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Author Topic: Is dark matter really needed?  (Read 4956 times)

Offline Mad Mark

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Is dark matter really needed?
« on: 28/05/2008 01:31:51 »
I read in journals that in order to explain the rotation of Galaxies and the speed at which their outer edge rotates you must have missing matter in order to explain their speed relative to the inner stars.
This is true when compared to a solar system with the inner planets rotating faster, but does the Galaxy spin? or does only the center spin but the whole Galaxy in the fabric of space Roll? And given enough time reach equilibrium.
If I was to get a sphere of water say with its inner core spinning faster than its surface and set the whole sphere rolling down a never ending hill would the ball not eventually reach a point where its entire contents moved in unison?


 

Offline DoctorBeaver

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« Reply #1 on: 28/05/2008 09:14:51 »
I think the point is that there is not enough mass to produce the effects we see.

As far as we know, only gravity can produce those effects; but we cannot detect sufficient mass to produce that amount of gravity. Therefore we can conclude, within the current limits of our knowledge, that there must be more mass somewhere but we cannot detect it.
 

Offline RD

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« Reply #2 on: 28/05/2008 18:22:48 »
There are alternative hypotheses which do not include "dark matter" ...
http://space.newscientist.com/article.ns?id=dn8631
 

Offline DoctorBeaver

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« Reply #3 on: 28/05/2008 18:29:48 »
Indeed, RD. There are other hypotheses but they have yet to gain mainstream acceptance.
 

Offline RD

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« Reply #4 on: 28/05/2008 18:55:53 »
Until I can buy a kilo of "dark matter" at Tesco I shall be sceptical about its existence  :).
 

Offline DoctorBeaver

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« Reply #5 on: 29/05/2008 12:41:34 »
Until I can buy a kilo of "dark matter" at Tesco I shall be sceptical about its existence  :).

Haven't you seen the large tins of Marmite, then?  :D
 

lyner

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Is dark matter really needed?
« Reply #6 on: 29/05/2008 14:57:16 »
They don't sell tins of neutrinos, either, but we accept their existence. None of us has what we could call personal experience of dark matter or neutrinos.
 

Offline RD

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« Reply #7 on: 29/05/2008 15:34:57 »
Until I can buy a kilo of "dark matter" at Tesco I shall be sceptical about its existence :).

let me rephrase my statement formally:
I will be sceptical about the existence of dark-matter until it has been observed directly by scientists working independently.

[and Marmite is disgusting  [xx(], a kilo of chocolate ice-cream would be much better].
 
 

Offline DoctorBeaver

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« Reply #8 on: 29/05/2008 17:01:39 »
Quote
[and Marmite is disgusting  dead, a kilo of chocolate ice-cream would be much better].

Indeedly-doodly!
 

Offline RD

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« Reply #9 on: 29/05/2008 22:57:00 »
Until I can buy a kilo of "dark matter" at Tesco I shall be sceptical about its existence :).

If dark-matter is discovered it should be very inexpensive, as the universe is allegedly 90% dark-matter.
Perhaps this will lead to the expression "dirt cheap" being replaced by "dark-matter cheap", as the latter is allegedly far more abundant :).
« Last Edit: 29/05/2008 23:01:33 by RD »
 

Offline lightarrow

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« Reply #10 on: 29/05/2008 23:22:09 »
Until I can buy a kilo of "dark matter" at Tesco I shall be sceptical about its existence :).

If dark-matter is discovered it should be very inexpensive, as the universe is allegedly 90% dark-matter.
Perhaps this will lead to the expression "dirt cheap" being replaced by "dark-matter cheap", as the latter is allegedly far more abundant :).
But there is a little problem: dark matter density is so small that you have to collect it for the entire solar system to have just a few of it (sorry don't know how much, but consider that the amount of dark matter in the solar system is so small not to change sensibly our solar system's dynamics computed in the usual way, that is, we can consider it negligible).
 

Offline LeeE

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« Reply #11 on: 30/05/2008 00:35:24 »
Does anyone know if anyone has calculated(estimated) the mass of our Oort cloud?
 

Offline DoctorBeaver

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« Reply #12 on: 30/05/2008 07:36:14 »
Does anyone know if anyone has calculated(estimated) the mass of our Oort cloud?


from http://www.solstation.com/stars/oort.htm

Estimates of the total mass of this Oort Cloud range from about 40 times that of Earth to greater than that of Jupiter.
 

Offline LeeE

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« Reply #13 on: 30/05/2008 13:54:12 »
Thanks for that DrB - only about 1/1000 of Sol's mass at most, so even if every star has it's own Oort cloud it will probably be essentially negligable in terms of the total dark matter required.
 

Offline DoctorBeaver

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« Reply #14 on: 30/05/2008 18:36:30 »
Thanks for that DrB - only about 1/1000 of Sol's mass at most, so even if every star has it's own Oort cloud it will probably be essentially negligable in terms of the total dark matter required.


That seems likely.

In case you're wondering, the total mass of Kuiper Belt objects is estimated at 1/10 the mass of the Earth. Not a lot there either.
 

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Is dark matter really needed?
« Reply #14 on: 30/05/2008 18:36:30 »

 

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