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Universal Expansion

  1. Mar 12, 2010 #1
    I have read theories that space itself is expanding, but if this were the case wouldn't our perception of this expansion be skewed by the very fact that we ourselves are part of the expansion?

    Has anyone pursued a theory that what we hold as constants, the power of the weak force, the power of the strong force or even the speed of light in a vacuum might not be constants at all? Perhaps instead they are ever changing values that from our relatively small set of observational data appear to be constant? This could explain a great deal of phenomena without the need for mysterious things like dark energy or dark matter.

    Is it possible that the accelerating expansion of the universe is only an illusion manifested from our poor perception? If any of these constants were changing infinitesimally would the precision of the observational data preclude detection of the changes? And might observation of objects from extreme distances in which some constant was different produce the observed results?
     
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  3. Mar 12, 2010 #2
    I'm no expert, but I think I can see a flaw in your initial statement.

    It would not be possible for our perception of the expansion to be skewed because the effect is negligible compared to the other forces we experience.

    My understanding is that it is only at a huge scale that the expansion becomes manifest because gravity overwhelms it when you are subject to the sort of gravitional fields created by clumped matter.
     
  4. Mar 13, 2010 #3

    Chalnoth

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    The basic issue is that the expansion is an average effect seen on large distance scales. Locally, if you work through how gravity behaves, the more overdense and underdense regions don't experience the same sort of effect. In fact, overdense regions like our own don't experience expansion at all.
     
  5. Mar 13, 2010 #4
    This is why I am questioning whether anyone has pursued a theory based on a changing set of "constants". We do not observe accelerated expansion locally, but instead only see the effect across vast distances or space/time.

    On cosmological scales our observations of distant objects are pictures of the past; what we observe of distant objects occured billions of years ago. We measure acceleration of distant objects through red shift (the doppler effect). If we consider the possibility that the speed of light in a vacuum is not a constant but is instead continually decreasing or increasing (specifically on an infetisimal scale) would the universe still be considered to be accelerating its expansion?

    Alernatively, if we consider the possibility that the nuclear forces are ever changing (again at a very slow rate) we might discover that light emission from distant stars (this is how we determine distance to stars more than 400 light years from us) might have been different in the past. As a result, what we assume to be an object billions of light years away may in fact be closer. This is significant because the pressence of "dark matter" is required to explain gravitational lensing of light from some distant objects. Instead of assuming the lensing effect is showing us an object that is further away, perhaps the object isn't as far as we think and thus there is no need to account for dark matter.

    The fact that so many theories rely on the presence of "dark matter" and "dark energy" to explain our observational data concerns me, it reminds me of theories of the past involving "the ether" which were later disproved. I think it would be an interesting thought experiment for someone with more time than I to consider the possibility that some of our assumed constants might be changing.

    Does anyone know if someone has pursued this?
     
    Last edited: Mar 13, 2010
  6. Mar 13, 2010 #5

    Chalnoth

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    Oh, of course! It's been examined, and it doesn't work. An examination of the issue was explained to a popular audience (in a completely different setting) here:
    http://www.talkorigins.org/origins/postmonth/aug06.html

    He references this review of the science surrounding the examination of how we know that the various "constants" of nature have actually been constant, to rather high accuracy, for the past 13.7 billion years at least:
    http://arxiv.org/abs/hep-ph/0205340
     
  7. Mar 13, 2010 #6

    marcus

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    There is no evidence that the space in our immediate neighborhood is expanding. The earth is not. Distances within our solar system are not. Distances within our galaxy with its billions of stars are not increasing.

    You read something. But somehow you got a strange idea. Maybe what you read was misleading, or unclear, or you didn't read the fine print.

    Even within our immediate group of galaxies, distances are not increasing.
    So you can't say "we ourselves are part of the expansion".

    What is expanding is large-scale distances (largescale like between different clusters of galaxies).

    It's kind of yucky to talk about "space itself" expanding. What is that supposed to mean? I don't say that because I don't like people to get the impression that "space itself" is some kind of material, a substance.

    General Rel, the basic dynamics of geometry, does not say space is a substance. It talks about changing geometry. Distance relations change, geometry evolves according to a law called Einstein field equation.

    In our universe largescale distances between objects which are stationary relative to the cosmic microwave background are currently increasing 0.007 of a percent every million years. Most galaxies have fairly small random motion with respect to Background, so one could just ignore their little random motions on average. And say that distances between widely separated galaxies increase 0.007 percent per million years, on average.

    Nothing to do with "space itself" or "natural constants changing" or our own bodies taking part in the expansion :biggrin: Chalnoth has been giving you the straight story.
     
  8. Mar 14, 2010 #7
    Thank you Calnoth for leading me in the right direction. I found that someone has recently considered the possibility that the speed of light is decreasing with time. http://arxiv.org/abs/1001.1561

    It's amazing how simple it is to find thing when you know where to look.
     
  9. Mar 14, 2010 #8

    Chronos

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    The speed of light is constant in all reasonably acceptable cosmological models.
     
  10. Mar 14, 2010 #9
    In other words you consider any discussion of a slowly decreasing speed of light "unreasonable".

    And yet theories to explain the observed accelerating universal expansion using these "reasonable" cosmological models requires the fabrication of mysterious "dark energy" to account for the accelerating expansion.

    Normally, when a theory's predictions do not produce the same results as observations it's time to find a new theory, but instead these theories have been augmented with "dark energy" to explain the discrepancy.

    I am not asking you to accept the concept that the speed of light is changing only to consider the possibility. It could allow for a theory to account for current observations without the need for dark energy.

    "If the facts don't fit the theory, change the facts." -Albert Einstein
     
  11. Mar 14, 2010 #10

    marcus

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    That's a 7 January 2010 letter to Astrophysics Space Science by someone named Alfonso-Faus

    Starts, I suppose many of us here, at least people who have been watching the scientific literature in cosmology for more than a few years, have "considered the possibility".

    It is not a new idea, the foremost advocate during the 1998-2003 years has, as I recall, been Joao Magueijo. The tag we saw used for this type of theory was "VSL", a "variable speed of light" cosmological model.

    It is very strange that your author Alfonso-Faus does not cite Magueijo's VSL papers.

    Magueijo published about it in peer-review professional journals, hardcopy.
    His first paper was http://arxiv.org/abs/astro-ph/9811018
    He published several papers and finally even a book about it in 2003. Then it died.

    Your guy Alfonso-Faus just published a letter, online.
    http://www.springerlink.com/content/j42v37037mpw3937
    It's naive. He has misconceptions. He does not make a strong case for VSL. And he does not seem to realize the prior research of Magueijo. So the indications are that this was not peer-reviewed, just a "letter to the editor" by someone who is not trained as a cosmologist.
    Anyway, that's my take. I wouldn't waste time on the Alfonso-Faus letter.

    You can look up Magueijo's papers if you want. He's a real cosmologist, smart and fairly prominent. He spent, I think, something like 5 years of his career studying and promoting VSL.

    VSL turns out to be a bad idea that doesn't work. But he put the hard work into it. Respectable.
    I wrote you a post that gives a clue why it doesn't work.
    If the speed of light were changing then ALL distances would be changing. But they are not. Cosmological expansion does not occur within our galaxy, or local group of galaxies.
    That basically kills any simple model.

    So then one can still play around with a speculative idea that maybe long long ago in the early universe (in the period when we think inflation may have happened) the speed of light was changing. This could be tried out as a substitute for inflation. Magueijo made a valiant attempt to replace inflation by early-universe VSL. As I recall it was like 1998-2003. He eventually gave up. The idea died.
     
    Last edited: Mar 14, 2010
  12. Mar 14, 2010 #11

    nicksauce

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    Interesting note: I recently had a problem set question that asked if the weak coupling constant G_F was different in the past, what constraints could we put on that using BBN. It turns out that you can only go down by a factor of .9905 or up by a factor of 1.0066 to stay within the 1-sigma limits of a typical BBN constraint.
     
  13. Mar 14, 2010 #12
    If Space is not expanding, how is it possible that galaxies recede from us with a multiple of c ?
     
  14. Mar 15, 2010 #13

    marcus

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    Hello Bartleby, welcome (your first post).
    Shall I try to answer? Is your question coming from something I said?
    =======
    My attitude is if other people want to think of expanding space that's OK. I personally think in terms of geometry being dynamic. Gen Rel is a theory of active geometry, interacting with matter. You have no right to expect distances to stay the same or to expect triangles to always add up to the same.

    That's just how it is. Gen Rel was first formulated in 1915 and it is basically our only theory of gravity that really works, it has been tested over and over and is amazingly accurate. So you either buy it, or you have no theory of gravity. So you have no right to expect geometry to remain static and passive.

    In Gen Rel there is no substance called space (there are quotes from Einstein in 1915 or 1916 where he says space has no objective physical existence). There is a web of geometric relations between events. There is a dynamic changing geometry.

    The geometry is called "the gravitational field".

    And cosmology is a mathematical science based on Gen Rel. It is not based on a logic of words and sentences like "space expands". It is based on certain simple equations derived from Gen Rel, and operationally speaking also based on the cosmic microwave background (CMB). The CMB facilitates defining basic concepts of time, distance and rest relative to background.

    We should all be talking about the same math model, whatever words we like to use to express our different intuitive takes on it. If you like to say "space expands" please do!

    In Gen Rel there is no rule against distances between two objects, both of which are at rest relative to background, increasing at a rate which is several times the speed of light. In fact MOST OF THE GALAXIES WE SEE, the proper distance to them is increasing faster than c, now today. The Hubble law recession rate is the rate of proper distance increase.

    There is a simple online calculator that this Iowa astronomy professor put up for her students which is a good way to learn about this, if you want. Google "cosmos calculator" or use the uni.edu/morgans link in my sig.

    When it comes up you are expected to type in the standard three parameters, in the boxes on the lefthand margin: .27 for matter, .73 for cosmological constant, and 71 for Hubble parameter. Then you can put in any redshift and it tells you distance then (when the light was emitted) and recession rate then, and distance now (as we receive the light) and recession rate now.
    Try it out. It is a simple hands on version of the standard cosmology model.

    Google "cosmo calculator" for a more complicated version of the same thing, put up by Ned Wright.
    Cosmos with the S will get you Morgan's, Cosmo without the S will get you Wright's

    Wright also has great computer animations of the balloon model universe, a 2D analog of our 3D world.
    All existence concentrated on the 2D surface of the balloon.
    Google "wright balloon model". I recommend watching it several times until you see how the galaxies are not moving (not changing longitude latitude) and the photons are moving between the galaxies, and changing color from blue to red as their wavelengths get longer.

    In my case, I imagine that the rubber of the balloon does not exist, only the geometry on the surface exists. The web of relation among the galaxies and the photons, in the animation. Then I imagine going up one higher dimension. But if you like to imagine a kind of rubber, instead dynamic pure geometry, that's fine. Whatever works for you!
     
    Last edited: Mar 15, 2010
  15. Mar 15, 2010 #14
    As i see distance as quality of space, please be so kind and give me a further explanation, what you mean by: "it's kind of yucky to talk about "space itself" expanding.".
     
  16. Mar 15, 2010 #15

    marcus

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    Well did you read my post #13? I was doing my best to explain my personal take.
    I said it would be fine with me if you want to say "space expands". A lot of people say that to themselves and it helps them.

    I do not say that. I focus on the math model. In the math model there is no substance called space. There are DISTANCES. The distances increase according to some simple equations.
    I focus on the changing geometry. I don't imagine an underlying "space substance" which after all is not part of the theory---it isn't in the model.

    I guess it's basically a matter of taste. What way of thinking about it work for you.

    But please do watch Wright's balloon model----just a few minutes, like 5 minutes, can help.
    And please do try one of the calculators.

    Like, we see lots of galaxies with redshift z = 6.
    At what rate were they receding back then when they emitted the light we are now receiving?

    At what rate are they receding now, today? Find out? If you get some answers I will check them to make sure you aren't having trouble with the online calculator.
    ================

    By "yucky" I mean distasteful. But i promise not to object if you decide that you like to think about the pattern of increasing distances as "space itself expanding". I will quietly to myself say "yuck" but I will not voice any objection. :biggrin:

    Look, there is no rubber. Hubble Law says v = Hd
    there is no "space" in it. It is about a type of distance called proper distance, and the current rate that distance d is increasing. It does not say anything about motion. It describes a pattern of geometry change.

    BTW proper distance is what you would measure by conventional radar or signal-timing if you could stop the expansion process at a certain moment. That is the kind of distance measure that the Morgan calculator gives you, and recession rates are rates of change of that distance.
     
    Last edited: Mar 15, 2010
  17. Mar 15, 2010 #16
    Thanks for all the answers. I think i am fairly acquainted with the balloon model.
    I do not look at spacetime as something "substantial", but i see what you mean now.
     
  18. Apr 18, 2010 #17
    If galaxies are moving away from each other does that mean there's an anti-gravity force that only exists between galaxies? Also if time is part of this fabric does it expand between the galaxies differently then time inside a galaxy?
     
  19. Apr 18, 2010 #18

    Chalnoth

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    No, just normal gravity.
     
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