Dark matter, dark energy & gravity

  • Thread starter brahma
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Dear Zz,

let me make clear that here my intention is not to attack any "well established theory".

If there are any non physicists in this forum then I just want to say them that with time our scientific theories are becoming more and more stronger and physics and for that matter any other science is much more close to truth than any relegious belief.

I would like to withdraw my arguments if anybody is feeling offended.


dear Zz again thanks for the discussion.
 

SpaceTiger

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brahma said:
I seriously feel that the quality of our observational data is not that good for making the statement about homogeneity and isotropy of the universe at all scales etc.
I'm not convinced you're familiar enough with the observational data to make that statement. Could you elaborate, please?


In the case of dark matter it is puzzling that why 90 % matter in the universe is unseen or dark.
Why? Is it not a philosophical prejudice that we should be able to see that which constitutes the majority of matter in the universe?


More puzzling is what it is: LSP, neutrinos, axions, MACHOS...
The fact that we can't see it means that it interacts weakly with ordinary matter, which means that it should be no surprise that we haven't identified it in the laboratory.


Even then, we cannot justify its existence only on the basis of the argument that we need it, without knowing what it is.
What do you mean, "can't justify its existence"? Nature need not justify herself to us.


It seems to me that people are so much confused about dark matter and dark energy that there is scope for questioning their existence.
There is always room for questioning in science, but as I said, you would have a very difficult time constructing a theory of gravity that explained all of the problems solved by dark matter. There are plenty of people trying to do it, but none have come up with anything particularly convincing.
 
Gravity, the force

brahma said:
Most of it is going above my head. I do not know what type of force you are talking about.

GR which is "the" theory of gravity so far which we have and it clearly syas that "gravity is the deformation of the geometrical structure of the space time by energy".

So where is the force you are talking about


:surprised
Gravity, the force.
There is no problem with your statement. But, as I see it; your statement
refers to the “effect” of gravity.
Brian Greene, Stephen Hawking and Edward Witten all report that gravity
is one of the four fundamental forces in nature.
They are named as the “Strong” force (particle is the “Gluon”),
Electromagnetic force (particle is the “Photon”), the Weak force (particle is “weak gauge bosons”) and the force of gravity (particle is the “graviton”).
The way I see it, if gravity depends on the messenger particle “graviton”,
then every atom in the universe would have to be surrounded by or
immersed in a cloud of gravitons from every other atom in the universe.:cry:
Brian Greene is currently working on a new “quantum” theory of gravity.
Joel
 
Dark matter gravity

brahma said:
Hi Friends,

I just want to know how many of you believe that there is dark matter and dark energy in the universe and how many of you believe that there is something wrong with our understanding of gravity.

:surprised
Dark matter gravity:shy:
One of the reasons for the requirement of dark matter gravity is that the
gravity of what's visible is insufficient to hold galactic clusters together.
Vera Rubin and Kent Ford noted that a spinning galaxy lacked sufficient
gravity to hold the faster stars and they should be flung away.
Their analysis required that the galaxy be immersed at the center a giant
ball of dark matter.
Assuming the dark balls spin is consistent with the imbedded galaxy, what
keeps the dark ball itself together without crushing the galaxy?
Joel
 
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joeljen said:
Gravity, the force.
There is no problem with your statement. But, as I see it; your statement
refers to the “effect” of gravity.
Brian Greene, Stephen Hawking and Edward Witten all report that gravity
is one of the four fundamental forces in nature.
They are named as the “Strong” force (particle is the “Gluon”),
Electromagnetic force (particle is the “Photon”), the Weak force (particle is “weak gauge bosons”) and the force of gravity (particle is the “graviton”).
The way I see it, if gravity depends on the messenger particle “graviton”,
then every atom in the universe would have to be surrounded by or
immersed in a cloud of gravitons from every other atom in the universe.:cry:
Brian Greene is currently working on a new “quantum” theory of gravity.
Joel
Hi,

Forget about the force. Ask Frank Wilczek (I have forgotten the reference but there is a good article writen by him for physics today; check his webpage). He will say there is nothing called force. All four are interactions. Can you write an expression for force on the basis of the standard model of particle physics for strong or for that matter for weak interaction. I hope you cannot. This is due to the excessive use of Newtonian mechanics in our life that we always look for the force. I hope next time you will use the term "fundamental interactions" in place of "fundamental forces".

:uhh:
 
check it out!

brahma said:
Hi,

Forget about the force. Ask Frank Wilczek (I have forgotten the reference but there is a good article writen by him for physics today; check his webpage). He will say there is nothing called force. All four are interactions. Can you write an expression for force on the basis of the standard model of particle physics for strong or for that matter for weak interaction. I hope you cannot. This is due to the excessive use of Newtonian mechanics in our life that we always look for the force. I hope next time you will use the term "fundamental interactions" in place of "fundamental forces".

:uhh:
Hi
OK:surprised
I'll check it out. Something new!
And try to keep an open mind, in spite of all those equations with the symbol "F".
more later
joel
 
17
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joeljen said:
Hi
OK:surprised
I'll check it out. Something new!
And try to keep an open mind, in spite of all those equations with the symbol "F".
more later
joel
Those "F" are for school kids ! :surprised
 
F better than 0

brahma said:
Those "F" are for school kids ! :surprised
I’ll quit being a school kid when I know all about it.
Frank Wilczek is a hard read; but I’m OK with a mathematician calling a force an interaction and a physicist calling an interaction a force. Just so the bridge don’t fall and somebody can play golf on the moon.
But back to your original question, I don’t believe in dark matter and I do believe there’s a need for a new idea about gravity.
joel
 

ZapperZ

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joeljen said:
But back to your original question, I don’t believe in dark matter and I do believe there’s a need for a new idea about gravity.
joel
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.
 
none

ZapperZ said:
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.
 
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Chronos

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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.
 
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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.
 
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.
 
yanniru said:
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.
 
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Chronos

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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

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Chronos said:
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 [itex]\Lambda[/itex]CDM model there still is a lot of dark baryonic matter. [itex]\Omega_{visible} \sim 0.003[/itex], [itex]\Omega_b \sim 0.04[/itex], 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
 
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

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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.
 
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joeljen said:
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.
 

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