Wednesday, October 05, 2016

A Natural Law for Rotating Galaxies… What Does This Mean for Dark Matter?

Distant galaxies, black holes, exotics worlds…these are not just the stuff of science fiction; they are also the stuff that makes up our reality. Our quest to understand the universe is thrilling, challenging, and often confusing. Even the basic question “What is the universe made of?” isn’t easy to answer.

Adding new pieces to this puzzle, researchers from Case Western Reserve University and the University of Oregon recently discovered a natural law for rotating galaxies. Their surprising result is the subject of a new paper coming out later this week in Physical Review Letters that could have serious implications for the theory of dark matter.

An image of the nearby galaxy Messier 81, created with data from NASA's Spitzer Space Telescope.
Image Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics). More information.
The planets in our solar system rotate around the sun because of gravity. In the early 1600s, astronomer Johannes Kepler worked out a mathematical relationship between a planet’s distance from the sun and its orbital period, the time it takes to go completely around the sun. This is known as Kepler’s third law of planetary motion, and it reveals that planets farther from the sun travel around it with slower speeds. Isaac Newton later generalized Kepler’s law for all orbital systems.

There’s a problem though. The law doesn’t appear to hold for stars orbiting the center of their galaxy. Astronomer Vera Rubin highlighted this discrepancy in the 1970s and since then, astronomers have verified time and again that stars on the outskirts of a galaxy rotate around the center at the same speed as stars closer the center. This result gave rise to the idea that galaxies have a halo of dark matter. If this is the case, stars do follow Newton’s law once the missing mass, made up of dark matter, is accounted for.

In this new research, work by astronomers Stacy McGaugh and Federico Lelli (Case Western) and Jim Schombert (Oregon) reveals that galaxies obey a natural law just like planets. It’s just not the same one.

The missing mass problem is the result of a difference in how we expect stars within a galaxy to behave and the way that they actually behave. Instead of assuming that these two should match, McGaugh, Lelli, and Schombert compared the observed orbital speed of stars within a galaxy (described by their radial velocity) to the predicted value that assumes what you see is what you get—no dark matter. Then they looked at the relationship between these results for a large number of galaxies.

Their work relied on data from NASA’s Spitzer Space Telescope, a space-based observatory that studies objects in infrared light. Over the years Spitzer has observed many types of galaxies, collecting information on how gas and stars are distributed within them. This information can be used to estimate the visible mass of a galaxy, which can be used to predict the radial velocity of its stars. Using 2,693 data points from 153 very different galaxies, the researchers modeled the radial velocity based on the visible mass of each galaxy.

Next, the scientists graphed these results against observed, published results of radial velocity for the same galaxies. When they got to that point, says McGaugh, “There was the result on the screen: Far better than we had ever hoped. It just fell right out.” There was a clear mathematical relationship between the observed radial velocity of a star in a galaxy to the distribution of visible matter.

If dark matter ultimately dominates the mass of galaxies, as many scientists think, then there’s no reason to assume that the amount of visible matter should be directly related to the amount of dark matter. But that’s just what this work implies. If two galaxies with very different properties have the same distribution of visible mass, the observed radial acceleration is the same. If you know one, you know the other. Not only is this surprising, it’s a big deal.

The authors suggest that their result can be explained in three ways.

  1. It represents the end product of galaxy formation.
  2. It represents new "dark sector" physics.
  3. It is the result of new dynamical laws rather than dark matter.

None of these explanations are completely satisfying, say the authors. Their paper doesn’t go into the details, but it falls right in the middle of an ongoing discussion about the validity of the dark matter model. This work demonstrates a regularity in the data, says McGaugh. “How you interpret that is a whole 'nother can o' worms.” There’s no doubt that scientists will be elbow deep in that can before long.

Kendra Redmond

17 comments:

  1. This doesn't say anything about dark matter. If the dark matter is sufficient to hold the galaxy together it is also sufficient to hold the location of the stars in their galaxy in a repeatable way, making these observations just another effect of dark matter.

    Dark matter has also been successfully imaged with gravitational lensing and that fact can't just be discounted.

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  2. Natural law definition: http://plato.stanford.edu/entries/natural-law-theories/
    The link between “natural law” and the physical laws of nature.
    http://www.westernfreepress.com/2016/03/13/scientific-proof-of-the-existence-of-natural-rights-found-in-the-constructal-law/

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  3. I am wondering whether the discrepancy between the gravitational laws observed for our solar system and for galaxies is different because of the distances involved. If gravitational effects occur instantly between tow bodies one would not expect to see differences based on distance. But if there is a lag in effect, even if very tiny, based on distance, one wonders if this could account for the difference. Is there a speed of gravity? Is there a very tiny delay in the manifestation of gravitational effects related to distance that would be unnoticeable because it is too small for the solar system, but would be noticeable in galaxies? This might be an analogy to relativistic effects which cause noticeable differences in Newtonian laws but only at large masses and velocities close to the speed of light.

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    1. tl;dr gravity does indeed have a speed, which is the speed of light.

      If we look at Newton's theory of gravity, the instant that the gravitational force is "cut off" or removed, the Earth would react instantly, we would no longer see light and we would be flung out. This instantaneous reaction is not what we observe.

      Rather, the answer lies in Einstein's theory of general relativity and an understanding of the speed of light. The term "speed of light" actually is the upper threshold of how fast information in general, not just light, can move. When we imagine spacetime, and this may be a little crude, the sun creates an "indentation" in spacetime, upon which the Earth travels. If the sun were to disappear, spacetime would return to being relatively flat. That change could be considered information.

      With general relativity, after 8.5 minutes (the time required for information to reach us from the sun), not only would we no longer see the sun (obviously) but we would be flung outwards tangential to our orbit. As per Einstein's theory of general relativity, the "speed of gravity" is the speed of light.

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    2. An interesting thought, but unlikely. I have a paper in submission showing the effect increases inversely to the mass (and hence size) of the galaxy; i.e. the smaller the galaxy (and distances) the greater the effect.

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  4. Dark matter fills 'empty' space. Dark matter strongly interacts with and is displaced by matter.

    What physicists mistake for the concentration of dark matter is the state of displacement of the dark matter.

    [0903.3802] The Milky Way's dark matter halo appears to be lopsided

    "the emerging picture of the dark matter halo of the Milky Way is dominantly lopsided in nature."

    The Milky Way's halo is not a clump of dark matter traveling along with the Milky Way. The Milky Way's halo is lopsided due to the matter in the Milky Way moving through and displacing the dark matter, analogous to a submarine moving through and displacing the water.

    What ripples when galaxy clusters collide is what waves in a double slit experiment, the strongly interacting dark matter which fills 'empty' space.

    Dark matter displaced by matter relates general relativity and quantum mechanics.

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  5. The implication is obvious. If dark matter exists at all, it must (1) either stick to normal matter, or (2) be generated by normal matter. The first could be light particles that are physically large, possibly pairs (or clusters) of bound tachyons (bound by a force that only affects tachyons) that also possess a charge that lets them stick to baryonic or electron matter. The second could arise from many worlds theory. If alternate versions of history exist in multiple time dimensions, they would occupy approximately the same space and there could be a weak gravitational interaction between alternate realities. Normal matter would appear gravitationally "smeared out" over a volume of many cubic light years, and have several times its own point gravity in total.

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  6. The gravity that the Solar system feels, is not only the centre of gravity of the galaxy but the gravity from all 200 billion stars and 200 billion stars worth of gas and dust. The galaxy is not rotating about a central point of gravity, but feel the gravitation pull of all the mass.

    Secondly, does not "time" have to be taken into account. The gravity that our solar system feels today, is the gravitation pull of the stars on the far side of the galaxy, 150,000 years ago. Where were they 150,000 years ago. Where was the central mass, 50,000 years ago.

    I don't think all of this has been taken into account properly.

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  7. I'm not a scientist, and if I played one on TV you would change the channel, but I have a theory about gravity that seems to fit in with explanation three:

    Maybe the force of gravity varies according to the density of the electromagnetic fields through which gravity works.

    As I understand it, gravity affects the path of electromagnetic radiation -- gravity bends the spacetime through which light travels, and this "bends" light.

    Is it possible that light in turn affects gravity? Do intense electromagnetic fields somehow strengthen gravity?


    Maybe, in the heart of a galaxy, where electromagnetic fields are most dense, the force of gravity is most powerful. Maybe the force of gravity is least powerful in deep, "empty" space, between galaxies that are moving away from each other -- where the fields of electromagnetic radiation are least dense.

    This is the third article I have read recently that calls into question the existence of dark matter. Maybe there really is no dark matter holding the spinning galaxies together. And maybe there is no dark energy pushing galaxies away from each other.

    Is this a testable hypothesis: The gravitational attraction between kilogram A and kilogram B, whose centers are ten meters apart, varies according to the density of the electromagnetic fields between them.

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  8. This is fascinating work to me and I will follow it. One thing I have never understood is how increasing the mass of the galaxy through dark matter would compensate for the orbital velocities of stars. In our own solar system (which the article does clarify is apparently not a good correlation to galactic orbits), the mass of an object is irrelevant to its orbital period. It is the object's radius from the star that is the defining factor.

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  9. the things we don't see, are symmetrical spiral systems with a missing arm.
    such a structure could exist if there were dark-mater; ea 96% is believed to be dark matter so a structure that doesn't contain visible matter (didn't catch) would have a high probability. In the great absence of such structures, i find it hard to believe dark matter exists. It could as well be an delay efect of mass distribution in the quantum 'foam'. virtual particles pop in and out of reality all the time (also creating particles from energy does cost a lot of energy) so i'm thinking more that this is a answer for dark energy and matter. More a flaw in the way how we think empty space behaves. Even empty space causes (mass) drag, as in a quantumworld space cannt be empty.

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  10. @Scott Manley -- the relevant mass for stars or gas clouds orbiting in a galaxy is the total mass inside their orbit. (In the solar system, the total mass inside a planet's orbit is that of the Sun plus all the other planets inside its orbit... which basically means just the Sun.) If you add up the mass of all the visible stars, gas, etc., inside the orbit of a particular star or gas cloud, it's not enough: the star/cloud is moving too fast, and should have escaped from the galaxy long ago. But if there's extra mass inside (i.e., dark matter), then it works out.

    One thing this article does not discuss is all the evidence for dark matter on larger scales. E.g., you need *extra* dark matter in groups and clusters of galaxies (in addition to the dark matter inside the galaxies themselves) to explain why the groups and clusters haven't flown apart long ago.

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  11. @Steve Maricic --
    Is it possible that light in turn affects gravity? Do intense electromagnetic fields somehow strengthen gravity?

    Yes, indeed. This is a well-understood part of General Relativity: energy in the form of light does have its own gravitational effect.

    However, it's a very weak effect unless the light is incredibly intense. In the inner regions of really massive stars, where the radiation is incredibly intense, this can actually have an effect on the star's gravitational stability. And for the first 45,000 or 50,000 years after the Big Bang, the gravitational effect of radiation was stronger than that of matter. But in the current era of the universe, the gravitational effect of light is almost effectively nil.

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  12. How can the fourth state of matter be so easily overlooked? It's plasma that makes up the so-called missing matter. The universe is chalk full of the stuff. It is unseen when not in glo-mode, but not being seen doesn't mean its not there. It's certainly more plausible than some mysterious unseen 'dark matter' that can never be observed, measured or falsified. But when gravity is your hammer everything looks like a nail.

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  13. As a science fiction author I like to try to understand what is going on in the universe.

    If I needed an explanation for this phenomenon I would have space time being dragged around the centre of the galaxy thus causing the stars to be dragged with it thus causing the orbital velocity to not be as predicted.

    Sadly it is probably not the answer.

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    1. Tony,
      Interestingly enough, "Frame Dragging" IS one of the phenomena that falls out of General Relativity. However, it's a relatively well-characterized one, and not enough to explain the observations currently attributed to dark matter.
      https://en.wikipedia.org/wiki/Frame-dragging

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  14. Spacetime? The Church of Einstein and their dogma. Do you believe? Do you have the faith?

    Space and time are concepts, space is a location in three dimensions, it is a non-physical construct. Time is a process, it is change and it too has no physicality. So, how can you take two non-physical concepts and weld them together to form a physical concept? You can't and there-in lies the mythology of GR for there is no such thing as the fabric of spacetime.

    Ahh, but Einstein needed something to replace the aether that was intrinsic to the competing theories of the day preferred by the likes of Maxwell, Tesla and some of the other electrical geniuses upon whose work formed the foundation for most of our technology today.

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