Do we consider Dark Matter on a small scale?

  • 3 Replies

0 Members and 1 Guest are viewing this topic.


Mike Garrard

  • Guest
Do we consider Dark Matter on a small scale?
« on: 02/11/2011 12:30:02 »
Mike Garrard asked the Naked Scientists:
Dear Naked Astronomers,

Thank-you for the article on dark matter and dark energy.  Your interviewee mentioned that 80% of the matter in the universe is dark matter, and how this successfully models many of the observable characteristics of the universe.  One of  these is the group rotation of stars in a spiral galaxy. Diagrams of our solar system put it about half way towards the edge of our galaxy, and thus is specifically one of those stars whose movement is greatly forced by this 80% of invisible mass.      Yet, from Kepler, to the NASA Pioneer probes, no dark matter was required.  If this really is an 80% gravitational effect, why is it not required at the scale of our solar system?  Why is our local region free from gravitational effects  that are so essential to maintain our relative position in the galaxy?


Mike G

What do you think?
« Last Edit: 02/11/2011 12:30:02 by _system »


Offline MikeS

  • Neilep Level Member
  • ******
  • 1044
  • The Devils Advocate
    • View Profile
Do we consider Dark Matter on a small scale?
« Reply #1 on: 02/11/2011 15:28:07 »
Good question Mike.


Offline Soul Surfer

  • Neilep Level Member
  • ******
  • 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Do we consider Dark Matter on a small scale?
« Reply #2 on: 02/11/2011 15:53:02 »
An even distribution of dark matter creates no gravitational influences. It is only uneven distributions that could have any detectable effect on the solar system.  On the galactic scale the solar system which is a few light hours across is very tiny where the nearest stars are light years away and galactic structure are on the scale of hundreds of light years.

Learn, create, test and tell
evolution rules in all things
God says so!


Offline Mike G

  • Jr. Member
  • **
  • 14
    • View Profile
Do we consider Dark Matter on a small scale?
« Reply #3 on: 05/11/2011 19:29:23 »
Hmm. Maybe I made the question too narrow.  I can see how gravitational effects might balance with an even distribution, but no detectable effects at all?  Let's assume a high end case where this 80% dark matter is clustered around the solar system.  I would expect friction on space probes, some interaction with solar particles, some reduction in the speed of light.

Thinking along here, with a solar system out to the last massive planet Neptune and the height of the Sun, data from Wikipedia:
Neptune orbit radius: 4.5*10^12m
Sun radius: 7*10^8m
Solar system mass: 2*10^30kg
Volume = pi.r^2.h = 9*10^34m^3
Average density = 22e-6kg/m^3 = 22mg/m^3
Dark matter density = 88mg/m^3

How would that impact the Voyager probe?  Voyager dish area = 10m^2 pointing back to earth so roughly the effective area of the spacecraft.  It's 2*10^13m away so this 700kg satellite swept through 2*10^14m^3 filled with around 2^10kg of dark matter.  That seems incompatible, unless the argument is that dark matter only interacts through gravity, in which case how is the LHC going to find it?

What about the low end case, where dark matter is evenly distributed out to the next star?  So the 4x solar system mass is spread out in a sphere half way to Alpha Centuri, 2 light years.
Volume = 4/3*pi*r^3 = 3*10^49m^3
Density = 7e-20kg/m^3
In this scenario Voyager would have impacted about 14mg of dark matter which wouldn't have bothered it much.  Let's generously assume that the dark matter orbits alongside the Earth, but what about the moon? 
Orbital radius 4*10^8m, Radius 1.7*10^6m, period 27 days, age 4*10^9 years,
Area = 9*10^12m^2
Swept area per orbit = 2*10^22m^3
Swept area lifetime = 10^33m^3
Swept dark matter = 7*10^13kg
Moon mass = 7*10^22kg
So this minimum density scenario seems to be required for dark matter not to interfere with local observations, but it begs the question, why would dark matter stay even? It has mass and is affected by gravity so any slight gravitational disturbance would result in clumping. 'Equal and opposite reaction' should cause it to fall into orbit.

On the large scale, if there is a halo around every galaxy then I would expect galaxies to start to collide before the visible matter interacts, yet the pictures and models from NASA appear to move in a way that only requires the visible matter.  I wonder what they would look like if all the matter was lit, it should be an easy thing to do in the model.

In summary, uneven doesn't work, and even distribution only works if you can explain how gravitationally affected matter stays even.