attributable to these minds. I believe they meant literally, a few centimeters in diameter.

    "Infinite" can be viewed as process. Space can be growing indefinitely and have recession speeds > c, which make its extent forever inaccessible, yet still be finite at a given moment. This space would be effectively infinite. That's how I see the surface of the balloon analogy (BA). But there are many views in the related sticky threads.
  5. Oct 8, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    It could be a miscommunication in the sense that he might have meant "observable universe", which does indeed get very tiny in volume as you go back in time. This doesn't mean that there wasn't any more "stuff" outside the observer's horizon, perhaps even an infinite extent of it.

    Note that it is quite easy to determine the centre of the observable universe - it's where the observer is.
  6. Oct 8, 2012 #5
    The universe tho did not prevent travelling faster than the speed of light o.o

    neutrinos broke the speed of light. So light is not necessarily the fastest thing :3.

    Also the dot expansion universe thing is just how i though of it in that theory i asked for feed back in lol.
  7. Oct 8, 2012 #6

    I'm only a novice myself, but I hope that this helps.

    Center of the Univerese: Do you remember the sceen in the programme where the "hall" floor was filled with ball bearings? Think of each ball bearing as being a small "observable universe". If you squeeze the bearings, the central point of each would then become closer and so the entire univerese is more dense, and it also turns out, much hotter. Imagine running a film of this "squeezing", focused on any particular point, you now have a sequence running to a point, but if you zoom out, no matter how far out you zoom, you never get a full view of an infinite univerese (the collection of all the ball bearings).

    Cosmic Speed Limit: As others have said, the objects are not travelling at the speed of light, the universe (space) in which they exist is expanding. Theory goes that very early in the history of the universe, the rate of inflation was very fast - the objects themselves did not move, the space between the objects expanded. Thus the speed limit was never exceeded and the laws of physics always applied - or so the story goes.


  8. Oct 8, 2012 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Neutrinos did not, and do not travel FTL.
  9. Oct 9, 2012 #8
  10. Oct 9, 2012 #9


    User Avatar
    Staff Emeritus
    Science Advisor

  11. Oct 9, 2012 #10
    Thanks all for the replies. I think I have understood my error in understanding now - in as much as the space expands rather than the mass moving.
    Whilst that makes sense the concept that something can get further apart without actually moving is a bit of a mind bender.
    It seems that the analogies offered in the program of an orange size after a trillionth of a second was actually misleading - and I do not remember it being mentioned that it was the observable universe as it is clear that the center of that would be from the point of observation.

    I also understand now that the rules of expansion most likely so not correspond to the rules that govern the universe now.

    Lino - yes, I remember the scene in the hall with the ball bearings, I thought that demonstrated Gravities requirement to the formation of the universe really well and clearly.
    I think I shall watch the program again to ensure I can grasp the concepts again...

    Thanks again,
  12. Oct 9, 2012 #11
    Learningmore, You are correct about the actual scene was used / discussed - I was just trying to use the same image in a different way.


  13. Oct 9, 2012 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    What do you mean?
  14. Oct 10, 2012 #13
    From your original explanation, I understood that Expansion did not follow the rules that the current universe, in respect of c being the speed limit.

    For me, this makes sense if I understand that as Expansion is not limited by c, as it is not moving through space, then is clearly cannot be governed by the same rules as things travelling from point A to point B through space.

    So, if the universe expanded to trillions of light years across in a small amount of time, then it is not limited by c, therefore not governed by the current rules?
  15. Oct 10, 2012 #14
    So, is current thinking, using the ball bearing / observable universe analogy to say that the matter expanded from each single ball bearing and not that the ball bearings all started from a single point?
  16. Oct 10, 2012 #15


    User Avatar
    Staff Emeritus
    Science Advisor

    Expansion is part of the current universe and follow the rules just fine. The rules allow for greater than c recession velocities due to the expansion of space. They don't allow for anything to travel faster than light through space. For example you could never outrun a photon.

    The two things are two different concepts and cannot use the same rules. (Though everything falls under the same "rulebook" if you get my point)

    Now we are talking about INFLATION. Inflation and "normal" expansion are two different things. (Although it's easy to get confused since both result in an expansion of space)
    Inflation was the very rapid expansion of the very early universe by a factor of at least 1078 that occurred over a tiny tiny fraction of a second.
    To quote wiki:
    Also, is it incorrect to talk about expansion and inflation in terms of lightyears unless you specify something to base that distance on. Expansion causes things to recede from each other at an increasing rate as the distance between those objects grows. This means that even at the start of inflation or expansion two objects that are 1 meter apart will increase in distance away from each other MUCH more than two objects that are initially 1 millimeter apart. So if expansion increases the distance between everything by a factor of 1000, then your objects are now 1 kilometer and 1 meter apart respectively. So the initial difference of 999 millimeters has grown to a difference of 999 meters.

    I hope that all makes sense.
  17. Oct 10, 2012 #16


    User Avatar
    Staff Emeritus
    Science Advisor

    The ball bearings in lino's example represent different parts of the overall universe. Imagine you have an observer at the center of each ballbearing. As time passes the ballbearings would grow larger AND move away from all the others. The growth of the ballbearings is simply due to the finite speed of light. The longer the universe has been around the more time light has had to get to us, so we can see further and further out from the center of our observable universe as time passes.

    The ballbearings are arbitrary representations of different pieces of the universe. Their exact placement doesn't matter. They could in fact overlap if you place an observer near another ballbearing. (Remember that these ballbearings represent a volume of space, not matter)

    I think in the original example on the video the ballbearings represent matter, not volumes of space, so it may be confusing to use the same example for two different things.
  18. Oct 10, 2012 #17
    Thank you.

    Apologies for not understanding the difference between inflation and expansion. Your explanation defines it far better than I did and I will revise my statement:

    I also understand now that the laws of Inflation do not correspond to the laws that govern travel through space.
  19. Oct 10, 2012 #18
    The video used the ball bearings to demonstrate that matter was not uniformly spread across the universe and how lucky we were that it wasn't in as much had it been, then galaxies etc would not have formed and we would not have existed.
    I did not correspond the two as being the same but agree it could be confusing.

    This leads to a question then - I believe the universe is around 14 or 15 billion years old and there is roughly 170 billion galaxies in it.. Is this based on the Observable Universe (OU) or has this been calculated by a different method?

    If purely on the OU, is it then possible that we cannot see all the galaxies yet as the Light hasn't reached us? This seems obvious to me but this thread has shown that my understanding is somewhat limited :(
  20. Oct 10, 2012 #19


    User Avatar
    Staff Emeritus
    Science Advisor

    Absolutely. There may be an infinite number of galaxies. We can only see the ones within our observable universe.
  21. Oct 10, 2012 #20
    That's right Learningmore. Hope it helps.



    [Oops, just saw the other related responses, so this is unnecessary. Sorry if it introduced confusion. One thing, I wasn't really tring to bring observables distance increases into it, it was just the growth of the ball bearings due to expansion / inflation.]
    Last edited: Oct 10, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook