|
|
Thread Tools | Display Modes |
#41
|
|||
|
|||
Black hole mass-sigma correlation
|
#42
|
|||
|
|||
Black hole mass-sigma correlation
In article , Ulf Torkelsson
wrote: The reasoning I gave would assume that in the very large scale, the universe is not homogenous. That may be a valid assumption, but according to existing observations it is fairly homogeneous on the largests scales that have been studied. The formula you gave, GM/(c^2 r), would give hints on how big a cosmological blob might become, as it should set a limit as GR forces grow strong. One would have to look beyond that limit to tell whether there is something beyond our local blob. That might be the case if the universe expansion is accelerating in later times (alternatively slowing down at large distances from us). If the acceleration rate is the same in all directions from us, one could not still assume that the universe is homogenous, as it might mean that we are in the center. It might mean, but that would put us in a very special position, which most of us would consider unlikely. If the expansion acceleration rate is the same in all directions. Are there observations confirming that? I have a vague memory that the cosmological principle you mention, that the universe looks the same everywhere was based on a homogenously growing (non-accelerating) universe. When so there is this acceleration phenomenon, one should be somewhat cautious about how that principle is carried over. No, the cosmological principle does not rely on any assumption about the time-dependence of the expansion. It does require that the expansion progresses at the same rate everywhere at the same time, but the time-dependence could be anything that could be generated by an equation of state and a dynamical theory such as GR. This is a problem, we do not know what the universe looks now, only what it looked like when the light was sent out. So I see how you can push through the argument by assuming that the universe we do not observe is uniform at any given time, but it accelerates with time, explaining what we actually can see. (Actually, I was well aware of this argument since before, but overlooked it as I was thinking too much on other explanations :-).) Hans Aberg |
#43
|
|||
|
|||
Black hole mass-sigma correlation
In article , Ulf Torkelsson
wrote: The reasoning I gave would assume that in the very large scale, the universe is not homogenous. That may be a valid assumption, but according to existing observations it is fairly homogeneous on the largests scales that have been studied. The formula you gave, GM/(c^2 r), would give hints on how big a cosmological blob might become, as it should set a limit as GR forces grow strong. One would have to look beyond that limit to tell whether there is something beyond our local blob. That might be the case if the universe expansion is accelerating in later times (alternatively slowing down at large distances from us). If the acceleration rate is the same in all directions from us, one could not still assume that the universe is homogenous, as it might mean that we are in the center. It might mean, but that would put us in a very special position, which most of us would consider unlikely. If the expansion acceleration rate is the same in all directions. Are there observations confirming that? I have a vague memory that the cosmological principle you mention, that the universe looks the same everywhere was based on a homogenously growing (non-accelerating) universe. When so there is this acceleration phenomenon, one should be somewhat cautious about how that principle is carried over. No, the cosmological principle does not rely on any assumption about the time-dependence of the expansion. It does require that the expansion progresses at the same rate everywhere at the same time, but the time-dependence could be anything that could be generated by an equation of state and a dynamical theory such as GR. This is a problem, we do not know what the universe looks now, only what it looked like when the light was sent out. So I see how you can push through the argument by assuming that the universe we do not observe is uniform at any given time, but it accelerates with time, explaining what we actually can see. (Actually, I was well aware of this argument since before, but overlooked it as I was thinking too much on other explanations :-).) Hans Aberg |
#44
|
|||
|
|||
Black hole mass-sigma correlation
In article , Ulf Torkelsson
wrote: The formula you gave, GM/(c^2 r), would give hints on how big a cosmological glob might become, as it should set a limit as GR forces grow strong. One would have to look beyond that limit to tell whether there is something beyond our local glob. This is rather the length scale at which the curvature of space-time becomes significant. If we use the critical density of the universe 1.88e-26 kg/m3, then this lenght scale becomes of order r = (c^2/G rho)^(1/2) = 3e26 m = 1e10 ly which is about the size of the observable universe. I can think of two possibilities: Either the mass density you quote remains constant throughout the universe, in which case I figure time-space will eventually (if the region where this is true is sufficiently large) curve so much that in effect it is a black hole, in effect the whole universe, as nothing can escape it. Or, moving outwards, the density must somewhere decrease, bent by GR effects. I recall that one turned the Hubble telescope to an area that formerly looked pitch black, and one found a lot of tiny galaxies there. I figure those galaxies are well within the limit you give above (otherwise we would probably have heard more about it in the newspapers :-)!). But perhaps better telescopes could admit one to look beyond that limit (a selling point for better telescopes?). I think that the redshift argument says that galaxies sufficiently distant will redshift so much that they cannot be observed. But if there is a local glob structure, one should be able to look beyond it to observe galaxies with less redshift. The big bang theory assumes that there is only one glob, plus that the expansion of visible matter is caused by a big bang. In a glob theory, one would still have to find an explanation of the expansion of visible matter. Hans Aberg |
#45
|
|||
|
|||
Black hole mass-sigma correlation
In article , Ulf Torkelsson
wrote: The formula you gave, GM/(c^2 r), would give hints on how big a cosmological glob might become, as it should set a limit as GR forces grow strong. One would have to look beyond that limit to tell whether there is something beyond our local glob. This is rather the length scale at which the curvature of space-time becomes significant. If we use the critical density of the universe 1.88e-26 kg/m3, then this lenght scale becomes of order r = (c^2/G rho)^(1/2) = 3e26 m = 1e10 ly which is about the size of the observable universe. I can think of two possibilities: Either the mass density you quote remains constant throughout the universe, in which case I figure time-space will eventually (if the region where this is true is sufficiently large) curve so much that in effect it is a black hole, in effect the whole universe, as nothing can escape it. Or, moving outwards, the density must somewhere decrease, bent by GR effects. I recall that one turned the Hubble telescope to an area that formerly looked pitch black, and one found a lot of tiny galaxies there. I figure those galaxies are well within the limit you give above (otherwise we would probably have heard more about it in the newspapers :-)!). But perhaps better telescopes could admit one to look beyond that limit (a selling point for better telescopes?). I think that the redshift argument says that galaxies sufficiently distant will redshift so much that they cannot be observed. But if there is a local glob structure, one should be able to look beyond it to observe galaxies with less redshift. The big bang theory assumes that there is only one glob, plus that the expansion of visible matter is caused by a big bang. In a glob theory, one would still have to find an explanation of the expansion of visible matter. Hans Aberg |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Jets Spout Far Closer to Black Hole Than Thought, Scientists Say(Forwarded) | Andrew Yee | Astronomy Misc | 6 | January 7th 04 11:49 PM |
The universe is expending. | sooncf | SETI | 24 | November 5th 03 03:24 PM |
VLT Observes Infrared Flares from Black Hole at Galactic Centre (Forwarded) | Andrew Yee | Astronomy Misc | 0 | October 29th 03 09:05 PM |
Universe Born in Black Hole Explosion? | Klaatu | Astronomy Misc | 0 | September 17th 03 09:54 PM |
Link between Black Holes and Galaxies Discovered in Our Own Backyard(Forwarded) | Andrew Yee | Astronomy Misc | 0 | July 17th 03 07:36 PM |