Gravitational Scalar and Elliptical Galaxies / Globular Clusters

On 2/3/2014 10:56 AM, Phillip Helbig---undress to reply wrote:
In , Jos Bergervoet
..
That of course changes the perspective! :-) But then I have
a practical question: what is the timescale for a globular
cluster to evaporate? More than the age of the universe?

I'm no expert in this field, but a) I think so and b) it should be easy
to find.

You are right! Just googling "globular cluster evaporation"
immediately links to Wikipedia which states that 10^10y is
the time involved, with references leading to:
Gnedin, O.Y., and Ostriker, J.P., 'Destruction of the galactic
globular cluster system', Astrophys. J., 474, 223-255, (1997).

[Mod. note: non-ASCII characters removed -- mjh]

This 10Gy is indeed almost the age of the universe. (Or at
least we can call it a draw.) But it still means that expecting
them to evaporate is correct (as opposed to expecting them to
collapse, as in OP's opening line in the thread!)

--
Jos

In article ,
David Staup writes:
what I found was self-interacting dark matter is but theoretical.

The simplest assumption is that dark matter self-interaction is
negligible; that's what the major simulations are based on (so far as
I know). That doesn't mean the interaction cross section is strictly
zero, just that at the relevant dark matter densities, interactions
are rare.

There have been suggestions that certain problems could be solved if
dark matter interacts with itself at a significant level. That
introduces another free parameter into the models, and I'm not sure
how much this hypothesis has been explored. It probably wouldn't
change the overall picture but could make a big difference in details
such as the mass spectrum of dark matter haloes.

Assuming the opposite would mean dark matter follows no exclusion
principle, would it not?

Depends on whether the particles are fermions or bosons, but in any
case, exclusion only matters at densities comparable to those in
atomic nuclei, far higher than any relevant to dark matter in
astronomy. I suppose there might be some effect in the very early
universe, but I have no idea what it would be. To first order I'd
expect any effect to be gone before the light nuclei were formed.

And by extension that dark matter is not matter because it has no volume.

I don't follow this at all. What do you mean by volume, and what
does it have to do with anything? Density, partical mass, and
interaction cross section would seem to be the relevant parameters
for WIMPs.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

On Mon, 03 Feb 14, Jos Bergervoet wrote:
This 10Gy is indeed almost the age of the universe. (Or at
least we can call it a draw.) But it still means that expecting
them to evaporate is correct (as opposed to expecting them to
collapse, as in OP's opening line in the thread!)

You are overlooking that tri-axial rotation is a MODEL and has never
been verified by observation. Indeed, if you draw the actual
trajectory of a "tri-axial orbiting" body, it follows a skewed path
which doesn't have enough angular momentum to avoid being sucked
rapidly into the cluster core -- 1+1+1 does not equal 3 rotationally.


I present an alternative model where the core's gravity is much
lessened, so a gentler kinder environment for the cluster members.
But once again, it is a MODEL. I "expect" things from OBSERVATIONS,
not models.

Eric

In article ,
Jos Bergervoet writes:
[globular cluster stars]
Occasionally one could reach escape velocity by gravitational
interaction with the others. So one by one they could escape,

This seems to be the standard model of globular cluster evolution.
The escaping star takes positive energy with it, so the net energy of
the remaining system decreases. The result of that is gradual
collapse of the cluster core.

As Jonathan wrote, the time scale for this is comparable to the
present age of the Universe.

A separate path for cluster destruction is tidal disruption when the
cluster's orbit takes it through the galactic plane or close to the
galactic nucleus.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA