The Naked Scientists

The Naked Scientists Forum

Author Topic: Why doesn't Dark Matter clump together?  (Read 4929 times)

Offline Jamey

  • First timers
  • *
  • Posts: 1
    • View Profile
Why doesn't Dark Matter clump together?
« on: 21/07/2010 18:16:11 »
Dark matter has mass, and interacts with 'normal' matter through gravity.  Since theory says there is 5x as much 'dark' as 'normal' matter in the universe why hasn't dark matter clumped together with normal matter through gravitation?  It would seem our sun should have 5x as much dark matter in it as normal matter, and then it's mass and gravity should be 6x what one would expect from the observed quantity of normal matter? 


 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why doesn't Dark Matter clump together?
« Reply #1 on: 21/07/2010 19:31:07 »
As a low pressure gas is compressed it heats up (remember your bicycle pump)  In order for any sort of matter to clump together it must be compressed a great deal and therefore even of it starts off quite cold it will get very hot and so needs to lose energy by radiating or possibly ejecting some of its mass at high velocities (ie high temperature) stars do this using electromagnetic interactions and bipolar flows as they are forming.  Dark matter does not interact electromagnetically.  The only way it can cool down is to radiate gravitational energy this is vastly more inefficient than electromagnetism and just does not create any significant cooling at the densities of matter in the universe now so dark matter can only clump together at relatively low pressures and very large lumps like the average densities of clusters of galaxies and galaxies.  This is probably why major galaxies and clusters are the size that they are.

The gravitational fields of stars and planets do create a tiny increase in the density of dark matter in their vicinity and it is only this increase that has any effect on the net gravitational field.  The average density of dark matter has no effect.   The effect of this tiny increase would be to cause planets and spacecraft to move very slightly as if the mass of the object was higher as you got further away  this is being looked for with very precise measurements but as far as far as I know the effect has not yet been detected with any degree of certainty.   The detection of this effect would act as confirming dark matter and give more information on its mean temperature.
 

Offline tommya300

  • Hero Member
  • *****
  • Posts: 655
    • View Profile
Why doesn't Dark Matter clump together?
« Reply #2 on: 21/07/2010 21:43:34 »
Dark matter has mass, and interacts with 'normal' matter through gravity.  Since theory says there is 5x as much 'dark' as 'normal' matter in the universe why hasn't dark matter clumped together with normal matter through gravitation?  It would seem our sun should have 5x as much dark matter in it as normal matter, and then it's mass and gravity should be 6x what one would expect from the observed quantity of normal matter? 

There may be some other explanation.

http://www.cosmosmagazine.com/news/3502/doubts-about-universes-dark-side

http://www.dailygalaxy.com/my_weblog/2010/06/are-dark-energy-and-dark-matter-myths.html
.
« Last Edit: 22/07/2010 02:06:49 by tommya300 »
 

Offline flr

  • Sr. Member
  • ****
  • Posts: 302
  • Thanked: 2 times
    • View Profile
Why doesn't Dark Matter clump together?
« Reply #3 on: 21/07/2010 23:39:57 »
 
 It could be a long story but to put it in one sentence:
 I don't believe there is any dark matter.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why doesn't Dark Matter clump together?
« Reply #4 on: 23/07/2010 23:42:58 »
flr  why should you consider that there is anything unusual about the existence of particles that interact only via gravitational attraction.  After al we have particles with charges that interact  electromagnetically, particles with no charges that interact via the weak interaction and composite particles that interact via the strong interaction surely one would expect one or more particles that interact only gravitationally.  these particles would constitute dark matter.  for many years before dark matter was named but the existence of the anomalous rotation characteristics of galaxies was known I have confidently expected that such particles must account for at least some of this anomaly.  To my mind it would be unusual if such particles did NOT exist.  It is just the way that certain sensationalist science reporters present these things that there is anything unusual about it.
 

The Naked Scientists Forum

Why doesn't Dark Matter clump together?
« Reply #4 on: 23/07/2010 23:42:58 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums