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Space Science

Galaxy Clusters' Stunted Growth Confirms Dark Energy 167

Posted by kdawson
from the glimmer-of-fur dept.
A new study of 86 galaxy clusters in the early universe has provided independent confirmation of the existence of dark energy. In its absence, gravity's pull should have caused the number of clusters to increase by a factor of 50 over the last 5.5 billion years. What is observed is a factor of 10 increase. "Together with earlier observations... the new data strengthen the suspicion — but do not prove — that dark energy is a weird antigravity called the cosmological constant that was hypothesized and then abandoned by Albert Einstein as a 'blunder' almost a century ago. If that is true, the universe is fated to empty itself out eventually, and all but the Milky Way's closest neighbors will eventually be out of sight. ... Adam Riess of Johns Hopkins and the Space Telescope Science Institute, said: 'If this was a fox hunt and dark energy was the fox, I think they have closed off another escape route. But there is still a lot of terrain left for the fox, and we've seen little more than a glimmer of fur.'"
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Galaxy Clusters' Stunted Growth Confirms Dark Energy

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  • by Anonymous Coward on Wednesday December 17, 2008 @02:35AM (#26142399)
    Galaxy Clusters' Stunted Growth Confirms Dark Energy

    "Together with earlier observations... the new data strengthen the suspicion â" but do not prove â" that dark energy is a weird antigravity called the cosmological constant that was hypothesized and then abandoned by Albert Einstein as a 'blunder' almost a century ago.

    Wait, what?
  • Re:Logic (Score:2, Informative)

    by Anonymous Coward on Wednesday December 17, 2008 @02:41AM (#26142425)

    Yeah, I often wonder about how I manage to breathe. There's all this stuff I can't see, and I'm not really sure it's really there.

    (Hint: Just because something doesn't interact with photons doesn't make it pseudo-scientific.)

  • by CodeBuster (516420) on Wednesday December 17, 2008 @02:49AM (#26142453)

    If that is true, the universe is fated to empty itself out eventually, and all but the Milky Way's closest neighbors will eventually be out of sight.

    Not only that, but depending upon the key value of state w, the ratio between dark energy pressure and its energy density, if the value of w is less than -1 then the universe will eventually be pulled apart as the rate of expansion begins to accelerate towards infinity. First the nearest galactic clusters will fade from view, then the nearest galaxies in our cluster, then the stars in our galaxy. Finally, approximately three months before the end, the solar system itself will become gravitationaly unbound, in the last minutes stars and planets will be torn apart, and finally, an instant before the end of everything individual atoms and their subatomic pieces will be ripped into ever smaller pieces until there is nothing left (i.e. the last bits just wink out of existence). The end, if it were to occur in this way, is around 50 billion years, or approximately 3.8 times the current known age of the universe, into the future. This hypothesis is known colloquially as the Big Rip [wikipedia.org].

  • Re:Logic (Score:3, Informative)

    by floodo1 (246910) <floodo1&garfias,org> on Wednesday December 17, 2008 @04:29AM (#26142869) Journal
    The part of logic that "necessitates" the existence of dark energy is inductive logic, which relies on inferences. Right now the evidence infers that dark matter exists, but as is the case with all inferences it could be wrong. Hence TFA stating that this discovery adds to the evidence but does not PROVE its existence (which would be deductive logic).

    In either case dark matter may not be necessary at all. This is because in logic necessary has a hugely different meaning than the way you used it :( To quote from wikipedia
    * possible if and only if it is not necessarily false (regardless of whether it actually is true or false);
    * necessary if and only if it is not possibly false;
    * contingent if and only if it is actually true (and so possibly true) and not necessarily true.


    So even if we can prove the existence of dark matter, we've only shown it to be contigent. We'd still have some heavy lifting to do to show that it cannot possibly NOT exist.
  • Link to full paper (Score:5, Informative)

    by Mwahaha (824185) on Wednesday December 17, 2008 @05:00AM (#26142997)
    For those interested the full paper is here [arxiv.org]. Apart from a couple of cosmological parameters they don't really improve previous estimates. It's still nice though that all the parameters agree very well with the previous (CMB + Supernova 1a) data with a completely independent method, hence the confirmation talk. I think though if there had been disagreement our understanding of clusters would have been blamed first. So in some senses this confirmed the current cluster models more than the cosmological constant, but that's not as 'sexy'!
  • by osu-neko (2604) on Wednesday December 17, 2008 @05:17AM (#26143073)

    Dictionary: confirm

    1. To support or establish the certainty or validity of; verify.
    2. To make firmer; strengthen

    See definition 2. Incidentally, in science, "confirm" always means 2. Certainty is impossible to establish using the scientific method. An experiment that produces the expected result confirms the theory, but certainly does not prove it.

  • Re:Fox Hunt? (Score:3, Informative)

    by Xest (935314) on Wednesday December 17, 2008 @05:20AM (#26143087)

    Seeing as fox hunting involves a bunch of extremely rich (inherited rich, never worked a minute in their life rich) people with a taste for animal blood riding horses around, sending a small army of dogs after a fox and ripping it to shreds just for the sake of it, I think your analogy is actually better.

    I'm not sure there are many rich physicists out there that ride horses round their labs wearing red jackets and joppers, nor am I sure how dogs would help track down dark matter but I am at least sure it's probably not a good idea to let a bunch of dogs try and rip some dark matter to shreds when we do find it.

  • by osu-neko (2604) on Wednesday December 17, 2008 @05:26AM (#26143107)

    The summary makes it sound like they actually proved that dark matter exist, not simply added to the inference of it's existence :(

    Science is not in the business of making provable claims. It's impossible to prove anything using the scientific method. Science makes falsifiable claims, and any experiment that fails to falsify them confirms the theory, but most certainly does not prove it. An experiment that "confirms" a theory is one that produces a result compatible with that theory under circumstances where a different result would have falsified it. Confirmation merely strengthens a theory, it cannot ever prove it.

  • by Anonymous Coward on Wednesday December 17, 2008 @07:45AM (#26143677)

    Let's not forget this man either:

    http://nobelprize.org/nobel_prizes/physics/laureates/1970/alfven-bio.html [nobelprize.org]

  • by ErkDemon (1202789) on Wednesday December 17, 2008 @08:52AM (#26143933) Homepage
    That timescale may not take into account the additional effects of the expansion on local rates of timeflow. You can have an extended version of the Hartle-Hawking bubble in which there's a larger conservation law between massenergy and spacetime, and the local rate of timeflow goes towards infinity as the background massenergy density goes towards zero (think of this as the inverse of the gravitational time dilation efect). [wikipedia.org]

    In that sort of model, the Hubble redshift is only proportional to the expansion ratio as a first approximation (whose range is roughly analogous to the range within the elastic limit of a spring).
    There then becomes an upper limit to the possible size of the universe, that corresponds to the total (finite) massenergy contained within it. As we approach that limit, things unravel. The resulting increase in atomic instability can then be expressed as an effect of decreased nominal inertial mass due to the reduced background field strength (nuclear stability is a function of inertia).

    But a decrease in local inertia also corresponds to an increase in the local rate of timeflow. The absolute end of the universe then represents a point in time where the nominal rate of timeflow is infinite (although, by then, there's nothing left to measure it with), so the period at which the universe nominally ends, measured in "insider-time", is in the infinitely far future. Okay, so its not quite infinitely far away, because the last proton evaporates at a finite time, but the timescale is effectively infinite to most intents and purposes, as far as we're concerned.

    The advantage of this form of time-scaling is that it tidies up the Hartle-Hawking model - it allows the "equator" of the H-H bubble to represent the apparent end of the universe for insiders, and to be totally smooth. This removes the messiness that we'd otherwise tend to get when the bubble reaches its maximum size and parts of it start to contract. Contraction implies reversed entropic timeflow, so the HH bubble has a problem in that an observer living through the expansion-contraction region might see some mightily strange things going on. Some regions might be seen to be ageing in opposite directions to others. But if the interior rate of timeflow goes to infinity at the equator (as the angle of "proper" time approaches the angle of axial time, and its angle with the radial time-parameter 'a' tends to 90 degrees), then interior detail is totally erased at the equator, and the apparent inconsistencies with observerspace physics disappear ... you can never survive a transition past the equator, and the event-meshes of each hemisphere are isolated from each other by the equatorial evaporation zone.

    The expansion and contraction phases of the bubble then both effectively belong to two separate universes, both of which think they're expanding, and both with opposite senses of proper time. The equatorial evaporation zone keeps both sets of causalities isolated, and prevents nasty messy phase transitions where the two "worlds" collide.

    If we look at the geometry of one hemisphere of the extended H-H bubble model, and we use axial time as our reference, or we take a tangent to a given zone and extend that zone's local sense of proper time as as a straight line to give us our time-reference for the rest of the bubble, then what we end up with is a description that seems to describe a "Big Rip" at a definite, finite time. Our projection tells us that the universe contents speed up and start to "fizz and whizz" at an increasing rate before finally disappearing altogether. But to physics performed inside that universe, things aren't hotting up, they're cooling down -- instead of matter mysteriously evaporating after few billion years, it's decaying more conventionally over rather vaster timescales.

    Cosmological timescales and reference systems

    The thing one has to be careful o

  • Re:blunder (Score:5, Informative)

    by ErkDemon (1202789) on Wednesday December 17, 2008 @09:47AM (#26144315) Homepage
    No, the earlier poster is right, as far as theory is concerned, Einstein's "Cosmological Constant" and "Dark Energy" perform different functions.

    Einstein invented his "repulsive" effect to explain why the universe was static, and neither expanding or contracting. Unfortunately for Einstein, Hubble's redshift observations a few years later indicated that the "static" property of the universe that Einstein's CC had been invented to reproduce within GR, wasn't correct.

    Dark energy was invented to explain why, when we take an expanding universe model decribed with general relativity, and try to compare it with reality, the numbers still don't appear to match up with the theory.

    ----

    Einstein's Cosmological Constant was an attempt to force GR to produce a wrong answer that Einstein (at that time) happened to think was a mathematically elegant one. The system seemed to describe a universe that would have to be expanding or contracting, and Einstein said ... "Well we know that THAT has to be wrong, so to make things nice and static, I'll write in an additional term for a necessary effect that I've just made up, that would exactly cancel the large-scale effect of gravity ... "

    The motivation, function, and results for the two hypothesised effects are different. Both effects are repulsive, and both of them are essentially "made up" as accounting fudges without any deeper physical or philosophical justification, to force a theory that generates one result to generate a different result that we like better, but that's about all they have in common.

    They're really different animals. Dark energy isn't an effect designed to explain why the universe is static. However, if you're inventing an arbitrary effect to bring your theory into line with experiment, the awkwardness of admitting that you're basically making stuff up to force the answer you want is reduced if you can claim some "provenance" for the idea, and present your "new" effect as if it's a logical historical development of an earlier idea by a Famous Physicist. That adds an air of legitimacy.

    But if we think that the DE idea is any good, then the idea that DE is a historical extension of Einstein's CC is phoney. Einstein's CC is dead and buried. The only way that DE might turn out to be able to claim descent would be if DE turns out to be a rotten idea too, in which case we could say that there's a common theme running through both bad ideas. :)

    But if the Dark Energy idea is good, then it's really not "bringing back Einstein's cosmological constant in revised form".

  • Re:blunder (Score:1, Informative)

    by Anonymous Coward on Wednesday December 17, 2008 @11:26AM (#26145555)

    You claim is not correct. The Greek atomic theory attributed to Democritus and his peers was more than metaphysical speculation. It was based on observations that matter could be divided, rings would wear from fingers and food could be smelled from a distance. Therefore stuff must be made of really small parts. That was the gist of their "theory". It's not as sophisticated as quantum mechanics but it's not purely metaphysical speculation either. By your argument all of Ancient Greek science was just metaphysical speculation.

  • by Anonymous Coward on Wednesday December 17, 2008 @01:24PM (#26147687)

    The thing one has to be careful of with cosmological descriptions is that they often use geometrically-convenient projective timescales that doesn't necessarily correspond (even approximately) to actual elapsed time, except over small regions.

    You've written quite a long-winded screed, but the problem with it is that astronomers usually do use proper cosmological time — i.e., the actual elapsed time — not coordinate time. Yes, sometimes it's easier to write down the metric in some kind of projective coordinates, but when people talk about "X years ago" or "Y years after the Big Bang", they almost always convert from coordinate to elapsed proper time. In particular, the Big Rip scenario uses a FLRW metric (albeit with an odd equation of state), and in the conventional FLRW coordinates the 't' coordinate is proper time, so no conversion is necessary.

    None of this has anything in particular to do with Hartle-Hawking quantum cosmology, by the way. It just has to do with coordinate time vs. proper time in ordinary general relativity.

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