Time to put my ‘nutter’ hat on. so please bear with me ..

Imagine a ‘box’ of arbitrary space-time …you could calculate the net gravitational potential of all the mass/energy within that box and come up with a figure.. This would be much less than the ‘observed’ gravitational potential in that box – and the difference is what we describe as dark matter. We’re saying there must be ‘more stuff’ inside that box than we can observe.

What if there was something ‘outside’ of the box, constraining the particles within the box .. something from a ‘higher’ (yech, i hate that term) dimension which was acting to ‘squeeze’ the box, keeping the particles within the box and making it look like there was more gravity going on than there was…

What if the areas where this ‘constraining’ happened tended to be areas where we observe matter clumping (say galaxies), so in effect describing galactic halos …

Have there been any observations made that would discredit this type of explanation (I won’t deign to call it a theory – cause a theory makes predictions)?

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/060726a.html

Q. Since string theory implies up to 11 dimensions, could dark matter be gravitons leaking from other dimensions into ours?

A. Actually, that’s exactly one possibility that’s being explored by brane-world theorists. Of course, the jury is still out on brane-world theories….

From the Cornell Chronicle: http://www.news.cornell.edu/stories/March06/Tye.brane.ws.html “In brane-world theory, the ends of strings are anchored in our brane, so the particles we see can only move within the brane. But the particles that carry the gravitational force, known as gravitons, are closed strings — little Cheerios — and can “leak” out of the brane. This explains why gravity is much weaker than the electromagnetic force and the strong and weak nuclear forces. It also offers a possible explanation for the “dark matter” that astronomers need to explain why the mass of the universe doesn’t agree with the observed objects. Dark matter could be in an adjacent brane, with its gravitons leaking into ours.”

————-
that’s all from there.

I look forward to sharing the nobel..

http://www.news.cornell.edu/stories/2006/03/does-our-3-d-world-hold-six-other-dimensions

this is the article linked to. the url in my post is 404.

I think the most likely candidate for dark matter is slow neutrinos. Since it is currently accepted that neutrinos have non-zero mass, they must travel at less than c, and therefore able to have any such speed (including zero). While their extremely low mass does mean that we only see high-speed neutrinos (and/or antineutrinos) from b-decay type processes, it shouldn’t be taken to imply that neutrinos can’t be slowed down. Neutrinos don’t interact electromagnetically so there would not be any scattering phenomena that would indicate baryonic matter or any charged particles.

On the whole, I prefer this explanation to any of the more exotic explanations such as string theory or particles that have never been observed in our particle-collider experiments.

What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

Dropbear said:

What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

No, no, you need to think outside the box :)

KJW, I must admit that I’ve never heard os the slow neutrino theory before. Are there any experiments set up to detect these?

Dropbear said:

What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

I think that may conflict with general relativity, in particular Birkhoff’s theorem.

I read something earlier on today where the author poo-pooed the idea of DM being neutrinos, because that many neutrinos in a very early galaxy would have overwhelmed gravity and stopped the early galaxies coalescing.

sibeen said:

KJW, I must admit that I’ve never heard os the slow neutrino theory before. Are there any experiments set up to detect these?

Sterile neutrino

It’s not entirely clear to me that this is exactly what I am referring to (neutrinos are peculiar objects that are unlike any other particle and particular semi-intuitive notions that may apply to other particles do not seem to apply to neutrinos).

Dropbear said:

I read something earlier on today where the author poo-pooed the idea of DM being neutrinos, because that many neutrinos in a very early galaxy would have overwhelmed gravity and stopped the early galaxies coalescing.

The fact of the matter is that there are no obvious candidates for dark matter because known physics doesn’t seem to be able to fully explain the observed properties of dark matter. But, I would put my money on less exotic physics than more exotic physics any day.

would there not be a hint in the standard model if something exotic was DM?

I reckon dark matter does not exist…

it does because we see the effects and no tweaking of gravity theories can account for these effects. afaik.

• it does because we see the effects and no tweaking of gravity theories can account for these effects. afaik.

That’s the problem. Observation doesn’t match prediction so instead of reassessing prediction, let’s just make something up that makes it fit…

KJW said:

Dropbear said:

What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

I think that may conflict with general relativity, in particular Birkhoff’s theorem.

While Birkhoff’s theorem is specifically about Schwarzschild geometry, in a broader sense, it is saying that the gravitation within a region is due to sources inside the region, not outside the region. For example, the gravitation inside a hollow spherical shell is zero.

furious said:

• it does because we see the effects and no tweaking of gravity theories can account for these effects. afaik.

That’s the problem. Observation doesn’t match prediction so instead of reassessing prediction, let’s just make something up that makes it fit…

Dark matter is definitely not due to a modification of gravitational law. The Bullet Cluster shows that dark matter is some form of material.

Observation doesn’t match prediction so instead of reassessing prediction, let’s just make something up that makes it fit…

observation came along after the prediction. DM postdates relativity and newtonian gravity. DM is an anomaly. same as dark energy. they have reassessed “prediction” anyway in the form of MOND. and they haven’t been successful. so DM is “something”.

I’m not an expert in such things but when a theory says one thing and then observation shows another I think filling the gap with an unobserved “other” is just a little bit dodgy…

furious said:

I’m not an expert in such things but when a theory says one thing and then observation shows another I think filling the gap with an unobserved “other” is just a little bit dodgy…

so what do you consider to be a sensible method of moving on with science when observation does not match prediction?

the thing is our gravity theories are pretty solid so it is more likely something else that is the cause.

of course our gravity theories still need work….

• so what do you consider to be a sensible method of moving on with science when observation does not match prediction?

Reassess the theory instead of moving the gold posts to fit the theory…

I don’t know how goal became gold…

furious said:

I’m not an expert in such things but when a theory says one thing and then observation shows another I think filling the gap with an unobserved “other” is just a little bit dodgy…

But the observations that led to dark matter came first and theory had to catch up.

• But the observations that led to dark matter came first and theory had to catch up.

I thought it was more like:

Gravity theory says this and this but now we look for it observation shows that and that therefore there must be something else we can’t see to make it fit so let’s call it dark stuff and give it a value to fit the theory…

furious said:

• But the observations that led to dark matter came first and theory had to catch up.

I thought it was more like:

Gravity theory says this and this but now we look for it observation shows that and that therefore there must be something else we can’t see to make it fit so let’s call it dark stuff and give it a value to fit the theory…

I don’t see the problem. As Dropbear pointed out, that’s how science develops. First we have observations, then we develop a theory to explain those observations. Then new observations come along that conflict with the new theory so we have to develop a theory to explain those observations. And the process continues. For example, during the 19th century, thermodynamics was developed, but when applied to blackbody radiation, it failed, and thus quantum theory was born.

But in this case they kept the theory the same and invented dark stuff to make it fit the observation…

it is one or the other and in this case, like i said earlier, our gravity theory is pretty good.

• our gravity theory is pretty good

Except for when observations didn’t match so dark stuff needed to be invoked to fill in the blanks…

but that shows our theories are good otherwise we would be looking at ways to change them. unless something really weird is in relativity that we haven’t seen…

While Newton’s gravitation theory was the accepted theory of gravitation, it was known for a long time that there was 43 arc-seconds/century that could not be accounted for in the perihelion motion of Mercury. When Einstein developed general relativity, the missing 43 arc-seconds/century was accounted for. While it’s not clear that Einstein intended to account for the missing 43 arc-seconds/century, he was aware of it, and included it in his calculations.

furious said:

But in this case they kept the theory the same and invented dark stuff to make it fit the observation…

But the term “dark matter” is a description of the observed phenomena.

• but that shows our theories are good

That’s what I can’t see…

• But the term “dark matter” is a description of the observed phenomena.

Which seemingly did not match the predicted phenomena…

furious said:

• But the term “dark matter” is a description of the observed phenomena.

Which seemingly did not match the predicted phenomena…

cool hey!! new science time

it is our particle theories that need looking at not our gravity theories.

• cool hey!! new science time

Everything is cool when I can make up numbers to fit my idea of reality…

cool hey!! new science time

time to sharpen the razor too.

furious said:

Everything is cool when I can make up numbers to fit my idea of reality…

But they’re not making up numbers. They’re measurements of the actual reality.

Everything is cool when I can make up numbers to fit my idea of reality…

that expression is so over used by “alternative science/my theory” proponents that i cringe when i see someone use it.

• it is our particle theories that need looking at not our gravity theories.

Its better to invent a new theory to match the phenomena rather than invent a new phenomena to match the theory..

Boris said:

it is our particle theories that need looking at not our gravity theories.

I’d be very wary of making a statement like that.

The ‘we know it all’ school seem to have been bitten on the arse on all occasions.

furious said:

• it is our particle theories that need looking at not our gravity theories.

Its better to invent a new theory to match the phenomena rather than invent a new phenomena to match the theory..

What’s the difference?

• that expression is so over used by “alternative science/my theory” proponents that i cringe when i see someone use it.

I’m not into making up theories only in this case I am into questioning re-sizing jigsaw pieces to fit the puzzle…

• What’s the difference?

Really?

i know furious, that wasn’t directed at you directly but more of an aside as to who usually say that, and a heads up as to how it comes across. for future reference. if you want.

:-)

furious said:

• What’s the difference?

Really?

Yes, really. The ultimate goal is to provide an explanation of the observed phenomena that can withstand the scrutiny of what is known to be true.

maybe “looking to…” would have been better sibeen. it is more likely to be a particle rather than the other stuff. if i was tom i’d give you odds.

• Yes, really. The ultimate goal is to provide an explanation of the observed phenomena that can withstand the scrutiny of what is known to be true

But phenomena remains the same regardless of the theory explaining it…

The ultimate goal is to provide an explanation of the observed phenomena that can withstand the scrutiny of what is known to be true.

and no matter who falls by the wayside.

furious said:

• Yes, really. The ultimate goal is to provide an explanation of the observed phenomena that can withstand the scrutiny of what is known to be true

But phenomena remains the same regardless of the theory explaining it…

But what is the phenomena of which you speak?

• But what is the phenomena of which you speak?

It works for all things but in this case we are talking about dark stuff. Theory said one thing, observation said another. Instead of changing theory, a new phenomena was invented to match the theory. I just think that theory should have been reevaluated…

furious said:

• But what is the phenomena of which you speak?

It works for all things but in this case we are talking about dark stuff. Theory said one thing, observation said another. Instead of changing theory, a new phenomena was invented to match the theory. I just think that theory should have been reevaluated…

I know we are talking about “dark stuff”, but what is the phenomena that remains the same regardless of the theory?

• I know we are talking about “dark stuff”, but what is the phenomena that remains the same regardless of the theory?

All phenomena remains the same regardless of the theory. Phenomena is the way that it is, theory is the way thatwe think that it is…

furious said:

• I know we are talking about “dark stuff”, but what is the phenomena that remains the same regardless of the theory?

All phenomena remains the same regardless of the theory. Phenomena is the way that it is, theory is the way thatwe think that it is…

So what you’re saying is that there ought to be no theories at all, and simply let reality be what it is without explanation?

• So what you’re saying is that there ought to be no theories at all, and simply let reality be what it is without explanation?

Not at all. What I am saying is that when the theory is proven wrong by observation then the theory needs to be rethought instead of inventing something to make it fit…

furious said:

Not at all. What I am saying is that when the theory is proven wrong by observation then the theory needs to be rethought instead of inventing something to make it fit…

But what theories have been proven wrong?

• But what theories have been proven wrong?

OK, you’re the physicist so you do know more than me but I see the idea of dark stuff growing out of observations not matching what was previously supposed. Theory says X but observation says Y so to make it fit there must be a -Z. All I am saying is that maybe X was wrong so -Z isn’t,t needed…

furious said:

OK, you’re the physicist

Actually, I’m not a physicist. I know physics, but that is not my profession.

• Actually, I’m not a physicist. I know physics, but that is not my profession.

Re

Stupid tablet posting randomly…

furious said:

• But what theories have been proven wrong?

I see the idea of dark stuff growing out of observations not matching what was previously supposed. Theory says X but observation says Y so to make it fit there must be a -Z. All I am saying is that maybe X was wrong so -Z isn’t,t needed…

But we don’t actually know if it’s X that wrong or if -Z is needed. That question is part of the exploratory process.

• But we don’t actually know if it’s X that wrong or if -Z is needed. That question is part of the exploratory process.

But from observation Y is true and so the only way to make X = Y is to apply -Z or rethinking X. I think rethinking X is more valid then inventing -Z…

furious said:

• But we don’t actually know if it’s X that wrong or if -Z is needed. That question is part of the exploratory process.

But from observation Y is true and so the only way to make X = Y is to apply -Z or rethinking X. I think rethinking X is more valid then inventing -Z…

No, the more valid is the explanation, whatever it is, that explains the observations it is intended to explain as well as all the other observations that have ever been made of reality.

• No, the more valid is the explanation, whatever it is, that explains the observations it is intended to explain as well as all the other observations that have ever been made of reality.

If the observations do not match the original theory then the theory needs to be rethought, don’t make up side theories that make the original observations match the original theory…

furious said:

If the observations do not match the original theory then the theory needs to be rethought, don’t make up side theories that make the original observations match the original theory…

But let’s not forget that the original theories exist because they fit other observations. Therefore, any new theory must be consistent with the original observations as well as the theory-changing observations.

Also, when it comes to changing theories, mathematics also has something to say: gauge theory cannot be ignored.

• But let’s not forget that the original theories exist because they fit other observations. Therefore, any new theory must be consistent with the original observations as well as the theory-changing observations.

In those cases, the theory was changed to fit the observation. No problem with that. In the case of dark stuff the theory was adjusted to the observation by introducing something not originally conceived of just so the theory remained…

>>>
What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

like a reverse gravity force of some kind?

• Also, when it comes to changing theories, mathematics also has something to say

I have nô problem with a change in theory I just don’t agree with creating an unknown to make an observation fit a theory…

Sorry.

CrazyNeutrino said:

>>>
What about the idea (stated in this thread) that DM isn’t anything in the box at all, it’s something constraining the box ;)

like a reverse gravity force of some kind?

A spherical shell of matter has no gravitational effect on its interior, be it normal or reverse.

furious said:

• But let’s not forget that the original theories exist because they fit other observations. Therefore, any new theory must be consistent with the original observations as well as the theory-changing observations.

In those cases, the theory was changed to fit the observation. No problem with that. In the case of dark stuff the theory was adjusted to the observation by introducing something not originally conceived of just so the theory remained…

But we don’t know what dark matter is. Some people think it represents a modified law of gravitation, other people think it is some form of invisible matter. Of those that think it is some form of invisible matter, these can be divided into those that think it is ordinary matter that is invisible because it is too small to see and nothing to illuminate it, and those that think it is exotic matter that we have not yet observed in our laboratories. Irrespective of the range of possibilities, dark matter is something specific and our hypotheses of what that is will need to conform to a vast body of knowledge about the dark matter observations as well as the entirety of physics.

I can’t see that it is anything exotic that we are unaware of. Nothing suggests that it is not ordinary matter that is yet to be accounted for…

furious said:

Nothing suggests that it is not ordinary matter that is yet to be accounted for…

The problem with it being ordinary matter is that dark matter doesn’t scatter the background light.

what does dark matter react with?

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

• The problem with it being ordinary matter is that dark matter doesn’t scatter the background light.

You are never going to see it from my point of view. I try to see the mainstream but it appears to be a lot of supposition to me. In this we can probably never agree. Thanks anyway.

KJW said:

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

By “observed gravitationally”, I mean by gravitational lensing effects.

KJW said:

KJW said:

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

By “observed gravitationally”, I mean by gravitational lensing effects.

Riff-in-Thyme said:

KJW said:

KJW said:

It is observed gravitationally, and does not interact electromagnetically.

By “observed gravitationally”, I mean by gravitational lensing effects.

Could it be said that dark matter is an unknown constraint within the gravity phenomena?

furious said:

You are never going to see it from my point of view.

Just because I disagree doesn’t mean I don’t see your point of view. For example, you said earlier:

furious said:

I just don’t agree with creating an unknown to make an observation fit a theory…

But it may surprise you that theoretically creating a new field is a part of gauge theory. The Higgs field is an example of such a field. Gravitation can be viewed as an artificial field created to explain the deviations from flat spacetime physics.

KJW said:

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

is this from experimentation/ observation or is a theory?

KJW said:

KJW said:

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

By “observed gravitationally”, I mean by gravitational lensing effects.

doesn’t ordinary matter affect light and lens it too?

Riff-in-Thyme said:

Could it be said that dark matter is an unknown constraint within the gravity phenomena?

The Bullet Cluster (mentioned earlier) appears to indicate that dark matter is some form of material and not a deviation from established gravitational law.

wookiemeister said:

KJW said:

wookiemeister said:

what does dark matter react with?

It is observed gravitationally, and does not interact electromagnetically.

is this from experimentation/ observation or is a theory?

This is from observation. The gravitational lensing is how we see it in the first place and the absence of electromagnetic interaction is known because it does not scatter background light.

wouldn’t dark matter be observable here too, if its everywhere wouldn’t it be here too?

I do not know why this particular theory bugs me as there are plenty of theories that I have never “seen” but when anyone pulls out an imaginary substance to explain something I automatically get on the defensive. I can’t see how this substance is more real than readdressing the original theory…

• wouldn’t dark matter be observable here too, if its everywhere wouldn’t it be here too?

I wonder that too and the answer is similar to why I’ve not yet seen a ghost…

wookiemeister said:

wouldn’t dark matter be observable here too, if its everywhere wouldn’t it be here too?

It’s not uniformly everywhere. However, there is the question of how we would know if we did detect it here. There would still be the question of whether what we detected here is the same as the dark matter observed elsewhere.

KJW said:

wookiemeister said:

KJW said:

It is observed gravitationally, and does not interact electromagnetically.

is this from experimentation/ observation or is a theory?

This is from observation. The gravitational lensing is how we see it in the first place and the absence of electromagnetic interaction is known because it does not scatter background light.

whats the difference between background light and light coming from a source such as a star which is presumably lensing the light?

just a thought

it makes me wonder if these effects are about a time distortion caused by the gravity of the thing doing the lensing rather than dark matter, its a false result – not that I know anything about this subject

KJW said:

Riff-in-Thyme said:

Could it be said that dark matter is an unknown constraint within the gravity phenomena?

The Bullet Cluster (mentioned earlier) appears to indicate that dark matter is some form of material and not a deviation from established gravitational law.

I would still leave room for other explanation. For instance does FoR play a greater role than we have measured? Lensing only suggests that various regions have greater available momentum than others.

• It’s not uniformly everywhere. However, there is the question of how we would know if we did detect it here. There would still be the question of whether what we detected here is the same as the dark matter observed elsewhere.

Its nice to have a random unknowable that works to fill any gaps that you might have at any one time…

wookiemeister said:

whats the difference between background light and light coming from a source such as a star which is presumably lensing the light?

I think you’re missing my point. Light passing through a gas of some sort scatters. This is what causes the sky to be blue and why there is a haze when we look at distant scenery.

Riff-in-Thyme said:

KJW said:

Riff-in-Thyme said:

Could it be said that dark matter is an unknown constraint within the gravity phenomena?

The Bullet Cluster (mentioned earlier) appears to indicate that dark matter is some form of material and not a deviation from established gravitational law.

I would still leave room for other explanation. For instance does FoR play a greater role than we have measured? Lensing only suggests that various regions have greater available momentum than others.

Observational evidence tends to suggest that candidates for DM style effects are in-fact manifestations of physical matter rather than quirky mathematical results.

diddly-squat said:

Riff-in-Thyme said:

KJW said:

The Bullet Cluster (mentioned earlier) appears to indicate that dark matter is some form of material and not a deviation from established gravitational law.

I would still leave room for other explanation. For instance does FoR play a greater role than we have measured? Lensing only suggests that various regions have greater available momentum than others.

Observational evidence tends to suggest that candidates for DM style effects are in-fact manifestations of physical matter rather than quirky mathematical results.

Sure, but does it have to be extra matter we can’t see or might effects of scale and opposing momenta come into it? It seems plausible to me that DM may have the same origin as say, the phenomena in which lightning will travel faster than artificially created arcs, to provide analogy?

Its nice to have a random unknowable that works to fill any gaps that you might have at any one time…

we look at rotation curves of galaxies and see that dm is evenly distributed. and not distributed like ordinary matter. so it is an observation not “just making shit up”.

KJW, have you been drinking, or perhaps ingesting hallucinogenic drugs?

It’s just that on some of your posts I’ve actually been able to understand what you’re saying :)

the phenomena in which lightning will travel faster than artificially created arcs

it does? never knew this. do you have something i could have a look at please?

It’s just that on some of your posts I’ve actually been able to understand what you’re saying

good isn’t it?

Boris said:

the phenomena in which lightning will travel faster than artificially created arcs

it does? never knew this. do you have something i could have a look at please?

Will track it down. The attempt to gain funding for the 200m high tesla coils was intended to investigate this area.

Boris said:

the phenomena in which lightning will travel faster than artificially created arcs

it does? never knew this. do you have something i could have a look at please?

+ shitloads.

I don’t believe that it occurs.

sibeen said:

Boris said:

the phenomena in which lightning will travel faster than artificially created arcs

it does? never knew this. do you have something i could have a look at please?

+ shitloads.

I don’t believe that it occurs.

this is the guy trying to investigate it

http://www.lod.org/

and this is a reference

Lightning is electricity, but operates very differently from electrical discharges at the human scale. As lightning forms, it breaks through the air up to ten times easier than small-scale electric arcs, using tricks we don’t yet understand. The problem of understanding “lightning initiation” still confounds world experts in the field.

http://www.lod.org/Projects/LightningFoundry/LightningFoundry.html

200 foot arcs? sibnie(sp) in russia holds the record i believe atm, and that place is now defunct.

woo site but good pics

http://www.abovetopsecret.com/forum/thread319602/pg1

sibeen said:

KJW, have you been drinking, or perhaps ingesting hallucinogenic drugs?

It’s just that on some of your posts I’ve actually been able to understand what you’re saying :)

Hehehe, but no.

I usually do try to be understood, but I also try to be accurate, thus being trapped by Bonini’s Paradox.

KJW said:

wookiemeister said:

whats the difference between background light and light coming from a source such as a star which is presumably lensing the light?

I think you’re missing my point. Light passing through a gas of some sort scatters. This is what causes the sky to be blue and why there is a haze when we look at distant scenery.

so light passing through clouds of dark matter aren’t scattered?

including a strong tendency to couple power wirelessly over large distances.

hmmmm like a radio? or is “great distance” just near field effects?

from

http://www.lod.org/Projects/LightningFoundry/LightningFoundry.html

>Lightning is electricity, but operates very differently from electrical discharges at the human scale. As lightning forms, it breaks through the air up to ten times easier than small-scale electric arcs, using tricks we don’t yet understand. The problem of understanding “lightning initiation” still confounds world experts in the field.

http://www.lod.org/Projects/LightningFoundry/LightningFoundry.html

wipes brow

I suggest that I can breathe easy and not have to worry to much about my field being overturned :)

http://teslamania.delete.org/frames/longarc.htm

wookiemeister said:

so light passing through clouds of dark matter aren’t scattered?

As far as I’m aware, no. Note that we have available to us the most distant source of background radiation, the CMBR, which allows us to see through the entire history of the universe all the way back to the last scattering event (which produced the CMBR).

so light passing through clouds of dark matter aren’t scattered?
—-

More or less by definition…

KJW said:

wookiemeister said:

so light passing through clouds of dark matter aren’t scattered?

As far as I’m aware, no. Note that we have available to us the most distant source of background radiation, the CMBR, which allows us to see through the entire history of the universe all the way back to the last scattering event (which produced the CMBR).

so light is bent but not scattered?

wookiemeister said:

so light is bent but not scattered?

In the loose sense that light is said to be bent by gravitation, yes (but in GR, gravitation doesn’t bend light)

KJW said:

wookiemeister said:

so light is bent but not scattered?

In the loose sense that light is said to be bent by gravitation, yes (but in GR, gravitation doesn’t bend light)

so if dark matter doesn’t scatter light does that mean it might not be able to absorb light

if you had light entering a gas cloud with normal matter the light would get scattered and absorbed by the gas

if it enters a dark matter gas cloud it doesn’t scatter and presumably has no interaction with light at all apart from being bent

dark matter still has gravity right?

dark matter still has gravity right?

yes, lensing and rotation curves show this.

so if dark matter doesn’t scatter light does that mean it might not be able to absorb light

we would see the DM “glow” if it absorbed light i presume.

well maybe its bending space but doesn’t actually have gravity as we know it

wookiemeister said:

if it enters a dark matter gas cloud it doesn’t scatter and presumably has no interaction with light at all apart from being bent

That’s why we know it isn’t baryonic (ordinary) matter… it doesn’t interact electromagnetically.

wookiemeister said:

dark matter still has gravity right?

It is a spacetime curvature (so yes). But the gravitational “bending” (lensing) of light is not an electromagnetic effect.

If dark matter doesn’t fit well with plain old light passing through it then why should it have all the other characteristics of matter?

KJW said:

wookiemeister said:

if it enters a dark matter gas cloud it doesn’t scatter and presumably has no interaction with light at all apart from being bent

That’s why we know it isn’t baryonic (ordinary) matter… it doesn’t interact electromagnetically.

wookiemeister said:

dark matter still has gravity right?

It is a spacetime curvature (so yes). But the gravitational “bending” (lensing) of light is not an electromagnetic effect.

why should only gravity create a “spacetime curvature”

does a blackhole ever spew out neutrons?

wookiemeister said:

well maybe its bending space but doesn’t actually have gravity as we know it

The curvature of spacetime and energy-momentum-gravity are the same thing (pushing the cosmological constant under the carpet for the moment).

does a blackhole ever spew out neutrons?

no. but the accretion disc outside the BH might.

so if you had a cloud of these neutrons would scattering still occur if light entered a gas cloud of neutrons

Boris said:

does a blackhole ever spew out neutrons?

no. but the accretion disc outside the BH might.

I would like to add that dark matter can’t be neutrons because free neutrons are unstable with respect to b-decay, the decay products being protons and electrons, which would interact with electromagnetic radiation, and antineutrinos.

can go a long way in 10 minutes.

;-)

what would happen if a photon struck a dark matter particle head on?

assuming that dark matter even conforms to the notion of particles, atoms

wookiemeister said:

what would happen if a photon struck a dark matter particle head on?

I think this question is quantum mechanically meaningless. There is no observed interaction between photons and dark matter. This means that the scattering cross-section for the interaction is zero.

KJW said:

wookiemeister said:

what would happen if a photon struck a dark matter particle head on?

I think this question is quantum mechanically meaningless. There is no observed interaction between photons and dark matter. This means that the scattering cross-section for the interaction is zero.

so if you had a planet made of dark matter the photon would pass through it

wookiemeister said:

so if you had a cloud of these neutrons would scattering still occur if light entered a gas cloud of neutrons

If we ignore the instability of neutrons that would prevent dark matter from being neutrons, and ask if photons can interact with neutrons, then even though neutrons are electrically neutral, they still possess a magnetic moment and therefore do interact with photons.

wookiemeister said:

so if you had a planet made of dark matter the photon would pass through it

Presumably. However, I don’t know what the density of a typical dark matter distribution is, so I don’t know if a planet made of dark matter is even a reasonable notion, or what effect of a planet-density distribution would have.

KJW said:

even though neutrons are electrically neutral, they still possess a magnetic moment and therefore do interact with photons.

One of the measurements used to establish a fundamental property of reality is that of the neutron electric dipole moment. A non-zero value for this indicates CP-symmetry violation. The current best upper limit is less than 2.9 × 10^–26e·cm. The known CP-violation would contribute about 10^–32e·cm to the neutron electric dipole moment. See neutron electric dipole moment for more details.

wookiemeister said:

If dark matter doesn’t fit well with plain old light passing through it then why should it have all the other characteristics of matter?

Have a look at the Bullet Cluster link I provided earlier. Basically, the dark matter is displaced relative to the ordinary matter (it appears as if the ordinary matter was impeded by the collision but the dark matter kept on moving), indicating that the dark matter is a type of matter and not a deviation of the gravitation law (which would always be centred at the ordinary matter).

wookiemeister said:

why should only gravity create a “spacetime curvature”

Gravity isn’t the only thing that is spacetime curvature. Energy-momentum is also spacetime curvature (however, this curvature is usually regarded as the gravitation internal to a energy-momentum distribution, as distinct from the gravitation external to the energy-momentum distribution). If we disregard the difference between the specific types of spacetime curvature (the four irreducible representations), then spacetime curvature manifests itself as synonymous to gravity.

> I think the most likely candidate for dark matter is slow neutrinos. Since it is currently accepted that neutrinos have non-zero mass, they must travel at less than c, and therefore able to have any such speed (including zero). While their extremely low mass does mean that we only see high-speed neutrinos (and/or antineutrinos) from b-decay type processes, it shouldn’t be taken to imply that neutrinos can’t be slowed down. Neutrinos don’t interact electromagnetically so there would not be any scattering phenomena that would indicate baryonic matter or any charged particles. On the whole, I prefer this explanation to any of the more exotic explanations such as string theory or particles that have never been observed in our particle-collider experiments.

That’s not a bad idea, KJW. I had been tending towards neutralinos but the LHC results and lack of neutralino results from the Milky Way have just about killed the neutralino possibility off. Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

mollwollfumble said:

Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

Maybe primordial neutrinos are slowed down by the expansion of the universe. Then as the ordinary matter gravitationally clumps to form galaxies, etc, the slow neutrinos clump with them.

KJW said:

mollwollfumble said:

Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

Maybe primordial neutrinos are slowed down by the expansion of the universe. Then as the ordinary matter gravitationally clumps to form galaxies, etc, the slow neutrinos clump with them.

What if the primordial neutrinos are slowed down by the dark matter itself?

CrazyNeutrino said:

KJW said:

mollwollfumble said:

Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

Maybe primordial neutrinos are slowed down by the expansion of the universe. Then as the ordinary matter gravitationally clumps to form galaxies, etc, the slow neutrinos clump with them.

What if the primordial neutrinos are slowed down by the dark matter itself?

That would require that neutrinos interact with themselves. I don’t know if they do via the weak force. Their very low mass and quite uniform distribution would probably prevent them from gravitationally clumping on their own, but the known clumping of ordinary matter would provide the seeds to clump with that. I chose the expansion of the universe because that does not require the neutrinos to interact with anything. However, I don’t know how fast 2.7K neutrinos are, or what the density of primordial neutrinos is.

KJW said:

However, I don’t know how fast 2.7K neutrinos are, or what the density of primordial neutrinos is.

I should remark that although the interaction of neutrinos with ordinary matter is very weak, they still interact with matter and this would allow the neutrinos to thermally equilibrate with the rest of the universe. Furthermore, the neutrino energy density would be comparable to the photon energy density of the CMBR (but note that the Planck distribution for black body radiation is based on Bose-Einstein statistics, whereas neutrinos are fermions obeying Fermi-Dirac statistics).

KJW said:

KJW said:

However, I don’t know how fast 2.7K neutrinos are, or what the density of primordial neutrinos is.

I should remark that although the interaction of neutrinos with ordinary matter is very weak,

What is involved in a neutrino weak force interaction?

Riff-in-Thyme said:

What is involved in a neutrino weak force interaction?

The basic processes are “>b</font-decay>.

KJW said:

Riff-in-Thyme said:

What is involved in a neutrino weak force interaction?

The basic processes are “>b-decay type processes.

Hope this fixes things. :-(

KJW said:

Riff-in-Thyme said:

What is involved in a neutrino weak force interaction?

The basic processes are beta-decay type processes.

KJW said:

mollwollfumble said:

Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

Maybe primordial neutrinos are slowed down by the expansion of the universe. Then as the ordinary matter gravitationally clumps to form galaxies, etc, the slow neutrinos clump with them.

I guess that could work.
According to https://en.wikipedia.org/wiki/Redshift#Highest_redshifts
the CNB (cosmic neutrino background) is predicted to have a redshift in excess of 1E10, and you’d expect that to slow them down quite a bit.

… … …

KJW said:

CrazyNeutrino said:

What if the primordial neutrinos are slowed down by the dark matter itself?

That would require that neutrinos interact with themselves. I don’t know if they do via the weak force. Their very low mass and quite uniform distribution would probably prevent them from gravitationally clumping on their own, but the known clumping of ordinary matter would provide the seeds to clump with that. I chose the expansion of the universe because that does not require the neutrinos to interact with anything. However, I don’t know how fast 2.7K neutrinos are, or what the density of primordial neutrinos is.

I doubt neutrinos interact much with each other via the weak force; for that matter neutrinos don’t seem particularly interested in annihilating with antineutrinos. But that’s ok, for the “DM=slow neutrinos” hypothesis as long as they interact with each other gravitationally.

The CNB temperature is predicted to be lower than the CMB temperature. Wikipedia calculates it to be roughly 1.95 K (compared to 2.725 K), although that calculation is strictly only valid for massless neutrinos.
From https://en.wikipedia.org/wiki/Cosmic_neutrino_background
Wikipedia said:

Before neutrinos decoupled from the rest of matter, the universe primarily consisted of neutrinos, electrons, positrons, and photons, all in thermal equilibrium with each other. Once the temperature reached approximately 2.5 MeV, the neutrinos decoupled from the rest of matter. Despite this decoupling, neutrinos and photons remained at the same temperature as the universe expanded. However, when the temperature dropped below the mass of the electron, most electrons and positrons annihilated, transferring their heat and entropy to photons, and thus increasing the temperature of the photons.

Dropbear said:

Imagine a ‘box’ of arbitrary space-time …you could calculate the net gravitational potential of all the mass/energy within that box and come up with a figure.. This would be much less than the ‘observed’ gravitational potential in that box – and the difference is what we describe as dark matter. We’re saying there must be ‘more stuff’ inside that box than we can observe.

Sort of. The old (pre-DM hypothesis) way to estimate the mass of a galaxy was to use the galaxy’s brightness to estimate the size of its stellar population, and then determine the mass of the stars it contains by assuming that the range of star sizes is roughly the same as what we see in our galaxy. And then add a small fudge factor to account for planets, dust and gas. So that method isn’t really calculating the net gravitational potential of all the mass/energy within the galaxy, but it was considered that it was a reasonable estimate… until it was realised that the visible mass distribution didn’t gel with mass estimates needed to explain galaxy geometry and rotation. And then gravitational lensing was discovered, which gave pretty definite proof that the visible mass estimates were far too low.

Dropbear said:

What if there was something ‘outside’ of the box, constraining the particles within the box .. something from a ‘higher’ (yech, i hate that term) dimension which was acting to ‘squeeze’ the box, keeping the particles within the box and making it look like there was more gravity going on than there was…

As KJW said, that hypothesis doesn’t appear to be consistent with the data from the Bullet Cluster. OTOH, if we reverse things & hypothesize a stretching force rather than a squeezing force, then it might be a reasonable hypothesis for Dark Energy.

PM 2Ring said:

As KJW said, that hypothesis doesn’t appear to be consistent with the data from the Bullet Cluster. OTOH, if we reverse things & hypothesize a stretching force rather than a squeezing force, then it might be a reasonable hypothesis for Dark Energy.

.

Hmmm. The universe is expanding at an increasing rate, isn’t it? How does that fit?

Michael V said:

PM 2Ring said:

As KJW said, that hypothesis doesn’t appear to be consistent with the data from the Bullet Cluster. OTOH, if we reverse things & hypothesize a stretching force rather than a squeezing force, then it might be a reasonable hypothesis for Dark Energy.

.

Hmmm. The universe is expanding at an increasing rate, isn’t it? How does that fit?

Dunno. Maybe the higher dimensional stuff is getting denser.

Seems to me that if the universe is expanding at an increasing rate, then there is a stretching force.

Michael V said:

Seems to me that if the universe is expanding at an increasing rate, then there is a stretching force.

Sure. And I’m hypothesizing that that stretching force is caused by attraction from stuff parallel to our space. OTOH, such an attraction would be perpendicular to our space. According to Newton, a force doesn’t act perpendicular to itself. But in relativity, that rule gets bent a little.

Michael V said:

Seems to me that if the universe is expanding at an increasing rate, then there is a stretching force.

It’s important to note at this point that the discussion of this thread is about dark MATTER, which is distinct from dark ENERGY.

Dark matter appears to be a material of some sort, whereas dark energy is likely to be a vacuum effect.

KJW said:

Dark matter appears to be a material of some sort, whereas dark energy is likely to be a vacuum effect.

To add to the “stretching” hypothesis, I would suggest that DM and DE are elastic dynamics of gravity waves and gravity.

Riff-in-Thyme said:

KJW said:

Dark matter appears to be a material of some sort, whereas dark energy is likely to be a vacuum effect.

To add to the “stretching” hypothesis, I would suggest that DM and DE are elastic dynamics of gravity waves and gravity.

DM and DE are different. And no, DE is not related to gravity waves.

KJW said:

Riff-in-Thyme said:

KJW said:

Dark matter appears to be a material of some sort, whereas dark energy is likely to be a vacuum effect.

To add to the “stretching” hypothesis, I would suggest that DM and DE are elastic dynamics of gravity waves and gravity.

DM and DE are different. And no, DE is not related to gravity waves.

Hmmm, no. I meant DM is related to gravity waves and DE phenomena related to amassing gravity wells. As usual I’ve thrown it out there before I’ve listed the dot points for debate.

For a constantly expanding (flat) universe, a certain proportion of the spacetime curvature is due to a form of energy-momentum that is indistinguishable from the vacuum (a vacuum energy, quite possibly the Higgs field as they are both scalar representations). However, as the proportion of the spacetime curvature is changed to below or above the proportion for a constantly expanding universe, the expansion of the universe becomes accelerated or decelerated.

KJW said:

For a constantly expanding (flat) universe, a certain proportion of the spacetime curvature is due to a form of energy-momentum that is indistinguishable from the vacuum (a vacuum energy, quite possibly the Higgs field as they are both scalar representations). However, as the proportion of the spacetime curvature is changed to below or above the proportion for a constantly expanding universe, the expansion of the universe becomes accelerated or decelerated.

Making a projection on a constantly expanding universe does not make a lot of sense if it is believed that the expansion rate has accelerated in a non uniform fashion since the BB?

Riff-in-Thyme said:

KJW said:

For a constantly expanding (flat) universe, a certain proportion of the spacetime curvature is due to a form of energy-momentum that is indistinguishable from the vacuum (a vacuum energy, quite possibly the Higgs field as they are both scalar representations). However, as the proportion of the spacetime curvature is changed to below or above the proportion for a constantly expanding universe, the expansion of the universe becomes accelerated or decelerated.

Making a projection on a constantly expanding universe does not make a lot of sense if it is believed that the expansion rate has accelerated in a non uniform fashion since the BB?

You appear to have misunderstood. A constantly expanding universe produces a specific proportion of the total energy being vacuum energy. Deviations from this specific proportion produces acceleration or deceleration. If the proportion is fixed, then so is the acceleration (or deceleration) rate. It is a simple matter to consider any expansion function for the universe. In all cases (independent of the expansion function) the spacetime is conformally flat, which limits the spacetime curvature to that of pure energy-momentum without pure gravitation.

KJW said:

Riff-in-Thyme said:

KJW said:

For a constantly expanding (flat) universe, a certain proportion of the spacetime curvature is due to a form of energy-momentum that is indistinguishable from the vacuum (a vacuum energy, quite possibly the Higgs field as they are both scalar representations). However, as the proportion of the spacetime curvature is changed to below or above the proportion for a constantly expanding universe, the expansion of the universe becomes accelerated or decelerated.

Making a projection on a constantly expanding universe does not make a lot of sense if it is believed that the expansion rate has accelerated in a non uniform fashion since the BB?

You appear to have misunderstood. A constantly expanding universe produces a specific proportion of the total energy being vacuum energy. Deviations from this specific proportion produces acceleration or deceleration. If the proportion is fixed, then so is the acceleration (or deceleration) rate. It is a simple matter to consider any expansion function for the universe. In all cases (independent of the expansion function) the spacetime is conformally flat, which limits the spacetime curvature to that of pure energy-momentum without pure gravitation.

I should point out that this is for a very simplified model of the universe that is isotropic and homogenous. The real universe is neither isotropic and homogenous except as an approximation at the cosmological scale.

A constantly expanding flat 3D-universe has the special property that it does not locally deviate from topological flatness and therefore can be considered as a “baseline” by which non-constantly expanding universes can be compared.

(This notion has no correspondence for 2D surfaces for which the only surface that does not locally deviate from topological flatness is a flat surface.)

PM 2Ring said:

According to https://en.wikipedia.org/wiki/Redshift#Highest_redshifts
the CNB (cosmic neutrino background) is predicted to have a redshift in excess of 1E10, and you’d expect that to slow them down quite a bit.

However, the neutrinos would have started out at a higher temperature, thus nullifying the greater redshift compared to the CMBR.

PM 2Ring said:

Before neutrinos decoupled from the rest of matter, the universe primarily consisted of neutrinos, electrons, positrons, and photons, all in thermal equilibrium with each other. Once the temperature reached approximately 2.5 MeV, the neutrinos decoupled from the rest of matter. Despite this decoupling, neutrinos and photons remained at the same temperature as the universe expanded. However, when the temperature dropped below the mass of the electron, most electrons and positrons annihilated, transferring their heat and entropy to photons, and thus increasing the temperature of the photons.

Eventually, the neutrinos must thermally equilibrate with the rest of the universe. However, I have no idea over what time-scale this would occur.

KJW said:

the spacetime is conformally flat

A conformally flat space is a space that can be mapped (at least semi-globally – topology may obstruct a fully global mapping) onto a flat space with a variation of scale over the location on the map. All smooth two-dimensional surfaces are conformally flat. For example, the Mercator projection is a mapping of the (spherical) surface of the earth onto a flat surface with a latitude-dependent scale (this also illustrates the topological obstruction to a fully global mapping). However, the spacetime external to a central mass cannot be mapped to a Minkowskian (flat) spacetime regardless of any variation of scale over the spacetime, and is therefore not conformally flat.

PM 2Ring said:

KJW said:

mollwollfumble said:

Slow neutrinos make sense, but how would they be slowed down when they don’t interact much with anything?

Maybe primordial neutrinos are slowed down by the expansion of the universe. Then as the ordinary matter gravitationally clumps to form galaxies, etc, the slow neutrinos clump with them.

I guess that could work.
According to https://en.wikipedia.org/wiki/Redshift#Highest_redshifts
the CNB (cosmic neutrino background) is predicted to have a redshift in excess of 1E10, and you’d expect that to slow them down quite a bit.

According to one of the astrophysics guys on xkcd, if we assume that electron neutrinos have a rest energy around 1 eV, then their current speed (in the CMB rest frame) is around 6000 km/s, which “is greater than the escape velocity of any galaxy, so cosmic neutrinos cannot be responsible for dark matter and are not bound to any galaxy or cluster of galaxies. This means that the CNB, if we are ever able to detect it, should be roughly isotropic like the CMB is, but more so. It decoupled from baryons and photons very early in the universe’s history, and so the density fluctuations imprinted on the CMB were not nearly as strong at that time.”

PM 2Ring said:

According to one of the astrophysics guys on xkcd, if we assume that electron neutrinos have a rest energy around 1 eV, then their current speed (in the CMB rest frame) is around 6000 km/s, which “is greater than the escape velocity of any galaxy, so cosmic neutrinos cannot be responsible for dark matter and are not bound to any galaxy or cluster of galaxies.

I suppose the possibility remains that they could be muon neutrinos or tau neutrinos, or perhaps the cold tail of the thermal distribution of electron neutrinos.

PM 2Ring said:

This means that the CNB, if we are ever able to detect it, should be roughly isotropic like the CMB is, but more so. It decoupled from baryons and photons very early in the universe’s history, and so the density fluctuations imprinted on the CMB were not nearly as strong at that time.”

The uniformity of the neutrino distribution is irrelevant because it is not expected for them to clump under there own gravitation, but rather clump with the ordinary matter.

KJW said:

PM 2Ring said:

According to one of the astrophysics guys on xkcd, if we assume that electron neutrinos have a rest energy around 1 eV, then their current speed (in the CMB rest frame) is around 6000 km/s, which “is greater than the escape velocity of any galaxy, so cosmic neutrinos cannot be responsible for dark matter and are not bound to any galaxy or cluster of galaxies.

I suppose the possibility remains that they could be muon neutrinos or tau neutrinos, or perhaps the cold tail of the thermal distribution of electron neutrinos.

I guess so. But that reduces the numbers (and hence the available mass) substantially.

KJW said:

The uniformity of the neutrino distribution is irrelevant because it is not expected for them to clump under there own gravitation, but rather clump with the ordinary matter.

Yeah, I expect they’d clump in the immediate vicinity of white dwarfs, neutron stars & black holes. But his point is that they aren’t going to clump at the galactic level, so they don’t behave the way DM does.

PM 2Ring said:

I guess so. But that reduces the numbers (and hence the available mass) substantially.

Bear in mind that muon neutrinos and tau neutrinos would form a substantial proportion of the total cosmic neutrino background. We only need their mass to be large enough for their mean speed to be below the escape velocity of galaxies but not too large that their population in the cosmic distribution be too greatly reduced.

KJW said:

Bear in mind that muon neutrinos and tau neutrinos would form a substantial proportion of the total cosmic neutrino background.

I guess that depends on how they were formed. OTOH, with neutrino flavour oscillation, it doesn’t really make a lot of sense to say that a given neutrino has one particular flavour.

KJW said:

We only need their mass to be large enough for their mean speed to be below the escape velocity of galaxies but not too large that their population in the cosmic distribution be too greatly reduced.

I don’t think we’re in luck.
Experiments that measure neutrino flavour mixing use a parameter
Δm_ij² = m_i² – m_j², where m_i and m_j are two pure mass states.
Δm_ij² is of the order of 10⁻⁴ eV².

See http://en.wikipedia.org/wiki/Neutrino_oscillation

PM 2Ring said:

I don’t think we’re in luck.
Experiments that measure neutrino flavour mixing use a parameter
Δm_ij² = m_i² – m_j², where m_i and m_j are two pure mass states.
Δm_ij² is of the order of 10⁻⁴ eV².

See http://en.wikipedia.org/wiki/Neutrino_oscillation

I thought that might be the case, but we still don’t know the mass of the electron neutrino.

KJW said:

I thought that might be the case, but we still don’t know the mass of the electron neutrino.

True. It’s not easy to accurately measure the rest mass of uncharged particles with a Lorentz factor of a million or so, as is typical of solar neutrinos.

PM 2Ring said:

True. It’s not easy to accurately measure the rest mass of uncharged particles with a Lorentz factor of a million or so, as is typical of solar neutrinos.

As far as I’m aware, there’s no direct evidence that neutrinos are anything but massless (neutrino oscillations hardly count as direct evidence, though the neutrinos-as-dark-matter hypothesis requires it).

I think that the difference between the masses of the different neutrino flavours would have to be quite small in order for neutrino oscillation not to violate the conservation of energy-momentum beyond what could be allowed by the uncertainty principle.

KJW said:

I think that the difference between the masses of the different neutrino flavours would have to be quite small in order for neutrino oscillation not to violate the conservation of energy-momentum beyond what could be allowed by the uncertainty principle.

BTW, I believe that neutrino oscillation implies an uncertainty relationship between mass and flavour.