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vincentm
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Can someone explain to me what is it or what is theorized to be dark matter. What effects does it have in regards to the cosmos.
Thank you,
vm
Thank you,
vm
Thanks to people who answered my question.vincentm said:Thanks lisa!
Surely they had some distribution in mind in order to suggest that it flattens rotation curves, right? I even hear recently that they think there may even be some DM in the center of galaxies. What did they use for a model to even come up with the suggesting that extra mass was needed?Mike2 said:So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect? Thanks.
Mike2 said:So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect?
Nacho said:Say you have a diffuse mass that has quite a bit more relative motion against the mass contained in an average galaxy. What would the gravitational effect of that mass be on that galaxy as it passed it by?
Mike2 said:Surely they had some distribution in mind in order to suggest that it flattens rotation curves, right?
I even hear recently that they think there may even be some DM in the center of galaxies. What did they use for a model to even come up with the suggesting that extra mass was needed?
I genuinely appreciate the help. So is the DM halo about 10 times as massive as the lumious matter?SpaceTiger said:There are a variety of theoretical models for the dark matter profiles. A simple one that gives a flat rotation curve is the "isothermal sphere":
[tex]\rho=\frac{v^2}{4\pi r^2G}[/tex]
where v is the rotation speed of the galaxy and r is the distance from the center. Numerical simulations imply a profile slightly different from this (called the NFW profile):
[tex]\rho \propto \frac{r_s}{r(1+\frac{r}{r_s})^2}[/tex]
where r_s is a free parameter (the scale radius). It's still not clear what the best fit model is, but it's almost certainly not as simple as an isothermal sphere. It's also not clear that it should be universal (i.e. that all dark matter halos should have the same profile).
Most models assume that there is some dark matter in the centers of galaxies (for example, the formulae I gave above), but that it's of much lower density than the luminous matter. To my knowledge, the only possible observational evidence for dark matter at the centers of galaxies is the excess gamma-radiation and the "haze" in the WMAP data near the galactic center. These are being interpreted as an annihilation signature of, perhaps, neutralinos. That's still very speculative at this point, however.
Mike2 said:I genuinely appreciate the help. So is the DM halo about 10 times as massive as the lumious matter?
I'd like to ask again if there is some evidence or proof that eliminates baryonic matter, such as too much scattering by baryonic matter for any distribution?
But you'd have to find the acceleration for the Unruh formula at a given radius from the galactic center. I suppose one could use Newton's inverse squared law as a good approximation.
If the major contribution of DM were photons, then it would not scatter light itself. But then I suppose you'd see a microwave signature of this effect, right? If 10 times the galaxy mass were evenly distributed throughout and around the galaxy, then I wonder how hot it would be?SpaceTiger said:Nucleosynthesis and the cosmic microwave background both suggest that the dark matter can't be baryonic. Also, there is some evidence of dark matter that is separated from luminous matter:
http://arxiv.org/abs/astro-ph/0312273"
That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.SpaceTiger said:Aren't these stars in approximate free-fall? I would think that the dominant effect that made their frame non-inertial would be rotation.
Mike2 said:If the major contribution of DM were photons
In the conclusion section of the paper you cite, one finds: "Adopting big-bang nucleosynthesis limits on the mean baryonic mass of the universe excludes most of this mass from being baryons in cold, condensed structures." They do not rule out an even distribution throughout space.
That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.
Oh, by the way, my thought for the day is to consider whether this Unruh effect may be the very cause of particle creation during inflation so that there is no need to suppose a Higgs boson and a false vacuum. We may already have all the physics we need. For particle creation during inflation is accompanied with very fast accelerated expansion so that horizons are small and the Unruh effect (=Hawking radiation) is more pronounced. Not only that but when the universe was small and expanding very fast, the gravitational field was more intense and perhaps this caused greater accelerated reference frames to produce a much higher Unruh effect.
As I understand the Unruh effect, accelerating objects feel a temperature because the ZPE is not invariant wrt acceleration. It's Lorentzian and invariant wrt to velocities, but not acceleration. OK. I simply thought it was fair to apply the equivalence principle wherein there is no distinction between acceleration and gravitation. I also think that there is nothing particular to the object that is causing the temperature rise, but it is the accelerating reference frame that "feels" the temperature whether there is an object in that accelerating frame or not. Correct me if I'm wrong, but it seems that DM is distributed as though the gravity it causes around it has some sort of weight itself, right? This argue for some sort of Unruh effect. I don't believe this Unruh effect produces permanant particles (unless there is an horizon) because an accelerating observer could feel a temperature and presume collision with particles. But if he were to stop and go back at constant speed, he would never see those particles again, right? So it seems applicable only to accelerating systems of which gravity is one.SpaceTiger said:Perhaps you can explain further. My understanding of the Unruh effect is that accelerating frames view the quantum vacuum as having a net temperature. Are you suggesting that the vacuum is viewing itself as having a net temperature and that space is curving as a result of the contributions to the stress-energy tensor? If so, why would the quantum vacuum not also be in free-fall?
Yes, I'm not real clear yet about all this. But it seems that if one region of space is accelerating wrt a second region, then the second would have to conclude that the first is feeling a temperature (due to the Unruh effect) so that the second should observe that the first has a higher ZPE than itself so that there must be some particles created in the second region to account for the temperature. I think it is equivalent to also think in terms of Hawking radiation. If the Universe is accelerating very fast, then there will be a much smaller cosmological event horizon which separates virtual particles out of the ZPE to create permatant particles during inflation.SpaceTiger said:Again, I'm prone to question your use of the term "acceleration" here. During inflation, space itself is undergoing accelerated expansion, but spacetime is still locally Lorentzian.
If the smaller mass (the galaxy) got close enough to the larger one, it would be tidally disrupted and you would likely see streams of stars and gas. Something like this:
Mike2, you may find this interesting:Mike2 said:That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.
Unruh's approach to gravitation and the vacuum are non-standard, and this 1993 paper (16 years after he proposed the effect named for him) is no exception.Unruh in paper said:Gravity is the unequable flow of time from place to place. It is not that there are two separate phenomena, namely gravity and time and that the one, gravity, affects the other. Rather the theory states that the phenomena we usually ascribe to gravity are actually caused by time’s flowing unequably from place to place.
Mike2 said:As I understand the Unruh effect, accelerating objects feel a temperature because the ZPE is not invariant wrt acceleration. It's Lorentzian and invariant wrt to velocities, but not acceleration. OK. I simply thought it was fair to apply the equivalence principle wherein there is no distinction between acceleration and gravitation.
Correct me if I'm wrong, but it seems that DM is distributed as though the gravity it causes around it has some sort of weight itself, right?
Just a moment,... now I'm thinking that DM can not be any sort of permanant matter whatsoever. For if it were, then it would gravitate towards the host galaxy and eventually concentrate there. But AFAIK we don't observe older galaxies with this central concentration of DM, but all seem to have the same distribution required to flatten rotation curve and not centrally concentrated DM which would not show this same flatness.
My point is that if DM were permanant particles, then it would all eventually gravitate to the center and there would be none around the edges. This would occur around old stable galaxies. Is this seen in the data?SpaceTiger said:There is a problem in which simulations have difficulty producing the exact central profile of the dark matter expected from observations, but producing flat rotation curves is not a problem in the standard CDM universe.
The DM particles, whatever they may be, would be in orbit around the galaxy as with any other massive object such as a star etc. If they did interact they could exchange energy and angular momentum and some filter into the centre where they would eventually be absorbed by the central BH, but that might take many Gyrs. For some to lose their orbital energy in this way others would have to be ejected.Mike2 said:My point is that if DM were permanant particles, then it would all eventually gravitate to the center and there would be none around the edges. This would occur around old stable galaxies. Is this seen in the data?
If DM orbitted the center like everything else, then they would orbit in a disk in spiral galaxies, like ordinary matter, right? Is this what the deflection maps show or the simulations that produce flat rotation curves?Garth said:The DM particles, whatever they may be, would be in orbit around the galaxy as with any other massive object such as a star etc. If they did interact they could exchange energy and angular momentum and some filter into the centre where they would eventually be absorbed by the central BH, but that might take many Gyrs. For some to lose their orbital energy in this way others would have to be ejected.
Garth
Mike2 said:If DM orbitted the center like everything else, then they would orbit in a disk in spiral galaxies, like ordinary matter, right?
Considering how relatively disperse matter is on the disk (how distant stars are from one another), it seems like the only thing causing the disk structure is gravitational attraction. What does photon coupling have to do with anything?EL said:In fact it would not, since it does not couple to photons (very much) and hence cannot loose energy through radiation. This means it will not distribute itself as ordinary matter does.
Mike2 said:Considering how relatively disperse matter is on the disk (how distant stars are from one another), it seems like the only thing causing the disk structure is gravitational attraction. What does photon coupling have to do with anything?
I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter. And we would see all sorts of crazy distribution from galaxies that collide. But what I'm understanding is that the DM halo is always just that, a halo, something always evenly distributed at the outer limits of galaxies in more of an evenly distributed manner. I suggest that this contradict DM as being massive particles. It seems more like DM is proportional to the gravitational field in and around galaxies. This suggests more of a phanomina associated with the acceleration of gravity, such as perhaps the Unruh effect. Again, I think the calculation would be comparitively simple in order to find out.Garth said:You have to model DM to try and emulate the observed rotation curves and other features of galactic dynamics such as the warp in the Milky Way's disk and its interaction with the Magellanic clouds. The general model is a spherical distribution of DM.
Garth
My problem with the nature of DM is that it is all so speculative. It has not been identified in the laboratory even after about forty years of intense investigation.Mike2 said:I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter. And we would see all sorts of crazy distribution from galaxies that collide. But what I'm understanding is that the DM halo is always just that, a halo, something always evenly distributed at the outer limits of galaxies in more of an evenly distributed manner. I suggest that this contradict DM as being massive particles. It seems more like DM is proportional to the gravitational field in and around galaxies. This suggests more of a phanomina associated with the acceleration of gravity, such as perhaps the Unruh effect. Again, I think the calculation would be comparitively simple in order to find out.
Mike2 said:I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter.
There are theories that suggest Dark Energy is a form of gravitation leaking through from other branes, situated in the other dimensions, but Dark Matter appears to be situated in this universe (brane), we just can't find it, perhaps the next generation of accelerators?-Job- said:I was going to propose that if there were more than 3 dimensions of space, then dark matter may be supposed to exist in there, but wouldn't radiation emitted from matter in a fourth or fifth dimension also propagate through our 3 dimensions? I'm thinking it would, so that doesn't solve anything.
vincentm said:Thanks for the info guys i have a better understanding of what dark matter is theorized to be. Also, is dark matter also factored into the Hubble constant regarding the expansion of the universe, mainly the space between galaxies that are receding from one another?