Dark matter and BH's

http://groups.google.com/groups?q=sci.physics,+%22Dark+matter+and+BH's%22&hl=en&sa=G&scoring=d

1 Ignor Ramus Dark matter and BH's maandag 19 november 2001 1:59
2 Uncle Al Re: Dark matter and BH's maandag 19 november 2001 18:51
3 Ignor Ramus Re: Dark matter and BH's dinsdag 20 november 2001 14:05
4 Steve Carlip Re: Dark matter and BH's dinsdag 20 november 2001 19:02
5 Etherman Re: Dark matter and BH's dinsdag 20 november 2001 23:46
6 Steve Harris Re: Dark matter and BH's woensdag 21 november 2001 6:01
7 Ignor Ramus Re: Dark matter and BH's donderdag 22 november 2001 15:56
8 Bruce Bowen Re: Dark matter and BH's vrijdag 23 november 2001 21:37
9 Nicolaas Vroom Re: Dark matter and BH's maandag 26 november 2001 17:23


1 Dark matter and BH's

Van: Ignor Ramus
Onderwerp: Dark matter and BH's
Datum: maandag 19 november 2001 1:59

If there are supermassive Black holes at the centre of every galaxy, to what extent could they account for the 'missing' mass in the universe (i.e. dark matter) ? Thanks, Ignor


2 Dark matter and BH's

Van: Uncle Al
Onderwerp: Re: Dark matter and BH's
Datum: maandag 19 november 2001 18:51

Ignor Ramus wrote:
>

If there are supermassive Black holes at the centre of every galaxy, to what extent could they account for the 'missing' mass in the universe (i.e. dark matter) ?

Negligible contribution. If you fully doubled *all* detectable mass equivalent you would still be short by 80%.

-- Uncle Al http://www.mazepath.com/uncleal (Toxic URL! Unsafe for children and most mammals) "Quis custodiet ipsos custodes?" The Net!


3 Dark matter and BH's

Van: Ignor Ramus
Onderwerp: Re: Dark matter and BH's
Datum: dinsdag 20 november 2001 14:05

Uncle Al wrote in message news:3BF9468A.9587BC4A@hate.spam.net...
> Ignor Ramus wrote:
> >

If there are supermassive Black holes at the centre of every galaxy, to

what
> > extent could they account for the 'missing' mass in the universe (i.e.
dark
> > matter) ?
>

Negligible contribution. If you fully doubled *all* detectable mass equivalent you would still be short by 80%. -- Uncle Al

Doh !! Should have realised that ! Thanks. Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter, would I be right in thinking that that 'only' leaves BB theory/models in support of the existence of dark matter ? Or is there other experimental or observational evidence I am not aware of ? (I am aware of at least some proposals /searches for what comprises d.m., such as Wimps, etc.)

Ignor.


4 Dark matter and BH's

Van: Steve Carlip
Onderwerp: Re: Dark matter and BH's
Datum: dinsdag 20 november 2001 19:02

Ignor Ramus wrote:

> Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

This isn't an unreasonable behavior for dark matter---it is, for example, the density dependence of an isothermal (that is, constant temperature) sphere of gas. But adding mass to the *center* of a galaxy won't help.

[Small mathematical note: in Newtonian gravity, the speed of a circular orbit at radius r is given by v^2 = GM(r)/r, where M(r) is the mass inside the orbit. There are small general relativistic corrections to this, but they're unimportant at galactic scales.]

Steve Carlip


5 Dark matter and BH's

Van: Etherman
Onderwerp: Re: Dark matter and BH's
Datum: dinsdag 20 november 2001 23:46

"Steve Carlip" wrote in message news:9te5sb$ci7$1@woodrow.ucdavis.edu...
> Ignor Ramus wrote:
> >

Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

>

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

Please provide evidence that we *need* dark matter to explain this behavior. Surely you meant to say that dark matter is sufficient to explain this effect.

-- Etherman

AA # pi

EAC Director of Ritual Satanic Abuse Operations

RAFC

AMTCode(v2): [Poster][TĘ][A5][Lx][Sx][Bx][FD][P-][CC]


6 Dark matter and BH's

Van: Steve Harris
Onderwerp: Re: Dark matter and BH's
Datum: woensdag 21 november 2001 6:01

Etherman wrote in message ...
>

"Steve Carlip" wrote in message news:9te5sb$ci7$1@woodrow.ucdavis.edu...

>> Ignor Ramus wrote:
>> >

Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

>>

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

>

Please provide evidence that we *need* dark matter to explain this behavior. Surely you meant to say that dark matter is sufficient to explain this effect.

It's the simplest explanation that is, consistent with already known physics. Is it necessary? No, blue fairies could be holding the stars in tighter orbits, mimicking exactly the gravity of extra matter we don't see.

SBH

-- I welcome email from any being clever enough to fix my address. It's open book. A prize to the first spambot that passes my Turing test.


7 Dark matter and BH's

Van: Ignor Ramus
Onderwerp: Re: Dark matter and BH's
Datum: donderdag 22 november 2001 15:56

Very clear and helpful. Thanks.

Steve Carlip wrote in message news:9te5sb$ci7$1@woodrow.ucdavis.edu...
> Ignor Ramus wrote:
> >

Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

>

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

This isn't an unreasonable behavior for dark matter---it is, for example, the density dependence of an isothermal (that is, constant temperature) sphere of gas. But adding mass to the *center* of a galaxy won't help.

[Small mathematical note: in Newtonian gravity, the speed of a circular orbit at radius r is given by v^2 = GM(r)/r, where M(r) is the mass inside the orbit. There are small general relativistic corrections to this, but they're unimportant at galactic scales.]

Steve Carlip


8 Dark matter and BH's

Van: Bruce Bowen
Onderwerp: Re: Dark matter and BH's
Datum: vrijdag 23 november 2001 21:37

Steve Carlip wrote in message news:<9te5sb$ci7$1@woodrow.ucdavis.edu>...
> Ignor Ramus wrote:
> >

Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

>

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

Flash of insight!! Isn't this (radial mass density) what one would expect from a mass background converging onto and (inviscid) flowing down a central hole? The mass contained in any thin radial shell is constant.

Conjecture: Dark matter is falling thru the galaxy radially (very little angular momentum) and with very little if any interaction with the light matter, and going down the central black holes!

I wonder if there is any way to measure various galactic central black holes rate of growth and see if it is consistent with radially infalling dark matter of a quantity consistent with conjectured radial densities?

> This isn't an unreasonable behavior for dark matter---it is, for example, the density dependence of an isothermal (that is, constant temperature) sphere of gas. But adding mass to the *center* of a galaxy won't help.

Gravitationally bound isothermal gas sounds good too. Again, is the rate of growth of the central black holes consistent with the conjectured dark matter density?

-Bruce


9 Dark matter and BH's

Van: Nicolaas Vroom
Onderwerp: Re: Dark matter and BH's
Datum: maandag 26 november 2001 17:23

Steve Carlip schreef in berichtnieuws 9te5sb$ci7$1@woodrow.ucdavis.edu...
> Ignor Ramus wrote:
> >

Next; if galactic BH's are responsible for the high rotational speeds of galaxies, offered as objective evidence for dark matter

>

They can't be, if they're in the centers of galaxies. The problem with galactic rotation is not that galaxies are rotating too fast, but that the rotational speed does not fall off fast enough with distance. If you put a large mass in the center of a galaxy, stars will rotate faster, but their velocities will still (at least roughly) follow Kepler's laws, with speeds varying as r^{-1/2}, where r is the distance from the central mass---just as the outer planets' velocities are smaller than, say, Mercury's. What we see instead is speeds that are nearly independent of r. To explain this, we need mass that's spread out throughout the galaxy. More precisely, we need the mass M(r) within a sphere of radius r to be proportional to r, or equivalently, the mass density to fall off as 1/r^2.

This isn't an unreasonable behavior for dark matter---it is, for example, the density dependence of an isothermal (that is, constant temperature) sphere of gas. But adding mass to the *center* of a galaxy won't help.

I do not understand why you need dark matter in order to explain the rotational curve of a galaxy.
What you need is a 3D model of a galaxy, with a bulge of roughly r = 10000 ly , a disk of r = roughly 40000 ly in the xy plane and consisting of only vissible matter. I have done the same See my home page http://users.pandora.be/nicvroom and check the information behind: 12 Darkmatter.

My point is that if you have calculated the mass for r = 40000ly with an almost flat rotation curve you need very little mass to make the r 10000ly larger with the same rotation curve.
If you want to understand why the speed of the sun is the way it is by assuming that all the mass is in one point is very misleading. You must start from a 3D picture. Excluding a possible BH the gravitational force in the center of a galaxy is zero, because (excluding z direction) there is even as much mass in the +x as in the -x direction. implying that v=0. (For the y direction the same logic applies)

As I started before first do a simulation with vissible matter and see if it matches the rotation curve.
If it does not you can add invissible matter, but I'am very doubtfull if you need.

A possible BH in the center will only marginal change the total mass of a galaxy (and the shape of the rotation curve)


Created: 28 November 2001

Back to my home page Contents of This Document