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Skeleton of the universe?

  1. Nov 3, 2009 #1

    Astronuc

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    Shedding Light on the Cosmic Skeleton
    http://www.eso.org/public/outreach/press-rel/pr-2009/pr-41-09.html [Broken]
    Does this imply a continuous network, web or skeleton of the universe? What are the cosmological implications of this discovery?

    Galactic Clusters - http://www.eso.org/gallery/v/ESOPIA/GalaxyClusters [Broken]
     
    Last edited by a moderator: May 4, 2017
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  3. Nov 3, 2009 #2

    marcus

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    "The most widely accepted cosmological theories predict that matter also clumps on a larger scale in the so-called ‘cosmic web’, in which galaxies, embedded in filaments stretching between voids, create a gigantic wispy structure....

    These filaments are millions of light years long and constitute the skeleton of the Universe: galaxies gather around them, and immense galaxy clusters form at their intersections, ..."

    It certainly is a very interesting discovery, and by no means a new one!
    We were seeing pictures of the wispy cobwebby structure already 10 years ago. And the formation of such structures is a central part of the accepted theory of structure formation in the early universe.

    Here is a 2001 press release about the observation of the wispy cobwebby structure, which was earlier predicted by models. This wouldn't be the earliest appearance of the idea, just a sample:
    http://www.eso.org/public/outreach/press-rel/pr-2001/pr-11-01.html [Broken]

    One of the important implications has to do with dark matter. People do computer models of the formation of structure, where matter (most of it dark) is allowed to condense from near-uniformity (by ordinary gravity) and they find the simulations give pictures that look pretty much like what we observe!

    The dark matter---because so much of it, it dominates---is what condenses into strands---and denser regions where strands intersect. Then the ordinary matter (only about 1/10 as much) is attracted to these concentrations of dark matter and collects along these strands and especially at intersections---and it is what we see.

    Structure formation computer modeling (and it's striking agreement with observation) is one of a several interlocking kinds of evidence supporting the assumption of dark matter.

    One of the places one can see these computer simulations of early universe structure formation is in the popular TED lecture by George Smoot. Google "TED Smoot". It is only 15-20 minutes and it's well worth watching the whole thing.

    =========================
    A further question is what can we expect to learn from this newly reported 2009 observation of a particular wisp at redshift z = 0.55?
    Presumably other things like this have been detected and they lend themselves to technical refinements of detail.
    Here is the Tanaka et al. abstract:
    http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361/200912929 [Broken]
    They say "The observed structure is among the richest ever observed in the distant Universe. They will be an ideal site for quantifying environmental variations in the galaxy properties and effects of large-scale structure on galaxy evolution."
    Here is the Tanaka et al. preprint:
    http://arxiv.org/abs/0909.3163
    The spectroscopically confirmed huge cosmic structure at z=0.55
     
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  4. Nov 3, 2009 #3
    2hoh8x0.jpg


    Each dot is a galaxy, the distance to the right and left are distances from earth above and below the galactic plane (the region we can see). I think this is one of the best cosmology pictures that exists, you can see this "web" very well.... amazing, really.
     
  5. Nov 3, 2009 #4

    marcus

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    Nice picture Mikey!

    Just a BTW comment, I was happy to see that the ESO press release used the present-day distance---the distance used in working astro and cosmology. They did not use "light travel time" interpreted as a distance. We should encourage this among ourselves.

    If you plug z = 0.55 into wright's calculator you get that the distance NOW to the structure is 6.7 billion lightyears.
    That would be the radar distance if you could freeze expansion today and send a signal to it.
    And ESO said "nearly 7 billion light years." Clearly they had that 6.7 in mind.

    On the other hand the calculator will also tell you that the light travel time was 5.4 billion years. Light travel time is not related in any simple fixed way to actual distance, because the expansion rate has varied considerably over time. Since they said 7 billion lightyears, they could not have had the travel time (of 5.4 billion years) in mind.

    There is a website that caters to middle-schoolers (and younger teens in general) that used to use light travel time as an expression for distance, this contributed to confusion---but I think they may have cleared that up somewhat recently. For astronomers and most of the rest of us, distance is normally actual distance (with expansion frozen at some definite time so that it can be well-defined.)
     
    Last edited: Nov 3, 2009
  6. Nov 3, 2009 #5

    Chronos

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    This, IMO, is misleading marcus. Submarines use sound travel time to compute the future distance and position of other ships. If this approach is flawed, how do they hit their targets? Sound travel time is used to compute the 'now' distance and predict the future distance. Light travel time is no different.
     
  7. Nov 3, 2009 #6
    Hey Mikey thanks for posting that image. As well I'd have to agree with Chronos... as long as it is made clear which distance method is used it won't make a difference... will it? (aside from the obvious difference in the numerical value)

    @OP
    Astronuc as you can see from the many posts here the web structure has been observed for quite sometime. A lot of simulations you can find on youtube show this web structure in 3D...




    When I first saw this simulation it blew my mind away.
     
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  8. Nov 4, 2009 #7

    Wallace

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    (emphasis mine)

    I'm sure ESO could have used any distance measure they wanted in the press release, all that matters is that it is in the Billions and a big number. But you're right, it looks they are using the transverse Proper distance as defined by FRW co-ordinates for this.

    But please don't use phrases like 'the actual distance' to describe this quantity. It is no more special, unique or correct than any other distance, and to say that it is is thoroughly misleading. It's perfectly acceptable, and probably true, to say that this might be the simplest, easiest to understand and most usefull distance to use at a pop sci level, but stop there!

    Let me make this clear, cosmologists do not use the transverse proper distance as defined by FRW co-ordinates when talking to each other! You would only use measureable distances for this, such as angular diameter distance, luminosity distance or redshift. What ESO uses to convert these into some distance for a press release should not be interpreted as saying anything about scientific discourse. For starters, you have to assume a set of cosmological parameters (I know you understand all this) which makes this distance ambigous and prone to revision in a way that measurable distances are not.

    I've read probably hundreds of cosmology papers and attended at least a dozen or so cosmology conferences. I've still yet to see a distance be communicated by the tranverse proper distance in co-moving co-ordinates, which would be surprising if indeed this was the correct 'actual distance'....

    (note to be clear: 'Proper distance' is a technical term, it doesn't imply a unique or correct description. In fact the definition of this distance requires arbitrary gauge choices to be defined).
     
    Last edited: Nov 4, 2009
  9. Nov 4, 2009 #8

    Astronuc

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    It was my impression that the web structure has been studied for sometime, although it's not something I follow closely. I believe SpaceTiger did his PhD on a certain large structure or set of large structures.

    I had read a Yahoo article from Space.com, so I went to ESO's site for the actual story. I thought it might be of interest here, and that more knowledgable folks here could provide more insight into the significance of the 'new' discovery and how it fits with the 'known' web/skeleton.
     
  10. Nov 4, 2009 #9

    turbo

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    There have been suggestions that the filamentous structures could imply a fractal distribution of matter, as well. Self-similarity might not play well with the observed near-by Fingers of God effect, nor with the Kaiser effect, which is observed on more distant systems. Both effects might point to an inconsistency or unexplained contribution to redshift that is not normally assumed by the effects expected from the Hubble distance/redshift relation, nor the contributions explainable by the Doppler effects resulting from the peculiar motions of the galaxies.
     
  11. Nov 4, 2009 #10
    Woah turbo I never knew you knew stuff about cosmology :smile:.

    @Astro. Sorry I thought you meant that the new information was suggesting our universe is of what structure.
     
  12. Nov 5, 2009 #11

    Wallace

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    On the contrary, the web like structure has long been a prediction from simulations assuming the standard model. The continued observational evidence for its existence is further evidence in favour of the standard model then, and not evidence for anomalies or inconsistencies as you suggest.
     
  13. Nov 5, 2009 #12

    turbo

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    Filamentous structure was discovered through surveys, as was the existence of "walls" of galaxies and voids. Once such structures are observed, cosmologists employ computer simulations to see if the existence of the structures can be accommodated within current models, including some hierarchical models of matter formation. Plausible accommodation should not be construed as "prediction".

    The apparent distortion of clusters is another matter entirely. When we map nearby clusters of galaxies, we find that galaxies near the center of the clusters are preferentially blueshifted WRT to the other cluster members. If we map the galaxies' distances using their redshifts, this gives us maps in which the cluster assumes the appearance of a wedge, with the central galaxies at the cusp, and pointed directly at us (the observers). This was not expected. It is assumed that in a virialized cluster, smaller members held in the gravitational sway of the larger members would have a wide range of peculiar motions, including some with substantial motion toward us or away from us (blueshifted and redshifted, respectively). This is not observed. Instead, we get the Fingers of God effect in which the redshift distributions of the cluster members make them appear to be distributed preferentially with respect to the observer. Central members closer to us (using redshift=distance model) and outlying members farther from us. The opposite effect (Kaiser effect) is observe in distant clusters, with the cluster maps assuming a flattened shape.
     
  14. Nov 5, 2009 #13

    Wallace

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    Sorry Turbo, but this is simply incorrect. N-body simulations were predicting the cosmic web a decade before we were able to see it is things like the 2DFGRS and SDSS surveys. It's a basic prediction of not just LCDM but more broadly any FRW like model with an effective tilt in the power spectrum of fluctuations in the range that we observe. The earliest simulations in the 1980's (e.g. Efstathiou and collaborators) showed this kind of structures, long before we had large galaxy surveys.

    This is also not at all true. The 'Fingers of God' simply refers to line of sight redshift space distortions, which are observed in surveys in much the same way as you get from simulations (or even just expectations from perturbation theor). We have even used these red-shift space distortions to learn some reasonably accurate things about dark energy. See for instance the Nature paper by Guzzo et al. There is no discrepancy between theory and observations along the lines of what you are suggesting. If you have any references to the contrary please share them, but I could give you dozens of references looking at redshift space distortions and their value to cosmology, and in no study I've seen has anything like this been shown to be a significant issue.
     
    Last edited: Nov 5, 2009
  15. Nov 5, 2009 #14
    The interesting thing about science is what constitutes the "standard model" changes over time. In the 1980's, there were two competing models for the cosmology. Cold dark matter and hot dark matter. If the dark matter is cold, then you should see very complicated structures like what we do see. If the dark matter is hot, then you shouldn't because the thermal movement of the dark matter would wipe out these structures. (Imaging throwing an ice crystal into boiling water.)

    Physics cage fighting. Two competing theories. Look at the observations. One theory dies. The other gets the "standard model" crown.
     
  16. Nov 5, 2009 #15

    Wallace

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    Depending on the temperature you assume for the hot dark matter, you'd still see filamentary like structures forming (unless it was really hot). Really, it is quite a generic prediction of non-linear gravitational structure formation in an expanding Universe, it doesn't actually tell you that much about the physics of say dark energy or dark matter etc since you get this kind of structure in a very wide parameter range.
     
  17. Nov 5, 2009 #16
    Also it's not just filament structure, but the *type* of filament structure. You run statistics on the the N-body simulations, get a number. Run statistics on observations, get another number. See if the numbers match.

    It's really important to use statistics to do these sorts of comparisons since "gee the pictures look the same/different" doesn't work very well.

    Also IIRC the statistics that you see are pretty clearly non-fractal, as the power spectrum doesn't show any self-similarity.
     
  18. Nov 5, 2009 #17
    Yes but the structures start looking very different. They start "melting".

    The fact that you get structures doesn't tell you very much. The detailed statistical properties of the structures look like tells you a great deal. One of the really important things about the standard model is that it doesn't merely tell you that you get structures, it tells you in pretty large detail what those structures are going to look like for a given set of input parameters.

    One probably with popular descriptions is that they try to avoid math, which means that the miss the point that the standard model can predict in a lot of detail not only that there are structures but what those structures look like.
     
  19. Nov 5, 2009 #18

    Wallace

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    Exactly right, we shouldn't overstate the importance of this whole filamentary structure thing, it comes up a lot in press releases (because it makes pretty pictures) but it's a very broad qualitative description of the density field. As you say, detailed statistical properties such as the power spectrum, bi-spectrum, halo mass function etc are the things that are studied in simulations and compared to observations.

    And yes you're right, we don't see a fractal bevhaviour or self similiarity in the distribution of material in the Universe. Turbo erronously suggested that the filamentary structures seen have some bearing on the Universe having a fractal distribution of matter (see post #9) which is spurious. You could have a web-like non-fractal distribution or a non-web like fractal one, the two are unrelated.
     
  20. Nov 5, 2009 #19
    One thing that someone could do (and someone really should do) is to set up some sort of web application in which people can punch in a power spectrum, and see what it looks like. At that point, you can do things like, "this is what the universe looks like", "this is what the universe would like like if it were five billion years old", "this is what the universe would look like if there were no dark matter."

    On problem with "eyeballing" a picture, is that you don't know whether things look the same or different because they really are different, or if it's because of the way you drew the picture. The other problem is that if you think that something looks similar, and someone else says that it looks different, then there's no real way of saying who is right.

    And I should point out that all of this stuff is really useful in finance. If you plot out stock prices, all you get are wiggly lines. There is a lot of statistics involved in telling whether one wiggly line behaves the same or different than another wiggly line.

    One thing that I've found is that people use "fractal" in popular language to mean something that it doesn't mean in mathematics. A "fractal" is a specific type of geometric structure. The distribution of matter in the universe is not "fractal" at all.

    Also once you understand what a "fractal" looks like, it's pretty clear that the universe is not one.
     
  21. Nov 5, 2009 #20

    turbo

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    Funny! When Geller and Huchra discovered the "Great Wall" in 1989, it came as quite a surprise. Somehow the existence of thin, wall-like structures wasn't "predicted" by the standard model. Until filaments were observed I doubt that they were predicted either. At the time of Geller and Huchra's survey, it was commonly believed that superclusters were the largest structures, and that they were distributed quite uniformly. That was only 20 years ago.
     
    Last edited: Nov 5, 2009
  22. Nov 5, 2009 #21

    turbo

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    Huh! I "erroneously suggested" that there appears to be a fractal-type distribution of matter in the universe? There is a large, well referenced body of work on this subject. It doesn't take a lot of research to pull up papers on this subject.

    Concordance cosmologists are pretty sold on an isotropic and homogeneous universe on very large scales, but that doesn't mean they are correct. Mapping of galaxy clusters and filaments is pretty much limited by selection-effects, since distant clusters are very difficult to get spectroscopy on, and their fainter members get undetectable pretty fast, leaving only the brighter outliers. Flattening of any fractal pattern at great distances could be a sign of homogeneity, or it could be a statistical artifact of selection effects in faint observations. We need more data.
     
  23. Nov 5, 2009 #22
    That's because what people defined as the standard model in 1989 isn't what it people defined it in 2009. In 1989, there were a number of viable cosmological models, some of which predicted large scale structures, some didn't.

    The one's that didn't became non-standard.

    And if that were the case, then what we now consider the standard model of cosmology just wouldn't work, and what we'd be calling the standard model would be some variant of hot dark matter.

    20 years is an eternity in cosmology. Stuff that is a year old is ancient history.
     
  24. Nov 5, 2009 #23
    Pull up three review papers that argue in favor of a fractal distribution. Every power spectrum that I've seen has been very non-fractal. Also you need to define "fractal-type". A fractal has a very well defined mathematical definition.

    If you are using the term "fractal-type" you need to define what you mean. If you mean a distribution with power at all scales, you are using terms in very non-standard ways.

    It's pretty obvious just looking at the distributions that they aren't fractals. The thing about fractals is they are scale invariant. If you magnify the a Julia set, it looks a lot like another Julia set. If you take one of the three-3 surveys of the universe, magnify it and then superimpose it on the original picture, it looks very different.

    Selection effects doesn't help you here.

    I think we have enough data to rule out fractal distributions. The thing about fractals is that they are self-similar. Take a fractal distribution of galaxies. Now put in some sort of selection effect. What you end up with is still a fractal. It's a different looking fractal, but it's still a fractal.

    Also non-fractal doesn't mean homogeneity.
     
  25. Nov 5, 2009 #24
    turbo-l: Mapping of galaxy clusters and filaments is pretty much limited by selection-effects, since distant clusters are very difficult to get spectroscopy on.

    But you can get rid of these sorts of effects by not looking at distance but instead looking at angular correlations.
     
  26. Nov 5, 2009 #25

    turbo

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    My point is that cherry-picking some models that happen to fit observations after the fact is not equivalent to "prediction", since there was no consensus on viability of the models prior to the observations. With enough freely adjustable parameters, you can make almost any model fit observations, though that is a shaky way to conduct science.
     
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