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Thread: Why does the galactic rotation curve start with low velocity

  1. #1 Why does the galactic rotation curve start with low velocity 
    Forum Freshman GreatBigBore's Avatar
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    Here is a picture of what I'm talking about. I've seen this many times and never been able to figure it out: close to the center of the galaxy, the stars are moving much more slowly than the stars further away. When people talk about dark matter, they always say that the rotation curve is flat. But it's not; it's flat after you get some distance away from the center. Seems like the stars closest to the center would have to have extremely high velocity to avoid falling into the center. Why would they move far more slowly than the stars further away from the center?


    The most exciting phrase to hear in science is not “Eureka” but “That’s funny...” -- Isaac Asimov
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  3. #2 Re: Why does the galactic rotation curve start with low velo 
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by GreatBigBore
    Here is a picture of what I'm talking about. I've seen this many times and never been able to figure it out: close to the center of the galaxy, the stars are moving much more slowly than the stars further away. When people talk about dark matter, they always say that the rotation curve is flat. But it's not; it's flat after you get some distance away from the center. Seems like the stars closest to the center would have to have extremely high velocity to avoid falling into the center. Why would they move far more slowly than the stars further away from the center?
    The velocity on orbits of Keplerian motion depends on the mass inside the orbit. In a planetary system like ours, about 99% of the total mass is in the central star. Therefore, the planetary orbits very well follow Newtonian mechanics of point-like masses. This is different in Galaxies. Here, the mass is distributed across the entire disk of a galactic spiral. So the relevant mass for inner orbits is much smaller than for outer orbits. This is the reason, why the orbital velocities close to the centre are comparably small. For an even distribution of mass, there is a radius, where the velocities should reach a maximum value and then slowly decline to the edge. But this is not observed for most Galaxies. This is why the Dark Matter was invented that should produce the additional mass that keeps the velocities up even at large radii.


    Figure 1: Orbital velocities due to Keplerian motion in the solar system. More than 99% of the entire mass is inside the sun.


    Figure 2: Orbital velocities due to Keplerian motion in Galaxies. The mass is broadly distributed across the galactic disks. "Dark Matter" was invented to explain the deviation from the theoretical behaviour.


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  4. #3 Re: Why does the galactic rotation curve start with low velo 
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    Quote Originally Posted by Dishmaster
    "Dark Matter" was invented to explain the deviation from the theoretical behaviour.
    You nailed it there. It was INVENTED to make something that doesn't work fit into what we think we know.
    Here is a postulation on the curvature of galaxies. (Not the graph curve spoken of in this thread). Could the apparent pinwheel effect of most galaxies be due to the speed of gravity? Gravity attracts in a linear direction but it takes time for it to travel and objects are not in the same place as when the gravity started pulling on it so it has a curving effect. That would make the shape of galaxies a snapshot of what gravity looks like. The ol' iron filings on a magnet trick.
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  5. #4 Re: Why does the galactic rotation curve start with low velo 
    Forum Freshman GreatBigBore's Avatar
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    Quote Originally Posted by Megadork
    It was INVENTED to make something that doesn't work fit into what we think we know.
    I wouldn't put it exactly like that. It definitely is an invention, a fiction, and everyone knows it. It's not like it's a secret or a conspiracy. Everyone knows that "dark matter" and "dark energy" are placeholders until we reach a deeper understanding. The word "inertia" is similar. We don't really know what that is, either. As for "what we think we know," again, everyone knows that what we call knowledge is subject to revision whenever new data becomes available. It's not like we're the ancient Catholic church, arrogantly saying that we know everything because God told us.

    Quote Originally Posted by Megadork
    Here is a postulation on the curvature of galaxies. (Not the graph curve spoken of in this thread). Could the apparent pinwheel effect of most galaxies be due to the speed of gravity? Gravity attracts in a linear direction but it takes time for it to travel and objects are not in the same place as when the gravity started pulling on it so it has a curving effect. That would make the shape of galaxies a snapshot of what gravity looks like. The ol' iron filings on a magnet trick.
    I wish I knew more about cosmology, because this sounds like a fascinating idea. But with thousands of observers and theorists studying these issues every day, one might assume that if it were a promising idea, someone specially trained in the subject would have thought of it by now. It's rather rare for big scientific advances to come from the laity.
    The most exciting phrase to hear in science is not “Eureka” but “That’s funny...” -- Isaac Asimov
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