Page 3 of 3 FirstFirst 123
Results 61 to 82 of 82

Thread: What is the difference between velocity in an expanding universe and a bomb?

  1. #61
    Join Date
    Mar 2008
    Posts
    5,278
    Again I was being sarcastic based on your oversimplification. You seem to be confusing the Universe with the portion of it that we can observe.

    Quote Originally Posted by WayneFrancis View Post
    Are you seriously asking what the universe looks like beyond the large scale which is basically our Hubble volume?
    You know I'm always weary of your questions like this because in the past you've done just what I've said in my last post. You quote mine and take things out of context.

    No one knows what the universe looks like beyond our Hubble volume but it is expected to look like it does here. Much like if you are in the middle of the Atlantic ocean you can
    ask what does it look like just beyond the horizon. Safe bet is that it pretty much looks like what you are looking at now. Sure there could be land just out of sight but there are
    more points that look like water everywhere then not. To take that a step further it would be like if we lived on water world and never saw land what is the odds that you'd run
    into it if you just went over the horizon.

    We have no indication that the universe would change so why say it does. What you are trying to do is come up with weird tommac physics just outside of our ability to view but
    some how still effect our whole observable universe so that you can say that the mainstream answers to cosmic expansion are wrong.

    The mainstream answer maybe wrong but its got a few things going for it that your ideas don't
    1) Observations that fit the model
    2) A model that break know physical principals like "non standard shaped black holes that cause a fan out tidal forces"

  2. #62
    Join Date
    Mar 2008
    Posts
    5,278
    Quote Originally Posted by 01101001 View Post
    Really, tommac? Was it a rhetorical question?
    Yes !

  3. #63
    Join Date
    Jun 2008
    Posts
    2,779
    Quote Originally Posted by tommac View Post
    Yes ... then my question is how we can assume that there is no spacial gradient?
    Because that makes no sense. Spatial gradianet of what ?

  4. #64
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by DrRocket View Post
    There is no such thing as a spatial gradient, so no you could not have one. What could "spatial gradient" mean ?
    You can blame me for that term. I use it to describe the slope of a gravitational well wall. Well actually I say gravitational gradient...

  5. #65
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by tommac View Post
    Well being sarcastic.... basically I am stating just because what we observe apears homogeneous doesnt mean that everything is homogeneous.
    And absolutely no one here is say that it is. Like it has been pointed out it isn't an absolute statement. Obviously if the universe was completely homogeneous we wouldn't be here...as we'd be indistinguishable from anything else in the universe.
    And just because we don't see invisible pink winged unicorns here doesn't mean they aren't out there hiding where we can't see them.

  6. #66
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by tommac View Post
    Yes ... then my question is how we can assume that there is no spacial gradient?
    What is this spacial gradient shaped like? What causes it? What question does it answer in a better manner then the current answer to that question?

  7. #67
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by tommac View Post
    Do you believe the universe is limited by our observable universe? If not then what does any of this have to do with anything?
    There you go hand waving again.
    Do I believe the universe is limited by out observable universe? No ... where do you get the idea that I would think that. There is a chance that dragons and fairies exist too but until someone produces observations that can't be better explained by something else then I'll ignore people yelling that they are going to be eaten by a dragon.

    If not then what does any of this have to do with anything? It has to do with why we don't build scientific models based upon something we can not observe nor ever observe. 30 billion light years from us the universe might turn to solid chocolate. The evidence we have for the that happening is the same as the evidence we have for a radically different universe from what we see around us. There could be dragons and daemons out there too or and infinite amount of other possibilities. As said before it serves no purpose beyond metaphysical pondering. The evidence gives no indication that the universe would change so if there is no evidence then why make stuff up?

  8. #68
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by tommac View Post
    Again I was being sarcastic based on your oversimplification. You seem to be confusing the Universe with the portion of it that we can observe.
    No I'm not. You seem to be confusing what we can say about something that is causally disconnected from us. You are the one that keeps making claims about stuff beyond our Hubble volume and their effects on not only us but stuff another 13.7 billion light years away.

  9. #69
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by tommac View Post
    Yes ... then my question is how we can assume that there is no spacial gradient?
    Ok Tommac, how about this. You come up with something using the language of physics that will warp space time in such a way that everything we see is not actually receding with an expanding space metric but moving through the manifold.

    Most of us know that your beating at your ATM idea here and your trying to be clever and probably getting sound bits that you can use in other post claiming we agree with your idea in some way when we really don't.

    The fact is the observations, of cosmic expansion, we have don't match, on SOOOO many levels, any kind of gravity well we can make with known physics. You constantly coming here and trying to fish for support that there might be something totally unknown and bizarre. Show us something beyond gut feels and misunderstandings of mainstream physic.

  10. #70
    Join Date
    Dec 2004
    Posts
    14,782
    Wayne,

    Do you agree that the Universe is roughly homogenous at the
    largest scales we can see, and then ends at the limit of our vision?

    -- Jeff, in Minneapolis

  11. #71
    Join Date
    Feb 2007
    Posts
    1,602
    Quote Originally Posted by Jeff Root View Post
    The Universe is homogenous as far as we can see, then it
    stops. Right?
    The universe is homogeneous as far as we can see, then we cannot see it any more.

  12. #72
    Join Date
    Dec 2004
    Posts
    14,782
    speedfreek,

    Wow. Is that how the original version of my post read? I changed it
    about five minutes later. Then you replied to the original wording
    more than four hours after that. It's no problem though!

    Do you agree, then, that the Universe is roughly homogenous forever
    and ever, trillions of light-years, even quadrillions and quintillions and
    sextillions of light-years away? And on and on beyond that?

  13. #73
    Join Date
    Feb 2007
    Posts
    1,602
    I'm not sure I understand you there, your post on page 2 still says what I quoted.

    How could I agree that the universe is roughly homogeneous "forever and ever", when we can only see a finite amount of the universe?

  14. #74
    Join Date
    Dec 2004
    Posts
    14,782
    Ah! You're right, of course. I asked the same question again,
    without realizing it was the same.

    Quote Originally Posted by speedfreek View Post
    How could I agree that the universe is roughly homogeneous
    "forever and ever", when we can only see a finite amount of
    the universe?
    I don't know.

    -- Jeff, in Minneapolis

  15. #75
    Join Date
    Sep 2003
    Location
    The beautiful north coast (Ohio)
    Posts
    49,459
    As this has evolved into an extended discussion, I've moved the thread out of Q&A and into Astronomy. If someone has a concern about either the move or the new location, please report my post.
    At night the stars put on a show for free (Carole King)

    All moderation in purple - The rules

  16. #76
    Join Date
    Dec 2004
    Posts
    14,782
    So, comments by Strange and Wayne led me to ask a question on
    page 2 of this thread (post #55). It did not get any response. Then
    Wayne made more comments saying the same thing, prompting me
    to ask the question again in post #70, though differently worded.
    I'm looking forward to replies from Wayne and Strange.

    -- Jeff, in Minneapolis

  17. #77
    Join Date
    Jun 2008
    Posts
    2,779
    Quote Originally Posted by speedfreek View Post
    The universe is homogeneous as far as we can see, then we cannot see it any more.
    The universe is NOT homogeneous as far as we can see. It is quite inhomogenious -- which is rather a good thing. If the universe were homnogeneous there would be neither stars nor planets.

    The assumption of homogeneity is made for several reasons:

    1) Without it there is no hope of solving the Einstein field equations to obtain some sort of large-scale model for the structure of the universe. With that assumption some exact solutions are known.

    2) I appears to be roughly correct as an approximation on the very largest of scales.

    3) It is philosophically satisfying to assume that the universe looks pretty much the same everywhere, and if the universe is very different elsewhere from we see locally we have absolutely no way of knowing that.

    But in the end the assumption of homogeneity (and isotropy) is just that, an assumption. It is called the "cosmological principle" but it is in fact an assumption.

    If you don't like that assumption you are free to pursue experimental evidence that it is wrong, or to make your own assumption and apply known physics to work out the consequences -- good luck.

  18. #78
    Join Date
    Mar 2004
    Posts
    13,440
    Quote Originally Posted by DrRocket View Post
    The universe is NOT homogeneous as far as we can see. It is quite inhomogenious -- which is rather a good thing. If the universe were homnogeneous there would be neither stars nor planets.

    The assumption of homogeneity is made for several reasons:

    1) Without it there is no hope of solving the Einstein field equations to obtain some sort of large-scale model for the structure of the universe. With that assumption some exact solutions are known.

    2) I appears to be roughly correct as an approximation on the very largest of scales.

    3) It is philosophically satisfying to assume that the universe looks pretty much the same everywhere, and if the universe is very different elsewhere from we see locally we have absolutely no way of knowing that.

    But in the end the assumption of homogeneity (and isotropy) is just that, an assumption. It is called the "cosmological principle" but it is in fact an assumption.

    If you don't like that assumption you are free to pursue experimental evidence that it is wrong, or to make your own assumption and apply known physics to work out the consequences -- good luck.
    While I agree with this, in general, there are some important points missing.

    First, cosmologists (or astrophysicists) have, indeed, made many attempts to look at GR applied to a non-homogeneous universe. One recent example is 'void cosmology', based on the idea that we live at, or near, the centre of a large 'void'. This was motivated by the idea that the 'dark energy' signal is, in fact, a result of this kind of inhomogeneity (references available, upon request).

    Second, the extent to which the observed universe approaches homogeneity, on large scales, can be estimated. From this some estimates of the GR-based consequences can be, and have been, made.

    Third, at the largest scale we can see, today - the CMB - the universe seems to be extremely homogeneous (~1 part per million?).

  19. #79
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,947
    Quote Originally Posted by Jeff Root View Post
    So, comments by Strange and Wayne led me to ask a question on
    page 2 of this thread (post #55). It did not get any response. Then
    Wayne made more comments saying the same thing, prompting me
    to ask the question again in post #70, though differently worded.
    I'm looking forward to replies from Wayne and Strange.
    So, your latest version was:
    Do you agree that the Universe is roughly homogenous at the
    largest scales we can see, and then ends at the limit of our vision?
    Are you suggesting the universe ends at the limit of our vision? It may do, although that seems unlikely.
    Are you suggesting homogoneity ends at the limit of our vision? It may do, although that seems unlikely. (Why should it suddenly change its nature just out of our range of visibility; although it may change at some point. Chi lo sa.)
    But ultimately we don't know. And can probably never know. As Wayne said, it may turn to chocolate at some point after that.

  20. #80
    Join Date
    Dec 2004
    Posts
    14,782
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Jeff Root View Post
    Do you agree that the Universe is roughly homogenous at the
    largest scales we can see, and then ends at the limit of our vision?
    Are you suggesting the universe ends at the limit of our vision?
    No, just asking whether you agree with that description.

    Quote Originally Posted by Strange View Post
    It may do, although that seems unlikely.
    Okay. Good.

    Quote Originally Posted by Strange View Post
    Are you suggesting homogeneity ends at the limit of our vision?
    No, just asking whether you agree with such a description.

    Quote Originally Posted by Strange View Post
    It may do, although that seems unlikely. (Why should it suddenly
    change its nature just out of our range of visibility;
    I have no idea. I agree with your assessment that it seems
    unlikely that it should change right at that particular place.

    Quote Originally Posted by Strange View Post
    although it may change at some point. Chi lo sa.)
    I think we may be able to figure it out from what we *can* see.

    In fact, as long as we keep working on it as we have been over
    the last hundred years, I'm fairly confident that we will figure
    it out (within some margin of uncertainty) within the lifetimes
    of people reading this today.

    With what we know right now, we can rule out -- with a pretty
    high degree of confidence -- the possibility that the Universe
    turns to chocolate beyond our horizon, at least on the largest
    scales that we are concerned with here. (We know that small
    regions of the visible Universe can turn to chocolate, so it
    seems reasonable that small regions of the Universe beyond our
    horizon may be able to turn to chocolate, too.)

    Since the conditions of the Universe change with time, and the
    Universe is of finite age, we can be certain that the roughly
    homogenous conditions we observe on the largest scales cannot
    extend throughout an infinite volume.

    So there are two possibilities consistent with what we have
    observed so far: Either the homogeneity never ends, and the
    Universe is closed in such a way that every straight-line path
    through space loops around and returns to regions it passed
    through before, or the homegeneity eventually ends, somewhere
    beyond our horizon.

    The best observations so far suggest that the volume of space
    within our horizon is "flat", so if the Universe is closed, it
    must be many times larger than the volume within our horizon.

    -- Jeff, in Minneapolis

  21. #81
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by Jeff Root View Post
    Wayne,

    Do you agree that the Universe is roughly homogenous at the
    largest scales we can see, and then ends at the limit of our vision?

    -- Jeff, in Minneapolis
    To be technical the limits of our vision represents the start, in time, of our universe. So in that sense the universe doesn't end at the limit of our vision but is just beginning at the limits of our vision.

    Homogeneous is a subjective term too so "roughly homogeneous" is a good term.

  22. #82
    Join Date
    Jun 2008
    Posts
    4,692
    Quote Originally Posted by speedfreek View Post
    I'm not sure I understand you there, your post on page 2 still says what I quoted.

    How could I agree that the universe is roughly homogeneous "forever and ever", when we can only see a finite amount of the universe?
    Agreed. I've heard more then one astronomer talk about the "universe" as a whole. One of my favourite examples it that while our observable universe might be flat the universe as a whole might be wrinkly like the surface of a raisin. In the end it doesn't matter our visible universe is roughly homogeneous at the large scale.

    The claims tommac keeps trying to make is that there is something beyond our Hubble volume that is not only effecting all the matter and energy between us and it but all the mass and energy on the other side of us. If he's not saying that then he's leaving out a a lot of observations out of his idea.

    The real problem is there would have to be some VERY exotic physics that is going on and would have to be VERY exact in nature.

    While we don't know what dark energy is conceptually it is very easy to describe and has the bonus that since it seems to be an intrinsic property of space there is no special geometric body pulling everything apart. It also doesn't have the problem of violating SR by mass travelling faster then c.

Similar Threads

  1. What is the difference between velocity in an expanding universe and a bomb?
    By tommac in forum Space/Astronomy Questions and Answers
    Replies: 73
    Last Post: 2010-Jun-11, 08:16 PM
  2. If space and the universe is expanding, is the matter in it expanding too?
    By pschroeter in forum Space/Astronomy Questions and Answers
    Replies: 87
    Last Post: 2010-May-30, 06:17 PM
  3. Escape Velocity of the Universe?
    By The_Radiation_Specialist in forum Astronomy
    Replies: 10
    Last Post: 2006-Jan-18, 12:47 AM
  4. Replies: 6
    Last Post: 2005-Nov-14, 07:35 PM
  5. Replies: 7
    Last Post: 2005-Jun-23, 11:50 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •