Dark Matter a query for the physicists...
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12-06-2013, 09:05 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 06:02 AM)zaybu Wrote:  Black Holes are what thery are because the mass is concentrated in a very small volume, not that they are necessarily more massive. For instance, if our sun's mass was shrunk to a radius of 3 km, it would be a Black Hole. It's not that it would be more massive but its density would be much higher.

Right. A black hole is not necessarily more or less massive than other cosmic objects. From a sufficient distance away, it behaves just as the star it used to be - the mass hasn't changed, so its gravitational effect can't either.


As to why dark matter - well, the other guys covered that. If the universe behaves according to the laws we know of (and that IS a big if), then its observed behaviour does not match its predicted behaviour.

We calculate more mass via observations of motion than we calculate via observations of electromagnetic emissions. If our theories are correct, then there must therefore be mass present which has no electromagnetic interaction. And so we call it dark matter.
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12-06-2013, 09:45 AM (This post was last modified: 12-06-2013 09:52 AM by ridethespiral.)
RE: Dark Matter a query for the physicists...
Right I get that the galaxies appear to be missing large quantities of mass, how black holes operate, how they are detected and why dark matter is a plausible explanation for the missing mass. I was just assuming that we are probably missing a heck of a lot of them(black holes), I mean gravitational lensing only works when there is a luminous object(s) behind the black hole although I guess that (non-rogue) black holes would generally be found near other visible matter.... I suppose the question isn't even limited to black holes, I assume there are a whole lot of dark (and still undetected by star luminosity graphs/wobble) planets circling various stars.

The original question still stands, is it the sheer amount of missing mass (and how is this conclusion reached)? The calculated position of said mass relative to the visible matter of a given galaxy? Both?

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12-06-2013, 09:54 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 09:45 AM)ridethespiral Wrote:  Right I get that the galaxies appear to be missing large quantities of mass, how black holes operate, how they are detected and why dark matter is a plausible explanation for the missing mass. I was just assuming that we are probably missing a heck of a lot of them(black holes), I mean gravitational lensing only works when there is a luminous object(s) behind the black hole although I guess that (non-rogue) black holes would generally be found near other visible matter.... I suppose the question isn't even limited to black holes, I assume there are a whole lot of dark (and still undetected by star luminosity graphs/wobble) planets circling various stars.

The original question still stands, is it the sheer amount of missing mass? The calculated position of said mass relative to the visible matter of a given galaxy? Both?

I would be more comfortable with the 'missing mass' being explained by unobserved normal matter like planets, brown dwarfs, debris, and gas. I haven't followed/understood the arguments for why this is likely not the case.

There has also been speculation that gravity is more complicated than we think and behaves 'oddly' in the realm of galactic distances and masses. Even that is more comfortable for me than dark matter. These ideas haven't gotten very far.

It seems that the universe doesn't care much about my comfort level. Sigh.

Skepticism is not a position; it is an approach to claims.
Science is not a subject, but a method.
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12-06-2013, 10:00 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 09:45 AM)ridethespiral Wrote:  Right I get that the galaxies appear to be missing large quantities of mass, how black holes operate, how they are detected and why dark matter is a plausible explanation for the missing mass. I was just assuming that we are probably missing a heck of a lot of them(black holes), I mean gravitational lensing only works when there is a luminous object(s) behind the black hole although I guess that (non-rogue) black holes would generally be found near other visible matter.... I suppose the question isn't even limited to black holes, I assume there are a whole lot of dark (and still undetected by star luminosity graphs/wobble) planets circling various stars.

The original question still stands, is it the sheer amount of missing mass (and how is this conclusion reached)? The calculated position of said mass relative to the visible matter of a given galaxy? Both?

It's both of those! It's the difference in observable mass to observable motion. Black holes are observable. And black holes are originally stellar matter, so it's 'drawing from the same well' in a sense - they don't explain additional mass!

If the mechanism for large scale, distant interaction is gravity - as we currently understand it, obviously - and we simulate behaviour with all the matter we can detect, we get incorrect results. What we predict should happen is not what we observe happening, IF all that is present is what we currently can detect, behaving in ways we currently understand.

The difference is dark matter.
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12-06-2013, 10:06 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 09:54 AM)Chas Wrote:  I would be more comfortable with the 'missing mass' being explained by unobserved normal matter like planets, brown dwarfs, debris, and gas. I haven't followed/understood the arguments for why this is likely not the case.

It's because those things aren't unobserved. They interact electromagnetically, and so we can tell, in a broad sense, if they're present. Spectral analysis is very good at determining composition and relative amounts.

If the 'missing mass' were accounted for by objects like that, distant galaxies (say) would emit far more electromagnetic radiation (because there'd be far more electromagnetic interaction). From the black body radiation alone they'd be much, much brighter.

(12-06-2013 09:54 AM)Chas Wrote:  It seems that the universe doesn't care much about my comfort level. Sigh.

Physics stopped making sense 108 years ago Tongue.
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12-06-2013, 10:50 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 10:06 AM)cjlr Wrote:  If the 'missing mass' were accounted for by objects like that, distant galaxies (say) would emit far more electromagnetic radiation (because there'd be far more electromagnetic interaction). From the black body radiation alone they'd be much, much brighter.

Gotcha. That answers my question and also further explains why the article in my OP is so darned intriguing.

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13-06-2013, 04:15 PM
RE: Dark Matter a query for the physicists...
I am generally distrustful of models that try to describe dark matter as one type of a particle. Of course, this is not the only hypothesis out there.

What if the reverse is possible? What if dark matter is so energetic, that it interacts the most with normal matter when it's slowed down the most? Then it would have to be massless like photons, or charge-less like neutrinos?

What if it radiates, but on radio bands? What properties does a hypothetical matter need to have if it radiates in radio bands and is not in form of a quasar or pulsar? My guess is, highly energetic.
I'm just wondering, does it even make sense in terms of physics?

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17-06-2013, 02:14 PM
RE: Dark Matter a query for the physicists...
(12-06-2013 06:08 AM)Chas Wrote:  
(12-06-2013 06:02 AM)zaybu Wrote:  Black Holes are what thery are because the mass is concentrated in a very small volume, not that they are necessarily more massive. For instance, if our sun's mass was shrunk to a radius of 3 km, it would be a Black Hole. It's not that it would be more massive but its density would be much higher.

The need of Dark Matter is to explain the velocities of stars in a galaxy. If we take our present theory without Dark Matter, those stars should be moving much faster, but they aren't. The only explanation for this observation is that the galaxy must be much more massive than what the counting of the stars and their estimated mass suggest. It's "dark" because we don't see it, hence we think it doesn't interact with the electromagnetic force - no light emitted, hence dark -- just with the gravitational force.

I think you have that reversed. The observed velocities of stars in galaxies and galaxies in clusters is too fast for the observed mass.

Right, my badBlush
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