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Dark Energy & Expansion of the Universe; Really ?

  1. Nov 24, 2011 #1
    Based on what I've gleamed from a couple NOVA type TV programs, it would appear based on our observations that all galaxies are moving away from us and each other at an accelerating rate. From what I can tell we infer the relative speed of galaxies based on observed light frequency shift which is more pronounced for galaxies more distant, and we infer distance based on the diminished assumed candle power of expected star types. So far so good, makes sense.

    But the general consensus conclusion that is being widely reported has me puzzled. The great 'mystery' of why the galaxies aren't slowing down due to their gravity on each other, and the conclusion that space itself must be expanding due to some unseen 'dark energy' seems like a more complicated explanation than is necessary.

    If our observable universe is surrounded by a large amount of mass either uniformly in a beautiful perfect surface of sphere (along with all the cool mathematics artifacts that so many people love) or more simply just other mass in clumps around us the effect would be the same. The accuracy of observations and sample size we have to date couldn't tell the difference. The galaxies closer to this other mass would be accelerating towards it faster than those farther away.

    Below are simple 2D representations of the 'expanding' universe as we see it.

    Wouldn't both case 1 and case 2 contain the same picture from our point of view for both time A and B ?

    What observations have been made that rule out case 2 or make case 1 more likely than case 2 ?

    Thanks.
     

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  3. Nov 24, 2011 #2
    First of all, the question of what dark energy is, is not really answered and people should try different perspectives on what could be the cause or explanation for what we call dark energy.

    Your proposal is sort of an explanation I guess, but it has large problems. With a larger than universe sized spherical mass (or lumps) you would get a fixed center of gravity, and it seems like it would be right here on earth! This should pull the total mass together (like other spherical masses like earth or sun with a clear center of gravity). A more reasonable model of the universe is the 3-sphere which means the shape of the universe is something like the surface of a sphere/baloon but with three spatial dimensions. See the sticky - ballon analogy, in cosmology section.
     
  4. Nov 24, 2011 #3
    Thanks for reply. But your conclusion regarding 'center' of gravity, I think may be mistaken ?
    If we assume the mass I was speaking of was large (orders of magnitude greater than the little specks we call galaxies), wouldn't every bit of matter regardless of where it is in our 'observable universe' be closest to one side or the other, and thus start being pulled to this large outside mass ? The closer a point is to this large mass the faster it would be pulled ?
     
  5. Nov 24, 2011 #4

    Nabeshin

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    Your model violates one of the fundamental assumptions about the universe -- that it is homogeneous. It assumes there is a special location in the universe.

    Beyond this, it is unstable. The gravitational 'ring' of matter you propose would itself collapse. Furthermore, mathematically the relation between distance and speed of recession would not be as predicted. That is to say, recession speed would not be linearly proportional to distance away (neglecting acceleration).

    Simply put, such a model would be have to extremely contrived and fine tuned in order to even come close to matching observational evidence. If you think the solution of dark matter or dark energy is inelegant, this is a thousand times more absurd (and lacking in observational evidence!).
     
  6. Nov 24, 2011 #5
    Uh, don't forget that the net gravity on the inside of a massive, uniform, spherical shell is 0. So, this idea really won't work at all.
     
  7. Nov 25, 2011 #6
    Actually it wouldn't.....

    What will happen is that the force gravity that pulls something toward one side of the sphere would be exactly canceled by the gravity that is pulling the object toward the opposite side of the sphere. So you'd get zero net acceleration.

    Now people *have* proposed that the acceleration is an illusion caused by the earth being in the middle of some sort of "bubble". This would be a weird coincidence, since we'd have to be *exactly* at the center of the bubble for this to work.

    The trouble with this is that we know how big the bubbles in the universe are, and as we get more data, then it looks less and less likely that we are in the center of one. Also, it's really unlikely that we are at the exact center of a perfect bubble, so people are looking for evidence that we are are *almost* at the center of a bubble that is *almost* spherical. If you can find some small deviations from spherical symmetry in the accelerations that would show that something is weird.

    There are about a few other things that we can figure out.

    Also, we can see all the way to the big bang, so if we are in a bubble, we can see past it. If the bubble is outside the "observable universe" then that means that it's not going to effect us and so that's not an explanation for the acceleration.

    One question that you have to ask in posing a theory is what accuracy and sample size we need to see something. If we need to build a telescope and make better observations to see something (like a local bubble) then we can build the telescope and make better observations.
     
  8. Nov 25, 2011 #7
    This is a cool freshman physics problem.

    Imagine a sphere. Now imagine a point inside the sphere. Now draw a double cone from the point to the sphere. You have one cone which has more gravity because it's close, but it covers a smaller part of the sphere. You have another cone which has less gravity because it's far away from the other side, but it's a bigger cone so it covers more area.

    It turns out that the gravity from these two cones exactly balance, and if you add up everything, you get a total gravitation pull of zero.

    O.K. maybe the big object isn't spherical. But if it isn't spherical then galaxies in different directions would be moving at different speeds.

    The other problem is that once you start assume lots of weird things that all sorts of things start to break. For example, we can calculate the "lumpiness factor" of galaxies, and if there is some huge object out there, that would affect the "lumpiness factor" in a big way. And then there is the about of helium in the universe.
     
    Last edited: Nov 25, 2011
  9. Nov 25, 2011 #8
    Thanks all, Yah I get it now. Thanks for the replies.
     
  10. Nov 25, 2011 #9
    Also the question that you ask are reasonable and are things that people have thought of. One problem with science shows is that they give you a ten minute explanation and they don't go into alternative ideas and the fact that people have spent years thinking of alternative explanations and why they are alternatives.

    One class of alternative explanations are "void models." Even though a spherical shell won't cause the objects to move differently, people have considered the possibility that the gravitational attraction of the shell would create a redshift that would look like cosmic acceleration. The magic google term is Lemaitre-Tolman-Bondi

    http://arxiv.org/pdf/1004.1493

    These models are "ugly" because you have to assume that the earth just happens to be in the middle of a void. However, just because something is ugly doesn't mean that it isn't true, and the question comes up about how we can show that we aren't in the middle of a void. People have spent years thinking about it, so if look at that paper you should see what we've come up with. Here are about 50 or so.....

    http://www.eprintweb.org/S/article/astro-ph/0712.0370/cited [Broken]

    Those models are not quite completely dead, but assuming we don't find anything unexpected, we'll be able to nail the coffin shut pretty soon (i.e. next two or three years).
     
    Last edited by a moderator: May 5, 2017
  11. Nov 25, 2011 #10

    * But what if....my 2D representation of surrounding mass was viewed slightly differently ?
    In my 2D representation of 'surrounding' mass, what if we think of the 2D image like we view a polar map projection. Picture a surface of a sphere where the 2D images of sample mass items I provided are sample masses close to the north pole. What if there is a very large mass at the south pole. To all mass items over the entire sphere's surface, would they not be accelerating towards the south pole along the curved space manifold ?.And according to standard gravity wouldn't the force applied to objects twice as close be 4 times as much ? So as long as we are currently on the ' northern hemisphere" all items would appear to be accelerating away from each other. All items. And those farthest from us, the fastest. So at some point in the 'future' when we (earth) are well into the 'southern hemisphere' we will observe that all items will be accelerating towards each other back to one point. Couldn't this apply to our 3D space by considering a 4th dimension that curves back on to itself like a sphere's surface is to 2D space grid?

    Are there any measurements we have currently that rule this out ?
     
    Last edited by a moderator: May 5, 2017
  12. Nov 25, 2011 #11
    Won't work. You can't specify the curvature of space and the mass distribution separately. Once you have a mass distribution then there is an equation (thanks to Einstein) that gives you the curvature of space. Now you could play with the possibility that this equation is wrong (and the article I gave you has a large section on alternative gravity theories). The trouble with alternative gravity theories is that we don't know if Einstein's equations are exactly correct, but we do know that they are close.

    Also coming up with a different theory of gravity is hard. It turns out that it's easy to come up with a curvature-mass equation that doesn't work.

    No. You end up with a gravity model that is very, very different from Einstein's and we can show that gravity works close to Einstein's equations.

    Tons. The basic problem with your approach is that it's not symmetric. We have a ton of observations (CMB data, galaxy count data, gravitational lensing) that indicate that at large scales the universe is isotopic (i.e. different directions are the same) and homogenous (one patch of the universe is the same as the other). So any viable model of the universe has to start out with the premise that all parts of the universe are more or less the same.

    Now you can play with the "more or less", but if you have a large mass somewhere, that destroys the symmetry that we observe. (And note that this is an observation that different parts of the universe are the same. It's not true for the milky way or the solar system.)

    Something that I'd recommend you do is to find an intro cosmology textbook, and go through the sections on "what we know about the universe." It's about twenty to thirty pages, but it will get you up to speed on how much data we have. One of the things that surprises people is how much we really know about the universe and where the big mysteries are. We know more about the big bang than we do about certain parts of the Amazon rain forest.

    Right now the big mysteries aren't really about the big bang. They are about galaxy formation. Other than the part right at the very beginning, we have a pretty good idea of how the universe formed because we have lots of data from cosmic microwave background. We really don't know how the Milky Way formed, because that was a "quiet" process.
     
  13. Dec 20, 2011 #12
    Since this thread got started, I want to propose a theory. Now mind you I am not a physicist by any stretch, rather a lover of physics and my mind wonders a lot. I have always wondered about the space-time sheet and even heard about that in “empty” space particles pop in and out of existence with anti-pair and annihilate themselves; happens so fast we cannot detect it… yet. So here is my caca-maniac idea. This goes with multiverse theory. What if you imagine two universes. One that has the laws of physics Gravity. Another is more Space-Time. Imagine two almost perfect spheres, actually now that I think of it does not have to be spherical, but for the imagination two balloons works.
    So the story goes like this these two balloons collide (Big Bang) the gravity from the other universe “wrinkles” the space of the other universe. Imagine you pinched a part of the inflated balloon. The “bruising” cause particles to appear; most of them annihilates themselves because of the anti pairs, but some are left. As the other universe recedes it stops giving off the gravity to this universe, and this universe is starting to stretch it back to smoothness (imagine the balloon is stretching back to original shape). This smoothing effect is essentially the “dark energy” pulling everything back the way it was before the collision. And explains the weak effect of Gravity because there is no more gravity coming from the other universe as it recedes.
    Has anyone thought this up already? If so cool, and wanted to know more. If not any comments?
     
  14. Dec 20, 2011 #13
    Physics is more than words. The language of Physics is mathematics. Your idea would have to be formulated in that language and it would have to be consistant with what we see in our universe then you can try to explain it words.

    The idea of gravity leaking out of this universe is not new anyway, but that belongs in the beyond the standard model section.
     
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