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Expansion of space vs stuff just moving away

  1. Mar 19, 2014 #1
    NOTE: I am not a cosmologist, so if any of my statements are not correct please tell me.

    When we observe distance galaxies we can measure how fast they move away using the red-shifting of their light. So how do we know space itself is expanding vs the galaxies are just moving away relative to ours. That is, if the galaxies are cars on a road, how do we know the cars are driving away from each other vs the road is just getting bigger between them?
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  3. Mar 19, 2014 #2


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    We cannot. Both interpretations are equally valid.
  4. Mar 19, 2014 #3
    If that is the case, why are all the recent stories about inflation talk about space expanding and not about objects moving away at near-light speed?
  5. Mar 19, 2014 #4


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    Because space IS expanding. Things are not traveling at > c in our reference frame, they are receding from us at > c. I'll leave it to bpowell to discuss his response.

    Things at the edge of our observable universe, for example, are receding at about 3c. They most certainly are not actually moving at 3c from any frame of reference. Nothing moves at speeds >c but recession velocities have no limit and break no speed limits.

    Let's assume an object X at the edge of our observable universe and an object Y that is half way between X and us and posit that Y is co-moving with us.

    To us X is receding at 3c and Y is stationary. According to Y, we are stationary and X is receding at considerably less than 3c because recession is distance dependent. If X were MOVING at 3c relative to us, it would also be moving at 3c according to Y. An object that was comoving with us but positioned out next to X would see X standing still.

    I am glossing over very slight proper motions of objects relative to us because the magnitudes are insignificant compared to the recession velocity.

    Google "metric expansion" for more discussion.
  6. Mar 19, 2014 #5


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    Actually, I think the galaxies really are receding from us at a velocity of 3c as viewed from our own frame of reference here on Earth. Also, galaxy Y is co-moving with us, which would require that galaxy X also be moving at 3c relative to it.
  7. Mar 19, 2014 #6
    I appreciate the replies on how fast the edge of space is receeding, but this does not answer the question: how do we know space itself is expanding and the objects are not just moving away really fast? Can the red-shift show a >3c expansion? Can we measure a greater-than-light recession, and therefore conclude space is expanding?
  8. Mar 19, 2014 #7


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    Yes, that's what I said. They just aren't MOVING relative to us (except for a small proper motion)

    No, that can't be right. If Y were NOT commoving with us, but receding, it would be receding at well under 3c AND it would see X receding at well under 3c (and us receding at the same speed in the opposite direction). X is NOT "moving" relative to us it is RECEDING relative to us and it will recede at a different rate for Y because Y is closer to it.
  9. Mar 19, 2014 #8


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    All galaxies outside the local cluster are moving away from us. AND they are red shifted. AND the farther away they are the more redshirted they are. Only expansion can explain all that. This is not the only reason. Did you google "metric expansion" ?
  10. Mar 19, 2014 #9
    There is one vital detail being overlooked. The above statement implies a preferred reference frame. All non gravitationally bound move away from each other equally. In simpler words take any number of coordinates. 3 or more, regardless of which coordinate you choose as the reference frame. You will measure the same rate of expansion to any other coordinate. However as Phinds pointed out the further away a coordinate is from the referencd frame you will measure a greater redshift.

    Unfortunately I am typing from my phone so I cannot post a specific post. However this related thread has an article "Redshift and Expansion" that will provide further details on the distance dependant recessive velocity Phinds mentioned with his 3c example.


    Another key detail is that when one describes an object as moving. You are stating that the object has inertia. In expansion the objects themselves have no inertia. The space between them is simply increasing.

    Edit there is one vital detail in regards to the chosen reference frame I forgot to add. The chosen reference frame must be at rest. A relativistic reference frame would not measure the universe as being homogeneous and isotropic. As his own movement would be a preffered loacation and direction
    Last edited: Mar 19, 2014
  11. Mar 19, 2014 #10


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    When we observe a redshift, does that tell us how fast the source is moving away NOW? Or does it tell us how fast it WAS moving away back when it emitted the light?
    (According to standard model of cosmos, the redshift does not correspond in a straightforward doppler way to either of those two speeds.)

    I think the choice of interpretation is partly a matter of making sense of the whole picture. For example we do not only observe galaxies, we observe the Ancient Light ("cosmic microwave background"). The physical explanation of that light is that it has to have been emitted when glowing hot gas filling space had cooled to around 3000 kelvin, because any hotter and ionization of the gas would dazzle and scatter the light. That 3000 kelvin is like a "threshold of transparency" when the fog clears.

    We know what mix of wavelengths gas emits at 3000 kelvin. So we measure the wavelengths NOW and we see that the CMB ancient light has been redshifted by a factor of 1000 (more exactly estimated 1090, but roughly 1000). Wavelengths are that much longer.

    The temperature and density of the CMB light NOW squares with the model that distances have increased 1000-fold and volumes increased 10003-fold, and temperature decreased 1000 fold, since the moment the hot gas filling space cleared (so light could pass freely).

    The simplest way to model the CMB is with an expanding distances model. What could be moving away, and when? to produce just that redshift of 1000? It's hard to think of an imaginary set-up that would reproduce what we observe with MOTION. Expanding distance works better (plus it is a prediction of GR, the version called Friedman equation that cosmologists use, and GR has been tested a lot.)

    Individually, just looking at galaxies (not the whole overall picture), you can go thru some mathematical gymnastics (there was a famous paper by Bunn and Hogg that did this) and explain the redshift as a series of a large humber of intervening doppler shifts. But it's simpler to just equate it to the factor by which the distance has grown while the light was in transit.

    It's not mathematically WRONG to look at redshift as the cumulative effect of hundreds of intervening little doppler shifts. It is just clumsy and inconvenient. And then you still have the CMB to explain.

    So maybe it just comes down to convenience---astronomers wanting the simplest best-fit model.
    Last edited: Mar 19, 2014
  12. Mar 19, 2014 #11


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    There is no single, easy answer. The current model of the universe is known as the ΛCDM (Lambda CDM) Big Bang model. In this model, the universe is described as expanding from a once very dense, very hot state to the cooler, less dense state that it is in now.

    Now, it's very important to realize that scientific models are not built upon only one or two pieces of evidence. Such is the case for the ΛCDM Big Bang model. Each piece of evidence, when viewed alone, could have a great many explanations. Only by looking and trying to explain ALL the evidence (or at least as much as possible) can we build the predictive models that science strives for.

    For example, a measurement of redshift, by itself, does not let us know whether or not galaxies are moving "through" space or whether space itself is expanding. Instead it is a combination of many different pieces of evidence combined to form a model that predicts and explains what we can and should see to a very, very high degree of accuracy.

    Per Ned Wright's Cosmology FAQ: http://www.astro.ucla.edu/~wright/cosmology_faq.html#BBevidence

    To add to this, I would say that we also have a theory of gravity, General Relativity, which is perfectly able to have an expanding universe in which space is expanding instead of galaxies moving through space. GR is a theory of geometry, and a basic explanation of how this is possible is that on the largest scales of the universe, geometry itself is "dynamic", meaning that it can change. This changing geometry is a very "intuitive" way of explaining expansion, meaning that expanding space falls very naturally right out of GR.

    Now, I know this isn't quite a direct answer to your question, but the fact is that the issue is, as far as I know, a little too complicated to explain with just a few pieces of evidence and ignoring the current model as a whole.

    Okay, if you use "recede" and "moving" as two different words, then sure.

    That isn't possible. If Y is co-moving with us then X MUST be receding at 3c relative to both of us. I think the problem here is that it isn't possible for Y to be co-moving with us because of the expansion of space.
  13. Mar 19, 2014 #12
    Roflmao... You have no idea how much of a headache it was to include Bunn and Hoggs work into the Redshift and expansion article. Thankfully PAllen's assistance and extreme patience stepped me through it lol.
  14. Mar 20, 2014 #13
    Could this be clarified?

    What is the relationship between the object and the expanding space that compels the object to move with or "stick to" the expanding space? How is inertia of objects no longer independent, but now dependent on the space metric?

    You are suggesting that objects subject to expansion offer no inertial resistance to the expansion, which is also a net acceleration, and a local accelerometer would show no indication... very similar to free fall in a gravitational field...?
  15. Mar 20, 2014 #14


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    So you think that recession velocity is independent of distance? That is just flat wrong.

    As a thought experiment, there is nothing wrong with having Y commoving with us and since recession velocity IS dependent on distance, X's recession velocity from Y will not be the same as X's recession velocity from us.
  16. Mar 20, 2014 #15
    Objects at rest require a force to act upon them to get them moving and a force to act upon it to stop that motion. The three laws of inertia apply on this case. Now consider the geometry involved in expansion metrics. All objects are increasing in distance from each other equally. (not gravitationally bound)
    This alone tells cannot be explained as the result of a kinetic explosion, you would have a centralized source. It also cannot be explained as a higher pressure/temperature flowing to a lower pressure/temp as this too would give a preferred location.

    In order for expansion to be explained by inertia would require some very unusual mechanism to be both homogeneous and isotropic. After all what force could act upon those objects that would fit a homogeneous and isotropic expansion? I certainly can't think of any.

    In expansion however the geometric distance between objects is simply put increasing. This increase in volume is both homogeneous and isotrophic. Think of it simply as an increase in geometric volume.

    google metric expansion for more detail
  17. Mar 20, 2014 #16
    The problem is applying the laws to pairs of objects... individual objects may appear to be each locally inertially at rest, but the distances between pairs are not simply increasing; the distances are accelerating. The mutual acceleration of distance between massive objects typically implies force.

    Acceleration is absolute, yet with expansion, two objects whose separation distance is accelerating are inertially at rest. What is the mechanism for that?

    I'm not seeking to explain expansion by inertia; I'm trying to see how expansion overcomes the inertia of objects. Saying that the objects don't move but rather recede is at the heart of it - that is saying objects resist changes in movement because of their inertia, but objects don't resist recession, which is an acceleration of distance.

    How do two objects accelerating their distance apart distinguish movement from recession, and so offer inertial resistance to the former but not the later?
  18. Mar 20, 2014 #17


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    No, that's not what I'm saying.

    I don't think its possible for a galaxy to be co-moving with us when it's at a distance where the recession velocity due to the expansion of space is greater than c. That would seem to imply that the galaxy would need to be moving "through" space at a speed greater than c in order to stay co-moving with us.
  19. Mar 20, 2014 #18


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    Yes, that was all already answered yes.
  20. Mar 21, 2014 #19
    Your question is pretty interesting, actually. It has been answered already and I don't think I can add much more.

    There are three types of redshift:

    Doppler redshift: This is caused by an object moving away from us. It's maximum value is 1.4. For a greater redshift, the object would have to be traveling faster than light or...
    Cosmological redshift: The redshift can be caused by the expansion of space itself that stretches the electromagnetic wave. That's how Hubble can measure redshifts higher than 8
    Gravitational redshift: This is caused by an object that stretches the wave due to its gravitational pull. This is only significant when the body is very dense, like a neutron star. This doesn't contribute much to the measured redshift of the galaxies

    correct me if I'm wrong
  21. Mar 21, 2014 #20


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    Doppler shift resulting from relative velocity is indistinguishable from other types of redshift and can have any value. Using the relativistic formula instead of the classical one gives you the correct values.
  22. Mar 23, 2014 #21
    per m3 the cosmological constant has a low energy density, its influence is easily overpowered. Why dark energy has such a large influence is the sheer amount of overall volume. As that volume increases the total amount of energy the cosmological constant gains becomes exponential. The density per volume is constant, what changes is the volume. hence the acceleration.

    treat the cosmological constant as a negative pressure influence among a positive matter influence, however as stated above its easily overpowered so its influence is seen mainly in the spaces between objects

    again the amount of influence per locality environment dictates the amount of measurable influence the cosmological constant has. One key note is that as the cosmological constant is homogenous and isotropic, (the same at any location/uniform) an object would have the same amount of influence on all sides and remain at rest. However the volume between two objects can increase.
  23. Mar 23, 2014 #22

    Your answers don't seem to address my questions; that may be because I'm having trouble making sense of them.

    I didn't ask the source of the acceleration, I asked about the effect of that acceleration on objects (the mechanism by which the acceleration of space acts to separate objects) - specifically why the objects being relatively accelerated apart from each other don't experience inertial effects... why is absolute acceleration not apparent?

    What do you mean by "...so its influence is seen mainly in the spaces between objects."? What is being influenced by an acceleration if not massive objects?

    If "The density per volume is constant..." then what does "...the amount of influence per locality environment dictates the amount of measurable influence the cosmological constant has." possibly mean?

    "...an object would have the same amount of influence on all sides and remain at rest." Yet two objects would also have the same amount of influence on all sides, yet recede (accelerate) from each other? What is the mechanism for that?

    "However the volume between two objects can increase." A restatement; is it simply that nobody knows the mechanism? Is there a purpose to using the word "volume" rather than "distance" to describe what is between two points?
  24. Mar 23, 2014 #23


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    The comparison with gravity is pretty good.

    Inertia says that an object wants to move in a straight line, which seems simple enough; it satisfied most classical physicists most of the time for most of several centuries. However, if you poke enough at it, you'll realize that we've just shifted the discussion to what a straight line is and how straight lines are related to one another.

    We can describe the attractive gravitational force in terms of initially parallel straight worldlines eventually intersecting... But suppose they diverged instead? Then we'd have the expansion effects that we observe at cosmological scales.
  25. Mar 23, 2014 #24
    fair enough, lets try another tack.

    I didn't ask the source of the acceleration, I asked about the effect of that acceleration on objects (the mechanism by which the acceleration of space acts to separate objects) - specifically why the objects being relatively accelerated apart from each other don't experience inertial effects... why is absolute acceleration not apparent?

    lets look at this portion first.

    according to the laws of inertia f=ma in order for an object to accelerate for rest the object must have a force act upon it, and in order to come to rest from being in motion it will also require a force to act upon it.

    now lets treat the cosmological constant as a force surrounding a star, or galaxy. Lets treat this as being in a region with no other influence such as gravity from other stellar objects.

    The energy density of the cosmological constant is the same everywhere, even in gravationally bound regions. Roughly 6*10-10 joules per m3.

    IN our standalone star/galaxy example the cosmological constant will act upon the star/galaxy equally from all sides of the star/galaxy. So the net balance of the force acting upon that object will be zero. As the net balance of forces acting upon said object is zero, the object will remain at rest.

    So given what I just described does the object gain inertia? The correct answer is no it doesn't, it will remain at rest. This will be true for all objects in space. As the cosmological constant is a uniform force,(vacuum pressure) its net balance of force on any object will be zero.

    So why do objects move apart????

    to answer that we have to look at the volume. If for example the volume of the universe stays static and cannot increase. IE a closed container (this an example only and not a descriptive of the universe). Then no object would separate from each other.

    However in our universe the volume is influenced by the relation of the cosmological constant and gravity. If gravity is stronger the universe collapses. If the cosmological constant is greater the universe expands.

    So the reason why objects appear to move apart is due to changes in volume, not due to momentum.

    If you think about the above those properties would be the same as two molecules inside a closed container.. The forces due to pressure on any molecule would be the same on all sides of the molecule. So the net balance of forces acting upon the molecule is zero. Therefore the molecule is at rest. If you increase the volume of the container though, then the molecules will separate to a uniform distribution.

    keep in mind the universe has no container walls or boundary lol however the same principles apply.

    The above should also answer why I chose to use volume instead of distance.

    hope this clarifies why we simply state the expansion of the universe in terms of changes in volume/distance as opposed to objects moving apart due to momentum.

    edit had to add a change, wrong value for energy density above.
    Last edited: Mar 23, 2014
  26. Mar 23, 2014 #25
    I’m so glad you’re having this particular discussion bahamagreen. This is one of the issues I’ve been mulling over for a week or so now, trying to find that missing piece. For me the pertinent question was

    In other words, “in order for the distance between our galaxy and a distant one to increase, the distant galaxy must somehow ‘grip’ onto the underlying space as it expands.” And if I’m reading it correctly this post from Mordred has just answered it.

    You see it’s all too easy to limit our thinking on this problem to just the two galaxies. But as Mordred implies, these galaxies do not exist in isolation. The same mechanism of expansion is also happening on the other side of the distant galaxy, holding it in position (as indeed it is on our own). Roughly speaking, “the pressure of expansion doesn’t move a galaxy because the pressure is being applied to the galaxy from all directions”. I hope that’s basically correct and that it helps (although I see now that Mordred has posted a more detailed explanation of this point).

    Finally I can free myself of considering space as fabric-like... Thanks all round
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