Later On

A blog written for those whose interests more or less match mine.

Maybe it’s not dark matter, just gravity leaking in

with 2 comments

Interesting idea:

The visible stars and other bodies do not move in the manner we would expect, given the gravitational pull of the matter visible to us. This has led to the theory of “dark matter” as the source of the remaining gravity, even though we don’t know what this dark matter is, and cannot even detect it beyond its gravitational effects on the celestial motions of visible bodies.

Perhaps it is not detectable because it is not there, at least not in this universe. When discussing extra-dimensional travel and its consequences on time travel (20 May, p 34), it was mentioned that in string theory the graviton, the hypothetical carrier of the gravitational force, is represented by a closed loop. As such it is not bound to this “brane” and is free to leave and travel into the 10-dimensional “bulk”. This is suggested as the reason why gravity is so weak compared to other forces: it leaks away out of this universe.

If gravity can leak out of this brane, then why shouldn’t it be able to leak in as well? If so, the dark matter which holds galaxies together could in fact be galaxies (or some other collections of mass) in other, neighbouring branes from which gravity is leaking into our own. This would explain why dark matter is undetectable and has a consistent pattern to its distribution.

But do we need dark matter at all? Maybe general relativity explains it all.

UPDATE: That last link references a paper that has serious problems. So we do need dark matter—or some other explanation—after all.

Written by LeisureGuy

10 June 2006 at 5:01 pm

Posted in Science

2 Responses

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  1. I’m pretty sure Einstein would say “God abhors dark matter!”
    I don’t like it either when new fudge factors are introduced everytime a measurement doesn’t accord with theory.

    The good thing about Cooperstock and Tieu is they got within spitting distance of explaining Galactic Rotation Curves whereas the Dark matter theory needed to increase the mass of the Universe by a factor of 10 to get therer.
    They should just try another metric without the non-analytic |z| quantity and they may get closer.

    On the otherhand, we need that extra 10 in mass to explain the expansion rate; so we would ideally like a single cosmological theory that explains both (accelerating) expansion and galactic rotation anomalies.

    I am working very much along the lines of Cooperstock and Tieu but I start out this way: Current estimates of the size and mass of the Universe place it entirely within its own Schwartzchild radius. That means we are in a gigantic black hole. The implications of this, namely the reversal of roles of time and the radial dimesnion, have not been adequately considered. I find it gives a perfect expalantion of initial inflation, current expansion rates, and accerlerating expansion leading to a final Big Rip as we hit the central singularity in perhaps 10 billion years. In effect, a Robertson Walker metric arises as an in-vacuo solution of Einstein’s equation, when the time/radius role reversal is factored in.

    Now, orbits follow geodesics of the spacetime, and these are calculable purely from the metric. So they will be the same as if the universe was filled with a perfect fluid, even though it’s not (because the Robertson Walker metric is that of a fluid-filled universe). Therefore we have to use Cooperstock and Tieu’s approach to solve Einstein’s equation for rotation curves given a galactic matter distribution AND a uniform homogenous filling of virtual fluid (non-rotating) on top. Haven’t done it yet, but am working on it.

    So, I postulate that the “Dark Matter” may be a “virtual” fluid, really only a mathematical equivalence to a real fluid, but it would actually be caused by real matter that is between us and the central singularity (or already there) and exerting its gravitational influence to influence to create an apparent RW metric as the interior metric of a black hole.. Here, “between us and the cemtral singularity” means it is adisplaced head of us in time, and that’s why we detect no interactions with it.

    Yet one more candidate theory. But at least no “exotix matter”! !

    Paul Dent

    19 April 2010 at 10:56 pm

  2. I have since solved the equations that Schwarzschild first solved to get the Schwartzschild metric, but in a virtual fluid filled universe. Namely, I derive the spherically-symmetric solution of Einstien’s equation with a non-zero matter stress-energy tensor on the right hand side. Then I take that metric and work out the geodesics for orbits. It definitely shows different galactic rotation curves, but the effect is now too extreme. The stars at the edge really fly off the handle!!
    By the way, excess galactic rotation equates to excess gravitational lensing. Both phenomena are based on geodesics that curve more severely than the visible mass would suggest. So you explain one, and you explain the other.

    By reducing the vitual fluid density, my solution could possibly be tamed, i.e. with the assumption that the universe was not 10 times more massive than its visible mass, but something less than ten times. However, I think the real problem is that we still don’t have the mathematical tools to properly predict the dynamics of a galaxy, because every method ignores some factor that probably cannot be ignored on a galactic scale. What I have ignored is the “drag effect” on one star of all the other orbiting stars. In my case, I need it to be a “braking” effect on the outer stars. If all the other stars were rotating with the same angular velocity, we could transform to a set of rotatating cordinates and then solve for the metric in these new cordinates.
    That has been done before but is invalid because not all rotating at the same speed, and anyway using cylnindrical rotation is invalid because it implies that stars out of the galactic plane would not be not be orbiting about the center of gravity. So it would only be valid for truly disc-like galaxies.

    Another approach I investigated was what happens to orbits if the gravitating mass reduces with time.
    I have found at least one paper by Peter D. Noerdlinger on this subject.
    Keplers law states that the mass swept out by the orbital radius is a constant i.e. w,r^2 = K
    But K depends on the gravitating mass holding the object in orbit; at least, it depends on the initial mass, but therafter it does not change even if the mass is radiated away to zero. At first I thought “Here is part of the answer. The rotation of stars around galaxies is in accodance with what the visible mass once was, not what the visible mass is today.” However, the interesting thing is that the orbital radius increases wtih reduction of mass such that the product Mr is constant. That means the w.r^2 that we would calculate based on today’s mass being the intial mass and never changing is exactly the same as we would have gotten with the initial mass and the initial radius. So God has conspired to conceal from us the history of the mass loss of galaxies.

    On a positive note, when the sun has lsot half its mass and become a red giant with radius of about where we are now, say 100 million miles, the earth will have moved out to 186 million miles, so we will be about as far away as we were before. Some further calculations may show we have a chance of surviving that. If not us, then Mars might be habitable at 242 million miles.

    Paul

    14 May 2012 at 8:10 pm


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