The true nature of dark matter

[Realitycheck]

With all of the stars throughout the universe over the past ten billion years or so that have cycled through their various elements to finally explode in a massive disarray of heavy elements, it certainly seems that there should be a large amount of bits and pieces of shrapnel and ejecta scattered throughout the universe that never re-assimilated into proto-planetary discs in star-forming regions. That's a lot of matter unaccounted for, exerting its aggregate momentum on the rest of the matter spinning around in galaxies or outward from the big bang.

 

I am referring to the wiki for dark matter which describes it as non-baryonic matter that is not detectable by electromagnetic methods. I find this highly read wiki to be highly and exceptionally suspect. Wouldn't all of this leftover unrecycled matter be too small and scattered out to detect after enough time had passed and it had cooled enough? With the exceptionally short lives that these types of stars live and the incredible number of times that these types of stars have potentially formed, it only makes sense that there should be lots of ordinary matter drifting around out there in space. Why should there be so much more exotic, unknown form of matter in the universe when I can't even find any on my planet?

 

What are your thoughts on this?

[Mr Skeptic]

I too think dark matter is highly suspect. However, at the very least it can give us data about gravity anomalies until we get a better theory. Of course if they can separate dark matter from regular matter then it would be more believable.

[Realitycheck]

http://en.wikipedia.org/wiki/Dark_matter

[pioneer]

If you look at the EM, weak and strong nuclear forces, when they lower potential, energy is given off. If gravity is also a force, like the other three forces, one would expect gravity would also give off energy when it lowers potential.

 

For example, if an electron lowers energy, a photon is given off. This photon can hit another atom and knock an electron to a higher energy level. The energy output has an anti-EM effect on other atoms.

 

If gravity is a force like the rest, when gravity lowers potential it should give off energy, which can increase the gravitational potential elsewhere. In the case of the universe, the summation cause it to expand.

 

Say we had a clump of matter in space that is collapsing to form a star. Since the gravity potential is lowering, the theory says energy will be given off at the same time it collapses. The combined effect would be collapsing and spinning.

 

The acceleration expansion of the universe would imply that the amount of matter in the universe, lower gravity potential into galaxies, stars, planets, blackholes, is increasing over time. Are there more galaxies and stars now than 10 Billion years ago, 5 billions years ago, etc.?

 

Since the matter of the universe tends to clump within galaxies, the bulk gravity energy output should have a connection to these largest bulk sources, causing the universe to expand relative to the galaxies.

 

Dark energy is the energy output stemming from gravity. Dark matter is predicted from the equivalence of matter and energy.

[Realitycheck]

I liked this other explanation quite a bit more, even though it was a howstuffworks article. Still does not quantify the total amount of unrecycled supernova ejecta. I have a hard time believing that gravity holds the ejecta in when there is no object to constitute gravity, it should be exploding indefinitely, without restriction through free space.

[Severian]

Why do you think you wouldn't be able to see this "shrapnel"? It would still scatter light. What you have in mind sounds very like Saturn's rings and we can see them.

[pioneer]

Relative to the shrapnel effect, one important consideration is entropy. Entropy needs to absorb energy to increase. As long as the shrapnel material is increasing degrees of freedom or entropy, some of the reflected energy signal will be absorbed. Matter will become more stealth because of entropy.

 

Let me give an example. Say we had a cylinder of compressed gas. We look at the cylinder using IR. We open the valve to expand the gas, increasing entropy. We would see a red shift as the gas and cylinder gets colder. The energy lost went into entropy and did not radiate to us.

 

I would assume the emptiness of space allows continued degrees of freedom. Once gravity begins to act, the entropy begins to lower. Lowering entropy will give off the energy that had been absorbed.

 

If the lowering of gravitational potential is given off an exothermic output to the universe, this may be what is driving the second law by offering universal energy for entropy to increase. The lowering of gravitational potential, will lower mass entropy. This will be exothermic and give off energy.

[Realitycheck]

Because it would just continue to scatter indefinitely? Most pictures of supernovae that we are familiar with, like Kepler's for instance, is only 400 years old. Sooner or later, doesn't it become too diffused and scatter into oblivion? But I'm kind of starting to see how that probably wouldn't measure up to the other 20% of matter, since only a small fraction of stars go supernova. It's tough to know for sure though, because when the universe was smaller and younger, I believe that supernovae were more prevalent.

[Janus]

With all of the stars throughout the universe over the past ten billion years or so that have cycled through their various elements to finally explode in a massive disarray of heavy elements, it certainly seems that there should be a large amount of bits and pieces of shrapnel and ejecta scattered throughout the universe that never re-assimilated into proto-planetary discs in star-forming regions. That's a lot of matter unaccounted for, exerting its aggregate momentum on the rest of the matter spinning around in galaxies or outward from the big bang.

 

I am referring to the wiki for dark matter which describes it as non-baryonic matter that is not detectable by electromagnetic methods. I find this highly read wiki to be highly and exceptionally suspect. Wouldn't all of this leftover unrecycled matter be too small and scattered out to detect after enough time had passed and it had cooled enough? With the exceptionally short lives that these types of stars live and the incredible number of times that these types of stars have potentially formed, it only makes sense that there should be lots of ordinary matter drifting around out there in space. Why should there be so much more exotic, unknown form of matter in the universe when I can't even find any on my planet?

 

What are your thoughts on this?

 

Here's just two reasons why this doesn't work.

 

1. When we look at far distant galaxies, we see them as they were billions of years ago. If dark matter were the remnants left over from earlier stars, the further the galaxy is away from us, the more of these earlier stars we would still see. In other words, as we look at further and further galaxies, we should detect more and more baryonic matter and less and less dark matter. We don't.

 

2. If all these earlier stars produced heavy elements, we would see a higher percentage of heavier elements than we do. It doesn't matter how much of this remnant formed latter stars, the mere fact of larger concentrations of heavy elements would change the relative abundance of heavy to light elements in the spectrum we get from second generation stars.

The ratio of elements that we measure in our observations precludes the existence of much more baryonic matter than we see.

[Akhenaten2]

If you look at the EM, weak and strong nuclear forces, when they lower potential, energy is given off. If gravity is also a force, like the other three forces, one would expect gravity would also give off energy when it lowers potential.

 

For example, if an electron lowers energy, a photon is given off. This photon can hit another atom and knock an electron to a higher energy level. The energy output has an anti-EM effect on other atoms.

 

If gravity is a force like the rest, when gravity lowers potential it should give off energy, which can increase the gravitational potential elsewhere. In the case of the universe, the summation cause it to expand.

 

Say we had a clump of matter in space that is collapsing to form a star. Since the gravity potential is lowering, the theory says energy will be given off at the same time it collapses. The combined effect would be collapsing and spinning.

 

The acceleration expansion of the universe would imply that the amount of matter in the universe, lower gravity potential into galaxies, stars, planets, blackholes, is increasing over time. Are there more galaxies and stars now than 10 Billion years ago, 5 billions years ago, etc.?

 

Since the matter of the universe tends to clump within galaxies, the bulk gravity energy output should have a connection to these largest bulk sources, causing the universe to expand relative to the galaxies.

 

Dark energy is the energy output stemming from gravity. Dark matter is predicted from the equivalence of matter and energy.

And if Gravity isn't a force?

[Severian]

And if Gravity isn't a force?

It is.

[Akhenaten2]

It is.

Don't worry, the vast majority of people agree with you. I dont', but I'm in a tiny minority. However that includes, besides me, Michio Kaku, Neil de Grasse Tyson, who have both said so on TV recently + a few other notables doing similarly + that other chap - er what's is name? I forget now! - oh yes that's it a Mr A. Einstein!!!

[ajb]

Don't worry, the vast majority of people agree with you. I dont', but I'm in a tiny minority.

 

Well, I am sure Severian is versed enough in physics to understand what the people you have listed mean by the statement that gravity is not a force.

 

It depends what you mean by a force. If I think about the relation between the Newtonian notion of a force and in particular acceleration then gravity is a force. Things accelerate in a gravitational field.

 

However, of you examine the Euler-Lagrange equations of a test particle in a gravitational field (viz general relativity) you see there is no term related to a classical force. In a sense, the non-trivial local geometry replaces this term.

 

So, is it a force or not? I would say yes, as gravity allows bodies to exchange momentum.

[Severian]

So, is it a force or not? I would say yes, as gravity allows bodies to exchange momentum.

 

I would go even further than that. Anything which causes an acceleration is a force, even if you are in a non-inertial frame. So a centrifugal force is still a force. It is not, however, a fundamental force.

[ajb]

I would go even further than that. Anything which causes an acceleration is a force, even if you are in a non-inertial frame.

 

Sure.

 

 

So a centrifugal force is still a force. It is not, however, a fundamental force.

 

So, via the equivalence principle would you say that gravity is therefore not a fundamental force? It is a bi-product of the geometry.

 

If I am honest, I don't worry if gravity is a force or not. We know how to deal with the motion of test particles under it's (classical) influence and that is what really matters.

[vordhosbn]

Sorry about the offtopic question, but is the "curving" of space-time by large mass bodies just another abstraction, usefull in explaining gravity, or is it something ... hmmm... real?

[ajb]

Sorry about the offtopic question, but is the "curving" of space-time by large mass bodies just another abstraction, usefull in explaining gravity, or is it something ... hmmm... real?

 

 

The philosophy that I try to keep is that mathematical models (aka physical theories) describe nature but are not nature. The only truly real things are what we can measure.

 

But then when you really look into physical theories it is difficult to keep this distinction. When you start to think mathematically things start to look mathematical.

 

Often the language used in theoretical physics does not make the mathematical world and physical world seem very distinct. Whatever one's philosophy you cannot doubt the power of mathematics in the physical sciences.

[Darkchilde]

Force is applied to objects; gravitational force has a meaning when applied to a mass. Gravity is actually a field; not a force per se. The Earth produces a gravitational field, and this field is what attracts my body.

 

So, gravity is not a force but a field. It becomes a force when the field interacts with a mass, and then a force is exerted on the mass which is equal to F=mg where m=mass and g=acceleration of gravity.

[Severian]

So, via the equivalence principle would you say that gravity is therefore not a fundamental force? It is a bi-product of the geometry.

 

No, I wouldn't subscribe to that. My the same token you could say that electromagnetism isn't a fundamental force - it is a bi-product of the U(1) gauge symmetry.

 

Though I do think your point has merit.

 

To be honest, I think it is simply too early to say for gravity. We need to understand it better.

[Radical Edward]

when you consider the volume of matter needed to account for gravitational phenomena such as the lack of falloff in rotation speeds, it would be very very visible. If it were normal electromagnetically interacting matter, then it would have a spectroscopic signature that we could observe. The fact of the matter is that there is no spectroscopic signature. What is more, from our understanding of the Big Bang, it simply could not produce enough baryons to account for this amount of matter without severe implications for what we do see. Whilst I do see reason for cynicism as regards WIMPS, suggesting baryonic matter isn't a solution either.

[ajb]

No, I wouldn't subscribe to that. My the same token you could say that electromagnetism isn't a fundamental force - it is a bi-product of the U(1) gauge symmetry.

 

Good point. Though I would say that electromagnetism is due to the geometry of U(1) principle (or associated vector) bundles.

 

Force is curvature!

 

For U(1) we can really make this concrete by adding the curvature to the symplectic form on the phase space of a charged particle and then derive the "free Hamilton's equations" for this modified symplectic form. This is then equivalent to the Lorentz force.

 

 

Though I do think your point has merit.

 

To be honest, I think it is simply too early to say for gravity. We need to understand it better.

 

The philosophical barrier with gravity is that it "happens on space-time" . (really one should think about natural bundles (or the principle bundles associated with these) over space-time, but as we have a functor from the category of smooth manifold to vector bundles the distinction is not really required) . The EM, weak and strong "happen on bundles over space-time". They don't mess with the arena of the physics. At least not in the same way.

 

I completely agree with you on the need to better understand gravity.

 

One thing to keep in mind is that a definition of force has it's roots in Newtonian ideas. If we require a post-Newtonian notion of dynamics then one would expect the notion of a force will also require modification. It makes me wonder if we should not really get too worried if gravity or the other "interactions" are really forces in any technical sense. We have accelerations or exchanges of momentum and that is enough.

[Severian]

It makes me wonder if we should not really get too worried if gravity or the other "interactions" are really forces in any technical sense. We have accelerations or exchanges of momentum and that is enough.

 

Yes, that is sort of the point that I intended to make.

[Akhenaten2]

Good point. Though I would say that electromagnetism is due to the geometry of U(1) principle (or associated vector) bundles.

 

Force is curvature!

 

For U(1) we can really make this concrete by adding the curvature to the symplectic form on the phase space of a charged particle and then derive the "free Hamilton's equations" for this modified symplectic form. This is then equivalent to the Lorentz force.

 

 

 

 

The philosophical barrier with gravity is that it "happens on space-time" . (really one should think about natural bundles (or the principle bundles associated with these) over space-time, but as we have a functor from the category of smooth manifold to vector bundles the distinction is not really required) . The EM, weak and strong "happen on bundles over space-time". They don't mess with the arena of the physics. At least not in the same way.

 

I completely agree with you on the need to better understand gravity.

 

One thing to keep in mind is that a definition of force has it's roots in Newtonian ideas. If we require a post-Newtonian notion of dynamics then one would expect the notion of a force will also require modification. It makes me wonder if we should not really get too worried if gravity or the other "interactions" are really forces in any technical sense. We have accelerations or exchanges of momentum and that is enough.

Hooray, Iv'e been trying to get the message over to physisists for 25 years or more, that despite A.E. describing it accurately to us 95 years ago, gravity continues to be still totally MIS-REPRESENTED as a FORCE.

It is of course, an acceleration (which means it mimics and can be calculated as if a force) but never-the-less is not a force. Once you accept that you can start making sense of everything.

 

"We have acceleration or exchange of momentum and that is enough"

What kind of complacency is this?

What about the billions of dollars and vast resources, being wasted on the futile search to unify gravity as a 4th force (the incredible weakness by comparison should be enough to realise it is not a force) with the other 3 forces of nature? What about all the mis-conceived experiments designed to find effects of a force which doesn't exist. Gravitons? - I ask you!!, Gravitinos? I ask you!!! - Total rubbish!!!

For example - Gravity waves exist, but there is no way they can be detected by deflecting mirrors or other 'mechanical' devices.

"LIGO" is a complete waste of time, as Iv'e I've already told them.

 

GRAVITY IS NOT THE PRIME MOVER OF THE UNIVERSE!! - it is merely an effect!!! - This is probably all I'm prepared to say at the moment.

Edited March 20, 2010 by Akhenaten2

[Simpleton]

Hooray, Iv'e been trying to get the message over to physisists for 25 years or more, that despite A.E. describing it accurately to us 95 years ago, gravity continues to be still totally MIS-REPRESENTED as a FORCE.

It is of course, an acceleration (which means it mimics and can be calculated as if a force) but never-the-less is not a force. Once you accept that you can start making sense of everything.

 

"We have acceleration or exchange of momentum and that is enough"

What kind of complacency is this?

What about the billions of dollars and vast resources, being wasted on the futile search to unify gravity as a 4th force (the incredible weakness by comparison should be enough to realise it is not a force) with the other 3 forces of nature? What about all the mis-conceived experiments designed to find effects of a force which doesn't exist. Gravitons? - I ask you!!, Gravitinos? I ask you!!! - Total rubbish!!!

For example - Gravity waves exist, but there is no way they can be detected by deflecting mirrors or other 'mechanical' devices.

"LIGO" is a complete waste of time, as Iv'e I've already told them.

 

GRAVITY IS NOT THE PRIME MOVER OF THE UNIVERSE!! - it is merely an effect!!! - This is probably all I'm prepared to say at the moment.

 

I am totally in agreement with you. I believe that gravity is the product of the consumption of background radiation by all that is mater.

Take note.

I realy am and feel as my name implies, when I dare to make a comment on this site.

[ajb]

"We have acceleration or exchange of momentum and that is enough"

What kind of complacency is this?

 

At the risk of repeating myself...

 

As we know how to deal with classical gravity, and specifically the motion of test particles I am not at all worried if gravity is a force or not in any technical sense.

 

I think that people realised that gravity is somehow different to the electromagnetic force soon after it was proposed. The dynamics of a test particle does not an explicit force term. It is all tied into the geometry. This is well known by everyone working on gravity.

 

(Geometry is also fundamental in the description of the other forces.)

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Merged post follows:

Consecutive posts merged

I wondered if this example is useful.

 

Let us consider a two dimensional manifold, and I will equip it with a pseudo-Riemannian metric. Let us employ local coordinates [math] \{ t,x \}[/math] such that the metric has the form

 

[math]g = - e^{2 \Phi(x)}dt \otimes dt + dx \otimes dx [/math].

 

This metric we would call static. I make no claim that it is the most general static metric. Also, as we are in two dimensions the Einstein field equations are trivial and we don't have any propagating degrees of freedom. Furthermore, all two dimensional Riemannian manifolds are conformally flat. But none of this is of concern in this example.

 

The nontrivial connection coefficients are

 

[math]\Gamma^{0}_{10}= \Gamma^{0}_{01} = \frac{\partial \Phi}{\partial x}[/math]

 

and

 

[math]\Gamma^{1}_{00} = \frac{\partial}{\partial x }\left ( \frac{1}{2} e^{2 \Phi}\right) [/math].

 

Then the geodesic equations are

 

[math] \frac{d^{2}t}{ds^{2}} + 2 \frac{\partial \Phi}{\partial x} \frac{dt}{ds}\frac{dx}{ds}=0[/math],

 

[math]\frac{d^{2}x}{ds^{2}} + \frac{\partial}{\partial x }\left ( \frac{1}{2} e^{2 \Phi}\right) \frac{dt}{ds} \frac{dt}{ds} [/math].

 

Now, it I am interested in the motion of massive particles I can reparametrise the geodesic with the time.

 

Then the geodesic equations become

 

[math]\frac{\partial \Phi}{\partial t } = 0[/math] which is trivial.

 

[math]\frac{d^{2} x}{dt^{2}} + \frac{\partial}{\partial x }\left ( \frac{1}{2} e^{2 \Phi}\right) =0 [/math].

 

This looks like Newton's law with the potential [math]U = \frac{1}{2} e^{2 \Phi(x)}[/math].

 

So, on this 2-d manifold time-like geodesics can be written "as a force" via a particular choice of coordinates. I'll stress that again, I have made a choice in the coordinates employed.

 

We can also look at null geodesics in these coordinates. We have

 

[math]dx = \pm e^{\Phi(x)} dt[/math].

 

The question now is how does this generalise to static metric on 4-d manifolds? I expect you can do something very similar. Now, we can use the field equations and I think that gives that our potential must be Harmonic (in vacua). I may look more into this...

 

Anyway, for static metrics with a lot of symmetry, like the Schwartzchild this construction generalises. Without the symmetry I'd expect the equations to be complicated and not so easy to interpret.

 

As an aside it also reminds me of dilaton theories of gravity, but is another story.

Edited March 22, 2010 by ajb
Consecutive posts merged.