Dark Matter

[Euracil]

I have an idea. If photons are 0% mass and 100% energy, what if dark matter was 100% mass and 0 % other energy. Maybe? Yes? No?

[Person012345]

mass and energy are the same thing.

[Anton P. Nym]

The big difference between "dark matter" and the matter we know well is simply that dark matter doesn't interact much (if at all) with electrical forces, so it doesn't interact (much, if at all) with photons. The only effects we can perceive are gravitational. I don't know much more about it than that.*

As noted above, mass and energy are different forms of the same stuff much like ice and steam are both forms of water.

-- Steve

* Still disappointed that dark matter turned out to be the explanation; I was rooting that its effects were the result of gravitation from masses in neighbouring branes, and thus some proof of string theory's validity. Oh well.

[Moon Goddess]

* Still disappointed that dark matter turned out to be the explanation; I was rooting that its effects were the result of gravitation from masses in neighbouring branes, and thus some proof of string theory's validity. Oh well.

This SOOOO much.

Though now that we know that dark matter is "matter" and not a math problem, I am eager to see if we can figure out if dark matter is effected by the other 2 fundamental forces. Though I'm not sure how we'd even figure that out.

mass and energy are the same thing.

Totally get what you are saying but mass and matter are not the same thing. OP is refering to the fact that photons are matter with a mass of zero. However you are correct that all matter is energy, so it's impossible to have matter than contains mass but no energy.

Edited August 19, 2013 by Moon Goddess

[jwenting]

for the moment "dark matter" is a mathematical trick to account for unexplained gravitational effects we don't yet know what causes them or how to fit them into our existing theories about how gravity works.

Wouldn't surprise me at all if in the not too distant future someone comes up with an amended theory of gravity that explains the universe without needing "dark matter" or "dark energy".

[Moon Goddess]

for the moment "dark matter" is a mathematical trick to account for unexplained gravitational effects we don't yet know what causes them or how to fit them into our existing theories about how gravity works.

Wouldn't surprise me at all if in the not too distant future someone comes up with an amended theory of gravity that explains the universe without needing "dark matter" or "dark energy".

Unfortunately I was hoping that to be true, but we've now been able to use gravitational lensing to 'see' where the dark matter is and map that it has a structure. If we were simply wrong about gravity it would be smooth, dark matter has shape thus showing that "something" is there that has gravitational pull but nothing else.

[Person012345]

This SOOOO much.

Though now that we know that dark matter is "matter" and not a math problem, I am eager to see if we can figure out if dark matter is effected by the other 2 fundamental forces. Though I'm not sure how we'd even figure that out.

Totally get what you are saying but mass and matter are not the same thing. OP is refering to the fact that photons are matter with a mass of zero. However you are correct that all matter is energy, so it's impossible to have matter than contains mass but no energy.

Mass is a measurement of the energy in a system. They are the same. The op is like saying "what if X was 100% weight 0% grams". It doesn't make any sense. Photons have a REST mass of 0 (theoretically although this isn't 100% confirmed, it's possible they have a tiny mass and that light is slower than c). If they have a rest mass of 0 then if they are at rest they presumably also have an energy of 0 (since, y'know...).

Edited August 19, 2013 by Person012345

[Moon Goddess]

Thank you, I was actually quite confusing on that matter, that makes a lot of sense.

[Diche Bach]

Unfortunately I was hoping that to be true, but we've now been able to use gravitational lensing to 'see' where the dark matter is and map that it has a structure. If we were simply wrong about gravity it would be smooth, dark matter has shape thus showing that "something" is there that has gravitational pull but nothing else.

Fascinating. This was the best I could find. After one quick read through I can't say it is crystal clear to me. Know of any better descriptions?

The thing(s) that puzzles me is:

1) is "dark matter" just basically asteroids, planetoids, dust, and the like which is so low albedo and/or so far from a luminous object and doesn't produce any other form of measurable radiation and is therefore simply 'invisible' to our telescopes?

Or is it something more special?

2) Obviously glass or other material through which photons can pass also bends light. Isn't it possible that the phenomenae responsible for gravity lensing of distant objects is behaving by refraction or reflection instead of actual mass?

3) Is mass-based gravity lensing of light something that has been directly measured in experiments, or is it simply something that has been inferred from things like the Hubble Deep Field?

4) Does mass-based gravity lensing apply to other forms of radiation? Can X-rays and gamma rays, etc., be bent?

5) It would seem to me, with my very naïve (but Oh so creative!) view of physical sciences, that, assuming that mass-based modification of light and/or other forms of radiation is a general and well-established phenomenon that it must have some rather profound engineering principles. Namely, if you can control energetic particles with nothing but mass, then it would seem you can potentially harness very large sources of energy for 'free' (meaning without having to expend energy.

For example, for hypothetical cases, lets say we put together an object that was as massive as the Earth (I know, not gonna happen, but bear with the childlike imagining). I would think that if we could shape into a specific shape and position such an object where we wanted, we could effectively harness and intensity radiation from the sun in such a way to concentrate it for some useful purpose?

Obviously, at the mega scale that is not going to happen. But if this mass-energy interaction effect extends downward in scale then perhaps it might really be a basis form something practical at the nano-technology scale?

A device in which some ambient source(s) of radiation (for example bananas, or 'spent' nuclear fuel, etc.) is structured with matching sources of mass to concentrate/magnify and redirect the energy into useful behavior?

ADDIT: Ah one last question!

6) Reading up about this on wiki a bit it sounds like there is at least one major assumption to thisweak gravitation lensing methodology: that the interstellar medium itself is not the source of the lensing.

If the aggregation of many background objects is being used and estimates of the intervening mass are being derived from that, there are, if I'm not wrong, two interpretations:

a) there is an intervening object of "Q" mass and X, Y, Z shape between us and those background objects, i.e., the assumption that seems to be typical.

there are intervening object(s) of "Q" mass and effectively no shape between us and those background objects.

What I mean by ( is: if light is traveling through several hundreds of millions or billions of light years of "empty" space (meaning space that does not have some sort of special "dark matter" in it) it would seem that it could nonetheless be bent by the diffuse matter and energy of the intergalactic medium? Because as we know the "empty" space in our solar system is not truly and completely empty. There are some atoms in the "vacuum" of the interplanetary medium (not to mention energetic particles), and it may be reasonable to postulate that there is also some matter and energy in the intergalactic medium?

Assuming that is the case, wouldn't it be possible for the sum total of all that diffuse matter and or/energy past which the light from the background objects is moving to cumulatively bend it as if the light had instead passed an aggregation of "dark matter" of the same mass?

. . . . Ahhh, now that I think this through a bit more I see that even if ( is the case, lensing is nonetheless revealing some sort of discrete concentrations of mass that is greater than the average distribution!

Currents and eddies in the intergalactic medium?

Edited August 19, 2013 by Diche Bach
Dreaming!

[Anton P. Nym]

1) That was an early proposal, but later discarded; alas I can no longer remember what disproved that theory.

2) If it was refraction/reflection we'd be able to see what was doing the refracting/reflecting; the process is never 100% efficient. We can't observe dark matter at all; it just doesn't seem to interact with photons, full stop.

3) Gravitational lensing has been measured, and even used. Wikipedia says it was first observed in 1979. That article also links to perhaps the most appropriate acronym in physics, the Optical Gravitational Lensing Experiment (OGLE).

4) All electromagnetic radiation acts as light does, indeed "light" is just a set of frequencies within that spectrum that we happen to be able to see with our eyes, so yes.

-- Steve

Edited August 19, 2013 by Anton P. Nym

[jwenting]

Unfortunately I was hoping that to be true, but we've now been able to use gravitational lensing to 'see' where the dark matter is and map that it has a structure. If we were simply wrong about gravity it would be smooth, dark matter has shape thus showing that "something" is there that has gravitational pull but nothing else.

unless we are wrong about gravity fields and those can be granular...

[Moon Goddess]

I think when you're arguing between a granular gravitational field not attached to any matter, and dark matter which exhibits gravitational interaction but no other interaction, I suspect we're just talking semantics.

[Yourself]

The smoking gun (excuse the pun) for dark matter and not some sort of modified gravitational law is the Bullet Cluster. What makes it so compelling is that there's actually a very large discrepancy between the location of the visible matter and the observed gravitational effects (i.e. the location of the dark matter). What allows that to happen is that visible matter tends to be very clumpy. The electromagnetic force causes normal matter to stick together and form things like stars and planets.

Since dark matter doesn't appear to interact through anything but gravity, there's nothing that allows it to stick together. It will still clump into gravitational clusters, but the clumping is generally pretty loose and fuzzy. So you end up with pretty low density (but nonetheless extremely massive) clouds of it. In the case of the Bullet Cluster it appears that a galactic collision managed to separate the dark matter from the visible matter by a measurable amount (which is unusual because gravity tends to keep all these things roughly in the same spot, it just took a very high speed collision to knock them apart).

[Person012345]

1) That was an early proposal, but later discarded; alas I can no longer remember what disproved that theory.

We know how many protons and neutrons there are in the universe apparently. Regular matter doesn't account for it.

[Person012345]

And yeah, we've seen gravitational lensing.

I recommend watching the whole talk, but starting there is the section relevant to dark matter (and if you keep watching he explains why they think the universe is flat and not open).

[Stochasty]

I listened to a very interesting talk at GR20 in Warsaw a month ago that made the claim that Dark Matter is actually an artifact of assuming that the Newtonian gravity approximation was valid on galactic scales. Basically, the claim was that, despite the fact that energy densities are small and velocities are non-relativistic, the Newtonian approximation was invalid because the total mass/angular momentum of the galaxy was not small and the non-linearities in the Einstein field equations add up to a total result that differs significantly from the Newtonian one.

The talk involved using a specific form of the metric to recompute mass distributions based on measured rotation curves for spiral galaxies, and found no need for a Dark Matter halo to explain the rotation curves. If this result holds up, it's rather a big deal, since rotation curves are one of the main reasons we came up with Dark Matter in the first place.

Of course, rotation curves are independent of lensing results such as the bullet cluster; however, it's interesting to note that we also use a Newtonian approximation to compute mass estimates from lensing results, so non-linear effects might be just as important here as they are in rotation curves.

Sadly, since that conference, I've taken a deeper look at the papers upon which the talk was based, and have identified at least one potentially fatal flaw - the density profile chosen for the model has a gradient discontinuity across the z=0 plane which results in the model failing to satisfy the Einstein equations on that plane (the model assumes a dust distribution for matter, but on the z=0 plane there is a delta-function term which appears in at least one of the pressure components of the stress tensor).

This flaw means that without further support for the model (such as a numerical relativity computation) I no longer trust the derived density profiles; however, the existence of this flaw doesn't change the main point of the paper, which is that neglecting the non-linearities of the Einstein equations by using a Newtonian gravity approximation for galaxies is probably a bad idea. As a result, the support for the existence of Dark Matter may be a lot less strong than we think it is. (There is still a third pillar of support for Dark Matter coming from CMB anisotropy measurements, and that support is presumably independent of the issues raised here, but when you kick two legs out from under a tripod it's understandable if the tripod begins to wobble a little.)

Edit: The original paper upon which the talk was based may be found here; see also more recent work by the same authors.

Edited August 20, 2013 by Stochasty

[K^2]

When you say "Newtonian," do you mean pure Newtonian, or something along the lines of linearized gravity? Because I can entirely believe gravitomagnetic effects to be quite significant on the scale of galaxies. But if they are actually saying that the problem is highly non-linear, that would be rather unexpected. I really would like to see some computations supporting that.

[Stochasty]

Pure Newtonian, to my knowledge. At least, the Virial mass is based on Newtonian mechanics. I don't know if anyone has applied a post-Newtonian or linearized gravity approximation and gotten the same results.

Regarding the Cooperstock paper, his claim is that the non-linearities in the Einstein equations matter. To quote from his 2007 paper: "It has been known since the time of Eddington that the gravitationally bound problem in general relativity is an intrinsically nonlinear problem even when the conditions are such that the field is weak and the motions are non-relativistic, at least in the time-dependent case. Most significantly, we have found that under these conditions, the general relativistic analysis of the problem is also nonlinear for the stationary (non-time-dependent) case at hand. Thus the intrinsically linear Newtonian-based approach used to this point has been inadequate for the description of the galactic dynamics and Einstein’s general relativity must be brought into the analysis within the framework of established gravitational theory."

As I said, after working through his model and finding the issue with the delta-function pressure term I'm no longer quite willing to believe the density profile results he presents without further evidence, but I think he has a good point regarding the non-linearities. I would like to see (and if I can find the time I'm considering attempting for myself) a numerical relativity computation of a spiral galaxy assuming a dust fluid for the disk and no Dark Matter. It would be interesting to see what happens.

[K^2]

Is this even doable numerically? There are no obvious symmetries to exploit, so this would be a full 4D sim. And I don't know if there might be a better way to do this, but the only thing that comes to mind is lattice, and 4D lattice computations are tricky. Even in QFT a considerable amount of trickery goes into it, and that stuff's all 100% linear.

[Stochasty]

There's still axial symmetry; the model assumes there's no dependence on \phi in the density or velocity terms, and that the disk is a smooth dust fluid (rather than a collection of point masses).

As far as whether or not it's actually doable; no idea. I've only scratched the surface of numerical relativity (by which I mean one of my lab mates was working on the initial data problem for binary black hole simulations when I was a grad. student, so I absorbed some of the principles by osmosis, but I've never tried such a calculation for myself).

[K^2]

Oh. Uniform? No arms or anything? Then it's uniform in time, as well, and that makes it a 2D problem. That's totally solvable. Still a hell of a mess, of course, since velocity field will depend on metric and vice versa, and that means the equations will have to be solved very precisely. But since the grid is only 2D, there are no hardware limitations. Just complexity of the code. This would be a lot of fun to implement. I can already see how it can be spread over a computing cluster nicely.

If you seriously want to try it and don't mind taking the brunt of the analytic work, I can work out all the numerics. There's probably a paper in it if nobody has done this with high enough fidelity before.

[Stochasty]

I'm definitely interested, if I have the time. I sent you a PM.

Regarding time uniformity: I think that must be a feature of the final analytic solution, but it may be better to keep time dependence in the numerics. The reason would be to show, starting from some given initial data, whether or not the solution converged to something that looked remotely like a spiral galaxy.

Edited August 20, 2013 by Stochasty

[Nuke]

sometimes i wonder if dark matter is really black holes in another universe poking holes in our universe. and they just appear diffuse because of differences in universal laws.

[Moon Goddess]

If your theory were true, the question is what they heck is wrong with that other universe. In our universe everything we can detect and sense makes up 4%, and dark matter makes up 23%. All the black holes in our universe make up a small portion of that 4%. So what's wrong with that other universe that it has so many more black holes?

[Nuke]

its not so much a theory as an idea that just sorta popped into my mind whilst eating questionable brownies.