Astronomy Without A Telescope – Dark Statistics


The hypothetical dark flow seen in the movement of galaxy clusters requires that we can reliably identify a clear statistical correlation in the motion of distant objects which are, in any case, flowing outwards with the expansion of the universe and may also have their own individual (or peculiar) motion arising from gravitational interactions.

For example, although galaxies have a general tendency to rush away from each other as space-time expands between them, the Milky Way and the Andromeda Galaxy are currently on a gravitationally bound collision course.

So, if you are interested in the motion of the universe at a large scale, it’s best to study bulk flow – where you step back from consideration of individual objects and instead look for general tendencies in the motion of large numbers of objects.

Very large scale observations of the motion of galaxy clusters were proposed by Kashlinsky et al in 2008 to indicate a region of aberrant flow, inconsistent with the general tendency in the motion and velocity expected by the expansion of the universe – and which cannot be accounted for by localized gravitational interactions.

On the basis of such findings, Kashlinsky has proposed that inhomogeneities in the early universe may have existed prior to cosmic inflation – which would represent a violation of the currently favored standard model for the evolution of the universe, known as the Lambda Cold Dark Matter (Lambda CDM) model.

The aberrant bulk flow might result from the existence of a large concentration of mass beyond the edge of the observable universe – or heck, maybe it is another adjacent universe. Since the cause is unknown – and perhaps unknowable, if the cause is beyond our observable horizon – the astronomical interrobang ‘dark’ is invoked – giving us the term ‘dark flow’.

To be fair, a lot of the more ‘out there’ suggestions to account for these data are made by commentators of Kashlinsky, rather than Kashlinsky and fellow researchers themselves – and that includes use of the term dark flow. Nonetheless, if the Kashlinsky data isn’t rock solid, all this wild speculation becomes a little redundant – and Occam’s razor suggests we should continue assuming that the universe is best explained by the current standard Lambda CDM model.

The apparent aberrant 'dark flow' (between the constellations of Centaurus and Vela) is alleged to show up in both close and distant galaxy clusters - where red is most distant, blue is least distant. This would suggest it is something that has been there since the universe was very young. Credit: Kashlinsky, NASA.

The Kashlinsky interpretation does have its critics. For example, Dai et al have provided a recent assessment of bulk flow based on the individual (peculiar) velocities of type 1A supernovae.

The Kashlinsky analysis is based on observations of the Sunyaev–Zel’dovich effect – which involves faint distortions in the cosmic microwave background (CMB) resulting from CMB photons interacting with energetic electrons – and these observations are only considered useful for identifying and observing the behavior of very large scale structures such as galaxy clusters. Dai et al instead use specific data points – being standard candle Type 1a supernovae observations – and look at the statistical fit of these data to the expected bulk flow of the universe.

So, while Kashlinsky et al say we should ignore the motion of individual units and just look at the bulk flow – Dai et al counter with saying we should look at the motion of individual units and determine how well those data fit an assumed bulk flow.

It turns out that Dai et al find the supernovae data can fit the general trend of bulk flow proposed by Kashlinsky – but only in closer (low red shift) regions. More significantly, they are unable to replicate any aberrant velocities. Kashlinsky measured an aberrant bulk flow of more than 600 kilometers a second, while Dai et al found velocities derived from Type 1a supernovae observations to best fit a bulk flow of only 188 kilometers a second. This is a close fit with the bulk flow expected from the Lambda CDM model of the expanding universe, which is around 170 kilometers a second.

Either way, it’s all down to a statistical analysis of general tendencies. More data would help here.

Further reading: Dai et al. Measuring the cosmological bulk flow using the peculiar velocities of supernovae.

61 Replies to “Astronomy Without A Telescope – Dark Statistics”

  1. OK assuming something beyond the universe horizon is pulling.
    How did it get there? And what prevents it form gravitationally collapsing since all matter beyond the horizon would get pulled by the other matter at the other side of the galaxy also beyond the horizon.

    1. It got there normally like our galaxy cluster got here. 😀

      There’s more galaxy clusters there that hold it all together and visible clusters hold it too. 😀 Visible galaxy clusters could be pulling on a galaxy cluster beyond the horizon. There really is no problem here, we just can’t see beyond the horizon.

      Was it really that difficult to answer? 😀

  2. How did it get there?
    It’s been there far longer than our observable universe…

    What prevents it from gravitationally collapsing since all matter beyond the horizon would get pulled by the other matter at the other side of the galaxy also beyond the horizon.

    This gravitational influence is not completely in our universe. We are only experiencing a small part of an adjacent dimension’s effect & the laws that govern it are slightly different from ours…

    1. As I understand it, there is a theoretical problem with having a big clumpy bit ‘out there’.

      While cosmic inflation does not preclude mass being flung out beyond the observable horizon, it should still be relatively isotropic (i.e. evenly spread) – with no greater clumpiness than we see in the observable universe.

    2. You make a couple of ad hoc claims that are confusing to me. For the purpose of the discussion of possible data on cosmology at large [sic!], maybe you can elaborate:

      It’s been there far longer than our observable universe…

      According to Kashlinsky an inhomogeneity has been there since before inflation, so before our observable universe became “observable”. But not before the inflating volume that would become our observable universe existed, or in principle could be observed at the time.

      So how should we read your claim? And does it have any relation to answering the question (“How did it get there”)? If so, I can’t see it.

      We are only experiencing a small part of an adjacent dimension’s effect & the laws that govern it are slightly different from ours…

      Well, the result of Dai point to exactly the problem with this.

      Our universe is for all purposes uniform in laws, and there is no reason to believe this uniformity doesn’t extend. So there is no adjacent universe (which arguably is the better description) but our observable universe is embedded in a larger universe, simplest and according to standard cosmology likely infinitely large.

      Now we have the question of how Kashlinsky et al result and/or hypothesis affect that. Preexisting inhomogeneities could be a result of non-uniformity in laws, which would warrant the description. But they could also be a non-uniform result of a process in a pre-universe with uniform laws; in fact, uniformity makes this a tempting simplest choice.

      Why did you dismiss this alternative out of hand? I can’t see that either.

  3. I tend to have somewhat skeptical view of this. For one thing the cosmological horizon is not the limit out to which we can observe. The CMB is out at z = 1000, and the velocity of receding galaxies is v = Hd locally. The cosmological horizon /\ (Lambda) determine the Hubble parameter H ~ sqrt{8pi G rho/3}, for rho the vacuum density of the universe. This determines the cosmological constant as

    /\ = (8pi G/c^2)*rho

    and this Hubble constant in terms of the cosmological constant is H ~ c*sqrt{/\/3}. .The cosmological constant /\ ~ 10^{54}cm^{-2}, and the cosmological horizon is at r = sqrt{3/ /\} which is at about 10^{10} light years. The CMB limit is beyond the cosmological horizon. Multiplying this by the relationship v ~ Hd gives v = c. The cosmological Doppler shift is then at the cosmological horizon z = 1.

    There are some departures from this, for the de Sitter metric involves an exponential and there are deviations from v = Hd for large enough d, but this is close enough. When we get these UT articles about the most distant galaxies observed with z = 7 or 8, these galaxies are being commoved by the expansion of space at about 7 or 8 times the speed of light! The CMB with z ~ 1000 is moving out at about that many times the speed of light. This is terribly confusing, and I see professionals, even some rather lionized ones, get tripped up on this. An observer who passes into a black hole has entered a region where they are being dragged by the isometries of space faster than light relative to things outside. Yet this observer will still be able to observe things outside the black hole before they crash into the singularity. What the observer is not able to do is to send a message to things beyond the black hole horizon. The same thing works with the cosmological horizon. We will never be able to send a message to any galaxy beyond it. We can however see everything into the past of the universe as far as our past light cone reaches.

    The question is then what is the observable limit to the universe? The early inflationary period expands the past light cone of the universe enormously. If there had been no inflation the past light cone would reach out only about as far as the cosmological horizon, or about 10 times that. The inflationary period of the universe expanded the cosmological volume by 63 e-folds. Why this is involves some issues a bit beyond the scope here. This means the limit to our observability is about 10^8 times the distance to the cosmological horizon, or 10^7 times the distance to the CMB limit. What we can “observe” at this distance are things with an incredible z ~ {10^60}. This means even superstring or Planck length scale physics at 10^{-31} to 10^{-33} cm lengths, would be expanded to the dimensions of the universe to around the cosmological horizon to CMB limit.

    This then leads to B-modes. Gravitons, which are states of the heterotic string, decoupled from the other forces, say IIB strings in matrix theory, in the inflationary early period of the universe. These gravitons then have a certain distribution in spacetime and will leave an imprint on the CMB, called the B-mode. The CMB anisotropies are a Gaussian system, but physics imprinted on there from the universe earlier than the formation of the CMB will be some over sigma or kurtosis in the statistical distribution of the CMB. What is then emerging is a battle of the CMB kurtosis. As the Planck spacecraft fill up more pixels of data on the CMB a picture will emerge as to what the universe has in store in this regard.

    As for dark flow, I think there are some funny ways that statistics are being used to argue the case. There has been no clearly substantiated and significant deviation from radial isotropic expansion in the large scale motion of galaxies on a cosmological scale. Arguments concerning the motion of galaxy cluster 10^8 to 10^9 light years out shed little light on vast cosmological scales out to or beyond the CMB.


    1. Seems to me the data was somewhat selectively gathered, I however will be the first to say this is likely as not is an artifact of my own ignorance and not in any way necessarily indicative of improper scientific method…we shall see.

  4. Frankly read this story, I am not convinced by the alleged science here. It seems to have far too many degrees of freedom for it to be plausible or testable anytime in the near future.

    Cosmology is in two main hugely divided camps; observational and theoretical cosmology. Articles like this gives observational cosmology a bad name and just feeds the crazy naysayers.

    …still waiting for one of our ‘dark’ EU/PC nutters to appear!

      1. I read the abstract, and it seems to follow the same reasoning with dark flow. Various velocity deviations in low z galaxies are observed and this is used to extrapolate out to cosmological scales. Unless I am missing something it seems improper.


    1. Never quite sure if people mean my article or the article/s I cite with these negative comments.

      I agree forming firm conclusions from data poor statistical analysis is inappropriate (like I said). Nonetheless, Kashlinsky’s idea is compellingly interesting if nothing else – even if only because you have to think a bit to build a case to refute it.

      I also liked how Dai et al came up with a counter-punch which uses the same data underlying dark energy and Lambda CDM. This is legitimate (albeit data poor and speculative) science.

      1. I meant nothing negative about your article or its balance covering the story. (Also isn’t sometimes critical debate is far different and more important than just being negative.)

        My concern just openly questions your statement. “Since the cause is unknown – and perhaps unknowable, if the cause is beyond our observable horizon – the astronomical interrobang ‘dark’ is invoked – giving us the term ‘dark flow’.”

        What does this actually mean? All I see is lots of degrees of freedom here!

        However, may of these cosmological debates are often just alternative options, which when argued for an against just tighten the overall aspects of cosmological theory. I have never really taken these very seriously — mostly as my mathematical skills are somewhat lacking when it comes to the proof. Not understanding the very nitty-gritty of the mathematical intricacies means my comments are probably more gut-instinct than comprehensively dead accurate.

        In the end negativity just means a little reserved doubt on the conclusion. It might be right or wrong, but the observational evidence is required is clearly unsubstantiated. Novices would likely not understand it, but those with cosmological credentials could have a good old memorable debate. For the rest of us, I just take it with a grain of salt…..

        My question to you would be; How useful do these ideas add to our knowledge of cosmology, astronomy and science in general?

        I’m still unconvinced that all of this is useful adding the idea to my memory bank…. but that’s just me. (Appreciate your effort though!)

        Note: As for the overall picture in my quoted part of the article that I’ve used above, well…. All Naysayers love this kind of language, because it can almost mean anything you like. I.e. davesmith_au EU/PC reply below. It is almost pointless arguing against them when the article can be “interpreted” many ways.

      2. Thanks SBC – fair enough.

        I enjoyed writing the article, because the Kashlinsky idea might really offer a way to know the unknowable (i.e. circumstantial evidence of things beyond the observable horizon).

        I don’t think the data are convincing in this case, but it’s a testable hypothesis and might lead to something interesting, should more convincing data become available.

        On the other hand, ‘dark flow’ may have registered with science blog readers, so it’s worth noting the current hypothesis is certainly not a consensus view – e.g. Dai et al offer a different data poor analysis that delivers a contrary outcome.

        So I think I am trying to help your cause here. I don’t particularly like the term ‘dark’ – less so the terms that get associated with it. If something is ‘dark’, it is a bit of hubris to suggest we know it’s mass, or know it’s energy, or know it’s flow. Dark just means we don’t know what the heck it is.

  5. Only idea that comes to mind for me is that we have particles colliding, planets and stars colliding, galaxies colliding, and even universes colliding, but the universes are the quantum black holes we live in is maybe gravitationally attracted to another mass of a black hole and is headed towards each other. As like the Milky Way galaxy is on target for the Andromeda galaxy. That’s the one rational idea to me. I just wonder what happens to all the info. in both of the black holes.

    1. Support
      More “total” bunkum.

      You haven’t a clue. Enough said.

  6. Interesting update on the Kashlinsky et al papers.

    Actually Wikipedia is on top of this, since they reference Keisler’s rejection where he discovered that the previous analysis was picking up their correlations from within the WMAP instrument itself: “We have demonstrated that the estimates for the kSZ signal are highly correlated between the different WMAP channels used in this analysis and that this correlation is caused by primary CMB anisotropy.”

    So Dai et al confirms that AFAIU, by promoting systematic effects the LCDM CMB anisotropy can be strengthened out of bounds.

    BTW, OB nitpick, or perhaps personal quirk:

    the cause is unknown – and perhaps unknowable, if the cause is beyond our observable horizon

    Sorry, but these sort of statements always look one-sided to me.

    If we leave the danger of extrapolations of statistics that LBC notes aside, and accept the testing of models as is, the set of possible causes will have a set of observable consequences (or we wouldn’t have an observable cause in the first place).

    They may or may not be distinguishable to leave us with a single cause. And there is always the possibility that some finetuning prevents this in the specific case even if the former is the general case. (I.e. that generally there is sufficient distinguishable differences; but we “unluck out” for our observable universe.)

    But here we have precisely the case that the negative result informs us on the same set of observable consequences. It is the assumption that uniformity holds that allow us to do this. But, fittingly, here be dragons! The same conspiracy of finetuning can make a general “dark flow” candidate look like uniformity.

    Now uniformity is a good assumption in most cases, so I wouldn’t go so far to say that this is a bigot position even in the case that “perhaps” is dropped. (Sorry, it is an ugly word IMO, but I couldn’t find a technically more fitting term.)

    But it is uncomfortably conservative in my opinion, we can know so much more by accepting quantifiable risk in our analysis.

  7. more untestable, unprovable cosmology? and it’s well beyond the average person’s comprehension levels?? darn! shouldn’t you scientists be working out how to make a longer lasting lightbulb? this is like the science communities’ version of spending a month sleeping in til noon and playing halo all day.

    1. Just some candy for the high rollers, my friend.
      As for Halo, I haven’t completed a mission yet! If you have, you’ve got one on me!

      1. wait wait wait… i’ve got one on THE resident UT super-genius HSBC!? Score one for Team Layperson!! Ok, im done. sorry. I’ll just go back to quietly self-inducing a brain aneurysm trying to understand the comments on this article, now. thank you.

    2. more untestable, unprovable cosmology?

      As well as uncertain, unreliable and unapproved of. That’s why they call it the universe, you know.

  8. There are a a number of things which have the fur on my back up with respect to this. The first is this business that the cosmological horizon is some black membrane of unobservability, like the surface horizon of a black hole. This is not at all the case. It requires that I crank through the connections and curvatures of spacetime in general relativity to make the case, which is beyond the scope of this blog — and frankly the time I have.

    The other things which is a bit strange is the idea that something can influence the local region of our world across an event horizon which is a barrier to such causal connections. If we were to assume the cosmological event horizon is a barrier to observability, this means no information or causal process beyond it can reach us. It does not matter whether this is electromagnetic or gravitational. So the basic idea seems rather suspicious at the outset.


    1. I always assumed that when they talked about not being able to observe beyond the the cosmic horizon that we just don’t have the detection technology to do it (yet).

    2. Isn’t the horizon observer dependent though?

      AFAIR even an observer falling into a black hole observes a horizon below them at all times.

      Consider. Three objects.
      Each of the objects has a 30 Gly horizon.
      There’s 20 Gly between the first and the second object.
      And there’s 20 Gly between the second and third.
      An observer at the first object can observe the second, but not the third.
      An observer on the second object can observe both the first and the third.
      But an observer on the third can only see the second object.
      So we have the situation where the second object can be influenced by the first and the third objects, but neither the first or the third objects can observe each other, even though they might be able to infer their existence through anomalous behaviours in 2nd.

      Maybe I’m just being naïve, but I don’t see how this is any different from any other situation where you have a reference frame under the influence of the external force (where from the reference frame of the first observer, the third observer is that ‘external force’ and vice versa)

      1. The more I look at this it seems the term “horizon” is being equated with the CMB region, about 45 billion light years out. This is not an event horizon, but a opaque region or limit that blocks any electromagnetic radiation from beyond, or equivalently earlier in the universe. However, this does not prevent neutrinos or gravity waves or possibly other neutral particles from passing from beyond.


      2. I know that sounded like a smart #$% remark but, if we on planet earth occupy no special place in our bubble, the CMB (if indeed it is the remnant of last scattering following inflation) should be everywhere including in our local neighborhood. Sorry, I keep getting hung up on this.

      3. So is that a yes or a no? 🙂

        Incidentally, I wasn’t refering specifically to the CMB with my comments.

      4. Every region of the universe looks out and back in time and the CMB is 45 billion light years away and 13.7 billion years in the past, or 380,000 years after the big bang.


      5. I do understand the further out the further in the past. I also understand ly distance vs. ly age per the accelerating expansion of space. I just can’t grasp that the CMB (if that is the most distant stuff we detect) is uniformly the same distance (“about 45 billion light years” as you say) in every direction from our spot.
        More lost sleep I fear.

      6. You are right to doubt, I don’t think that view is correct. It is implies the CMB originates from the inner surface of an expanding sphere. If this were correct, it should come as a flash of radiation and then pass us by.

        Better to think that a small 380,000 year old universe suddenly glowed with light throughout its entire volume – and it’s still glowing now except this same universe has been stretched out so the remaining photons of that glow (that haven’t yet crashed into something) are red-shifted into the microwave frequencies.

      7. Thank you Steve and LC, so, the light (photons) from the scattering is everywhere, we are just looking at the coldest (~2.7 K) surface or the farthest (stretched) region.

      8. I am sorry folks, I don’t mean to turn UT into my own personal tutor and I do appreciate all that give their time to enlighten me and others.

      9. CMB is everywhere, but now and here we can only see that light that was emitted from points currently located on a sphere centered around us with a distance of 45 billion light years. And only those photons that happened to be pointing directly at the point where we now are.

      10. Thanks Frogstar,
        I might be getting the picture. The CMB are the photons that escaped re-ionization and were moved out by the expansion (calculated to be 45 bly based on the 2.73 K ) and the others were all absorbed.

    3. I must agree, I see no mechanism which would allow for causal interaction. It is almost as if some sort of “relay” function were being implied to allow us to observe this phenomena. It makes about as much sense as building a high speed moving transmitter to launch radio waves from so they might reach their receiver quicker!

    4. I don’t think that is what they propose. Rather that a non-uniformity of the universe has set up a gradient over the observable universe. (Whether we roll back some of it as we speak or look at the future cosmological horizon.)

      So no more causation than that which set up the gradient and possibly part of which was showed beyond the horizon by inflation. But remaining correlation, if you will.

      This is, as I see it, a similar effect to the reason why diminishing statistics of lower frequencies (longer wavelengths) of cosmological observations or theories are beset by “cosmic variance”. I think Steve already noted the data poorness inherent in these problems.

  9. First I’d like to applaud UT for providing us with news on cosmology. It is what they do (news) and IMO they do it well. I don’t think I’ve seen any slant at all from their perspective or writers. There is a reason any article on cosmology sparks so much debate and comment. it’s because we all are so very interested in it. Since there were no eyewitnesses, what we humans are left with is teasing out any circumstantial evidence we can through measurements ands instruments and using our minds (math) and observational capabilities. This isn’t easy work. I can’t do it. No skill set and not enough neurons. Kind of like a CSI reconstructing a crime with no witnesses. Hopefully we can find a reasonable and logical result before we to become extinct. The physics of this universe has “for better or worse” locked us onto this blue earth and the fact is we will never get much beyond it. All we can do is observe, try, test, and find the best fit. Its good to know that there are researchers and private parties and governments (NASA) who are willing to do the work and provide the support for this process. And news providers like UT willing to tell us about it. So I would like to thank UT and their staff of writers for being willing to provide us readers any news on this stimulating subject of cosmology. And it’s free. They’ve always got my interest. dbob.

  10. I remember something very similar to this from an issue of Scientific American. I believe it was the December 2009 issue but I’m not 100% on that.

    The crux of the article was that certain galaxies were moving as though they were being drawn toward a huge mass. An explanation for this was that this is what we would expect to see if our universe were part of a multiverse, and our universe and another were in the process of merging. [i]The multiverse is a theory that our universe is just one of many that have or will pop off with different initial conditions set during the big bang. In some gravity would be too weak and the whole thing would fly apart, others too strong and it would collapse in on itself, and others that manage to hold together like ours yet have different parameters. (ie different speed of light, differences with the flow of time, all anti-matter, etc, etc.)[/i] Interestingly enough a close encounter with another universe some time in the past would also explain an unusual region of the CMB.

    I’m sure someone around here recalls that issue and probably still has it laying around. I hope I did an adequate job of summarizing, I used to have several years worth of issues saved. Unfortunately I decided to part with most of my magazine library when I moved around this time last year.

    1. It was probably a review of work along the lines of this update on the ongoing work to observe multiverses. Cosmologist Sean Carroll invited a guest blogger describing the advances in this as of yet fruitful field:

      “In the past four years, a few groups have tried to understand if it is possible to confront this radical picture of a “multiverse” with observation. The idea is to look for signatures of a collision between another bubble universe and our own. Even though the outside eternally inflating spacetime prevents all bubbles from merging, there will be many collisions between bubbles. How many we are even in principle able to see depends in detail on the underlying theory, and given the proliferation of theories, there is no concrete prediction.


      While we didn’t make any clear detections of bubble collisions, we did find four features in the WMAP data that are better explained by the bubble collision hypothesis than by the standard hypothesis of fluctuations in a nearly Gaussian field.


      While identifying the four features consistent with being bubble collisions was an exciting result, these features are on the edge of our sensitivity thresholds, and so should be considered only as a hint that there might be bubble collisions to find in future data. The good news is that we can do much more with data from the Planck satellite, which has better resolution and lower noise than the WMAP experiment. There is also much better polarization information, which provides a complementary signal of bubble collisions (found by Czech et. al. – arXiv:1006.0832). We’ll be gearing up to analyze this data, and hopefully there will be more to the story then.”

      This is as sexy as physics can be, it offers alluring glimpses of beauty, and if the evidence comes forth it will be hot! And yeah, of course it can be another dud(e) in drags. Don’t get too excited as of yet.

      1. I thought about including this in the discussion on statistical deviations in the anisotropy distribution of the CMB. I decided against it because I thought it might introduce unneeded complications, where a lot of people are obviously struggling with this topic. As I indicated yesterday the domain of observability is about 10^8 times the distance to the horizon or 10^7 times the distance to the CMB. The observable universe is a bubble of nucleation which emerged in a space with an enormous quantum vacuum energy density. This bubble emerged after about 63 efolds (e^{63} expansion) of this space by inflationary pressure, where the vacuum in this bubble “crashed” to a small value. It is possible that early on some of these bubbles interacted with each other.


  11. It seems like the basic players are doing good science. They are coming up with theories that should be capable of disproof. They are coming up with the theories before the data is in, which is also good, because we know they cannot have tweaked any figures to fit. The UT team have reported it fairly, and that’s good too, good be on you all.

    Outside of UT, the general reporting of topics like this seems to be driven by some creepy need to have something dark, mysterious, unmeasurable and ineffable out there, shaping the universe for us. I don’t think this is a science versus religion thing. While I don’t, personally, believe astronomers will extrapolate from measurements in our universe to the -1th second and get a fuzzy image of an old man in a white smock lighting a big firework; I also believe if any evidence like that were found, the astronomers would tell us about it as soon as they could, whether they were religious or not. It seems some people prefer a mystery.

    Apologies for the oo-eee touchy-feely trick-cyclist stuff. Not my usual style. Blame it on the clocks going forward.

  12. mr. nerlich,

    i’d like to request further posts/updates on this topic as they become available to you. most of the nitpicking in the above comments still show general curiosity towards this subject, which i myself also share.

  13. This is very interesting. I do wonder that if there are massive objects pulling the observable universe apart, would it not eventually collide with our “universe”?

    Also, how far away are these galaxies that are supposedly being affected? If they are extremely far away, that means they are quite young, as we are seeing them as they were, not as they are.

    I’m having problems visualizing this, as it seems to indicate that the gravitational waves started effecting them long ago, and as gravity waves apparently travel at the speed of light, should we not have also “felt” it by now. Plus at the time the galaxies were being pulled, the universe was smaller (or more dense depending on how you look at it.) Shouldn’t that also mean the external flow would be distributed across the universe more evenly?

    1. I recommend LC comments above on this.

      Short version, as I understand it: if we identify the observable universe with what we will eventually see as time goes to infinity (horizon), nothing much will “pull” in the remaining volumes to be uncovered. There may be large scale non-uniformities (but observations such as described in the post suggests not, so far).

      However, there is something that could collide with our universe. That would be other universes, since (independent) universal expansion isn’t bound by inflation and/or the horizon.

      They could “sneak up” on us, and we would never know until it is too late. In some cases it is believed the other universe can act as a great rubber eraser and “reset the universe” (reheat it). Mind, it hasn’t happened in ~ 14 Gy, so it isn’t a very likely thing to happen within our lifetimes.

      1. The cosmological horizon at 10^{10}ly is again a barrier to our being able to communicate a signal to any galaxy beyond the limit, and where that galaxy is commoving outwards with the expansion. We can send a signal outwards, but that galaxy will always recede outwards faster than our signal. Remember, space can evolve in ways which permit the frame dragging of particle faster than light: Black holes do this inwards, the universe does this outwards.

        We can observe signals emitted in the past from an arbitrary distance beyond the horizon. The question might then be asked if there is a barrier way out there. The bubble of nucleation which defined the vacuum of our “pocket universe” may have a boundary, where beyond that boundary space has a different scalar inflaton or dilation vacuum value. There may be then been other bubbles of nucleation formed in the early universe that were other pocket universes. The Matt Johnson result might indicate that our bubble or pocket interacted with another in the early phase of its development.


      2. Interact with another one in its early phase of development. That is one cool thought!

        I am just guessing, there is no basis in my claim:
        I know that virtual particles spontaneous appear into existence and out of the existence. Could something have gone wrong back then in the early universe so that 2 universe bubbles got started?

        (I also want to note that having something pulling gravitationally does not automatically mean that dark energy is none-existing.)

  14. Layperson here. There is something nearly or possibly beautiful about the universe keeping its secrets. Einstein was frustrated by his inability to come up with a complete model but it’s amazing how accurate he seems to have been on the whole. Where does a little human brain come up with anything that can stand up to the universe? But again I’d say that if the universe was in fact constructed that its designers did a fine job of instilling enough mysteries to keep mortals occupied with thoughts both great and small. Well done.

    1. To paraphrase Homer Simpson ‘It’s just a bunch of stuff that happened’.

      I’m sure we can figure it all out eventually if the data is still accessible.

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