Dark Matter

[coldcreation]

This brings to mind something I’ve been meaning to ask you, as one of scienceforum’s recognized deep-thinkers-about-cosmology, coldcreation: are you aware of, and if so, what do you make of Cooperstock and Tieu’s recent paper claiming that the galaxy rotation problem (which you’re describing here) can be resolved without the need for MACHOs or MANCHOs* “simply” by a correct formalism using GR in place of Newton’s Universal Gravitation?I can’t resist a quiz! Water (and air, as well) spins so quickly around the eye of a hurricane because: 1) the eye contains an area of very low air pressure, causing the surrounding, higher-pressure areas to accelerate toward it; and 2) the disorderly motion of the air surrounding the air makes it much more likely that the inrushing air will not move straight into the eye, but be deflected by other inrushing air into a spiral path, producing the characteristic swirl pattern we see looking down on a hurricane. It is conceivable that a hurricane system might involve winds blowing straight in, then straight out, of its center (like waves from a stone dropped in a pool, or the motion of stars in a globular cluster) but so unlikely as to never occur.

 

:) In other ways, a galaxy is not like a hurricane. Even making many circuits around it, higher pressure air eventually reaches the eye of a hurricane, in far less time than it takes a hurricane to dissipate. Why do hurricanes not dissipate much more quickly than they do?

 

* MANCHO (my own acronym, I believe) = Non-Compact (possibly uniformly spread throught space) MACHO.

 

 

Thank you CraigD for posting the article by Cooperstock and Tieu. I had not known about it, but have seen someting along similar lines, e.g., Gómez-Flechoso, M. A., and Domíniguez-Tenreiro, R. 2001, On the Stability of Quasi-Equilibrium Self-gravitating Configurations in a Tidal Field, ApJ, 550:703, 2001 April 1. They are less concerned with GR and more with pressure. They argue that the projected configurations “orbit inside steady state, spherical halos, as those that presumably surround compact galaxy groups and galaxy clusters.” So, solutions for large-scale quasi-equilibrium have been found.

 

They suggest, “one of the main shortcomings of galaxy models appearing in the literature is that, in most cases, galaxies are built up as if they were isolated.” The general results show that the bulk of the models are conserved alone 12.5 Gyr of evolution, irrespectively of whether circular or eccentric orbits are chosen. The results also suggest that the configuration of galaxies is fixed at its formation, determined by its mass, energy content, and the environment at that time. When the configuration is on a circular orbit, the authors used the Jacobi integral to define zero-velocity surfaces in the configuration space.

 

I agree with Gómez et al, but also with Cooperstock and Tieu: that pressure is of vital (tidal) importance, and so too is GR.

 

A couple points quickly: The paper you bring to attention is mostly concerned with dark matter halo’s (that surround galaxies) and not SMBHs (supposedly at the core). I think that Clay’s thread is about SMBHs (even though the title evokes otherwise, and even though black holes have already been extensively pondered in a thread called black holes).

 

That aside, as I’ve pointed out somewhere above, all these issues are related, both locally on scales compatible with galaxies (smaller too) and with large-scale (compatible with at least the visible universe. Why be concerned about dark matter locally and not globally? They are one and the same subject. If there is no non-baryonic dark matter locally than where is it hiding on larger scales? Cooperstock and Tieu point this out in the last paragraph, in different words.

 

Their use of GR rather than Newtonian mechanics is ingenious as it is obvious. It is time, high time, to place Newtonian gravity, as a linear attractive force aside, and replace it with Einstein’s general postulate and non-linearity gravity as a non-Euclidean curved spacetime phenomenon (at all scales). And not for massive extrapolations to infinite densities of the type that lead to dubious SMBHs, or even BHs, but to explain empirical evidence. That is in fact how GR came about to begin with, at least according to its maker.

 

This has been the brunt of my stance from the outset, my first reply, since my first post here at science forum. GR is the theory of choice, not simply to explain-away dark matter in galactic halo’s or nuclei, or on the largest scales visible, but to explain redshift z as the result of relativistic nonlinearity, material creation, the essence of the physical universe and its evolution in time.

 

These are the most important topics that need to be addressed in peer-review papers and cosmology forums especially, rather than ignored.

 

(Some of the issues can be preliminarily resolved, even if the goal is too ambitious).

 

Coldcreation

 

Something has only just begun

[questor]

if the compression of mass causes heat in the earth's core, why are'nt black holes the most heated and luminous objects in the universe?

[cwes99_03]

In theory they are. However, how do we measure it's heat? No information in any format can escape black holes. Nothing as theorized anyway.

[cwes99_03]

BAM! A thought just hit me. Please correct my thinking.

 

Nothing escapes a black hole.

 

Gravity travels at c.

 

Gravity of black hole is used to theorize their existence.

 

Which one of these must be wrong?

[coldcreation]

I've edited this post because it showed a lack of respect for the individual it was directed at.

 

There was no lack of respect. Who was it directed at (?) CraigD (?). I like him (her). He (she) is one of the more interesting gentlemen (women) on this forum.

 

And what was wrong with WMAP (this one had no sexual connotations). There is nothing wrong with a little humor mixed in with a subject that often alienates: cosmology. Be a swell guy infamous, put them back in...

 

Proof of my respect for CraigD was the response to his link. Did you see that?

 

Acronyms are fun. Here are a couple more harmless ones: natural elements radical density (NERD), Stephen Hawking’s university-time un-plugged (SHUTUP), golly an irreversible expansile (GAIE), solar isotope spectrometer yank (SISY), gee wiz a muon (GWM).

 

Joviality is a game of children.

 

CheersCreation

[Jay-qu]

BAM! A thought just hit me. Please correct my thinking.

 

Nothing escapes a black hole.

 

Gravity travels at c.

 

Gravity of black hole is used to theorize their existence.

 

Which one of these must be wrong?

 

well if your somehow suggesting that gravity cant escape a blackhole, i dont think so... that would mean that somehow gravity is unable to escape itself :)

[Bo]

BAM! A thought just hit me. Please correct my thinking.

 

Nothing escapes a black hole.

 

Gravity travels at c.

 

Gravity of black hole is used to theorize their existence.

 

Which one of these must be wrong?

 

a black hole curves spacetime; this curvature doesn't bother anything about the schwarzschild radius. tha paths particles follow are described by so called geodesics, and the statement "nothing escapes a black hole" actually means that geodesics that start within the Schwarzschild radius will never leave it. It is true that gravitons follow these geodesics, but that doesn't mwan that outside the S-radius the presence of the black hole is unnoticed.

 

Bo

[cwes99_03]

geodesics are curved paths, the shortest distance between two points on a curved surface is an example of a geodesic (literally earth division) or if you were to cut the earth, your cut would follow a curved path through the outer crust. Commonly used to describe the gravitational field of an astronomical object since gravity radiates in all direction from a point mass interpretation of space (simplistic).

Re: a planet orbiting a black hole

Yes, a planet can orbit a black hole. An orbit

is achieved when the tangental velocity outward

and the gravitational pull are equal.

 

Quick links:

http://en.wikipedia.org/wiki/Orbit

http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/formulas.html

http://www.fourmilab.ch/gravitation/orbits/

http://www.astro.ku.dk/~cramer/RelViz/text/geom_web/node3.html

 

The gravitational force of a black hole is

relativity easy to figure out. From there you

simply figure the distance and the velocity

required to maintain orbit.

 

The Swartzchild radius can be found with:

R = 2Gm/C^2

 

This is the radius at which the black hole

pulls at C. From there it's up to you,

but you can apply the inverse square law or

any number of other things to figure out

that there's an infinite number of possible

combinations of circular orbits, not to

mention eliptical ones.

 

When working on the math of something like

this it is far more convenient to think in

terms polar coordinates, with the origin as

the center of force.

 

Yes there would be time dilation. Look at

Mercury for a small example of what would happen.

GR is a lot easier to use for this because it is

a non-inertial frame of reference. Remember that

sizes change too, so the object being pulled in

will also appear to stretch along a curved path.

 

Depending upon the distance of the orbit,

the effects of GR would different. At distances

approaching infinity, the effects would reduce

to zero. So we could all be within the well of

a black hole and not see any real effects.

 

We are not certain if a black hole is in the

center of our galaxy or not.

This I believe is what you were saying. Anything outside the event horizon or swartzchild radius still has a chance to escape the pull of the black hole. However, anything within the black hole cannot escape, as the gravitational pull is so strong that even light traveling at c cannot escape it (escape velocity becomes greater than c).

 

You are infering that there are geodesics inside of the swartzchild radius, which honestly I have never heard someone say before. The reason being that in order for something to orbit the center inside the event horizon it's velocity from my crude estimation and understanding would have to be traveling at v>c. Of course, as you pointed out time dilation effects take hold as well, so i cant get my mind wrapped around who's reference frame we'd have to be in to define what's happening.

 

My question is more based on the idea that all objects warp space-time. GR says that light will be bent to some degree whenever it passes near mass and that the degree of 'bending' is directly related by the geodesic gravitational field (correct?). Of course anything traveling at the speed of light (gravity included) can escape that field because it is relatively small, but how does gravity "escape" the itself if the escape velocity brought about by a black hole (the matter within the swartzchild radius) has to be greater than c? Is there something that allows for gravity to do this in GR or SR?