Dark Matter a query for the physicists...
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11-06-2013, 01:53 PM
Dark Matter a query for the physicists...
I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

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11-06-2013, 01:55 PM
RE: Dark Matter a query for the physicists...
(11-06-2013 01:53 PM)ridethespiral Wrote:  I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

I've read it's just the sheer amount of missing matter. I must note that I'm in no way an expert on the subject, just an interested layman.
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11-06-2013, 02:07 PM
RE: Dark Matter a query for the physicists...
(11-06-2013 01:55 PM)ghostexorcist Wrote:  
(11-06-2013 01:53 PM)ridethespiral Wrote:  I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

I've read it's just the sheer amount of missing matter. I must note that I'm in no way an expert on the subject, just an interested layman.

Ya know I guess that you could actually reach that determination fairly easily now that I think about it...We know approximately how many stars of any given size could have existed at any given stage of the universe and you need a star to make a black hole, therefor we probably know with a fair level of accuracy the mass of all the black holes in the universe.

I'd still like a physicists input but it's making more sense to me already.

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11-06-2013, 02:23 PM
RE: Dark Matter a query for the physicists...
(11-06-2013 02:07 PM)ridethespiral Wrote:  Ya know I guess that you could actually reach that determination fairly easily now that I think about it...We know approximately how many stars of any given size could have existed at any given stage of the universe and you need a star to make a black hole, therefor we probably know with a fair level of accuracy the mass of all the black holes in the universe.

I'd still like a physicists input but it's making more sense to me already.

The book A Universe From Nothing discusses this. Several people on here have read it. I'll try to dig my copy out later if no one gives you an answer.
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11-06-2013, 03:17 PM
RE: Dark Matter a query for the physicists...
(11-06-2013 02:23 PM)ghostexorcist Wrote:  The book A Universe From Nothing discusses this. Several people on here have read it. I'll try to dig my copy out later if no one gives you an answer.

I actually read it, although admittedly I hacked the DRM (I bought the damn thing) and listened to it with text-to-speech on a plane (me + actually reading + moving/flying vehicle = barf), so I know I zoned out for a few pages in there somewhere. I recall a lot of discussion of dark matter and the need for it to be present to account for the mass of the galaxies but I don't think the good prof Krauss ever went into why black holes where ruled out.

Also on a side note I just watched this today: http://www.youtube.com/watch?v=uSwJuOPG4FI ...Hamza just keeps arguing semantics while them Islamists eat it up *sigh* You can just see Krauss' frustration.

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11-06-2013, 05:36 PM
RE: Dark Matter a query for the physicists...
I'm no physicist, but I think the conclusion comes from a number of directions. First that models of galactic rotation and the like require extra mass to be distributed throughout a galaxy rather than at point locations. Also, we have relatively direct observations of mass distributions that match these models due to gravitational lensing effects[1][2][3].

That said what we are seeing is gravity effects that we are attributing to mass which we are attributing to particles. I still wouldn't be surprised if we were to find that dark matter is some gravitational effect that didn't require actual particles.

[1] http://scienceblogs.com/startswithabang/...sing-show/
[2] http://arxiv.org/abs/1001.1739
[3] http://en.wikipedia.org/wiki/Dark_matter...al_lensing

Give me your argument in the form of a published paper, and then we can start to talk.
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11-06-2013, 07:52 PM
RE: Dark Matter a query for the physicists...
Hafnof pretty much hit it. Our observations of galaxies just don't match with the mass that we can estimate to be there, so there is something new going on here. The observations are more consistent with there being a whole bunch more mass than what we estimate galaxies to have. Short of tweaking the law of gravity (hey, maybe we should give that 'Intelligent Theory of Falling' a shot. teach the controversy, right?), positing dark matter seems the simplest solution.
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12-06-2013, 02:40 AM (This post was last modified: 12-06-2013 02:58 AM by Adenosis.)
RE: Dark Matter a query for the physicists...
(11-06-2013 01:53 PM)ridethespiral Wrote:  I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

My opinion is...

Dark matter either shows the limits of General Relativity, in explaining the orbits of stars in the outer parts of galaxies, much like the limits of Newtons law of gravity were shown by observing mercury's orbit.

Or there actually is mass out there that doesn't interact with the electromagnetic field in a way that would cause it to give off radiation allowing us to see it like ordinary matter. This matter mustn't have a strong enough gravitational field to bend light to a noticeable degree, which is what black holes would do. If this dark matter was black holes, I'm sure we would see excessive gravitational lensing (within our own galaxy) when we look through our dear Hubble (or any sufficiently powerful earth based telescope as well). We also don't have problems seeing behind the dark matter we expect to be there, but we could not see through black holes dotted throughout the the galaxy. This rules out black holes, I'm sure we could come up with more reasons.

This is my favorite theory for dark matter that I have read (the one you linked).

Edit: Basically what Hafnof said.

2.5 billion seconds total
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12-06-2013, 06:02 AM
RE: Dark Matter a query for the physicists...
(11-06-2013 01:53 PM)ridethespiral Wrote:  I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

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.
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12-06-2013, 06:08 AM
RE: Dark Matter a query for the physicists...
(12-06-2013 06:02 AM)zaybu Wrote:  
(11-06-2013 01:53 PM)ridethespiral Wrote:  I was reading about this really elegant new dark matter theory: http://phys.org/news/2013-06-simple-theory-dark.html and it reminded me of a question that bugs me whenever I read lay physics...

No need for an extraordinary amount of detail, I just want to know why the consensus is with the existence of purely theoretical dark matter and not good 'ol black holes....

I mean we now know that black holes are way more common and way more massive than we ever thought... I don't understand why the plethora of undiscovered black holes out there can't account for the excess mass? Is is is the gravitational patterns of the galaxies? The sheer amount of missing matter? or something else?

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.

Skepticism is not a position; it is an approach to claims.
Science is not a subject, but a method.
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