AskAstro March 2013 follow-up about dark matter

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  • Member since
    March, 2002
  • From: 59d24.3'N17d49.1'E
AskAstro March 2013 follow-up about dark matter
Posted by boberglund on Thursday, March 07, 2013 10:41 PM

After reading the item on dark matter in AskAstro I would like to know the following:

Since dark matter is supposed to interact only through gravitation, how come it has not sunken to the center of galaxies and vanished into the black holes there?

It seems like that would be the end result of a "halo" around the galaxy rather than it just staying there...

Bo B
Sweden
(N59d24.27' E17d49.12')

  • Member since
    August, 2007
Posted by Primordial on Friday, March 08, 2013 8:43 AM

Boberglund : consider this, the gravitational is the weakest of the interactions.  You might compare that to relative density ; a sort of specific interaction( buoyancy), similar to the manner with which the stars balance their interactions during their life, but on a lower dinsity scale over a larger volume. A form of quasi equilibrium.  This is NOT to say, a possibility of range of relative density within the substance could not exist. We really do not have enough infornation about this effect to really understand its structure.  

I think the "halo" would just be the result of a vector analogy ( similar to the center of gravity ), plus the quanity of dark matter involved. Just my opinion.

  • Member since
    March, 2002
  • From: 59d24.3'N17d49.1'E
Posted by boberglund on Friday, September 13, 2013 1:18 PM

Since the last time on this thread I have taken the Stanford on-line course in Cosmology and in one of the lectures professor Susskind gave the reason for why the dark matter chunks do not collapse.

The first of 10 lectures is here:
Susskind Cosmology lectures

It is believed that the dark matter consists of a kind of elementary particles only interacting extremely weakly with other particles. The only significant interaction is by way of gravity.
This means that particles have no way to lose their potential energy so they will just obey the Newton gravity laws and orbit the common center of gravity forever.

In order for a particle to "sink" to the center it must get rid of the potential energy it has at the distance it starts out and there is no dissipation method available so it keeps the energy, which converts to velocity if it gets closer to the center in its orbit. That velocity in turn brings it back up the gravity potential well on the other leg of the orbit.

So these dark matter clouds are expected to have an extremely long life, much longer than the current age of the universe and thus never form black holes by themselves.

"Normal" matter on the other hand interacts with other particles  plus the CMBR photons via the electromagnetic force and this causes them to gradually lose their potential energy so they truly sink towards the center of the gravity field they are immersed in.

Bo B
Sweden
(N59d24.27' E17d49.12')

  • Member since
    July, 2013
Posted by Harvey Marshall on Wednesday, January 01, 2014 3:15 PM
Maybe dark matter is very large. electrons the size of Pluto, atoms the size of Neptune, molecules the size of Jupiter or the Sun, see where I am going here?? Isn't Science wonderful, or in my case Fantasy.

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