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

The expansion of the universe

  1. Aug 27, 2009 #1
    The general theory of relativity leads to the conclusion that the fabric of space time is ever expanding . This has indeed been proved by Hubble when he found out that the galaxies are moving away from each other.In fact all stellar bodies are moving away from each other.
    My question is this : If space time in itself is expanding, how is it possible that we are able to observe such an expansion since we too are a part of this space time. Let us consider a coordinate sytem in which each axes are rubber rods. Now let there be markings at regular intervals on these rods to help us measure length .Now if this coordinate sytem expands relative to some other coordinate system it would mean that the rubber rods have been stretched. Now if we consider these rubber rods as representing space itself , then any stretching of rubber rods would not be visible if a person were to use these markings on the rods as a measure of length , because though anything in this space would have expanded, the rods too would have been stretched accordingly so something that would have been 2 units of length will still be 2 units of length


    My concepts of relativity and 4 dimensional space time are extremely poor . This question probably is baseless , but can someone explain what is the problem in the above reasoning.
     
  2. jcsd
  3. Aug 27, 2009 #2
    My suggestion is to look anew at Gravity, present, widely accepted teories do not 'explain' what it really is!
    Given a quantum structure to Space/Time, there exists the possibility for a totally different mechanism for the so called, ‘Force of Gravity’. This is a subduction (or tunneling) effect, in which space/time quanta are forced to tunnel to other parts of the universe due to the vibration of matter on a quantum scale. Such a phenomenon would exhibit exactly the same inverse square law of Gravity over short to moderate astronomical distances, however over large astronomical distances Gravity would average to zero (as nothing is lost!). Thus in areas of the universe nearly devoid of matter, Gravity would be negative as these areas would be expanding. This expansion would be by addition, not by the ‘stretching’ of space/time.
    Such a theory predicts that Gravity would increase, beyond the total of the mass, as the density becomes extremely high and this has been observed! The theory would allow for the self-destruction of super-massive black-holes by a runaway gravitational effect and positive feedback by ‘gravity-waves’. Thus producing mini(?) ‘Big Bangs’.
    If this is how Gravity actually works it would explain the difficulty in obtaining a ‘Grand Unified Theory’ because Gravity would be an effect and not a force and thus not subject to the same restraints.
    Can anyone give a conclusive reason (other than the unprovable, red shift* = expansion, mantra) why this could not be so?
    *I am well aware of the observable ‘Doppler’ shifts due to the movement of the emitting matter but there is no real, obtainable evidence that the red shift is due to expansion, this is just the present, widely accepted Theory. Unfortunately this theory is never prefixed with the word ‘apparent’!
     
  4. Aug 27, 2009 #3

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  5. Aug 28, 2009 #4

    RUTA

    User Avatar
    Science Advisor

    Space is expanding, not spacetime. In order for spacetime to expand, you'd have to introduce another temporal dimension.

    "There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as moving through space-time, or as following along their world-lines. Rather, particles are just in space-time, once and for all, and the world-line represents, all at once, the complete life history of the particle." R. Geroch, General Relativity from A to B (University of Chicago Press, Chicago, 1978), pp. 20-21.
     
  6. Aug 29, 2009 #5
    Thank You for the replies , but still my question seems unanswered . As George Jones , suggested , I read through the thread , but still I did not find any satisfactory answer there . Refering to your specific post , you mention that the universe is homogeneous and isotropic at a large scale but not at small scales . I agree- that may be true , but how does that explain my question ?

    And regarding RUTA's post , that it is space that is expanding and not spacetime, I would like to make 2 points here :
    1. I have heard in many places that it is space time and not just space that is expanding - but what I heard may be wrong .
    2. Even if it was only space that was expanding , assuming that your definition of space is the whole coordinate system that is used to represent space ( now this coordinate system may be curved / euclidean - or something else ) , still within this coordinate system there is no way to measure the expansion of this coordinate system .

    I think maybe as climmb1 said , we should look anew at gravity .

    If we are able to observe that galaxies are moving away from each other , then what I believe is that we are not observing any expansion of space or space time , but we are rather observing an expansion within space or spacetime . And hence space would then have to be infinite .. wouldn't it ?
     
  7. Aug 29, 2009 #6

    RUTA

    User Avatar
    Science Advisor

    The individual galaxies do not expand, so the picture is that of pennies glued to a balloon. The expanding balloon carries the pennies away from one another but the pennies do not expand themselves. Astronomers measure this expansion via redshifted spectra z and luminosity standards (Cepheid variables, brightest globular clusters, supernovae, etc). You use z to get a recession velocity (for small z, v = zc works for all cosmology models) then use luminosity (apparent and absolute) to get distance (not trivial, assumptions required). Make a plot of velocity vs distance and the result is linear, just like an expanding balloon (the analogy is precise in this case). The galaxies serve as "the coordinate system," so this is in fact a measurement of "the expansion of the coordinate system."
     
  8. Aug 29, 2009 #7
    It's not terribly relevant that we are in spacetime while observing the effects of an expanding universe at great distances. As noted in post # 6, in our own galaxy, for example, gravity is more than sufficient to hold planets, black holes and stars in orbit...the weakness of the cosmological expansion is dwarfed by the strength of gravity on such "small" scales. But over vast instellar distances, over many many galactic distances, space does appear to be expanding...and in fact accelerating. That was NOT expected and so speaks well at least for objectivity in the experimental findings....

    The evidence for an expanding cosmos is rather strong, what with Hubble's observations, redshift, etc, but I prefer to keep an open mind about the possibility for alternative explanations and interpretations of such phenomena. Any theory that explains current phenomena and which makes new testable predictions deserves attention.
     
  9. Aug 29, 2009 #8
    Ok, speaking relatively, shouldn't the 'expansion' of the universe be able to be modeled so that the universe remains stationary and all the masses and bodies in the universe are simply contracting towards many many different centers? For example, our super-cluster is contracting toward it's center, leaving behind a large vast field of empty space. I say relatively because it seems that all reference frames should be equally valid, including the reference frame of the entire universe itself.

    In the absence of external forces, if person A and person B push away from each other, person A sees person B moving away from them at velocity V, while they feel as though they are standing sill. Person B sees person A moving away from them at velocity V, while they feel as though they are standing still. Person C, who was right next to them, basically in between, sees them both moving away from him with velocity V/2.

    Now lets go to a different scenario, a person is standing inside a balloon (or a bubble) and starts to "blow it up", by introducing more pressure (exactly how is utterly irrelevant), and so he sees the balloon expand out away from him. Only, while the person "thinks" he is "blowing the balloon up" the balloon thinks that IT is the one exerting the pressure, and that the person is being compressed into a smaller space...

    Relativity, or at least that's how I've always defined the word. Sure, from our standpoint the universe is "expanding", but what about from the rest of the standpoints? Let's say there's an organism that is composed of superclusters of galaxies (just hypothetically speaking), how would this organism view the phenomenon that we call "expansion"?
     
  10. Aug 29, 2009 #9

    RUTA

    User Avatar
    Science Advisor

    Our local cluster (Local Group) is collapsing (M31 is moving towards the Milky Way at 0.001c), but our supercluster is expanding. Nonetheless, could it be that what we see as overall expansion is due to localized collapses as you suggest? No, because objects on the far side of a collapse center would be moving towards our collapse center and that would make velocity vs distance nonlinear in opposition to what we observe, i.e., Hubble's Law (v = Hd).
     
  11. Aug 30, 2009 #10
    Well I still do not agree that space or spacetime is expanding .
    Let us consider your picture of the universe as an inflating balloon with pennies that are like the galaxies .( In case of penny , it is mechanical forces that keep it from expanding , but for galaxies it is gravity - but still we can use the analogy) . Now you say that an observer on the surface of the balloon ( for him space is 2 dimensional and curved ) will see that the pennies are moving away from each other. I completely disagree here . If the surface of the balloon was the entire 2 dimensional space for this observer , then I dont think he will observe the pennies moving away from each other at all .
    Why you say that he should see the pennies moving away from each other is because you are observing the surface of the balloon from outside ( i.e from a 3 dimensional space in which the balloon is inflating - i.e in which the surface of the balloon that is the 2 dimensional space is expanding) . But for him his space is just the surface of the balloon , there is no way in which he can see any expansion at all .


    Just consider a grid over the surface of the balloon ( just like latitudes and longitudes around the surface of the earth. As the balloon expands this grid system expands along with it . hence a point with a given latitude and longitude with respect to some origin , will continue to have the same latitude and longitude during the entire expansion.

    What the observer on the surface would see is that the pennies are infact contracting . Now you may say that there is a catch here . You can argue that had the balloon not been inflating then the as per my discussion , the observer on the surface of the balloon would not see the pennies contracting - so there must be something wrong here . But what I believe is that when comparing the case where balloon is inflating and when it is not , then that would manifest itself as 2 different laws of physics in the 2 dimensional curved space of
    the observer. In one case the laws of physics would be formulated for the observer in such a way that causes contraction of the pennies . In another case the laws of physics would be such that there would be no contraction of pennies. ( Why should the laws of physics be different in the 2 cases - well because this 2 dimensional space is just a construct within a larger 3 dimensional space - and in this larger 3 dimensional space the laws of physics are one )
     
  12. Aug 30, 2009 #11
    hello srijithju.

    Does the observer's meter stick, which is ultimately his method of measurement, expand or not? It matters. Regarding your gridlines on the balloon, in other words a coordinate system, are they expected to stay scaled to the meter stick. In you example, if the pennies remain the same size then I assume that the meter stick must remain the same in relation to them and so the coordinate system is not scaled to it as you might reasonably expect it to be. Size and distance are the measurements on the meter stick and so how it behaves matters fundamentally.

    I suppose the question boils down to:- is space expanding or is space and everything embedded in it expanding. If space and meter stick both expand pro rata, how can you tell.

    Matheinste.
     
  13. Aug 30, 2009 #12

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Yes he will see their angular size decrease if he watches long enough. But what he can see immediately is the Doppler shift of the signal, indicating that the distance to them increases.
    You can always make up some more complicated laws to match the same observation, like contracting galaxies with different light spectrums. This doesn't disprove simpler theories giving the same prediction.
     
  14. Aug 30, 2009 #13
    Well regarding the meter stick being the same in relation to the pennies , I have already said that the observer will see the pennies contracting , so the meter stick is indeed scaled with the expansion .

    I completely agree with you here . What I have been trying to say from the beginning is that everything embedded in space is expanding and not space itself.
     
  15. Aug 30, 2009 #14
    With this I agree 100%. Too often I see theory thrown around as scientific fact, which works it's way out exponentially when explaining the universe to people like me, who don't understand the mathematics behind the theory. I do have a pretty good grasp on mathematics though, so I can understand when we are relying on the human interpretation of the mathematics.

    It's difficult to separate theory from fact sometimes. This would be particulary relevant when it comes to the big theories, like expansion, acceleration, SR, and the Big Bang. All of these observation are inherently bound to each other and are what make up the universe. Misineterpret one, and they all go sideways in a hurry.

    Looking at the pure facts of the observations of say expansion, all we know is the red shift. Exactly how that defines expansion and acceleration is not known. I say the inverse is also possible, and current mathematics would support this model.

    Lookng at SR, or the spacetime continuum, all we know for certain is that nuclear clocks lose timing at V^2/C^2. Time has never been experimentally verified. There is evidence to support the hypothesis, but it's not a proven fact that there is any meaning in that just yet. And until we physically put a human being into a spacecraft, and send him off at speeds in the range of C to see what happens, it's just another theory.

    The Big Bang, what can I say. This is almost always portrayed as a virtual fact. Do a quick search on Big Bang, and the word theory is often completely void of the context. We simply don't know if there ever was a Big Bang.

    In essence, no one knows for certain what's going on out there, and the entire model is based on a tangled web of highly complex theory. The mathematics that support these theories is even more complex, where only a handful of people in the entire world are even capable of understanding the calculations with intrinsic depth. So when you put all these together, you have to understand that a percentage of the ideas are based on highly complex mathematical calculations, which in turn leads to a human explanations to form thoeries. While the mathmatics are for the most part, fact, the bridge that links the math to understanding is human.

    My point is this. There is no definite conclusion of just about anything when you break down what we actualy know with certainty. Very little has been proved conclusively. Yes, bodies are moving away from one another at an accelarated rate according to the red shift, but that is all we really know with certainty. From that point, it's purely human interpretation of the observations. Have one human interpretation wrong, and the entire model skews out of control exponentially.
     
  16. Aug 30, 2009 #15
    Here's one of the biggest problems with the current definition of expansion that I see, which makes me suspect something is seriously wrong with the model.

    If you fill a small balloon with water, then grab the outer edge and start expanding that balloon, what's going to happen according to our understanding of physics?

    Well, for one, the water will separate into a gas. Eventually, the water molecules themselves would separate into hydrogen and oxygen. Eventually, those atoms themselves would come apart.

    Never has science observed anything differently, unless the balloon was going the other way, and the energy in that balloon was condensing back into water.
     
  17. Aug 30, 2009 #16
    Remember that filling a balloon with water doesn't reflect the universe model. The model is more equivalent to the 2D surface of the balloon as a representation of 3D space.

    The balloon analogy filled with water doesn't take into account gravitation, the water evaporates because the electromagnetic forces of the hydrogen and oxygen easily overcome the gravity between such small particles. Also, there are pressure differentials inside and outside the balloon.

    In space, there IS no outside, no pressure ddifferential. When discussing galaxies in the same terms as pennies or water molecules, we need to consider gravity.
    Gravity has been observed and proven to act on various objects including light.

    The Big Bang is a theory, but an alternative is quite unlikely. The red shift of CBMR (Cosmic Background Microwave Radiation) permeating the entirety of the visible universe has cooled from an earlier time, indicating that in the past all was much, much hotter.
    This has been observed.

    Distant objects, such as galaxies, are receeding at an equal rate, calculated, again, by redshift of emitted radiation. Therefore in the past, not only were things much hotter, but also, much closer together.
    The fact that galaxies in opposite directions are receeding equally and our modesty to not assume the earth/solar system/milky way is at some universal centre, shows that the entirety of the universe was closer together.
     
  18. Aug 30, 2009 #17
    I'm not trying to say it isn't true, merely pointing out that the entire model is based on a tangled web of highly complex theory, not scientific fact. While my simplistic example of a baloon filled with water is definitely incomplete, so would pennies attached to the outside. A big bang is violating our understanding of physics in profound ways, and that is a fact. Doesn't make it wrong, but human interpretation of what we think is going on isn't necessarily right either. To say, "In space,there IS no outside, no pressure differential," is not fact. A recent theory suggested the opposite.

    Quote"The Big Bang is a theory, but an alternative is quite unlikely." What if the explosion was an implosion? Instead of heating up a relatively large and expanding segment of space, it heated up a very small and receding segment of space. Everything would look exactly the same as we see now, but our perspective would be inverted.

    My point isn't that any of these theories are right or wrong, my point is they are theories. Extreme caution should be excersized when stating them as fact versus speculation. It really messes up people like me, which make up the masses. After looking at this hard over the past 12 years, one thing has become abundantley clear. Very little is actually proven conclusively, and even less is understood with any sense of certainty. A lot of it makes great science fiction, but we really don't know in the end. If you can't pinpoint gravity, it's very hard to guess the rest. While we understand a great of the physical properties, we are truly missing enormous parts of the puzzle. Gravity would be a good example. We understand many of the physical properties, but not all, and we don't really know what it is exactly. That is true for nearly all the forces. We're missing the mechanisms to complete the puzzle. We're guessing, and speculations turns into theory, and those theories become engrained in our culture. This violates the principals of science, which requires emperical evidence. We simply don't have emperical evidence on many of the largest pieces of the puzzle. We're guessing.
     
  19. Aug 30, 2009 #18
    I do absolutely agree with you.

    To my knowledge, in fact, Black holes are still theoretical, we (astronomers/cosmologists) have observed very likely candidates, I'm not aware of irrefutable proof of such a thing being known to exist, yet many textbooks discuss them as actualities.

    Much of the research nowadays, for example in unification, is so heavily theoretical lacking opportunities to make observations, or to extract equations (even complete such equations) that can lead to observable qualities, so they are largely accepted as basis for further theories until something else comes along, or there is proof one way or another.

    It's not very scientific overall, especially when these theoretical ideas are passed on as facts, this kinda exascerbates the situation. Maybe it's good to step back and take a good look once in a while to make sure we are all aware of what's fact - fact, and what isn't!

    I will say, though, that were it not for the 'thinking outside the bo' with regards to some theories, we would likely not have such a good understanding of what we are looking at / for.

    Incidentally, I am intrigued, Andreas, by yyour mention of theories regarding the differential pressure. Do you have a link for this? Is it related to Dark Energy or casimir effect like negative pressures?
     
  20. Aug 30, 2009 #19

    RUTA

    User Avatar
    Science Advisor

    “It is therefore not surprising that Einstein and his successors have regarded the effects of a gravitational field as producing a change in the geometry of space and time. At one time it was even hoped that the rest of physics could be brought into a geometric formulation, but this hope has met with disappointment, and the geometric interpretation of the theory of gravitation has dwindled to a mere analogy, which lingers in our language in terms like "metric," "affine connection," and "curvature," but is not otherwise very useful. The important thing is to be able to make predictions about images on the astronomers' photographic plates, frequencies of spectral lines, and so on, and it simply doesn't matter whether we ascribe these predictions to the physical effect of gravitational fields on the motion of planets and photons or to a curvature of space and time. (The reader should be warned that these views are heterodox and would meet with objections from many general relativists.)” Steven Weinberg, Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity (John Wiley & Sons, New York, 1972), p 147
     
  21. Aug 30, 2009 #20
    They would be moving toward our collapse center, but if their contraction were allowed to continue indefinitely, this would not necessarily be what we observe. Let's say you have a circle representing a two-dimensional "collapse zone", with the center being the collapse center. Now, on two opposite points on the circle, place two smaller circles, contained entirely within the collapse zone, each representing smaller collapse zones. The large collapse zone may be bringing them closer together, but as the smaller collapse zones collapse toward their own respective centers (along with every measuring rod and clock contained within them), there may be any of a number of observations: if the two smaller collapse zones collapse fast enough (while not perceiving their collapse), they will look at each other and proclaim that space is expanding. If they are collapsing at just the right rate, they will appear to each other to stay the same distance from each other, and would proclaim that space is at a standstill. If they collapse too slow, they'll proclaim that space is contracting and wait for the inevitable collision of the two collapse centers.

    In our local cluster, it could be said that each galaxy is collapsing slow enough relative to the collapse of the local cluster, that the collapse of the local cluster is bringing the galaxies closer together. In our supercluster, it could be said that the local clusters are collapsing fast enough relative to the collapse of the supercluster that the supercluster seems to be carrying everything away from everything else.

    Most of space is far less dense with energy than it was at the time of the Big Bang. But the universe, by the laws of physics currently held to be true, is a closed system. There is no "outside", so if the universe is not expanding "into" anything, and there is no pressure differential, then when we consider the universe as a whole, then the density is constant (over time), and if the density is constant when considering the universe as a whole, but less in a local area, it must be more dense in some other local area, and thus the energy must be able to be considered to be collapsing locally.
     
    Last edited: Aug 30, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The expansion of the universe
  1. Universal Expansion (Replies: 3)

  2. Universal Expansion? (Replies: 30)

Loading...