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Thread: Ring of galaxies darting away from us much faster than predicted

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    Ring of galaxies darting away from us much faster than predicted

    Sorry but this is another $100 question but an important one.

    - What is the relation between galaxy speed and GR being wrong?
    https://phys.org/news/2017-03-einstein.html

    - Does that officially move GR out of the mainstream, making place for ATM theories?
    Last edited by philippeb8; 2017-Mar-16 at 02:59 PM.

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    No, this doesn't officially move GR out of the mainstream. It is certainly interesting and may end up contributing to furthering our understanding of gravity, but no, GR has not been "proved" wrong by this new finding. Before getting too excited we should wait for this study to be well verified, at least.

    There is a truly huge mass of evidence supporting GR. To date there has never been a well verified experiment or observation that has contradicted GR. Is GR complete? Almost certainly no, as far as we can tell. But, GR is very unlikely to ever be shown to be just completely wrong. Like GR improved and extended beyond Newtonian physics, but did not make it obsolete or show it to be completely wrong. Whatever comes after GR is likely to add to the regime-space that can be explained and modeled, but it will also have to explain and model everything that GR already does at least as well as GR does. And GR explains and models a lot of things very well indeed.

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    Quote Originally Posted by Darrell View Post
    There is a truly huge mass of evidence supporting GR. To date there has never been a well verified experiment or observation that has contradicted GR.
    Not all ATM experiments have been done yet. PM me for more details, if not then let's put a blind eye and wait a few years.

    Is GR complete? Almost certainly no, as far as we can tell. But, GR is very unlikely to ever be shown to be just completely wrong. Like GR improved and extended beyond Newtonian physics, but did not make it obsolete or show it to be completely wrong. Whatever comes after GR is likely to add to the regime-space that can be explained and modeled, but it will also have to explain and model everything that GR already does at least as well as GR does. And GR explains and models a lot of things very well indeed.
    I agree. GR is certainly not 100% false but probably isn't 100% right either.

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    Quote Originally Posted by philippeb8 View Post
    Not all ATM experiments have been done yet.
    I think the acronym ATM is superfluous in that sentence. Yeah I agree, all experiments have definitely not been done. They never will be. Lots of well supported hypotheses and theories were ATM at one time. The problem with ATM isn't that the idea is literally ATM, it's that so many proponents of ATM ideas don't seem to understand the rigorous science that needs to take place for any new idea to become well verified enough to become mainstream, which takes time.

    There are a couple of reasons, at least why it takes time and lots of science. 1) Because of how easy it is to fool ourselves we've learned to be cautious and that the process has to be followed if we want useful results. Exciting new fundamental discoveries that instantly result in rewriting well established science and enable immediate, amazing new technology happen so rarely that it's pretty fair to say, "only happens in the movies.

    2) New hypotheses and theories have to explain and model things at least as well as the ones they purport to replace, and that is often a lot of ground to cover.

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    Quote Originally Posted by Darrell View Post
    I think the acronym ATM is superfluous in that sentence. Yeah I agree, all experiments have definitely not been done. They never will be. Lots of well supported hypotheses and theories were ATM at one time. The problem with ATM isn't that the idea is literally ATM, it's that so many proponents of ATM ideas don't seem to understand the rigorous science that needs to take place for any new idea to become well verified enough to become mainstream, which takes time.

    There are a couple of reasons, at least why it takes time and lots of science. 1) Because of how easy it is to fool ourselves we've learned to be cautious and that the process has to be followed if we want useful results. Exciting new fundamental discoveries that instantly result in rewriting well established science and enable immediate, amazing new technology happen so rarely that it's pretty fair to say, "only happens in the movies.
    This sounds like a "giving up" approach.

    2) New hypotheses and theories have to explain and model things at least as well as the ones they purport to replace, and that is often a lot of ground to cover.
    Like I said: I cannot talk about ATM stuff and we're drifting away from the original question so PM me to discuss this further, I will not waste your time.

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    No, it really isn't. It is a way to move forward with warranted confidence approach.

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    Quote Originally Posted by Darrell View Post
    No, it really isn't. It is a way to move forward with warranted confidence approach.
    You chose not to PM me either so that's subjectivity.

    Anyway I'm still interested to know:
    - What is the relation between galaxy speed and GR being wrong?

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    Quote Originally Posted by philippeb8 View Post
    Anyway I'm still interested to know:
    - What is the relation between galaxy speed and GR being wrong?
    If we assume that the galaxies were once satellites or neighbours of our galaxy and Andromeda and if we assume that a single simple sequence of events was responsible for imparting their velocity to them, and if we make some assumptions about how the DM halos interacted, then the connection is that they don't have a model for how a dynamical event could have generated the distribution of galaxies they see. Lots more work needed.

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    Quote Originally Posted by Shaula View Post
    If we assume that the galaxies were once satellites or neighbours of our galaxy and Andromeda and if we assume that a single simple sequence of events was responsible for imparting their velocity to them, and if we make some assumptions about how the DM halos interacted, then the connection is that they don't have a model for how a dynamical event could have generated the distribution of galaxies they see. Lots more work needed.
    Ok thanks again Shaula; I'm guessing they tried with an advanced simulator as well, given the complexity.

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    Quote Originally Posted by philippeb8 View Post
    Ok thanks again Shaula; I'm guessing they tried with an advanced simulator as well, given the complexity.
    I am not so sure they did to be honest - I think that is why more work is required. Looking at the paper (https://arxiv.org/pdf/1506.07569.pdf) their approach is a particle model with some twists. They don't cover baryonic interactions or take into account interactions with other galaxies in the local group, among a number of other simplifying assumptions. They do cover the possible sensitivity of their results to these factors but it is hard to get a feel for that based on the numbers they give. Many body gravitational systems are notoriously sensitive to initial conditions and interaction history. I am certainly not saying it is a bad piece of work - it is very interesting. But (as usual) the magazine article on it oversells it.

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    Quote Originally Posted by philippeb8 View Post
    You chose not to PM me either so that's subjectivity.
    No, I won't be PMing you. Absolutely nothing personal, just too few hours in a day to following up on everything, even if it seems interesting. One has to edit.

    Quote Originally Posted by philippeb8 View Post
    Anyway I'm still interested to know:
    - What is the relation between galaxy speed and GR being wrong?

    The researchers' model for how the satellite galaxies in question came to have the motion and location observed today requires that gravity works differently than GR describes / models. The satellite galaxies are moving faster than would be predicted using GR to model the gravitational interactions, and granting all of their starting assumptions, the other parts of their model, are accurate.

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    Quote Originally Posted by Darrell View Post
    No, I won't be PMing you. Absolutely nothing personal, just too few hours in a day to following up on everything, even if it seems interesting. One has to edit.
    Your choice. I won't be spamming either because I rationalise my efforts so no worry.

    The researchers' model for how the satellite galaxies in question came to have the motion and location observed today requires that gravity works differently than GR describes / models. The satellite galaxies are moving faster than would be predicted using GR to model the gravitational interactions, and granting all of their starting assumptions, the other parts of their model, are accurate.
    Ok so rotating galaxies in a cluster was found to be abnormal since the 1950's I think but this time dark matter is interacting as well because of their proximity.


    Thank you both!
    philippeb8
    Last edited by philippeb8; 2017-Mar-16 at 07:24 PM.

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    Quote Originally Posted by philippeb8 View Post
    What is the relation between galaxy speed and GR being wrong?
    https://phys.org/news/2017-03-einstein.html
    This is the paper Dynamical history of the Local Group in ΛCDM – II. Including external perturbers in 3D
    The title hints at an issue. The Local Group is not that affected by the expansion of the universe (Λ) because it is bound gravitationally, e.g. Hubble's law is not well matched until we look at galaxies outside of the Local Group (ETA: actually outside of the Virgo Cluster). However the abstract is mostly about dark matter.
    Then we read the paper and see ΛCDM being used in their previous 2D model, e.g. "The expansion rate of the Universe is quantified by the Hubble parameter..." and in this 3D model. Thus their results are dubious. They have explanations of their results from convectional astronomy, e.g. galaxy flyby. They only mention MOND which is known not to work except for galaxy rotation curves and is not modified GR!

    In summary: That paper may be dubious and the authors are speculating about modifications to GR.

    ETA: Anisotropic Distribution of High Velocity Galaxies in the Local Group by the same authors has the galaxy flyby explanation:
    We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than in a 3D dynamical model of it based on Λ CDM, the standard cosmological paradigm (MNRAS, stx151). 5 out of these 6 galaxies are located very close to a plane with root mean square thickness of only 88.2 kpc despite a radial extent of almost 1 Mpc. This plane also passes within 140 kpc of both the Milky Way (MW) and M31 and just 6 kpc from their mid-point. The orientation of the plane is such that the MW-M31 line is only 20 ∘ from lying within it.
    We develop a basic model in which a past MW-M31 flyby encounter forms tidal dwarf galaxies that later settle into the recently discovered planes of satellites around the MW and M31. The MW-M31 orbital plane required by this scenario is oriented similarly to that of the LG dwarfs with anomalously high RVs. The fast relative motion of the MW and M31 at one time would lead to LG dwarfs being flung out via gravitational slingshot encounters. These encounters would likely be most efficient for objects flung out close to the MW-M31 orbital plane. This suggests a possible dynamical reason for our findings, which are otherwise difficult to explain as a chance alignment of isotropically distributed galaxies (probability < 0.01).
    GR has passed the tests of general relativity. The only significant test left is a direct observation of an event horizon to confirm that black holes exist.
    The modified versions of GR used to explain dark matter have not passed the tests of general relativity and some are invalid in themselves, e.g. TeVes predicts stars with a lifetime of < 1 second.
    Last edited by Reality Check; 2017-Mar-17 at 12:19 AM.

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    Quote Originally Posted by Shaula View Post
    They do cover the possible sensitivity of their results to these factors but it is hard to get a feel for that based on the numbers they give. Many body gravitational systems are notoriously sensitive to initial conditions and interaction history.
    Quote Originally Posted by The Paper
    We constrain their masses using distance and radial velocity (RV) measurements of 32 LG galaxies. To do this, we follow the trajectories of many simulated particles starting on a pure Hubble flow at redshift 9....
    Yeah, I just started reading this, and I don't even get what they're saying here. They can't possibly start some well chosen configuration of bodies at z=9 and reproduce a near exact version of what we see today, can they? I could see running the current galaxies' motions backward, but it's a long way to z=9! Seems like the current distance and velocity measurements would be so imprecise, as you say, Shaula, sensitivity to initial conditions would probably yield nonsense results long before that.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    Yeah, I just started reading this, and I don't even get what they're saying here. They can't possibly start some well chosen configuration of bodies at z=9 and reproduce a near exact version of what we see today, can they? I could see running the current galaxies' motions backward, but it's a long way to z=9! Seems like the current distance and velocity measurements would be so imprecise, as you say, Shaula, sensitivity to initial conditions would probably yield nonsense results long before that.
    I think they basically Monte Carloed it and looked for statistical measures of similarity between the velocity distributions over the cluster - but you are absolutely right. 13 billion years is a lot of time for things to diverge over!

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    Quote Originally Posted by Cougar View Post
    Yeah, I just started reading this, and I don't even get what they're saying here. They can't possibly start some well chosen configuration of bodies at z=9 and reproduce a near exact version of what we see today, can they? I could see running the current galaxies' motions backward, but it's a long way to z=9! Seems like the current distance and velocity measurements would be so imprecise, as you say, Shaula, sensitivity to initial conditions would probably yield nonsense results long before that.
    Yep I felt the same when I started reading it -it's a bit of a fantasy.

    On the other hand, here is an article about a possible past encounter between the MW and M31, which uses the distribution of satellite galaxies as evidence:

    https://darkmattercrisis.wordpress.c...dromeda-flyby/

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    This is interesting stuff - by coincidence we had a talk at our local astronomy society from Indranil Banik just a few weeks ago (and one from Dr Hongsheng Zhao a few years ago).

    Admittedly Indranil's talk was aimed more at us amateur astronomers, so left out a lot of the more complex maths, but he covered a number of other pieces of evidence as well as the local dwarf galaxy motions, all of which contributed to a case for there being more to gravity than our current understanding. Dr Zhao's talk a few years ago didn't really convince me at the time that there was real evidence for any modified gravity theories, but the latest information has at least got me thinking that they may have a case.

    At the very least, there are some interesting new observations which need to be explained...

    (NOTE : I've seen a bit of media hype already about "overturning Einstein", but that's very far from what they're proposing. Most of Newton and Einstein's theories will still be valid, but it's almost inevitable that there are additional "wrinkles" to be added to improve the models.)
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    Quote Originally Posted by philippeb8 View Post
    - What is the relation between galaxy speed and GR being wrong?
    https://phys.org/news/2017-03-einstein.html
    That article said:

    "...researchers... found a gigantic ring of galaxies darting away from us much faster than predicted. This 10 million light year-wide ring made up of small galaxies is expanding rapidly like a mini Big Bang."

    I searched for reference to this is the arxiv paper and found nothing! What gives?
    Everyone is entitled to his own opinion, but not his own facts.

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    We are once again seeing a case of caveat lector, that is, let the reader beware. It appears that certain writers are running wild with interpretation of raw data that have large uncertainties.

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    I am failing to understand what is in question here. Are the claims that 1-the current model of Gravity is wrong. 2- the current model of GR is wrong. Or 3 claiming that a new Big Bang has started?

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    Quote Originally Posted by bruno View Post
    I am failing to understand what is in question here. Are the claims that 1-the current model of Gravity is wrong. 2- the current model of GR is wrong. Or 3 claiming that a new Big Bang has started?
    The issue is that a science journalist used language to make a relatively uninteresting observation sound like everything we know is wrong... and there are people yearning to witness a major upheaval in science who want to run with it. Gravity and GR are still great models, and no new big bang has started ... and it is very misleading to describe these little galaxies as "darting away", but the journalist went to the thesaurus and found the most animated way to describe the situation possible.

    Concerning the story itself, it is nice that observations confirm that little galaxies created by a tidal interaction of two larger galaxies are, after the event, moving away from the larger galaxy in what might be described as a ring.
    Forming opinions as we speak

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    Quote Originally Posted by antoniseb View Post
    Concerning the story itself, it is nice that observations confirm that little galaxies created by a tidal interaction of two larger galaxies are, after the event, moving away from the larger galaxy in what might be described as a ring.
    It may be "nice", but it's also "interesting", and maybe even "unexpected"...

    I'm not convinced that any of the MOND or modified gravity theories are correct, but this is another piece of evidence which lends some support to them. As such it needs to be investigated and understood, rather than brushed off.
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    What would be evidence supporting the already invalid MOND or the flawed modified GR theories would be running the model in the paper using those theories and finding a match to the observations.

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    Quote Originally Posted by Hornblower View Post
    We are once again seeing a case of caveat lector, that is, let the reader beware. It appears that certain writers are running wild with interpretation of raw data that have large uncertainties.
    Thank you for the very good summary of a chronic problem.

    Regards, John M.

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