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.

Calilasseia wrote:Before I launch into my question, I'll run through my understanding of the reasons for postulating the existence of dark matter, which centres upon galaxy rotation.

When the standard model of gravity is used to simulate galaxy behaviour, the results do not match our observations of actual galaxy behaviour. Out of curiosity, scientists tried re-running the simulations with extra mass added, and when this was done, the results dovetailed exquisitely with observation. On this basis, scientists postulated that the missing mass from their models was a real entity. This led to the question of why this missing mass hadn't been observed before, which led to the postulate that the missing mass constituted a new form of matter, a form that doesn't engage in direct interaction with the electromagnetic force (i.e., photons). This absence of interaction with the electromagnetic force, led to the missing mass being termed "dark matter", in contrast to ordinary matter, which interacts with photons on a grand scale.

So far, so good. Dark matter halos around galaxies makes the models work. Furthermore, on the basis that said matter interacts via the gravitational force, it's possible to predict what sort of space curvature effects such matter will have, and how this will alter the paths of photons reaching us from distant parts of the universe, and then look for observed instances of those effects, Again, so far, so good. When those effects are observed, we have confidence that the dark matter hypothesis is something more than the product of the televisions inside our heads.

However, one feature of gravity, is that it attracts masses to each other. Consequently, if dark matter actually exists, it should be subject to the same attraction. As a corollary, I an temporarily at a loss to understand how dark matter and light matter becomes segregated, in the manner needed for the models to work. If the only force acting upon dark matter particles is attracting those particles to each other, and to light matter as well, then surely the two should become intermingled?

I'll hand this over to the physics experts.

hackenslash wrote:
General Relativity

surreptitious57 wrote:

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

The Standard Model references gravity as one of the four fundamental forces so what is your reason for disregarding it ?

surreptitious57 wrote:That explains what causes gravity but does not invalidate it as being defined a force in its own right. So saying that it is a force that acts between two bodies is still true. And at some point General Relativity may have to be modified or rejected to accommodate a theory of quantum gravity which in turn will reference a Theory Of Everything. G R in other words is only an approximation at this point in time as indeed are all theories

Calilasseia wrote:Before I launch into my question, I'll run through my understanding of the reasons for postulating the existence of dark matter, which centres upon galaxy rotation.

When the standard model of gravity is used to simulate galaxy behaviour, the results do not match our observations of actual galaxy behaviour. Out of curiosity, scientists tried re-running the simulations with extra mass added, and when this was done, the results dovetailed exquisitely with observation. On this basis, scientists postulated that the missing mass from their models was a real entity. This led to the question of why this missing mass hadn't been observed before, which led to the postulate that the missing mass constituted a new form of matter, a form that doesn't engage in direct interaction with the electromagnetic force (i.e., photons). This absence of interaction with the electromagnetic force, led to the missing mass being termed "dark matter", in contrast to ordinary matter, which interacts with photons on a grand scale.

So far, so good. Dark matter halos around galaxies makes the models work. Furthermore, on the basis that said matter interacts via the gravitational force, it's possible to predict what sort of space curvature effects such matter will have, and how this will alter the paths of photons reaching us from distant parts of the universe, and then look for observed instances of those effects, Again, so far, so good. When those effects are observed, we have confidence that the dark matter hypothesis is something more than the product of the televisions inside our heads.

However, one feature of gravity, is that it attracts masses to each other. Consequently, if dark matter actually exists, it should be subject to the same attraction. As a corollary, I an temporarily at a loss to understand how dark matter and light matter becomes segregated, in the manner needed for the models to work. If the only force acting upon dark matter particles is attracting those particles to each other, and to light matter as well, then surely the two should become intermingled?

I'll hand this over to the physics experts.

Horwood Beer-Master wrote:I think I get what Cali is trying to ask here. I keep wondering the same thing myself. If both dark and 'normal' matter produce gravity and feel it's effect, and if dark matter is surrounding all galaxies, then why doesn't normal matter pull it in? Why doesn't (for example) our planet attract it's own halo of dark-matter (and if it does, why isn't the earth's gravity stronger than we can account for)?

Even if dark matter does not experience any other force that familiar matter does, and therefore does not interact with it in any other way, I'm still unsure what's preventing any passing dark matter from falling into, and accumulating in, our planet's gravity-well (or that of any other 'normal matter' object).

theropod wrote:Stars do "fall in" to galactic black holes, so orbital mechanics doesn't cover the lack of effect on dark matter.

RS

theropod wrote:Not my point. If normal gravitational effect played with dark matter the same way as normal matter it would, eventually, fall into the gravity well of a black hole. Since we do see stars fall into these supermassive black holes it seems, to me, that dark matter isn't effected by gravity in the same manner.

What am I missing?

RS