Dark matter, dark energy & gravity

[joeljen]

none

You can "believe" anything you want, but since this is physics/astronomy, such a statement is worthless unless one can back it up with sound theoretical formulation and/or valid experimental observation. You might as well say you believe in the tooth fairy.

PF Guidelines against over speculative posting can be found here:

https://www.physicsforums.com/showthread.php?t=5374

Zz.

My goodness!
-Chill out-
I just answered Brahma’s question, he didn’t ask for details.
Joel
PS
Just so you’ll know; my mom is the tooth fairy.

 

[jagyb]

Could someone comment on Jakob Bekenstein's last few papers which attribute Dark Matter to theoretical artifact; http://arxiv.org/abs/astro-ph/0412652, http://arxiv.org/abs/astro-ph/0509519, http://arxiv.org/abs/astro-ph/0602266?

My comment is an obvious one. Bekenstein believes he is pursuing a valid approach that would obviate the need for DM in explaining some astronomical observations. He's trying to offer an alternative to DM.

 

[Chronos]

Just a loosely constructed comment by Joel, IMO, Zz. We generally prefer more content than 'I believe' in posts here, Joel. But feel free to believe, just 'show the math' [specific reasons why you think the way you do] so we have something to chew on [scientists feed upon each other's flesh].

Hi jagyb and welcome to PF [and Joel too]! I've read those papers and find them puzzling. Bekenstein has done some very nice work on black holes, but, I find his papers on dark matter too ad hoc for my taste.

 

[yanniru]

Chronos or anybody, is it true that there is no DM associated with spherical galaxies? I ask Chrnons because he mentioned that different amounts of DM are expected for different galaxies.

 

[jagyb]

Hi chronos and big thanks for the welcome.

I live in Aspen, CO so I get to hang out with physicists even though I'm not one. Hopefully somewhere between "I believe" and fancy math there's some level of discussion I can participate in here.

The last visitor here who did a public presentation was John Womersley did a lecture called "The Quantum Universe". Mr. Womersley was quite good; matter of all kinds and how it's put together is clearly what he knows a lot of math about. I got the impression he's happy to look for DM with new tools at CERN but that it's not something he would have come up with on his own in a million years.

The only other general comment I have about DM is that if Einstein's GR finally starts to show some age effects around it's 100th b-day that the need for DM may go out the window about then too. That's what Milgrom and Bekenstein seem to anticipate.

 

[kmarinas86]

Chronos or anybody, is it true that there is no DM associated with spherical galaxies? I ask Chrnons because he mentioned that different amounts of DM are expected for different galaxies.

Low surface brightness galaxies (LSBs) and dwarf galaxies tend to have more spacetime curvature than expected from their visible mass, whereas denser galaxies such as large ellipticals tend to have hardly any anomalous space-time curvature. The spacetime curavature determines the velocities of the stars, and in low-density galaxies, this becomes more obvious. Also there are regions in very low-density space between the galaxies where the is more spacetime curvature than expected, hence the formation of hyrdogen clouds that seem to be too hot without something really heavy bonding it to that region. It seems that the anomalous spacetime curvature is inversely proportional to the density of the visible mass. Either the anomalous curvature is due to dark matter, MOND, or perhaps a very fundamental flaw about our understanding of the curvature and optical properties of spacetime outside our solarsystem which makes sparse areas look sparser optically, where as denser areas are made to look sparser as well, but not to as high of a degree.

 

[Chronos]

Nice summation, kmarinas86. It is difficult to explain why CDM has an affinity for different morphologies. I think it is either a selection effect or a modeling problem. The one thing that bothers me is the typically huge masses of spherical galaxies. Is there some kind of back reaction that expels their dark matter halos? Is CDM an illusion, or do we suffer from hideous artifacts in observational evidence [I lean that way]?

 

[Garth]

The one thing that bothers me is the typically huge masses of spherical galaxies. Is there some kind of back reaction that expels their dark matter halos? Is CDM an illusion, or do we suffer from hideous artifacts in observational evidence [I lean that way]?

In the standard $\Lambda$CDM model there still is a lot of dark baryonic matter. $\Omega_{visible} \sim 0.003$, $\Omega_b \sim 0.04$, i.e. over an OOM more invisible baryonic matter than visible stuff.

The masive sperical/elliptical galaxies are ~ one OOM more massive than the spirals, and the spirals have massive dark halos some of which must be baryonic in nature (if not all as in the FCM), so could it be that for some as yet unknown reason the ellipticals are just more efficient in converting dark baryonic matter into stars and HII regions?

Garth

 

[Juan Casado]

Another explanation for SNe Ia faintness

This explosions would seem farther away than they really are (were) because of a small negative curvature of space. In a slightly hyperbolic Universe, the wave front of light is spreading out faster than in a flat one (the light cone resembling a horn) so that luminosity distances would appear longer than they are.
In such scenario no dark energy would be needed.

 

[selfAdjoint]

Mr. Casado, you seem to be posting this same text (or maybe very slight variants) wherever anyone mentions the accelerated expansion hypotheses. May I suggest you start a thread where your idea can be discussed? Such a discussion would be OT in many of the threads you have responded to.

 

[brahma]

Brahma
Gravity as a force just doesn?t seem to want to fit in very well with what is observed
and with what experiments that science is capable of now.
The problem, as I see it, is that the scientific paradigm of our day (even the last 200 years) is based on the idea that gravity is a force that not only holds the universe together but determines it?s configuration (or geometry).
With this view, then, is the requirement for something unseen and so
mysterious that it cannot be detected; although we, right here, are
immersed in it.
No one wants to change a comfortable paradigm, even when it doesn?t
work anymore.
I think what is needed is a new theory of gravity where it is not a force!

By the way that theory in which gravity is not a force is already there called general theory of relativity in which gravity is considered as a distortion of space time in place of a force.

What seems to be ignored is that gravity causes two or more objects to
accelerate toward each other without overcoming inertia or any
expenditure of energy.
The accelerating objects must be, technically, at rest in spacetime!
Therefore; it is distance between objects that is diminishing that results in
the effect of gravity.

This is what general theory of relativity says i.e., if two geodesics are diverging then the curvature of space-time is -ve and if they are converging then it is positive.

But such a new concept would require an entire revamping of the
paradigm, no big bang, no gravitons and the instant propagation of the
effect of gravity. Ironically, most of Newton and Einstein?s equations would
remain.
Joel

What are you talking about ? once you include relativity there is no instantenious interaction.