Dark Gravity discussion

[PB666]

I was just watching Tyson on Charlie Rose, it is interesting that he did not want state WIMP were responsible for Dark Matter.

His statement is that particle physicist want it to be a particle simply because they are particle physicist. It appears that he wants to relabel it as Dark Gravity, reduction of it to its most basic property.

Since the moderator pulled the plug on the other important topic hopefully we can have a non-fringe discussion on the topic, so logically I'm restarting the discussion with a fringe name.

In all seriousness though, however, i don't agree with Tyson on many things, but I agree with him on this, because bad science is when it is led with a foregone conclusion. Its better to start with an open mind and the best experiments should pull the hypothesis.

Tyson is talking right now about how Journalist like to present fringe science.

Edited December 1, 2015 by PB666
retagging

[K^2]

I was just watching Tyson on Charlie Rose, it is interesting that he did not want state WIMP were responsible for Dark Matter.

His statement is that particle physicist want it to be a particle simply because they are particle physicist. It appears that he wants to relabel it as Dark Gravity, reduction of it to its most basic property.

Well, here is the deal, more or less. At the base level, what we are talking about is not so much gravity or acceleration, but curvature. Each star within a galaxy or galaxy within a cluster is in free-fall. From perspective of differential geometry, it moves along the closest thing to a straight line in curved space-time - the geodesic. By watching these objects move we can plot out the geodesics and deduce the curvature. This is not how we actually compute these things, but it's easier to picture this way. So in effect, we are measuring curvature.

This is where General Relativity comes in. We know how curvature corresponds to stress-energy density. The relation is given by Einstein Field Equation. We plug in our observations and obtain some sort of stress-energy distribution. It's a rank two tensor at every point. However, due to certain properties of GR, we can select a coordinate system, at least locally, that puts that tensor in diagonal form. Woot! So what do we get? We get two quantities. One that scales like energy and one that scales like pressure.

Parts of energy and pressure density that we observe we can account for with luminous matter, which has both energy and pressure. However, majority of these, by far, is unaccounted for.

And this is where things turn for the confusing, for you see, the Dark Energy is the unaccounted for portion of the pressure term. And Dark Matter is unaccounted for portion of the energy term, because this is the E = mc² situation.

Tyson is absolutely correct in pointing out that we don't really know that Dark Matter is some sort of a particle. And really, the "Matter" here is just to indicate that it corresponds to the same part of stress-energy as normal matter does. Whereas Dark Energy corresponds to the part that you don't usually associate with matter. Not in these sort of quantities. Is the naming confusing? Yeah. I suppose, Dark Stress-Energy would have been better, but it's a mouthful. But I also don't like the idea of calling Dark Matter portion the Dark Gravity. It really doesn't reduce the confusion, since both attraction and repulsion are part of gravity here. Even though repulsion is not something we usually associate with classical gravity.

Anyways, people who work in relevant fields are used to this, and people who don't will probably be confused regardless. I don't think it'd be worth changing.

[PB666]

We plug in our observations and obtain some sort of stress-energy distribution. It's a rank two tensor at every point. However, due to certain properties of GR, we can select a coordinate system, at least locally, that puts that tensor in diagonal form. Woot! So what do we get? We get two quantities. One that scales like energy and one that scales like pressure.

Parts of energy and pressure density that we observe we can account for with luminous matter, which has both energy and pressure. However, majority of these, by far, is unaccounted for.

And this is where things turn for the confusing, for you see, the Dark Energy is the unaccounted for portion of the pressure term. And Dark Matter is unaccounted for portion of the energy term, because this is the E = mc² situation.

I don't really want to reply to this in this thread cause I was hoping to give it a clean start. Agree 100% with the matter equivilant aspect. However what you said about the pressure tensor does not jive with the paper i presented yesteday.

There are either two things happening, Dark Energy is very weak acting between particles in our galaxy, or Einstien was correct in his assessment of a pressure constant, but the pressure only applies to mass.

Having thought about this a bit, if there is a pressure constant that is independent of local energy density (mass if you are stuck on a planet), and it would appear that dark energy is more powerful than Einstiens constant, Then there is a local problem. The vector is supposed to be in th meV range, which means it has a field intensity that we can measure. If that is true, and I start assembling lead balls in space the attraction to those balls And i carefully measure th mass going into the balls then when I start measuring attractions, the calculated G should be smaller for small objects but quickly plateau at the observed constant as ball size increased, because DEF would be proportionally smaller compared to mu, if the constant was space-time dependent. The problem is that the latest gravity experiments show a fluctuation in G, but its in the 10-6/-7 range, much smaller than it should be to explain Dark energy. The latest experiment shows no deviation in acceleration down to the um/sec2 range with an attractive differential. So dark energy is spreading things apart only in a mass dependent manner, with no dependency on space at all, a mass-time dependency.

This means that the cosmological constant cannot be constant across space-time because mass is not consistent across space-time. Therefore it would appear that the press release (key word being appear) that dark energy is either not acting between particles in our galaxy, but between galaxies or inserting my alternative, interacting only with mass-time but in fact appears to be inflating space time, lol. The other possibilty is that dark energy preferentially interacts with dark energy tensor, and matter is reflected in its gravitation interaction with the energy tensor. This creates another set of problems in galactic shape distortions and in particular favors dark energy tensor accelerating away from mass to the edge of the inflaton. The only way that doesnt happen is if the tensor is a field that travels at Close to C, dumping small bits of momentum to small particles it passes until it returns to the rest state. Also not supportable.

Thus it would appear that dark energy remains elusive, but I don't see how we can assess the pressure tensor in our galaxy and distinguish it from G if dark energy is acting on mass-time dependency, it would be the same as saying 5 billion years ago G was much higher and suddenly fell, which entirely changes what a type 1a supernova is. So I have to bias that it is not acting between matter in our galaxy, it can only spread vacuum space, its inflation.

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[PB666]

The issue of threads was brought up here. This article is relevant.

http://arstechnica.com/science/2015/11/scientist-says-huge-clumps-of-dark-matter-may-lie-just-beyond-the-moon/

I really dislike the presentation of science in this article, it begins with heavy speculation to defend something that is speculative.  I have to say that if dark gravity threads existed so close to earth our space observaties would probably have detected momentarily anomolies in the star field in the same way the sun difracts starlight during a solar ellipse. 

 

Edited December 1, 2015 by PB666

[K^2]

Agreed. Dark matter must either be uniformly distributed as a yet undetected field, or be predominantly found far from stars. (Hence the whole WIMP or MACHO thing.) Anything else would be reasonably easy to detect. Even if Sol was somehow unique, we would have seen clumps of dark matter near other stars during our exoplanet search efforts. Although, this does remind me of that "Alien Megastructures" thread. If MACHO hypothesis is valid, we might simply be looking at an exception to the rule, where the huge amount of non-luminous matter is located close to the host star rather than far from it. A close pass of another star could have caused this if there was enough debris orbiting the star to begin with at great distance. For all we know, that's all there is to it. Of course, it's still puzzling that we are having trouble detecting it. Could Sol be "special" anyways? Could something have stripped majority of our halo mass, making us make such incorrect predictions about other star systems? I do recall some stellar evolution simulations showing way more stuff orbiting a Sol-sized star than what we see out there.

[manaiaK]

1 hour ago, K^2 said:

Of course, it's still puzzling that we are having trouble detecting it.

Not to me. To me, we're hardly trying to look. We need to get out to the Oort cloud (out a light-year or so from Sol), where the gravitational gradient is smaller and we can build bigger detectors. Heck, even building telescopes past Saturn would be a start.

[PB666]

1 hour ago, manaiaK said:

Not to me. To me, we're hardly trying to look. We need to get out to the Oort cloud (out a light-year or so from Sol), where the gravitational gradient is smaller and we can build bigger detectors. Heck, even building telescopes past Saturn would be a start.

Com'on seriously, you don't think our super fancy telescopes would see the gravimetric distortion created by a filament passing while taking pictues of the horsehead nebula. 

[K^2]

1 hour ago, manaiaK said:

Not to me. To me, we're hardly trying to look. We need to get out to the Oort cloud (out a light-year or so from Sol), where the gravitational gradient is smaller and we can build bigger detectors. Heck, even building telescopes past Saturn would be a start.

Gravity isn't the limiting factor. We can't build and launch a mirror of sufficiently large diameter to make a dramatic difference against Hubble, and for an interferometer, even with a solid frame and microgravity, you'll need optical tracking and adjustment that can work just as well in LEO. The only thing Saturn's distance gives you is reduction of sunlight, but you can get that in L2 just as easily and for way less. L2 interferometers have been proposed and even briefly planned. Then budget got cut. Go thank your senator.

But that's kind of a moot point. An interferometer would allow us to take an actual look at exoplanets. Perhaps even do spectrography to detect atmospheric gases. Finding a more Earth-like exoplanet would be interesting. Finding one with gaseous oxygen lines in its spectrum, that would be the biggest discovery mankind has ever made bar none.

Looking for Oort cloud objects, though? Not that helpful. If MACHO holds, the dominant objects in the cloud would be large enough for Hubble to find. You just have to point it at one. And having a bigger telescope with better sensitivity and resolution won't make it easier. Just the opposite. A 1ly sphere around the Sun has surface area of well over 10ly². Our Sun is about 5 light seconds across. Plastered across that 1ly sphere, you could fit approximately 550,000,000,000,000 Suns. And we are looking for mass that accounds for, maybe five Suns.

Telescope won't help you. Spotting these objects by direct observation is statistically impossible. What we can do, and what we are looking for, are indirect indications of these objects there. If there is that much junk out there, and if there are just a few massive enough bodies floating about, they are going to knock things loose. And we should be seeing these things come visit. And we do. But we aren't seeing quantities or sizes we expect. Of course, our expectations are based on a lot of assumptions. There might not be any large objects out there. Just a ton of little ones. That would be very strange and indicate very serious flaw in our understanding of Sol's formation. Or there could be one really big object, which managed to clear out a chunk of the cloud already and get rid of any competition. That's completely plausible, but that's the sort of thing we should be able to find from periods of comets, etc. People are trying with no success so far.

We are looking. We are finding zilch. We can't really be looking any harder with tech we have, and probably won't be able to even with tech we can invision. Though, an interferometer might help us detect such a heavy cloud around other stars. These things are easier to see from without than from within. At any rate, there are indirect indications we expected and did not find. All it means that we still have questions about it.

[*Aqua*]

After so many years of guessing by scientists what dark matter and dark energy could be, could it be possible that they don't exist? Maybe our idea of gravitation isn't right or it's not complete.

I'm only a layman but my experience tells me that if something unexpected happens the basic assumption is most likely wrong. Think about how Kepler's laws of universial gravitation was superseded by Einstein's general relativity. I assume there could be an even 'greater' theory which doesn't need dark something we just didn't discovered (yet).

What do both of you think about that?

Edited December 1, 2015 by *Aqua*

[RainDreamer]

Off topic: I find it hilarious how scientist came up with names like WIMP or MACHO and have them go against each others. I am not going to believe that someone DIDN'T spend an evening coming up with these names just for them to appearing in distinguished scientific journals as a joke.

Edited December 1, 2015 by RainDreamer

[K^2]

15 minutes ago, *Aqua* said:

After so many years of guessing by scientists what dark matter and dark energy could be, could it be possible that they don't exist? Maybe our idea of gravitation isn't right or it's not complete.

I'm only a layman but my experience tells me that if something unexpected happens the basic assumption is most likely wrong. Think about how Kepler's laws of universial gravitation was superseded by Einstein's general relativity. I assume there could be an even 'greater' theory which doesn't need dark something we just didn't discovered (yet).

What do both of you think about that?

General Relativity is the most precisely tested theory perhaps with exception of Quantum Electrodynamics. If you add to that the fact that both GR and QED are based on the same underlying field theory, the evidence becomes completely overwhelming. We have tests ranging from dynamics of individual electrons to physics of neutron stars fitting the theory to 12 decimal places.

Sure, many assumptions could be wrong. Yes, theory might be incomplete and might require corrections further down the line. But discrepancy between ordinary and dark matter being due to faulty theory is a statistical impossibility at this point. Which is why scientists are perfectly happy imagining all sorts of crazy new particle fields as possible source of dark matter. As crazy as any of these sound, it's not nearly as crazy as an idea that our theory of gravity is that far off.

[cfds]

Did something change since I studied physics? Five years ago MAssive Compact Halo Objects were suspected to be located in the galactic halo (to provide this observed additional spherical mass distribution), so looking in the Oort Cloud would not help a lot...

[K^2]

I dunno. The way I've seen it is that distribution of DM roughly matches distributions of stellar bodies in the galaxy. That includes old, dead stars of the galactic halo. In contrast, luminous matter is primarily confined to the disk, where new stars are generated. Obviously, simply having way more dwarfs, neutron stars, and black holes out there would do it, but that doesn't seem to match with evolution of other galaxies. Most of these objects must have been stars at some point, and that would be apparent from "young" galaxies we see far from us. If that's out, then star systems themselves must simply be way heavier. Natural way to have that is to have compact objects in debris circling the star.

I could easily be out of date on that, though. If you have a reference for research that suggests that DM distribution is very different from star distribution, I'd be interested in reading it.

[YNM]

DM distribution have been established as rather more spherical, in the form of dark matter halo that reach out firther than stellar halo. Even so, multiple types of dark matter have been proposed, even one (look up double disk dark matter) pretty much adds a second equally dark matter...

That being said, most papers now rather focuses on the nature of them, including things like dark matter annihillation.

 

Edited December 1, 2015 by YNM

[Findthepin1]

Is it possible that there's nothing physical there? We haven't found any dark matter and we can only detect it through its gravity. It's not dark in the way that you could see it with a light, it literally seems to only affect the universe with gravity. Maybe it's just little bits of gravity floating around . Maybe it's in another dimension. 

Edited December 1, 2015 by Findthepin1
*dimension

[K^2]

Quote

DM distribution have been established as rather more spherical, in the form of dark matter halo that reach out firther than stellar halo.

Hm. I'll have to read up on that. That does put a different spin on things.

2 hours ago, Findthepin1 said:

Is it possible that there's nothing physical there? We haven't found any dark matter and we can only detect it through its gravity. It's not dark in the way that you could see it with a light, it literally seems to only affect the universe with gravity. Maybe it's just little bits of gravity floating around . Maybe it's in another dimension. 

That's almost exactly what WIMP hypothesis is. That dark matter consists of kind of particles that have a lot of mass, but (almost?) do not interact with normal matter by any means other than gravity. It could make such matter entirely undetectable other than through its gravitational interaction. Basically, picture neutrino, only without even the weak charge, and a bit heavier. It'll pass right through your detector, you, your assistant, walls of the lab, and fly off merilly into space without anyone being the wiser.

And with gravitational interaction being the only source of "drag" on this stuff, it wouldn't exactly stick to planets or stars, but would concentrate around galaxies.

Still, even such a weird thing would have some sort of "hydrodynamics" to it. I wonder if we could detect eddies in it. They'd have some very interesting gravitational properties.

(Edit: The new editor sucks. And apparently, IPB does not provide a way to turn it off. Can we please have a mod installed for that? This is terribly annoying.)

Edited December 1, 2015 by K^2

[justidutch]

25 minutes ago, K^2 said:

Still, even such a weird thing would have some sort of "hydrodynamics" to it. I wonder if we could detect eddies in it. They'd have some very interesting gravitational properties.

Who is Eddie?

Please forgive me, I couldn't resist the Arthur Dent reference!

Edited December 1, 2015 by justidutch

[PB666]

1 hour ago, Findthepin1 said:

Is it possible that there's nothing physical there? We haven't found any dark matter and we can only detect it through its gravity. It's not dark in the way that you could see it with a light, it literally seems to only affect the universe with gravity. Maybe it's just little bits of gravity floating around . Maybe it's in another dimension. 

Gravity is the ability to curve space-time, its generally a combination of energy in the form of rest mass [quarks(massive but miniscule), gluon (no rest mass-all mass comes from interaction energy), W and Z bosons that interact with the higgs field, and electron]. Simplifing,  the interactions give mostly massless fields mass which, in turn, curves space time. mostly its the energetic interactions that create mass, and most of the interactions are measurable. There are also x and y bosons, theoretically, and self-interacting gluons, called glueballs.

So either empty space has the ability to warp space time, or some type of interaction of fields  is occurring in that produce inertia curving space time.  The problem with fields that only interact with quantum vacuum (ie the virtual particles created and obliterated therein) is that such fields propogate at the speed of light, thus would not stay in a place as to create a noticable gravity. So to have a force that say holds a galaxy together it needs to linger in and around the galaxy for a bit of time. So guage bosons and gluons are massless, but they get mass via interactions which slow them down very quickly, light on the other hand travels great distances, shows no mass while traveling but imparts momentum leaving and arriving, neutrinos exhibits mass episodically and are to energetic to be captured explaining dark matter. Thus for dark gravity to be generated by a wave instead of a massive particle, it would have to propogate slow enough that it is mostly captured (orbits) by visible matter stuff, Something that interacts between light and gluons with itself or something else. IOW it would be a field or interaction of fields that exhibits both energy and mass, it would lack charge or spin unlike e- and p+ and would not interact with protons via nuclear forces like a nuetron. 

[K^2]

2 hours ago, justidutch said:

Who is Eddie?

Please forgive me, I couldn't resist the Arthur Dent reference!

https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics)

Erm, sorry, didn't see the small text. But I'm going to leave the link up just for sake of clarity.

Edited December 1, 2015 by K^2

[PB666]

5 hours ago, K^2 said:

Hm. I'll have to read up on that. That does put a different spin on things.

That's almost exactly what WIMP hypothesis is. That dark matter consists of kind of particles that have a lot of mass, but (almost?) do not interact with normal matter by any means other than gravity. It could make such matter entirely undetectable other than through its gravitational interaction. Basically, picture neutrino, only without even the weak charge, and a bit heavier. It'll pass right through your detector, you, your assistant, walls of the lab, and fly off merilly into space without anyone being the wiser.

And with gravitational interaction being the only source of "drag" on this stuff, it wouldn't exactly stick to planets or stars, but would concentrate around galaxies.

Still, even such a weird thing would have some sort of "hydrodynamics" to it. I wonder if we could detect eddies in it. They'd have some very interesting gravitational properties.

(Edit: The new editor sucks. And apparently, IPB does not provide a way to turn it off. Can we please have a mod installed for that? This is terribly annoying.)

Thats how i imagine dark matter, as material that circulates in elliptical orbits around gravitationally defined systems, it flows rapidly around peak densites and lingers in between and around these peak densities. 

[K^2]

Plausible. Locally, such a fluid would behave as an ideal gas, because it interacts so weakly, but in bulk of gallactic scale, it would behave as a fluid with very small, yet finite viscosity. If that's true, we could make some conclusions about it. I wonder if anyone ever tried to model something like that.

[PB666]

It has to have some viscosity (like friction in accretion) otherwise it would not confine itself proxi-galactic sufficient to alter the gravity of galaxies? Has anyone modeled yet the movement of dark matter between galaxies to see what the preferential non-cohesive distribution would look like.

 

[K^2]

Yes, the fact that it hangs around galaxies suggests some viscosity. Also, whatever it is, it must at very least interact gravitationally, which also leads to conclusion that there will be at least some viscosity. In fact, it should be possible to derive the density, at least as a function of a few parameters like density and temperature, which we have decent estimates for. I'll see if I can remember enough fluid dynamics to get an estimate.

[YNM]

I'm rather wondering about the possibility that, if they really substitute ~ 20% of the Universe's mass-energy, why do we only model it so simple ? When the "rest 5%" can have up to 12 kind of particle, maybe these dark things have so many variations within themselves, that we can't simply model them as a homogeneous inert matter ?

People have put up some evidence that these matter might be more varied than we thought :

- Dark matter dominated galaxies. Mostly some sparse spiral that spins really fast.

- Dark matter annihilation. Most papers point out an anomaly of the galactic center in γ-rays that it's brighter than it should be, and not in the direction of central black hole. Lately, some dwarf galaxies have similar phenomenon where it glows brighter in γ-ray than expected.

 

Edited December 2, 2015 by YNM

[K^2]

Well, we tend to describe things from perspective of, "How does it interact with things we care about." It's been shown that at long ranges DM interacts very weekly both with ordinary matter and with itself. That leaves only two possibilities. It's either compact or weakly interacting. If we are looking at something more fluid-like, it has to be the later.

Mind, it can still interact in ways ordinary matter doesn't. It can have an entire spectrum of particles with completely new sets of symmetries and charges. But if it interacts with ordinary matter so weakly as to be undetectable, none of it matters for our physics. If the only appreciable interaction with matter is gravity, then the only parameters we care about are its density and viscosity, which we can study without needing information about actual particle composition.

As for annihilation being source of these gamma rays, I find it somewhat dubious. Basic field theory. Annihilation process is equivalent to scattering. If particle X annihilates with antiparticle X' to produce photon, then particle X can scatter a photon. In other words, it would imply that DM interacts with light. And that'd make it anything but weakly interacting.