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Question about MOND and gravity 
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#19
May212, 08:57 PM

P: 6,863

One other thing about arguments toward elegance is that different people can weight things differently. If someone looks at the data that modified gravity requires less tweaking than dark matter, it can be hard to argue otherwise because these are somewhat subjective. 


#20
May212, 09:07 PM

P: 6,863

The other problem is that Krupa seems to misunderstand the applicability of LCDM. The idea behind LCDM is that the big bang produced large scale clumps and that these clumps influences where galaxies form. How galaxies actually form is outside of the theory, so LCDM really says nothing about things at small scales.
No one has been able to reproduce the cosmological observations with only modified gravity (lots of people have tried). Once you assume that some dark matter is necessary, then it becomes easier to assume (unless you have some reason otherwise) that it's all dark matter. 


#21
May312, 08:59 AM

PF Gold
P: 1,159

A star on the edge of a galaxy is treated by MOND as being very weakly accelerated as a whole by the galaxy, so the MOND rule applies. However, if you consider the component atoms of the star, they are all within the gravitational field of the star itself, so the overall gravitational acceleration on those atoms would be expected to be much greater than a0, which means they would obey Newtonian gravitation and be "immune" to MOND. It is difficult to see how the atoms of a star can accelerate in one way but the star as a whole accelerate in a different way. Similarly, a system of masses such as a binary star or a star and planets at the edge of the galaxy is also treated by MOND as a single object in the lowacceleration regime, even though the components are clearly subject to higher accelerations from each other. Note however that the MOND force is quite tricky to work with anyway, in particular because it is not linear in the source mass. 


#22
May312, 12:27 PM

P: 553




#23
May312, 03:21 PM

P: 553

I was looking at the MOND equation, it looks like the adjustment is 'hidden' at smaller scales allowing newtonian mechanics to work on our scale as the function brings it's value to 1 while adjusting to increased values for 'a' as the effects of gravity would become weaker as distance 'r' is increased.
I can not figure out how the function μ(a/a0) actually works except that a0 becomes more significant with respect to an increase in the value for 'r' as it reduces 'a' to a lesser value than a0 = 1^9m/s^2, a very tiny value. So the equation has terms in it I am unfamiliar with: ∇  ??? ρ  this is a function for the spread of mass in a galaxy is it not? If so how does it work? I don't see the symbol to the right for gravitational potential as written in the function Any thoughts? 


#24
May312, 03:55 PM

PF Gold
P: 1,159

$$ \left ( \frac{\partial}{\partial x}, \frac{\partial}{\partial y}, \frac{\partial}{\partial z} \right ) $$ For example, if you apply the gradient operator to the gravitational potential, you get the vector field describing the gravitational acceleration. The symbol ρ in the MOND article is simply the local density of mass (in mass per unit volume). 


#25
May312, 04:09 PM

P: 553




#26
May312, 07:41 PM

P: 1

Sorry if I'm in the wrong section to ask this question.
I'm trying to find out when astronomers discovered that the solar system oscillates through the galactic plane. I just can't imagine the Mayans having the ability to determine that it actually occurs on a 26,000 (or whatever) year period. Thanks for the consideration 123 mark 


#27
May312, 08:19 PM

P: 6,863

You have a function X. You have a set of rules to convert that function into another function Y. The problem is here is that learning what those rules are is a one semester course in calculus. Look at 18.02 on MIT OCW. 


#28
May512, 12:22 PM

P: 553

With DMT I was told by an astrophysicist that the dark matter is put into the model and essentially is an adjustment to the mass to have the stars match newtons gravitation formula. Is that right? Is Gm/r^2 modified in any way? 


#29
May612, 02:56 AM

PF Gold
P: 1,159

The Newtonian acceleration is then calculated in the usual way by integration (summing the effect of all the mass). With spherical symmetry, Newtonian gravity would simplify to being equivalent to having all the mass inside a given orbit concentrated at the center, but for galaxies the shape is more complicated. The MOND acceleration can then be calculated in terms of the Newtonian acceleration. A very weird feature of the MOND rule is that for a wide range of galaxies it correctly predicts the velocity distribution based only on the distribution of visible matter. If dark matter is the real explanation, this suggests that the distribution of the dark matter in galaxies must somehow be strongly linked with the distribution of the visible matter in such a way as to reproduce the MOND result, but so far there is no theoretical explanation for this. 


#30
May912, 12:03 PM

P: 553

Sorry it took a few days to get back to you, had finals last couple days.
I was told by an astrophysicist that it has only been recently that papers were published changing the model from a spherical density to a more disc like shape, I found this surprising as well. 


#31
May912, 03:29 PM

PF Gold
P: 1,159




#32
May912, 09:05 PM

P: 553

I am going to quiz the proffesors at school again and see if I can get a more complete answer. I was told by one it was basically the same as you stated originally; the mass is essentially summed and put into the center and then calculated. That seems overly simplified and frankly I don't see how that could calculate anything properly. 


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