A Dark Matter and the Energy-Momentum Tensor

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How do astrophysicists accurately account for all of the energy and pressure within a galaxy? How is it tabulated? My understanding of general relativity predicts that space-time curvature is a consequence of mass, energy, and pressure as expressed in the Energy-Momentum tensor.

The accepted explanation of the excessive rotation speed of stars in the outer periphery of galaxies is due to some kind of dark matter that accounts for the needed mass necessary to keep the stars in orbit.

But could it be that there has not been an adequate accounting of the total pressure and energy, across the full electromagnetic spectrum, including energy associated with quantum particles and states, that is contained within a galaxy to account for the additional curvature or gravitonal force necessary to keep the stars in their orbits?
 

phinds

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... could it be that there has not been an adequate accounting of the total pressure and energy, across the full electromagnetic spectrum, including energy associated with quantum particles and states, that is contained within a galaxy to account for the additional curvature or gravitonal force necessary to keep the stars in their orbits?
Yeah, it's possible that all the thousands of physicists who have studied this issue never thought of that and just made up "dark matter" to cover their ignorance. But I doubt it. Also, you'd need to show how that accounts for the gravitational lensing that is clearly caused by dark matter, plus other things like the bullet cluster.
 
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Yeah, it's possible that all the thousands of physicists who have studied this issue never thought of that and just made up "dark matter" to cover their ignorance. But I doubt it. Also, you'd need to show how that accounts for the gravitational lensing that is clearly caused by dark matter, plus other things like the bullet cluster.
OK. Take the Andromeda galaxy as an example. What is the total amount of energy and pressure in Joules associated with that galaxy? Where can this be looked up?
 

phinds

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OK. Take the Andromeda galaxy as an example. What is the total amount of energy and pressure in Joules associated with that galaxy? Where can this be looked up?
I have no idea but someone else here may.
 
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My understanding of general relativity predicts that space-time curvature is a consequence of mass, energy, and pressure as expressed in the Energy-Momentum tensor.
In general, that's correct. However, for the specific case you're asking about, a galaxy, energy (other than rest mass) and pressure are completely negligible as sources of gravity; the only significant source is rest mass. The only situation in which energy other than rest mass and pressure are significant as sources of gravity is inside neutron stars.

including energy associated with quantum particles and states
If you're intending this to be a separate category, in addition to energy (other than rest mass) and pressure, it isn't. "Energy associated with quantum particles and states" is just another way of saying "energy and pressure".

could it be that there has not been an adequate accounting of the total pressure and energy, across the full electromagnetic spectrum, including energy associated with quantum particles and states, that is contained within a galaxy to account for the additional curvature or gravitonal force necessary to keep the stars in their orbits?
No. There are several lines of reasoning involved here. First, there is what I said above, that the only significant source of gravity for a large system like a galaxy is rest mass. Then there are the following additional items:

We can count stars in nearer galaxies, and we can measure the luminosities of more distant galaxies. Over large numbers of stars, we can get accurate estimates of the total mass in stars from the total luminosity.

We can get good estimates of how much mass in the entire universe is contained in baryons (i.e., in the kind of matter that stars are made of) from measures of light element abundances, which put fairly tight constraints on how many baryons were present at the end of Big Bang nucleosynthesis (when light elements--mainly deuterium, helium, and lithium, with a small smattering of a few others--were made).

Comparing the two estimates above tells us that practically all baryonic matter in the universe is contained in the stars and galaxies we see. There isn't any room for significant baryonic mass that isn't visible. That means that whatever the invisible stuff is that is keeping stars contained in galaxies whose visible matter isn't enough to hold them in, it can't be anything that is accounted for by our current theories. That's why cosmologists have postulated dark matter.
 
26,229
6,848
Take the Andromeda galaxy as an example. What is the total amount of energy and pressure in Joules associated with that galaxy? Where can this be looked up?
Evidently you didn't try Google. I suggest that you try it.
 
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In general, that's correct. However, for the specific case you're asking about, a galaxy, energy (other than rest mass) and pressure are completely negligible as sources of gravity; the only significant source is rest mass. The only situation in which energy other than rest mass and pressure are significant as sources of gravity is inside neutron stars.



If you're intending this to be a separate category, in addition to energy (other than rest mass) and pressure, it isn't. "Energy associated with quantum particles and states" is just another way of saying "energy and pressure".



No. There are several lines of reasoning involved here. First, there is what I said above, that the only significant source of gravity for a large system like a galaxy is rest mass. Then there are the following additional items:

We can count stars in nearer galaxies, and we can measure the luminosities of more distant galaxies. Over large numbers of stars, we can get accurate estimates of the total mass in stars from the total luminosity.

We can get good estimates of how much mass in the entire universe is contained in baryons (i.e., in the kind of matter that stars are made of) from measures of light element abundances, which put fairly tight constraints on how many baryons were present at the end of Big Bang nucleosynthesis (when light elements--mainly deuterium, helium, and lithium, with a small smattering of a few others--were made).

Comparing the two estimates above tells us that practically all baryonic matter in the universe is contained in the stars and galaxies we see. There isn't any room for significant baryonic mass that isn't visible. That means that whatever the invisible stuff is that is keeping stars contained in galaxies whose visible matter isn't enough to hold them in, it can't be anything that is accounted for by our current theories. That's why cosmologists have postulated dark matter.
Ok. Thanks for your explanation.
 

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