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posted by Dopefish on Sunday February 23 2014, @08:00AM   Printer-friendly
from the where's-carl-sagan-when-you-need-him? dept.

pjbgravely writes:

"Scientists use gravity lensing measurements to determine mass of galaxy clusters. Anja von der Linden, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California, is using the Subaru telescope and the Canada-France-Hawaii telescope, both on Mauna Kea in Hawaii. The results are 40% higher than the measurements done by the Planck collaboration.

I guess there goes Douglas Adams' theory that the missing mass was in the packing material of the scientists' equipment."

 
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  • (Score: 5, Informative) by kebes on Sunday February 23 2014, @02:58PM

    by kebes (1505) on Sunday February 23 2014, @02:58PM (#5187)
    The science of dark matter is indeed not settled (in particular, even if our measurements of its distribution are correct, we don't know what it is made of; e.g. what particle(s) are responsible). Although nothing in science can ever be considered 'settled', the evidence for dark matter is actually quite strong.

    With respect to the counter-evidence, I should note that the MIT Technology Review [technologyreview.com] article is flawed. It basically says that measurements of Dark Matter influence on the orbits of the planets in our solar system are negligible, and then claims that this is a blow against dark matter as an explanation. However, if you calculate the magnitude of the effect we expect [scienceblogs.com], it is indeed small and would be nearly impossible to measure at the scale of the solar system. The density of dark matter that we measure (by motion of galaxies, fitting the CMB [scienceblogs.com], etc.) is really quite low (at this point in the history of the universe). It adds up to large amount of mass over large-scales, but it's highly diffuse and so imperceptible over short distances.

    Alternative explanations (MOND [wikipedia.org]) also do not fit [scienceblogs.com] as much of the data as the dark matter explanation. (Sorry to keep linking the same blog-author; but he does a good job of summarizing the state-of-the-art in astrophysics.) Of course, it's possible that with further study, an alternative gravitational theory that fits the data will be found. But one has not yet been found.

    There's always the old equivalence principle problem... We only ASSUME that the laws of physics we observe locally, apply to the rest of the universe as well.

    This is indeed a central philosophical issue in science more broadly. And difficult to ever settle. In the case of modern cosmology, our best theories (which include dark matter) are able to very accurately fit data from the very early universe (the Cosmic Microwave Background [wikipedia.org]) under the assumption that the laws of physics are the same at those times as they are now. This gives us confidence that we're on the right track...

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  • (Score: 3, Interesting) by evilviper on Monday February 24 2014, @12:42AM

    by evilviper (1760) on Monday February 24 2014, @12:42AM (#5400) Homepage Journal

    In the case of modern cosmology, our best theories (which include dark matter) are able to very accurately fit data

    Well OF COURSE it fits observations. It was a theory created to fit the discrepancy between accepted theories and actual observations. That just shows the discrepancy is uniform, but not really proof of anything by itself.

    I wasn't trying to suggest MOND or some other existing theory is accurate, just that dark matter and dark energy are a "fudge factor" without a firm theoretical basis, nor any evidence or observations seen on a local scale.

    I'm not opposed to it being found and quantified, but in the mean time, it's just a bit of a stretch to assume that a fudge factor is a real, physical object floating around in space, even if it might be.

    --
    Hydrogen cyanide is a delicious and necessary part of the human diet.
    • (Score: 1) by khchung on Monday February 24 2014, @01:52AM

      by khchung (457) on Monday February 24 2014, @01:52AM (#5435)

      In the case of modern cosmology, our best theories (which include dark matter) are able to very accurately fit data

      Well OF COURSE it fits observations. It was a theory created to fit the discrepancy between accepted theories and actual observations. That just shows the discrepancy is uniform, but not really proof of anything by itself.

      You wrote this as if our current theories are just a bunch of curve-fitting functions with a whole bunch of free parameters we can tune. The truth cannot be farther than that!

      What makes the current physical theories "beautiful" is the fact that these theories often start with very simple and clear premises, and then much of everything flows logically from them, with almost no room for tuning or "fudging".

      Take Special and General Relativity as example, the premises "Every observer is the same", "the speed of light is constant for everyone", "you cannot distinguish between an accelerating lift and a lift in gravity", etc, are all either measurable, or would make the universe quite incomprehensible if wrong ("every observer is the same"). Then you work out the math and got the equation, and if observation did NOT fit, BAM! the theory is wrong.

      If the precession of Mercury was not exactly what GR predicted, there is nothing else you can play with without causing it to fail in other areas.

      Sure, you can postulate that it was GR that was wrong instead of the existence of some dark matter. But until you can come up with another theory of gravity that worked as well as GR AND do away with dark matter, then having one workable theory (GR) + one unknown (dark matter) seems better than having no workable theory at all.

      • (Score: 1) by evilviper on Monday February 24 2014, @12:38PM

        by evilviper (1760) on Monday February 24 2014, @12:38PM (#5759) Homepage Journal

        the premises "Every observer is the same", "the speed of light is constant for everyone", "you cannot distinguish between an accelerating lift and a lift in gravity", etc, are all either measurable, or would make the universe quite incomprehensible if wrong

        It doesn't have to invalidate the entire theoretical basis of GR/SR, just because the equations need a fudge factor to work. Just consider the Cosmological Constant... Or consider how it falls apart with black holes or other quantum phenomena.

        But until you can come up with another theory of gravity that worked as well as GR AND do away with dark matter, then having one workable theory (GR) + one unknown (dark matter) seems better than having no workable theory at all.

        It's not an either-or. You can keep using it, but you should acknowledge that there seems to be something "wrong", rather than acting like it's an entirely solved problem. Think of Newtonian mechanics before GR... Nobody invented exotic new theoretical types of matter to make the equations match more closely than they do.

        Better yet, just TRY to consider the possibility that in the next couple decades there will be numerous experiments trying to identify and study dark matter / energy, and it's possible they'll all simply come back with a negative result. Then you still have numbers that work, but without the conceit that they might be theoretically sound.

        --
        Hydrogen cyanide is a delicious and necessary part of the human diet.