Re: Dark Matter / Energy

Thanks OG. That is by far the most helpful contribution for a layman
like myself, who is a stranger to all this Newtonian balistic business
and the rest. Having said that iIt raises more questions however. Why
is this dark matter supposedly in a spherical halo evenly distributed
in and around our galaxy? Why doesn't it clump like all other matter
into larger mass objects? And whatever the physical reason for it not
having a tendency to clump what makes it hang around galaxies then? If
it has no tendecy to condense together what is its attraction to
visible matter, and why does this attraction not cause it to clump
closer to individual pieces of visible matter? I'm confused! I need a
drink.


OG wrote:
"Paulus" <paul476@xxxxxxxxxxxxx> wrote in message
news:1139573082.343497.292920@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have recently become interested in this subject, however there is
something that troubles me that no-one seems to bother discussing or
explaining. If there is someone out there that can I would appreciate
it.

As I understand it dark matter and energy have been theorized due to
the observed behaviour of galaxies. Both internally and between each
other galaxies appear to behave in a way that contravenes Newton's
inverse square law without the addition of extra mass.

What I don't get is that if this extra 'dark' mass is there why isn't
it here? Why is it that the planets within our solar system appear to
be obeying Newton's inverse square law? Why arn't we seeing the effect
of additional mass within the solar system? Why is it only on the large

scale of the behaviour of galaxies?

If anyone can explain I would appreciate it.

As I understand it, it is because the dark matter isn't very lumpy. Put
simply, The earth goes around the Sun because that is where most of the mass
of the Solar System is concentrated.

It is a well known principle (from Newton's original calculations I believe)
that if matter is distributed with spherical symmetry, there is only a net
gravitational force from matter between a body and the centre of the sphere.
i.e. evenly distributed matter outside the orbit of the Earth can have no
effect on the orbit of the Earth. However, it is also demonstrable that the
gravitational force from the matter 'inside the orbit' is identical to the
force that would apply if the matter were concentrated at the centre.

Were the Sun's mass to be spread evenly throughout the Solar system (say, to
the radius of Neptune's orbit), less than one part in a hundred thousand
would be inside the earth's orbit - so the Earth would not have an orbit as
at present, as the central gravitational force would be 100,000 times less.
However, for a planet just outside the orbit of Neptune, the gravitational
effect of a 'distributed' Sun or a 'condensed' Sun would be identical -
because all the matter is 'inside' the orbit.

As far as Dark Matter is concerned, there is good evidence to suggest that
this is evenly distributed through and around the galaxy - hence there is
no concentration within the solar system to affect the Earth's orbit around
the Sun. However as the Dark Matter is centred on the Galaxy centre, it does
affect the orbit of the Sun (and other stars) around the centre of the
Galaxy.

Hope this helps.

.

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