Timeline for How Does Hubble's Expansion Affect Two Rope-Tied Galaxies?
Current License: CC BY-SA 2.5
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| Feb 4, 2011 at 7:28 | history | edited | Luboš Motl | CC BY-SA 2.5 |
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| Jan 16, 2011 at 20:35 | comment | added | Luboš Motl | But you can't see this curvature in a thin enough rope. After all, the internal geometry of any thin enough stretched cylinder is indistinguishable from a thin cylinder in a flat space, up to second-order terms. When it comes to pairs of galaxies only, there's no difference between the expansion of the Universe and a simple increase of distance between 2 objects. You just can't say which of the galaxies is "at rest" - but this is a basic principle of relativity, even special relativity. All galaxies are on equal footing but their relative distances still grow. | |
| Jan 16, 2011 at 20:32 | comment | added | Luboš Motl | Hi EdP, thanks for your question. Well, I would say it's a drag between two galaxies, not "space" and a galaxy, but maybe I still misunderstand your actual point. If you only cut 2 somewhat distant galaxies and a relatively thin cylinder around the rope, and you may extend this cylinder in time, you get a piece of spacetime that is totally identical to the case when the two galaxies are receding from each other in an ordinary flat space because of their different initial velocities. It's only the rest of the spacetime outside the cylinder that is "curved". | |
| Jan 16, 2011 at 19:51 | comment | added | EdP | Thanks, Lubos, for the answer. This is a conceptual rope... and my interest is really how the expanding space makes the galaxies move. My basic textbook in astronomy uses raisin-bread analogy to explain Hubble's Law; clearly, if the raisins were tied with a string before baking, the distance between them would not increase. This analogy implies a drag between the dough and raisins, so is there a drag between space and matter? | |
| Jan 16, 2011 at 19:43 | history | answered | Luboš Motl | CC BY-SA 2.5 |