Cosmos TV show

[kenevie]

I'm new to doing this so please bare with me.
A recent COSMOS show was a discussion on dark matter and energy. Neil said the stars on the outer rings of a galaxy move at the same speed as those near the center due to the presents of dark matter. I disagree because for a galaxy to be and look as it does it needs to have a gravitational force in the center. This force makes objects orbit faster as they approach the center. If there were no force accelerating objects toward center they would wander all over. Has anyone been watching COSMOS ?

[neufer]

A recent COSMOS show was a discussion on dark matter and energy. Neil said the stars on the outer rings of a galaxy move at the same speed as those near the center due to the presents of dark matter. I disagree because for a galaxy to be and look as it does it needs to have a gravitational force in the center. This force makes objects orbit faster as they approach the center. If there were no force accelerating objects toward center they would wander all over. Has anyone been watching COSMOS ?

The same circular orbital speed as those near the center due to the presence of dark matter.

[kenevie]

That is what Neil claims but it does not make sense in the nature of physics.

[BDanielMayfield]

That is what Neil claims but it does not make sense in the nature of physics.

Think of what would happen if the stars in the disks of spiral galaxies moved in just the same way that planets orbit in a stellar system, with the planets all moving slower with increasing distance from the central star. In galaxies the inner stars would move much faster than the outer stars and the spiral arm structures would get smeared out over time. This is known as "the wind up problem." The theorized pressence of a great deal of dark matter especially in the outer halos of galaxies is the most physically logical way to resolve this problem.

[Chris Peterson]

I'm new to doing this so please bare with me.
A recent COSMOS show was a discussion on dark matter and energy. Neil said the stars on the outer rings of a galaxy move at the same speed as those near the center due to the presents of dark matter. I disagree because for a galaxy to be and look as it does it needs to have a gravitational force in the center. This force makes objects orbit faster as they approach the center. If there were no force accelerating objects toward center they would wander all over. Has anyone been watching COSMOS ?

Regardless of the mass distribution in a galaxy, the center of mass is in the center, and the stars orbit in nearly circular orbits around that. The orbital speed of any individual star depends on the total mass inside its orbit. We can measure the orbital speed of stars, and use Kepler's third law to determine the interior mass: M = r³ / P², where r is the orbital radius and P is the orbital period. The mass can also be inferred from the intensity. The problem is that the two don't yield the same numbers, which means that a lot of the mass must not be luminous. And that mass must be distributed very differently than the luminous mass. The luminous mass is very dense at the center, and falls off rapidly with radius. If that's all there was, we'd observe the outer stars orbiting much more slowly than the inner ones (as the planets in a solar system do, for example). But if the dark matter is distributed more uniformly, then the orbital period will increase much more slowly (because of the increased interior mass) with radial distance. And that's exactly what is observed for outer stars- a shorter period, or higher orbital velocity than we'd expect from the mass distribution suggested by just the luminous matter.

[neufer]

That is what Neil claims but it does not make sense in the nature of physics.

Think of what would happen if the stars in the disks of spiral galaxies moved in just the same way that planets orbit in a stellar system, with the planets all moving slower with increasing distance from the central star. In galaxies the inner stars would move much faster than the outer stars and the spiral arm structures would get smeared out over time. This is known as "the wind up problem." The theorized pressence of a great deal of dark matter especially in the outer halos of galaxies is the most physically logical way to resolve this problem.

The only way THAT might work would be if the velocities INCREASED proportionally with radius (such that the RPM was constant).

The velocities don't decrease (as one might expect) but they don't increase either.

Spiral arm structure is a density wave effect.

[BDanielMayfield]

The velocities don't decrease (as one might expect) but they don't increase either.

Spiral arm structure is a density wave effect.

Yes, I knew that, but I was attempting to help kenevie see a reason why we know star motions inside galaxies aren't as simple as the orbits of planets around stars.

[kenevie]

The spiral arms around galaxies flow into the center indicating they are being drawn in by a force. Laws of physics dictate that objects will accelerate as they get closer to the center of mass. If objects were at constant speed they would merely orbit as do the rings around Saturn or the planets around the sun. On the other hand, there is friction even in space and maybe if there were enough objects orbiting the sun they would appear as they were flowing into the sun. Would not dark matter act the same in a micro scale as our solar system as it acts on a macro scale as in a galaxy ?

[Chris Peterson]

The spiral arms around galaxies flow into the center indicating they are being drawn in by a force.

No, the spiral arms don't flow into the center. Nothing in a galaxy is flowing in towards the center. The stars in a galaxy are in essentially circular orbits, and their radial distance from the galaxy center doesn't change significantly over billions of years.

They are in orbit because of a force. The force of gravity, which is created by all the mass inside their orbits.

If objects were at constant speed they would merely orbit as do the rings around Saturn or the planets around the sun.

Which is exactly what they do.

On the other hand, there is friction even in space and maybe if there were enough objects orbiting the sun they would appear as they were flowing into the sun.

There is no friction in space. When the density of material is high enough (as with an accretion disc, for example) you get collisions, and that means that the system can be described by fluid dynamics. There is a large amount of momentum transfer, individual particles can lose energy, and show orbital decay. But that isn't going on in solar systems, and it isn't going on in galaxies.

Would not dark matter act the same in a micro scale as our solar system as it acts on a macro scale as in a galaxy ?

Probably, if our solar system had the same kind of distribution of dark matter around it as galaxies do (i.e. a spherical halo). But that's probably not the case.

[geckzilla]

Re: Spiral arms and density waves. Came across this paper tonight:
http://arxiv.org/pdf/1403.1135v2.pdf

Also learned a new term, Lindblad resonance.

[Six_Strings]

The orbital speed of any individual star depends on the total mass inside its orbit.

That being accepted, why when modeling dark matter in galaxies is the majority of that dark matter in a distant "halo"?
Why would this gravitationally interacting "invisible" matter not be distributed within the baryonic matter? Or is it?
If so, and dark matter allegedly accounts for 4 times as much mass as baryonic matter don't we see effects on a local scale?

I've gotta say, I'm very suspicious of the alleged dark matter being something so exotic. We cannot visibly resolve planets outside our own solar system, (there may be exception to this, but not in general) why jump to such conclusions that the missing mass must be exotic invisible matter? I understand we believe some of the measurements done leave mass missing. The orbital velocities are conflicting, the luminosity tests conflict, the lensing effects etc. But, what assumptions are made in those measurements, tests, and calibrations? Have we empirically proven any of these measuring resources? There's a degree of assumption that is required in all these measurements from what I understand.

It seems very ad-hoc, and I'm suspicious lol...

[owlice]

why jump to such conclusions that the missing mass must be exotic invisible matter?

So what's your definition of invisible, then?

[Chris Peterson]

That being accepted, why when modeling dark matter in galaxies is the majority of that dark matter in a distant "halo"?
Why would this gravitationally interacting "invisible" matter not be distributed within the baryonic matter? Or is it?

It is. The "halo" is actually a "blob".

If so, and dark matter allegedly accounts for 4 times as much mass as baryonic matter don't we see effects on a local scale?

Because it's too uniformly distributed. It's been suggested that the local density of dark matter is an order of magnitude or more greater than in the galactic halo. But even so, the effect this has on planetary orbits, perihelion precession, and the like is still below our ability to reliably measure.

I've gotta say, I'm very suspicious of the alleged dark matter being something so exotic.

What's your definition of "exotic"? Simply something we don't understand well? Something that behaves contrary to intuition? Are neutrinos "exotic"?

We cannot visibly resolve planets outside our own solar system, (there may be exception to this, but not in general) why jump to such conclusions that the missing mass must be exotic invisible matter?

Because we are perfectly capable of detecting ordinary matter by its IR emission, and would readily do so in the case of dark matter if that's what it was made up of.

It seems very ad-hoc, and I'm suspicious lol...

Well, most physicists who study this sort of thing are not.

[Six_Strings]

It is. The "halo" is actually a "blob".

Then why keep calling it a "halo" if that's not the true alleged characterization of its distribution?

Because it's too uniformly distributed. It's been suggested that the local density of dark matter is an order of magnitude or more greater than in the galactic halo. But even so, the effect this has on planetary orbits, perihelion precession, and the like is still below our ability to reliably measure.

Why if it gravitationally interacts would it be so uniform and not clump and mass together as baryonic matter? Why would it not have measurable effects, especially considering its mass is alleged to account for 4 times that of baryonic matter? And yet, the dark stuff itself has yet to be detected, either directly, in particle physics laboratories as a new subatomic particle.

What's your definition of "exotic"? Simply something we don't understand well? Something that behaves contrary to intuition? Are neutrinos "exotic"?

Point taken, however even neutrinos have been detected (indirectly?). For the sake of simplicity I will refrain from calling invisible matter "exotic" since the definition of exotic is not really of my concern...

Because we are perfectly capable of detecting ordinary matter by its IR emission, and would readily do so in the case of dark matter if that's what it was made up of.

This is where I feel there's a lot of room for discrepancy and why I previously asked & stated (and yet to be clarified):

I understand we believe some of the measurements done leave mass missing. The orbital velocities are conflicting, the luminosity tests conflict, the lensing effects etc. But, what assumptions are made in those measurements, tests, and calibrations? Have we empirically proven any of these measuring resources? There's a degree of assumption that is required in all these measurements from what I understand.

Thank you for your input!

[Chris Peterson]

It is. The "halo" is actually a "blob".

Then why keep calling it a "halo" if that's not the true alleged characterization of its distribution?

Because "halo" and "blob" are synonyms in this context.

Why if it gravitationally interacts would it be so uniform and not clump and mass together as baryonic matter?

Baryonic matter clumps together because of electromagnetic interaction. If particles don't interact except by gravity, there is little to bring material tightly together. It just exists in separate orbits around a common center of mass. That's also why the distribution is an approximately spherical halo, and not a disc- without some kind of fluid interaction, there's no mechanism to create a disc, no mechanism to create coalescence.

Why would it not have measurable effects, especially considering its mass is alleged to account for 4 times that of baryonic matter?

Because even at several times the total mass, the very low density doesn't produce any significant gravitational effects except at a very large (galactic) scale. If you took four times the entire mass of the Solar System in the form of baryonic matter, atomized it and distributed it throughout the Solar System, it wouldn't noticeably affect the orbits of the planets.

And yet, the dark stuff itself has yet to be detected, either directly, in particle physics laboratories as a new subatomic particle.

It's extremely difficult to detect a particle that has no EM interaction.

[neufer]

Why would it not have measurable effects, especially considering its mass is alleged to account for 4 times that of baryonic matter?

Because even at several times the total mass, the very low density doesn't produce any significant gravitational effects except at a very large (galactic) scale. If you took four times the entire mass of the Solar System in the form of baryonic matter, atomized it and distributed it throughout the Solar System, it wouldn't noticeably affect the orbits of the planets.

If you took four times the entire mass of the Solar System in the form of baryonic matter, atomized it and distributed it throughout the Solar System as defined by the Oort cloud, it wouldn't noticeably affect the orbits of the planets.

[Chris Peterson]

Why would it not have measurable effects, especially considering its mass is alleged to account for 4 times that of baryonic matter?

Because even at several times the total mass, the very low density doesn't produce any significant gravitational effects except at a very large (galactic) scale. If you took four times the entire mass of the Solar System in the form of baryonic matter, atomized it and distributed it throughout the Solar System, it wouldn't noticeably affect the orbits of the planets.

If you took four times the entire mass of the Solar System in the form of baryonic matter, atomized it and distributed it throughout the Solar System as defined by the Oort cloud, it wouldn't noticeably affect the orbits of the planets.

Actually, I think you could distribute it inside the spherical volume of the planets and still not detect it by its gravitational influence.

[geckzilla]

So do you think our solar system's "sky" would glow or otherwise be detectable by any means if there was a few solar system masses worth of atoms confined within it? It should be, if you say the only reason we can't see dark matter is because it doesn't interact with light.

[Chris Peterson]

So do you think our solar system's "sky" would glow or otherwise be detectable by any means if there was a few solar system masses worth of atoms confined within it? It should be, if you say the only reason we can't see dark matter is because it doesn't interact with light.

I think it might show up as an IR signature. If you distributed the mass of the Solar System into a sphere with the radius of Neptune's orbit, you'd have a density of about 10¹² atoms per cubic centimeter. That's what you get with a decent ordinary vacuum pump, and it's about the density in low Earth orbit. Even if we couldn't detect that by its thermal emission, we could detect it directly from space with any number of solar wind monitoring instruments we have operated.

[geckzilla]

Oh, yeah, maybe it would affect the trajectory of a long term probe enough that we'd notice. A few missed comets, asteroids, or a Pluto might raise some eyebrows.

[Chris Peterson]

Oh, yeah, maybe it would affect the trajectory of a long term probe enough that we'd notice. A few missed comets, asteroids, or a Pluto might raise some eyebrows.

I've seen calculations that show it's not enough to account for the (already explained) Pioneer anomaly, which was itself extremely small. So no, I don't think it would show up in trajectories due to any gravitational effects. It would show up from direct aerodynamic effects, however, just as we see an effect from the thermosphere (which is about the same density) on low Earth satellites.

[geckzilla]

Yeah, not from the gravity. I thought it would slow them down slightly enough to matter. It seems to matter after a few thousand low Earth orbits.