Is dark matter real?

Chipper Q
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Thanks Ben, and thanks

Thanks Ben, and thanks Solomon; I'm afraid I interpreted the picture (and caption) backwards, and took 'hot gas' (and 'most of the normal matter') to include all the stars and the cores of both galaxies, as though a merger had taken place. So the regions of dark matter appeared to me like they were displaced from halos into lobes. (Do clusters even have cores at all similar to spiral galaxies?)

Is it correct to say, then, that the hot gas didn't fill the space between the galaxies as a result of gravitational attraction? Would this whacking of the gases be attributed as resulting more from electromagnetic interactions? Why didn't the normal matter and dark matter "fall" towards each other at the same rate?

I don't mean to interrupt Tullio's (rather good) questions:
If dark matter exists, which is its temperature? Does dark matter obey the laws of thermodynamics?

And I'll guess that it exists, obeys the laws of thermodynamics, and is probably close to absolute zero.

Odysseus
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RE: I thought the rate of

Message 38323 in response to message 38317

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I thought the rate of expansion was determined by measuring how fast galaxies are receding from each other, by measuring respective red shifts, and distances (i.e., motion of objects through space). Doesn't a gas expand to the size of its container? Expansion isn't the same as inflation, is it? What does it mean, then, to say the universe is expanding?

As this is a bit of a side or background topic to the thread, let me just refer you to Ned Wright’s Cosmology Tutorial: these should be relevant places to start on expansion vs motion (scroll to the top for the FAQ index) and inflation. They’re brief but have links to more thorough or graphic material.

Mike Hewson
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RE: Huh i've just thought

Message 38324 in response to message 38318

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Huh i've just thought of this - while earth is moving around the sun the new space is appearing all the time between them so the actual gravity force must be a bit stronger than we measure (measured gravity force) because it has to compensate for space enlargement that we do not take into consideration in our measurements (we think of constant space between earth and sun when measuring the force).


Yes, if all space is expanding ( as distinct from motion of objects within it ), then it ought to be happening hereabouts too. The trouble is that during our lifetimes ( or all of human history for that matter ) the fractional change is too small to be measured in 'realtime'. The good news is that because of the finite speed of light then we can collect 'old news' from far far away that is arriving in the here and now.
If light was infinitely fast then we'd have simultaneity in the old Newtonian sense, and our deductions would be more sort of 'archaelogical'. While we can't, here and now, view Brutus et al. stabbing Julius - someone/somewhere in theory can. They would have to be on the surface of a sphere centered on the ( old ) position of the Earth with a radius of approx. 2K light years. Note that this is well within or very nearby our Milky Way galaxy, and Andromeda hasn't yet seen any clever apes. This is the origin of 'time-travel', meaning: that to catch up with old emitted information ( the cut n' thrust of the Roman Senate scene ) we would have to travel fast enough to gain upon and overtake those photons that left a long while ago. Yes, you guessed it, the speed of light would need to be exceeded to achieve a refresh on Julius' last moments.

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It would also be interesting to know if the same amount of new space appear near gravity sources and far away from any gravity sources ?


I think that's pretty well assumed by the modelling, probably because it's the simplest, moreover if not then what's a sensible/helpful/testable alternative? Good thought!
Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Solomon
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RE: RE: RE: Is it

Message 38325 in response to message 38317

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Is it possible that the predominance of matter over antimatter is symmetric with a predominance of negative charge over positive charge?

Not in any serious quantity. Otherwise, there would be measurable effects that we would already be aware of.

Not so much a measurable effect as an observation: there's more matter than antimatter in the universe; seems like either a violation of symmetry (or conservation); I understand they're created in equal quantities in the lab, but I was thinking about initial conditions prior to inflation.

There is an asymmetry between matter and antimatter; and this does require a violation of what is called "CP symmetry" early in the universe. However, since there are both positively and negatively charged particles of ordinary matter, a CP violation does not automatically mean that the universe has a non-zero net charge.

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Do galaxies have a net negative charge? If so, could that be what's responsible for an accelerated expansion?

The expansion of the universe is not a motion of objects through space. So, things that would cause a net force between galaxies aren't relevant. Additionally, for such a force to be large enough to have significant effect, there would need to be enough extra charge that we would see its effects locally.


I thought the rate of expansion was determined by measuring how fast galaxies are receding from each other, by measuring respective red shifts, and distances (i.e., motion of objects through space). Doesn't a gas expand to the size of its container? Expansion isn't the same as inflation, is it? What does it mean, then, to say the universe is expanding?

As debugas explained, the expansion of the universe means that space itself is stretching. To put it a different way, two objects which are both stationary in space will find the distance between them increasing, because the actual amount of space between them increases.

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Could dark matter be vast quantities of electron plasma?

Electrons interact with light. Dark matter does not.

I thought it might be possible that a very cold electron plasma might be a nearly perfect medium for transmission of light, in regards to being transparent to the extent of appearing as if there's nothing there (except for the mass, of course).

The real problem with this scenario is that the ability to interact with light leads to a sort of friction, where the particles can easily shed kinetic energy. This would lead to large halos around massive objects shrinking, in much the way that gas clouds can condense and, ultimately, form stars.

Solomon
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RE: Thanks Ben, and thanks

Message 38326 in response to message 38322

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Thanks Ben, and thanks Solomon; I'm afraid I interpreted the picture (and caption) backwards, and took 'hot gas' (and 'most of the normal matter') to include all the stars and the cores of both galaxies, as though a merger had taken place. So the regions of dark matter appeared to me like they were displaced from halos into lobes. (Do clusters even have cores at all similar to spiral galaxies?)

Is it correct to say, then, that the hot gas didn't fill the space between the galaxies as a result of gravitational attraction? Would this whacking of the gases be attributed as resulting more from electromagnetic interactions? Why didn't the normal matter and dark matter "fall" towards each other at the same rate?

I don't mean to interrupt Tullio's (rather good) questions:
If dark matter exists, which is its temperature? Does dark matter obey the laws of thermodynamics?

And I'll guess that it exists, obeys the laws of thermodynamics, and is probably close to absolute zero.

There's no reason dark matter should follow the laws of thermodynamics (although, being that it's weakly interacting, one would expect the relaxation time to equilibrium states should be quite long).

The best work that's been done on dark matter suggests that the largest component is what's called "cold dark matter." In this context, "cold" means that the dark matter particles have less kinetic energy than rest energy (remember that temperature is just a measure of the average kinetic energy of the particles in a large system). Most often this is used to mean that the particles are quite massive.

In a more absolute sense, dark matter can't be too cold, since, the less kinetic energy a particle has, the smaller a distance it can escape from the local gravitational well. In other words, the size of dark matter halos can be used to learn about the temperature of the dark matter in the halo.

Mike Hewson
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RE: .....In principle you

Message 38327 in response to message 38319

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.....In principle you probably could still make a consistent modified gravity theory, but since it was originally proposed to get rid of dark matter Occam's razor says forget it. For that matter, since modified gravity now needs three extra things tacked on to explain different observations, that's more for the razor.....


Ahh .... I see the predicament much better now!
Scrub my earlier comments about 'effective' theory. I was not aware of the other data.... the dark matter scooting away with the galaxies ... hmmm...
Geez thanks for the terrific explanations, Ben!

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being that it's weakly interacting, one would expect the relaxation time to equilibrium states should be quite long


Yeah!
Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

hockeyguy
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RE: Cosmic collision

Message 38328 in response to message 38313

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Cosmic collision reveals dark matter
It looks like dark matter does indeed exist. By measuring the effect of gravitational lensing of a collision of huge clusters, in addition to x-ray observations, direct evidence of the dark matter is observable; furthermore, there are no modified Newtonian gravity theories that would work to satisfactorily explain the observation. Dark matter looks real, whatever it is...

yeah, i was pretty pissed after reading this. i really didnt think dark matter was real.

Chipper Q
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Thanks for the link to Ned's,

Thanks for the link to Ned's, Odysseus. It's hard to consider one piece of the cosmological puzzle without trying to fit it in with all the rest.

Solomon mentioned:

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There's no reason dark matter should follow the laws of thermodynamics (although, being that it's weakly interacting, one would expect the relaxation time to equilibrium states should be quite long).


Some reasons I considered were principles of invariance and conservation; if these are valid for dark matter, wouldn't thermodynamics necessarily be valid also?

Could dark matter be superfluid helium? Interesting article about how it can act similarly to vacuum, while increasing the moment of inertia of an object, as compared to vacuum. More inertia is like more mass. How would this affect the rotation curve, when scaled up to the size of a galaxy?

tullio
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RE: Thanks for the link to

Message 38330 in response to message 38329

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Thanks for the link to Ned's, Odysseus. It's hard to consider one piece of the cosmological puzzle without trying to fit it in with all the rest.

Solomon mentioned:

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There's no reason dark matter should follow the laws of thermodynamics (although, being that it's weakly interacting, one would expect the relaxation time to equilibrium states should be quite long).

Some reasons I considered were principles of invariance and conservation; if these are valid for dark matter, wouldn't thermodynamics necessarily be valid also?

Could dark matter be superfluid helium? Interesting article about how it can act similarly to vacuum, while increasing the moment of inertia of an object, as compared to vacuum. More inertia is like more mass. How would this affect the rotation curve, when scaled up to the size of a galaxy?


I think he meant "There's no reason dark matter should NOT follow the laws of thermodynamics". A double negation is an assertion.
Tullio

Keck_Komputers
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RE: RE: Cosmic collision

Message 38331 in response to message 38328

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Cosmic collision reveals dark matter
It looks like dark matter does indeed exist. By measuring the effect of gravitational lensing of a collision of huge clusters, in addition to x-ray observations, direct evidence of the dark matter is observable; furthermore, there are no modified Newtonian gravity theories that would work to satisfactorily explain the observation. Dark matter looks real, whatever it is...

yeah, i was pretty pissed after reading this. i really didnt think dark matter was real.


Me too. I always thought it was just regular matter that was not seen, not something entirely different.

BOINC WIKI

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