Evolving wrote:Right, I've had a sift through my textbooks to see whether I had missed anything (and obviously I knew I had, but I meant anything relevant to this discussion); and I think this is all a semantic misunderstanding.

Now I learned in chapter one of "An Introduction to Galaxies and Cosmology" (unfortunately only second year stuff, sorry about that) that in the Milky Way, at any rate, the dark matter is gathered together in a spheroid (a stretched, elliptical version of a sphere), and we call it the "dark matter halo". That seems to be all we can say about how it is distributed, since it is (by definition) not luminous and can only be detected by its gravitational effects, i.e. (as hack alluded to) observing the rotation of the galaxy, working back through Kepler's laws and determining how much other, otherwise unobservable stuff must be contained in the galaxy.

With the luminous part of the galaxy (stars, dust, gas) we refer to the disc, the bulge and the halo, with the stellar halo being a sparsely populated and in comparison not very luminous component in the whole structure. By convention, any star that is in the disc shaped bit or in the bulge shaped bit is designated as belonging to the disc or the bulge, as the case may be, whereas anything outside these regions is referred to as the halo.

Linguistic symmetry suggests that, if dark matter subsists solely in a halo, it must be missing from the disc and the bulge; but I don't think this is actually what we think is the case. I think that - considering how hard it is to observe at all - all we can say about it is that, firstly, it is there, and secondly, it forms more or less a spheroid shape; but we are not saying that it is outside and surrounding the luminous part of the galaxy. On the contrary: it almost certainly permeates the entire galaxy; and it may well (this is me speculating, now) form a disc and a bulge just like the luminous bits: we just can't observe enough about it to be able to tell.

Calilasseia wrote:Of course, there's still the same problem applicable to any matter in that ellipsoidal halo, namely, how does it stay in the halo over the long term.

One possible solution is for matter at high declination relative to the galactic disc, to be in more eccentric elliptical orbits about the galactic centre, but for sufficiently high eccentricities, this brings those particles into potential collision with interior, rather than exterior, galactic disc material.

For that matter, this makes me wonder what's going on in elliptical galaxies, regardless of whether dark matter exists or not ...

Calilasseia wrote:It seems several have missed the point of my question here, which is why I was careful with my introduction, and laid down therein some of the reasons for the postulating of the existence of dark matter in the first place.

My question, quite simply, is why hasn't the dark matter in the halo been attracted into the galaxy proper by gravity?

Even susu missed this basic point I'm making, which surprises me.

If the mass of a dark matter particle is positive, then gravity will act on it to attract it to other positive masses, including positive mass light matter particles. What keeps those particles from moving toward the galactic centre?

Evolving wrote:Right, I've had a sift through my textbooks to see whether I had missed anything (and obviously I knew I had, but I meant anything relevant to this discussion); and I think this is all a semantic misunderstanding.

Now I learned in chapter one of "An Introduction to Galaxies and Cosmology" (unfortunately only second year stuff, sorry about that) that in the Milky Way, at any rate, the dark matter is gathered together in a spheroid (a stretched, elliptical version of a sphere), and we call it the "dark matter halo". That seems to be all we can say about how it is distributed, since it is (by definition) not luminous and can only be detected by its gravitational effects, i.e. (as hack alluded to) observing the rotation of the galaxy, working back through Kepler's laws and determining how much other, otherwise unobservable stuff must be contained in the galaxy.

With the luminous part of the galaxy (stars, dust, gas) we refer to the disc, the bulge and the halo, with the stellar halo being a sparsely populated and in comparison not very luminous component in the whole structure. By convention, any star that is in the disc shaped bit or in the bulge shaped bit is designated as belonging to the disc or the bulge, as the case may be, whereas anything outside these regions is referred to as the halo.

Linguistic symmetry suggests that, if dark matter subsists solely in a halo, it must be missing from the disc and the bulge; but I don't think this is actually what we think is the case. I think that - considering how hard it is to observe at all - all we can say about it is that, firstly, it is there, and secondly, it forms more or less a spheroid shape; but we are not saying that it is outside and surrounding the luminous part of the galaxy. On the contrary: it almost certainly permeates the entire galaxy; and it may well (this is me speculating, now) form a disc and a bulge just like the luminous bits: we just can't observe enough about it to be able to tell.

Pulsar wrote:The difference between dark matter and ordinary matter is precisely due to the fact that dark matter only interacts gravitationally, whereas baryonic matter is subject to the electromagnetic force as well. Quantum fluctuations in the early universe led to regions with slightly higher / lower densities than average. The overdense regions then collapsed to into haloes, but they cannot collapse indefinitely, because as the regions contract, the kinetic energy of the particles increases.

If gravity is the only relevant force (which is the case in dark matter haloes), then the particles essentially don't interact with each other: the individual masses of the particles is so small and gravity is so weak, that gravitational forces between neighbouring particles is negligible. Instead, the orbits of the particles is only determined by the overall gravitational potential of the entire system. Such systems are called collisionless, and eventually they
will reach an equilibrium state where the average kinetic and potential energy are balanced in a specific relation, known as the virial theorem. Conservation of energy and angular momentum of each particle prevents them from getting closer together, in the same way as the planets don't spiral into the sun.

Baryonic particles however do interact with each other, due to the electromagnetic force (and nuclear forces). The higher the density and kinetic energy, the more they interact ('collide'). In these interactions, the particles do exchange energy and angular momentum, and importantly, energy is dissipated in the form of radiation. In other words, systems of baryonic matter can cool down. The energy loss allows the baryonic particles to clump together and form smaller structures.

Bottom line: baryonic matter interacts and can lose energy, dark matter cannot.

hackenslash wrote:This is a good post, Yves. My understanding is that dark matter permeates the entire galaxy, but it dominates in the halo, while ordinary matter dominates in the interior of the galaxy.

hackenslash wrote::clap:

Horwood Beer-Master wrote:One further query following-on from Pulsar's post; since we aren't yet certain what most of dark matter is, how can we know for sure it's particles don't interact with each other, albeit via forces other than the four known fundamental forces?

If any such forces between dark matter particles were to exist, it'd surely be very difficult for us to know about them; since the 'normal' matter we're familiar with would presumably be "dark" with respect to those forces just as most 'dark matter' must be with respect to the weak, strong and electromagnetic forces.

hackenslash wrote:...unless the known particles are invisible to this force in the same way that dark matter is invisible to electromagnetism, which is certainly a possibility.

Calilasseia wrote:...
What keeps those particles from moving toward the galactic centre?

surreptitious57 wrote:

DavidMcC wrote:

surreptitious57 wrote:
It is not currently known what dark matter actually is but the best conjecture is that it is a composite of weakly interacting massive particles from the Big Bang known as WIMPS. The prime candidate is the neutralino which is stable enough to still exist after thirteen billion years. But in order to determine its actual - as opposed to just hypothetical - existence it has to interact with something else. That something else is any atom with large nuclei such as germanium or xenon or silicon. But so far none has yet been detected. It is not an easy process anyway because of interference from cosmic rays and false signals caused by natural vibration. And which is why it can only be detected underground where such rays and vibrations cannot permeate. Dark matter is actually relatively ubiquitous - a billion WIMPS pass through the brain every second - but unless it interacts with the aforementioned something else it leaves zero trace

All the known gravitational matter in the Universe should give an Omega reading of 1 . 0 but instead it is only 0 . 3 and so something else has to account for the missing 0 . 7 and that something else is dark energy. Although like dark matter it is not actually known at this point in time what it actually is [ although it has the properties of repulsive gravity ]

Chapter One : The Missing Universe : 13 Things That Dont Make Sense - Michael Brooks

But all this is only the "best conjecture", as you said, from the "top guys" -
he opinion formers. Their word is taken as "the bible" in some parts, it seems

This is rather disingenuous David. Scientists are not actually interested in opinions as such but testable hypotheses that can determine the validity of a truth claim by repeatable and verifiable means. In science what one thinks is not as important as what one can demonstrate. Nothing in it is absolute because it is an inductive discipline relying on evidence - as opposed to a deductive discipline relying on proof - like mathematics. Also the history of science shows that conventional wisdom was not always right. Which is why an opinion has zero scientific merit until it can be tested. Nothing is set in stone. The best science can do is provide as accurate a representation of the natural world as possible. But knowledge is never complete so nothing can ever be deemed absolute. But as close to as possible to with regard to the pre existing knowledge of the time

Calilasseia wrote:
No electromagnetic interactions means, in effect, no collisions. So these particles can, in effect, pass right through solid (standard) matter and not be halted. in short, there could be thousands of these particles passing right through me at this
very moment, and I wouldn't notice?

hackenslash wrote:
(I exclude gravity because it's far from clear that it can properly be described as a force)