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Cosmological expansion vs. stretching

  1. Jul 9, 2012 #1
    Cosmological expansion vs. "stretching"

    From a relativistic point of view, how can we tell the difference between a universe that’s expanding (outward) from a singularity versus one that is being sucked INTO a singularity? In the case of the latter, it seems that those objects closer (than us) to the singularity would be accelerating more rapidly and thus be red-shifted. Also, we would be accelerating more rapidly toward the singularity than those that are farther from it, so those would appear to be red-shifted too.

    The physicists from whom I’ve been able to elicit a response counter with, “This can’t be true because the universe has no center.” I understand that that the universe has no three-dimensional center. But I don’t see why a singularity of DESTINY is different from a singularity of ORIGIN. In other words, if the conventionally accepted singularity (the origin) doesn’t represent a 3D center of the universe then why would the one we’re (putatively) falling INTO have to represent a 3D center?

    In such a scenario, my uses of “closer,” “farther,” “toward,” and “away” refer not to 3D space but to some higher order construct where at least one other dimension (e.g., time) obfuscates the “position” of the singularity. In fact, one might even consider that, relativistically speaking, “expansion” is not a meaningful term; the universe is, more correctly, experiencing STRETCH; whether that is an outward stretch or an inward stretch would seem difficult to determine. The expansion of the 2D surface of a balloon is often used as an analogy for the 3D expansion of the universe. I envision instead the 2D surface of a vortex, whereby the direction representing “down” (e.g., down into a whirlpool) represents a 4th (or higher) dimension not perceived by us as “space.”

    Notably, the stretching apart of galaxies as they accelerate into this putative singularity would account for the new data that appear to indicate acceleration of the universal “expansion,” right? And, if we apply Occam’s razor to this data, the current explanation doesn't hold much water. The “dark energy” required to fuel such an acceleration seems much more contrived and convoluted than would a descent into a singularity, especially in the lack of any (other) evidence for this energy.
     
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  3. Jul 10, 2012 #2

    Chalnoth

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    Re: Cosmological expansion vs. "stretching"

    A collapsing universe would have further objects being blue-shifted, not red-shifted.
     
  4. Jul 10, 2012 #3
    Re: Cosmological expansion vs. "stretching"

    Yes, but I wasn't asking about a collapsing universe. Hence, the word "stretching." Please read the post again.

    Perhaps it would be best to scale things down to the vicinity of a black hole as an analogy. Imagine your spaceship has been captured by the gravitational field of a black hole without the requisite velocity/trajectory to remain in a stable orbit or escape. As the ship accelerates in toward the black hole, objects farther away would appear to be red-shifted because your point of reference is receding from them.

    Suppose there is a gargantuan singularity capable of casting a gravitational field across the visible universe. This is where my mathematical understanding of the current models breaks down. Is the asymptotic flatness derived from equations that contain an assumption or two? Or does it fit real observation? And if so, how does one explain the "peculiar motion" of the Milky Way? Indeed, how does one explain the Shapley supercluster?! Don't answer that-- I know it's explainable. But it's also VERY intriguing! We don't really know what's going on there, do we...?
     
  5. Jul 10, 2012 #4

    Chalnoth

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    Re: Cosmological expansion vs. "stretching"

    Then you're not talking sense. The visible universe is smooth and uniform as far as we can see (which is quite far). You can't get a significant redshift from local objects like black holes that looks like a cosmological redshift in such a universe.
     
  6. Jul 10, 2012 #5
    Re: Cosmological expansion vs. "stretching"

    That's not strictly true, is it? The red shift is not uniform. And there are whole superclusters of galaxies that appear to be aggregating rather than moving apart. How do we know that this is due to a local effect such as gravity rather than aberrations in the fabric of the expansion? And even if it is gravity, we have no idea what's responsible for it, right? It seems to differ from black holes quantitatively enough to seem like a qualitatively different phenomenon.

    And as for "(which is quite far)," what about the Zone of Avoidance? How can we know what's going on behind that curtain?

    I don't envision anything "local"; or even an "object," really. I've seen erudite and confident statements on this forum asserting that a singularity is not necessarily a black hole. What I'm trying to wrap my imagination around is a distortion of space-time that, for whatever reason, we don't perceive spatially. Going back to the tried-but-true balloon analogy, imagine the balloon is tied off and no more gas is going in to expand it. Now imagine a pencil is being shoved into the balloon (eraser first, of course!). The disks attached to the balloon in that vicinity are moving farther apart because the balloon's material surface is stretching.* Sure, that indentation doesn't look flat from our vantage point; but it's my understanding that the two dimensions of the balloon's surface are held to be analogous to the three dimensions of what we perceive to be space, such that the third dimension of balloondom-- that through which we see its curvature-- is time. Or do I have that wrong? If not, I suppose a Flatlander living on the surface of the balloon would not really see its curvature, including the curvature of the indentation caused by the pencil.

    *Disks attached to the opposite side of the balloon are moving apart because that part of the surface is actually expanding as the insertion into what was previously a sphere displaces the gas inside. But I think we can pretend that doesn't happen with regard to the universe. Suppose that instead of being a closed system, the space and matter disappearing into the singularity are escaping our universe altogether-- transiting into another universer, perhaps. (Besides, an observer in the indentation wouldn't be able to see the other side of the balloon anyway.)
     
  7. Jul 10, 2012 #6

    rbj

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    Re: Cosmological expansion vs. "stretching"

    Chalnoth, my understanding of how the question is posed is like there is some singularity (with a lotta mass) out at the point of infinity. sorta like saying the complex function

    [tex] f(z) = \frac{1}{z} [/tex]

    has a "zero" at [itex]z=\infty[/itex].

    from a Euclidian POV, that's a lotta different z's. so, instead of saying that 1/z has a singularity at z=0, is the OP saying that [itex]f(z)=z[/itex] has a singularity out there at infinity? so as galaxies get closer to that, they accelerate faster toward it?

    i can't visualize it either and i dunno that i'm guessing at the OP's point correctly.
     
  8. Jul 10, 2012 #7

    Chalnoth

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    Re: Cosmological expansion vs. "stretching"

    Redshift certainly is uniformly related to distance, which is the picture of a universe which is the same everywhere and in all directions but expanding. Having some singularity out there would produce a preferred direction (towards the singularity), and no such preferred direction exists.
     
  9. Jul 11, 2012 #8

    Drakkith

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    Re: Cosmological expansion vs. "stretching"

    This sure seems like wild speculation to me...
     
  10. Jul 11, 2012 #9
    Re: Cosmological expansion vs. "stretching"

    Distance AND direction of travel, you mean. Because there are most certainly galaxies that are blue-shifted with respect to the Milky Way and other galaxies. So to say that the red-shift is uniform suggests "uniform after correction for peculiar motion." But it would seem that such correction is only possible for objects whose motion can be validated by some means other than red shift (otherwise would be begging the question). So I suppose it only holds for galaxies close enough that their motion can be confirmed by parallax or line-of-sight observation. Sure, there's Tully-Fisher, but that's subject to pretty big error, right? Like as much as 50%?

    Now, I recognize that a blue-shifted galaxy in a field of other galaxies that are shifted as predicted can be considered an anomaly-- a case of peculiar motion-- regardless of distance or independent confirmation of the peculiarity. But when one considers a whole supercluster that is SUPER-red-shifted, does it not give one pause?
     
  11. Jul 11, 2012 #10
    Re: Cosmological expansion vs. "stretching"

    Well, yeah-- touche'. But its undeniably helpful to think outside the box now and again.
     
  12. Jul 11, 2012 #11
    We know this scenario can't be true because of the cosmic microwave background. The CMB is the radiation left from the recombination. The recombination was the point 380,000 years after the big bang when the universe reached a specific, cool, temperature of 3000 degrees Kelvin. Prior to this, the universe was too hot for electrons to be held into atoms. Their kinetic energy was far too high to be held in orbit by the electromagnetic force. Since these elections weren't bound by atomic nuclei, they were free to run around and do their own thing. More precisely, they scattered photons. So, no light could be released. The entire universe was an opaque plasma, and we can not see farther than this point. When the electrons were captured by atoms at the recombination to form hydrogen, photons were free to be emitted.

    So, we have a prediction of the big bang model as compared to yours. The big bang model predicts that we will observe this radiation today - as being homogenous everywhere, having a very high redshift (because of Hubble's law. Since this radiation is reaching us from far away, it will have a higher redshift), having a blackbody spectrum, having certain polarizations for the photons that compose it, and having slight anisotropies that would be evenly distributed, to give us the even distribution of galaxies we see.

    We observe this through BOOMERanG, COBE and WMAP, with all of these characteristics.

    On top of that, your scenario doesn't predict a homogenous distribution of galaxies as the big bang model does. We do observe such a distribution.
     
  13. Jul 11, 2012 #12

    Chalnoth

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    Re: Cosmological expansion vs. "stretching"

    This is only true of a small number of nearby galaxies. Basically, there is an overall effect (the expansion), plus small local random motions. To give a sense of scale, typical galaxy motions top out at around 1000km/s, while the current Hubble rate is around 70km/s per Megaparsec, meaning that beyond about 14 Megaparsecs (about 47 million light years), the recession velocity always wins out. Most of the universe that we can observe is more than a hundred times further than that, so that it is perfectly reasonable to ignore the local velocities most of the time we're talking about the expansion. Local velocities are important for understanding the errors in our estimates of the expansion and other parameters, but don't account for much otherwise.

    So yes, effectively the redshift is uniform with distance, once you go out beyond a few million light years.
     
  14. Jul 11, 2012 #13
    To add to what Chalnoth said, the only galaxies that are blue-shifted are some within our galactic cluster. Because the galaxies are sufficiently close together, the mutual gravitation of the galaxies is enough to counter the effect of expansion.
     
  15. Jul 11, 2012 #14
    Re: Cosmological expansion vs. "stretching"

    Well, if you mean "blue-shifted" in an absolute sense, yes. But one can also consider "blue-shifted relative to the expected red shift." In other words, wavelengths that are longer than they'd be in a static universe but still shorter than those from galaxies at an equal distance. And I believe there are plenty of those outside of our cluster.

    And it's my impression that "mutual gravitation" is not the only thing that makes the Milky Way's motion peculiar. We are moving relative to the CMB in a way that is not consistent with the center of gravity of our local cluster, right? Isn't it the case that we appear to be moving generally toward the Great Attractor and the Shapley supercluster beyond that? In fact, the Dark Flow seems to indicate a very large portion of the visible universe is sashaying its way nonchalantly in that general direction (Kashlinsky et al., 2008). It's my understanding that the visible mass-- and its attendant mutual gravity-- of those peculiar clusters is insufficient to account for the Dark Flow, though that appears to be a subject of ongoing debate.

    So, there seems to be agreement that there is red shift and that it follows Hubble's Law, but... that it is not uniform. This is essentially what a vortex model would predict, no? The anomalies seem subtle relative to the distance-magnified, exponential rate of the Hubble effect; but that also seems to fit a vortex model.
     
  16. Jul 11, 2012 #15
    Re: Cosmological expansion vs. "stretching"

    But that's only because the peculiar motion of those very distant galaxies gets proportionally dwarfed by the Hubble effect. That doesn't make the red shift uniform-- it just makes anomalies more difficult to detect/appreciate. Right?
     
  17. Jul 11, 2012 #16

    Drakkith

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    Re: Cosmological expansion vs. "stretching"

    To think outside the box, one must first understand what's inside the box. As an example, your claim that certain galaxies are blue shifted is true, but those galaxies are very very close to us on a cosmological scale. Everything past a hundred million light years or so is completely red shifted.
     
  18. Jul 11, 2012 #17

    Drakkith

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    Re: Cosmological expansion vs. "stretching"

    The redshift isn't 100% uniform, as galaxies orbit around each other, just like anything else in space does. This results in some moving away slightly faster or slower than the average due to their velocities through space. On average the redshift is uniform with distance.
     
  19. Jul 11, 2012 #18
    Re: Cosmological expansion vs. "stretching"

    Perhaps this post crossed in the ether with the one where I explained that I was talking about RELATIVE blue shifts rather than ABSOLUTE blue shifts. Check that, then hit me back, please.
     
  20. Jul 11, 2012 #19
    Re: Cosmological expansion vs. "stretching"

    Yeah, I know about local clusters and local gravitational effects. That's not what I'm talking about. And I think Kashlinsky would quibble with you about the "average redshift" being uniform.

    For those of you who keep harping on uniformity, I would be sincerely interested in your take on the following refs:

    A. Kashlinsky, F. Atrio-Barandela, D. Kocevski, and H. Ebeling (2008). "A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications". Astrophys. J. 686: 49–52.

    A. Kashlinsky, F. Atrio-Barandela, D. Kocevski, and H. Ebeling (2009). "A measurement of large-scale peculiar velocities of clusters of galaxies: technical details". Astrophys. J. 691: 1479–1493.
     
  21. Jul 11, 2012 #20

    Drakkith

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    Re: Cosmological expansion vs. "stretching"

    Yeah that's pretty much what we are saying. Galaxies don't fit the redshift curve exactly but are spread around it. I don't know much about this vortex model you mentioned though, so I can't comment on it.
     
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