Simulation Predicts Clumps of Dark Matter Within Galaxies

A team of researchers has simulated the gravitational interaction of dark matter particles over the course of a hypothetical 13.7 billion years. They found that the particles tended to form clumps large enough to assist in the formation of galaxies. The results contradicted observations from previous, smaller studies, but they lent support to an unrelated simulation of how the Milky Way formed. UCSC's press release is also available. Quoting ScienceNews: "The clumps of dark matter in the simulation have densities that are remarkably similar to densities that a University of California, Irvine research group found when simulating the formation of the Milky Way and its satellite dwarf galaxies, says James Bullock, the astrophysicist who leads the UC-Irvine group and was not involved in the new study. 'This is a remarkable success of the particular model simulated and adds strong support to the idea that the dark matter is made up of particles that are "cold." There are a number of planned experiments aimed at detecting the dark matter that are betting on it being cold, so this is generally good news for the community,' Bullock says. And, [study co-author Piero Madau] notes, larger simulations that might help constrain the nature of dark matter even more are already in the works."

Comments

Matrioshka brains?

[Anonymous Coward]

It's probably just stars converted into mbrains or something.

Re:Matrioshka brains?

[moteyalpha]

I wish you mbrains would quit posting anonymously so I could mod you up. And if you didn't hide your home planets, I for one would welcome our new mbrain overlords.

Re:Matrioshka brains?

[Anonymous Coward]

Who's hiding? Improve your gravitometric scanner strengths, you incompetent baboons.

PS you can mod up an AC, I do most of the time.

And in other simulations:

[Anonymous Coward]

Clumps of dark matter were predicted around Uranus.

Re:And in other simulations:

[REJOSU]

Well, its better than in my simulation where they found dark matter in Uranus.

Re:And in other simulations:

[Joebert]

Too much dark matter could be the origin of 'roids.

Dark matter particles are cold?

[skotay]

Well... colder then light matter particles?

Re:Dark matter particles are cold?

[BrentH]

Not necesarily (and this 'Cold' refers to something like mean velocity of the particles, not temperature), but current astrophysicists think that Dark Matter is predominantly cold. Otherwise it couldn't really clump together (it would 'boil' away so to speak) and we would be able to explain why we appear to detects clumps of unseen mattert (like in the Bullet cluster).

Re:Dark matter particles are cold?

[Phaedrus420]

(and this 'Cold' refers to something like mean velocity of the particles, not temperature)

I understood that to be the definition of temperature.

Re:Dark matter particles are cold?

[BrentH]

Astrophysicisists, on the topic of Dark Matter, use Hot, Warm, or Cold Dark Matter to refer to fundamentally different models, in which the Hot/Warm/Cold refers to the average velocity the Dark Matter particles have. For structure (like 'star'-systems) to be gravitationally bound, the average velocities of its member particles can't be above some threshold (something like the escape velocity of the system or clump), otherwise they just fly away. Cold Dark Matter is therefore the stuff that actually can clump together.

Re:Dark matter particles are cold?

[Kjella]

Astrophysicisists, on the topic of Dark Matter, use Hot, Warm, or Cold Dark Matter to refer to fundamentally different models, in which the Hot/Warm/Cold refers to the average velocity the Dark Matter particles have. For structure (like 'star'-systems) to be gravitationally bound, the average velocities of its member particles can't be above some threshold (something like the escape velocity of the system or clump), otherwise they just fly away. Cold Dark Matter is therefore the stuff that actually can clump together.

Sounds pretty much exactly how you'd describe how water is steam, liquid or a solid depending on the temperature to me. I'm sure that within the field "cold" or "warm" have particular meanings but no more than say a car mechanic speaks of a "cold" or "warm" engine which doesn't necessarily mean the same as when we're talking about the weather. Anyway, from what I've understood there doesn't seem to be enough hot/warm dark matter so cold dark matter is the big mystery "dark matter".

Where's the evidence?

[Nazlfrag]

I doubt this simulation did more than let them see what they wanted to see. "The researchers note that the simulation does not model any forms of normal matter such as stars or planets." Hardly a complete picture they drew.

Why the obsession dark matter? Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales? Why do they think MOND is for cranks and crackpots? What of a static non-expanding universe and alternate redshift paradigms? Are they not just as feasible as exotic matter that only interacts gravitationally?

That's a lot of questions, so I'll break it down to one. I'm just curious as to why dark matter is so widely supported, is it merely because breaking the standard model makes physicists too uncomfortable?

Re:Where's the evidence?

[Anonymous Coward]

>breaking the standard model makes physicists too uncomfortable?

yup, even though the model is known to be wrong (that's why they are looking for quantum gravity)

Re:Where's the evidence?

[Anpheus]

I Am Not A Physicist, but the problem with MOND is that, well, it explains only one problem of gravity or cosmology, rather: galactic rotation. It fails to explain how spectacularly successful tests of general relativity have been. For example, where does MOND predict frame-dragging? Answer: it doesn't.

MOND is what you get when you have a problem posed to amateur mathematicians and physicists, and they answer that problem (galactic rotation) with the simplest solution (let's just tweak this equation) without considering the fact that their modified theory is inconsistent with well-established theories that currently exist. MOND does not predict certain things that we see in nature, but this isn't seen by proponents. All the proponents see is, hey, it solves this problem. Well, yes, but it causes a whole lot more that you neglect to mention. Chiefly among those is, pretty much every quirk of relativity.

Re:Where's the evidence?

[timmarhy]

because MOND hasn't been verfied by any pyshical tests. it's MOND that lacks evidence not dark matter. all dark matter is, is visible gravitational effects of matter we can't see just yet. it IS crazy to go modifying a verified theory of newton's laws to fit our results without being able to explain why.

Re:Where's the evidence?

[Anonymous Coward]

First I want to clarify what dark matter is. It is just matter that cannot be seen directly, because it does not emmit light or radiation. But it can be observed by the gravitational effects it has on objects that can be seen. To an alien Saturn would be dark matter. Earth would be dark matter.

So we have two options:
1. We assume that our planets are the only ones in the universe, every other matter is light up and we can observe it by its radiations. Then we have to conclude that there is a fundamental error with Newtons theory of gravity.

2. We assume that Newton is actually a quite accurate model of gravity, and there just exists lots of matter that does not radiate anything we can observe directly.

What does sound more reasonable?

Re:Where's the evidence?

[jabithew]

All physics is most likely wrong. Some of it is *less* wrong than the rest of it. Even if we know of problems within the standard model, we don't currently have much better to explain things.

Re:Where's the evidence?

[Tablizer]

Sounds like a fun debate brewing here. Let me juice it a bit: MOND is for vi users and dark-matter is for emacs users. Go!
     

Re:Where's the evidence?

[Hektor_Troy]

Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales?

It's not about being scared - it's about the scope of that theory. Using "stuff falls down" as a theory of gravity works, but only in very specific situations. It's not something you can use as a usable replacement for any of the accepted theories, like GR and Newton.

Talking about scientists being scared of MOND is silly. Especially when you bring in Newtonian mechanics, which have long since been proven to fail in a large number of ways. The reason we still use Newtonian mechanics is that it's "good enough" for most things. Just like "stuff falls down" works great here on Earth, but not so well inside the ISS.

Re:Where's the evidence?

[hypomorph]

Strictly speaking, the "standard model" does not include anything along the lines of WIMPs or other sorts of crazy non-baryonic matter that might be dark matter. Only the "supers symmetric" models claim to have the ability to sort out dark matter. This is the sort of theory that the Large Hadron Collider is supposed to help sort out (in the limit of energy that the standard model _breaks down_). Maybe (?) we'll some answer's after the LHC comes online in a few months.

Re:Where's the evidence?

[Anonymous Coward]

At the very least they need a hell of a lot assumptions to simulate the distribution of matter at galactic scale for billions of years. To get results in reasonable time, their spatial resolution must be measured in light years and their time steps are probably at least years if not millennia. There's a lot that could go wrong there.

Re:Where's the evidence?

[amirulbahr]

I do have mod points, but thought I'd have a crack at this one. I look forward to be corrected.

Saturn and Earth interact with light, and so an alien may be able to observe them through their effect on radiation coming from the sun. Dark matter would have no such effect and would be observable only by gravitational effects it may have on light and/or nearby objects.

So to sum up, an alien would not need to use the idea of dark matter to account for Saturn or Earth.

Re:Where's the evidence?

[sgbett]

No way - vi is far more pluasible than emacs, so it should be for the dark-matter users!

Re:Where's the evidence?

[TapeCutter]

"I doubt this simulation did more than let them see what they wanted to see..[snip]..Why do they think MOND is for cranks and crackpots?"

The first bit is wrong, to understand why it is wrong you need to answer the second bit yourself.

Re:Where's the evidence?

[mazarin5]

Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales?

Nobody uses MOND for the same reason we don't use epicycles anymore: it's an unnecessary adjustment to an outdated system. Newton is a good approximation for low-speed and low-mass systems, but hasn't been considered perfect during the lifetime of anybody here.

What of a static non-expanding universe and alternate redshift paradigms?

Because it doesn't meet the observations. If you have to throw out everything else that does meet the observations to force fit your pet theory, then you're doing science incorrectly. Also, alternate redshift paradigms? Redshift is a very, very basic thing; it would take a lot of phenomenal evidence to change anything related to it.

I'm just curious as to why dark matter is so widely supported,

Dark matter is supported because it seems likely, fits the data, and doesn't contradict other observations. All it means is that we think there's some more mass out there, and we haven't seen it yet.

is it merely because breaking the standard model makes physicists too uncomfortable?

I get the willies just thinking about it! Or not.

Re:Where's the evidence?

[Nazlfrag]

Why crazy to modify the laws if the empirical evidence deviates? I'd say that's a damn good reason to modify the laws.

Re:Where's the evidence?

[Nazlfrag]

Neither. As I asked initially, why the faith in dark matter? If there is only gravitational influence, where's the lensing?

Re:Where's the evidence?

[Nazlfrag]

I guess I should have used 'sacred' instead of 'scared'. It is good enough for things whithin a lightyear or less, but why should what we know about the small scale apply equally to the large scale? It doesn't work the other way round, perhaps there's an AU level quanta effect prevailing, perhaps many things.

The blind dedication to the dark matter cause seems superfluous to the realisation that our local models of physics don't scale galacticaly.

Re:Where's the evidence?

[Nazlfrag]

Our models need to account for 90% more mass than empirical evidence proves exists. This only applies when considering galactic scales. Either our models are wrong, or there is a large mass of hidden matter that violates all the known properties of matter except one.

I believe our models are wrong, and this study is flawed. Don't get the willies, just try to broaden your perspective.

Re:Where's the evidence?

[Anonymous Coward]

Because you can't modify the laws until you're certain that it's the only explanation. I can't think of a better example than this - if there IS dark matter (something we can't see but has gravitational force) then Newton's Laws may very well be correct on large scales, and therefore MOND is wrong.

Re:Where's the evidence?

[Hektor_Troy]

If you read up on dark matter, you'd realise that it's merely a placeholder. We don't know WHAT it is, we just know it's there.

Re:Where's the evidence?

[Nazlfrag]

Yet empirical evidence says this is wrong, by a large factor. If there is an explanation that breaks what we know of as matter it should be condemned, not encouraged.

Re:Where's the evidence?

[Goaway]

Why the obsession dark matter?

How about because MOND completely and utterly fails to describe the Bullet cluster?

Re:Where's the evidence?

[Nazlfrag]

The only thing there is an inadequacy among our own models. Nothing more.

Re:Where's the evidence?

[Nazlfrag]

And when the analysis is wrong, we cling to the old models and advance fabrications as science? I think not. A single iota of evidence has never been forthcoming from the dark matter camp. Innuendo passes for affirmation. Please, all I asked for initially was a validation of dark matter theory in any way, and none has been forthcoming. Modifying the laws seems the only logical avenue.

Re:Where's the evidence?

[Nazlfrag]

Explain to me then how the experiment let them see more than which they analysed.

The first experiment was wrong because they never even calculated the known variables, they just coaxed the unknowns into a verifiable pattern. Little science happened here, and a shitload of speculation took its place.

Please give me your genuine answer for the second question, have you even considered MOND for yourself as an alternative, or are you just repeating that which has been foisted upon you?

Re:Where's the evidence?

[Anonymous Coward]

I doubt this simulation did more than let them see what they wanted to see. "The researchers note that the simulation does not model any forms of normal matter such as stars or planets." Hardly a complete picture they drew.

I do not understand why simulating only dark matter is a problem to the completeness of the description of the universe, since it represents more than 20% of the observable matter in the universe, most of the remaining matter/energy is dark energy. The stellar and gas constituents are the minor part of our universe.

MOND is a patch to a already patched (with relativity) theory. Introducing things "by hand" in closed physical theories is not comfortable for anyone. MOND has a great dependence in the dimensions and its parameters require a fine tuning to function well, in most of the cases this is not the way strong physical theories are build.

Re:Where's the evidence?

[Anonymous Coward]

The problem is the distance. Sure, Earth and Saturn reflect the light of the sun. But so does every other piece of dark matter. We just can't observe these effects because it is too far away and the reflection far to weak. If we would get close enough to dark matter it would just look like ordinary space dirt.

Re:Where's the evidence?

[vertinox]

What of a static non-expanding universe and alternate redshift paradigms?

A personal theory of mine is that redshift is affected by gravity and lack thereof.

Gravity's effect is more apparent the close you are to a body. The closer you are to the earth the more you are affected by earth's gravity and the closer you are to the moon the more you are moon's gravity and so on.

Hence, seeing light is affected by gravity, there might be something going off with the hand off between galaxies and other galaxies around it.

Re:Where's the evidence?

[wanerious]

I don't think "uncomfortable" is the right word. Certainly there is a huge upside to overturning the standard way of thinking, but also a huge risk in supporting ideas that seem untenable.

Dark matter is inferred from a number of observations and calculations, including excess rotation speed of stars around galactic centers, excess speed of members of clusters of galaxies, and lensing of background galaxies not associated with luminous matter. While one may fiddle with MOND to possibly fit the first two phenomena (for a specific case, moreover --- one adjustment to Newton's laws had better account for *all* rotation curves), the last group of observations really seem to argue strongly against any reasonable form of MOND.

If you google the Bullet Cluster image and description, you'll note that the blue region (most lensing of background) is tracing the highest density of matter, where the pink traces the density of luminous (here, X-ray cluster gas) matter. It is clear that there is a huge component of matter not identified with luminous matter. To account for this with MOND, it is not enough to increase the *strength* of the gravitational field, but also now the *direction*, since we observe the lensing in a direction roughly perpendicular to the axis of the cluster. Writing down such a gravitational law to account for this case would make any astrophysicist uncomfortable indeed.

Now, especially since we can work backwards and calculate the distribution of dark matter needed to cause each observation, dark matter seems to be the *much* simpler explanation. Besides, we already have examples of matter that interact only gravitationally (and possibly weakly) --- neutrinos. As hard as it is to detect them, there may well be others we don't know about yet.

Re:Where's the evidence?

[wanerious]

In case you don't get my response above, google for Bullet Cluster. There you'll see the lensing.

Re:Where's the evidence?

[wanerious]

Why do you suppose DM violates all known properties of matter (save one)?

Re:Where's the evidence?

[Nazlfrag]

It only interacts gravitationally right? Otherwise why is is dark?

Re:Where's the evidence?

[Nazlfrag]

Why do you attribute that to dark matter? It is an anomaly in Einsteins field equations sure, but evidence of unseen matter I doubt.

Re:Where's the evidence?

[wanerious]

and perhaps weakly. Why is that of concern? It would be composed of neutral particles of small cross-section.

Re:Where's the evidence?

[wanerious]

What?? Where in the field equations is there an anomaly? If it *were* some anomaly in the field equations, we'd see really bizarre effects everywhere else there are strong gravitational fields. We don't.

The point is that it's not a strange theoretical solution, but a real, physical deflection of light by a gravitating mass. It is certainly evidence of unseen matter --- were there a cloud of unseen matter, this is exactly the effect it would have on passing light. To attribute it instead to a non-specific flaw in a theory we know *very* well is far more speculative than what you criticize others for.

Re:Where's the evidence?

[pipingguy]

On the other hand climate computer models that predict catastrophe are usually pretty reliable.

Oh wait, we're not supposed to talk about that anymore.

Re:Where's the evidence?

[Ambitwistor]

Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales? Why do they think MOND is for cranks and crackpots?

Who's this "they"? There is a really unpleasant meme running around Slashdot that reactionary scientists scorn and mock anything that isn't mainstream. In reality, MOND is not a mainstream theory (and for good reason), but it's still discussed seriously. Google around for Sean Carroll's presentation a year or two ago on MOND vs. dark matter (can't remember when that was when he was at U. Chicago or Caltech). This is a good summary of his for why dark matter is likely to exist.

What of a static non-expanding universe and alternate redshift paradigms? Are they not just as feasible as exotic matter that only interacts gravitationally?

No, not even remotely. They are vastly less plausible than even MOND, which has problems of its own. But since this is about dark matter and its competitors, I'll stick to those.

I'm just curious as to why dark matter is so widely supported, is it merely because breaking the standard model makes physicists too uncomfortable?

Yeah, it's because theoretical physicists hate new theories. No theoretical physicist ever got fame and tenure by coming up with a new theory. They gotta stick to the old ones to survive.

Seriously, tone down the paranoia. Dark matter also breaks the Standard Model by introducing new kinds of particles. (Well, unless you think it's axions, which are arguably part of the Standard Model). It's not like nobody has ever thought of an alternative gravity theory before. You can carpet a small moon with all the alternative gravity theories out there; scalar-tensor gravity, vector-tensor gravity, conformal gravity, chiral gravity, supergravity, and so on.

The simple facts are that it's really hard to muck around with gravity in a way that simultaneously agrees with observations on tabletop, stellar, planetary system, galactic, and cosmological scales. With MOND it's easy to reproduce galactic rotation curves, but not much else. MOND also contradicts relativity; it's strictly Newtonian. There has been an attempt to correct that in the form of TeVeS (tensor-vector-scalar gravity). But TeVeS requires you to introduce two new gravitational fields, plus couple them together in just the right way, and even then it's far from certain whether it can explain dark matter evidence on all different scales (galaxies, clusters, cosmology, etc.). Furthermore MOND has difficulty explaining rather direct evidence of dark matter like the Bullet Cluster. Even if MOND is correct, it seems likely that you still need dark matter to explain everything.

After all that, MOND looks far more ad hoc than just postulating the existence of a new kind of particle, especially since most of the new particle theories out there predict some kind of dark-matter like particle anyway for completely independent reasons. It's not like weakly interacting particles are terribly bizarre in the first place; neutrinos are dark matter, although they're too light to be most of the dark matter. The main difference between most of the dark matter and neutrinos is mass, and what's so odd something weak like a neutrino, only heavier? Such a particle, predicted by many theories, can (unlike MOND) simultaneously explain all the astrophysical phenomena which point towards dark matter.

Re:Where's the evidence?

[Ambitwistor]

That's not a personal theory, that's a real theory: redshift is affected by gravity. (Google "gravitational redshift".) The explanation of cosmological redshift can be interpreted that way (expanding universe implies curved spacetime which implies gravitational redshift). But you can't explain cosmological redshift using a non-expanding flat universe by appealing to galaxies' gravity alone. It doesn't explain the distance-redshift relation, the luminosity-redshift relation, it would show a strong correlation with galactic/stellar mass which is absent in the data, and so on.

Re:Where's the evidence?

[Ambitwistor]

The blind dedication to the dark matter cause seems superfluous to the realisation that our local models of physics don't scale galacticaly.

There is no "blind dedication to dark matter". And in fact, not scaling properly is exactly what's wrong with MOND. If you apply it to galaxies, it doesn't work for cosmology, and so on.

Re:Where's the evidence?

[Ambitwistor]

Dark matter is matter that doesn't interact electromagnetically (or does so very weakly). It can interact by any other interaction other than electromagnetism, including but not limited to gravity.

Re:Where's the evidence?

[Ambitwistor]

Yes, they're reliable enough for policy recommendations. What's your point?

Re:Where's the evidence?

[Ambitwistor]

The Standard Model arguably includes axions, which were postulated to solve the strong-CP problem in quantum chromodynamics. They are dark matter candidates (although I think astrophysical constrains favor SUSY.) However, like the Higgs boson, they have not yet been observed.

Re:Where's the evidence?

[Abcd1234]

Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales?

Because results like the Bullet Cluster have left even MOND theorists admitting that there must be at least some dark matter out there in order to explain the observations? Seriously, the least you can do is keep up with the latest science on the topic.

Re:Where's the evidence?

[Anonymous Coward]

This is wrong. Dark matter, as far as we know, ONLY interacts gravitationally and (hopefully) via the weak interaction. Reflecting light would require it to interact electromagnetically, which it does not.

Also, dark matter isn't "too far away." It's right here. It's just that we can only right now see its gravitational effects, which means we need to look at entire galaxies.

I say "hopefully" for the weak interaction because that's our best bet for directly detecting it on Earth. Many of the big detector projects use a tank of liquid xenon or argon and wait for a WIMP to come by and bump into it, exchanging a virtual Z boson and causing the nucleus to scintillate.

Re:Where's the evidence?

[Urkki]

Since it's known that some of the dark matter is regular baryonic matter such as MACHOs, then if MOND is the right way to go, shouldn't it be possible to determine from the galaxy rotations plus MOND, how much of the effect is due to MACHOs, and how much is due to theory being wrong?

I mean, to me it sounds a bit fishy, if there is no observable difference between effects of invisible dark matter without MOND, and MOND with just baryonic dark matter...

Re:Where's the evidence?

[mazarin5]

Our models need to account for 90% more mass than empirical evidence proves exists.

It's not necessarily as high as 90%, but that's one of the more extreme estimates in one of the models. Also, you have to understand that this is what the empirical evidence suggests exists.

This only applies when considering galactic scales.

Not especially true; dark matter, in many of its postulated forms, would be around us locally, but in concentrations (or distributions) that are inconsequential on the local scale. That is, its effect would be below our margin of error.

Either our models are wrong, or there is a large mass of hidden matter that violates all the known properties of matter except one.

Or there's something entirely at work that hasn't been considered yet. Dark matter is one of those possibilities. It doesn't violate anything - space is dark and things are very far away. This makes it hard to see things that aren't burning brightly on their own. That's all it takes to make dark matter - mass where we can't see it. If it were just dust, it would likely block out light from other stars, so that implies that the unseen matter is either very, very big or very, very small. It could just as well be brown dwarfs bouncing around the outskirts of the galaxy. Somebody thought it may be one of several possible exotic particles, that would explain things nicely. We actually have a better chance of detecting the exotic particles than the rogue planets, so some people are focusing on that (and getting the most coverage).

I believe our models are wrong,

I disagree. I would however, like for you to continue explaining what you think needs to change, why, and how it reconciles with other areas of knowledge.

and this study is flawed.

It very well may be at this stage. I'm sure that it's more detailed than described in this article, but simulations like these tend to get run in stages, or are considered rough cuts. Take it as a thought experiment that a human brain couldn't handle.

Don't get the willies, just try to broaden your perspective.

Don't you pretend to not catch my sarcasm! Anyways, there's no physicist alive today that wouldn't love to be the one that blew away everything we thought we knew and usher in a new era of scientific knowledge. They would have half a dozen theorem, laws, and units named after them. It would be sweet. Trust me when I say that nobody is ignoring some truth out of their own insecurity - thinking otherwise is paranoid and silly.

Re:Where's the evidence?

[TapeCutter]

You missed my point. The first thing your post did was disparage others for "seeing what they wanted to see". You then went on to tell us how you can see MOND making sense despite the fact that scientist can't get it to work.

Re:Where's the evidence?

[Ambitwistor]

We actually have a better chance of detecting the exotic particles than the rogue planets, so some people are focusing on that

It's the other way around. Dark matter in the form of brown dwarves and such has actually been detected through gravitational microlensing observations. That's how we know that most of the dark matter isn't brown dwarves and other compact astrophysical bodies; it's there, but there isn't enough of it to explain the necessary gravitational effects.

Re:Where's the evidence?

[amirulbahr]

Wrong again. Is this the same AC that I replied to or another one? Please don't add to the confusion about why there is a concept of Dark Matter. I call it a concept, because we don't know what it is. Perhaps a better term would be "Matter that has gravitational effects but DOES NOT interact electromagnetically", so as not to confuse people like yourself. THERE IS A FORM OF MATTER THAT DOES NOT INTERACT WITH LIGHT. They call it dark matter.

I am not a physicist, but please do look into the matter (excuse the pun) before you make authoritative sounding comments like "The problem is the distance".

Re:Where's the evidence?

[Anonymous Coward]

MOND was basically ruled out by NGC 4736.

Re:Where's the evidence?

[anandsr]

Since MOND does revert to GR in high Gravity situations so it will support frame dragging. Anyway we have not much observations in the low gravity situations. The only problem with MOND is that it is not a proper theory. There is now a theory that models MOND called TeVeS which does support Frame Dragging, etc. The theory has not yet been studied to the extent that the really big questions like inflation, nucleosynthesis, WMAP, etc can be worked out.

But one thing is for sure that TeVeS cannot be ultimately correct. We know that the final theory must atleast bring GR and QM together. TeVeS cannot do so.

Re:Where's the evidence?

[anandsr]

Well Bullet Cluster or any other cluster is a problem for MOND. But the Bullet Cluster is also a problem for Dark Matter. The velocity of the Cluster is much faster than GR can provide, which MOND can easily handle.

I actually liken MOND to Keplers laws. It shows how the matter moves. If GR cannot reproduce it, then there is a need for a new theory. TeVeS is a patch which will not survive for very long.

The other problem is that GR and QM don't work well. Possibly when we a proper Quantum Gravity theory then we will find out how MOND comes about. MOND is just an indication of the problem.

I don't disbelieve in Dark Matter, but I am sort of convinced that there is something about MOND. It could be that both MOND and DM are correct. DM may exist in significant quantities in Clusters while MOND is sufficient at the Galactic levels.

Re:Where's the evidence?

[Ambitwistor]

Well Bullet Cluster or any other cluster is a problem for MOND. But the Bullet Cluster is also a problem for Dark Matter. The velocity of the Cluster is much faster than GR can provide, which MOND can easily handle.

(GR doesn't have anything to do with it per se; Newtonian gravity predicts the same thing as GR on galactic scales.)

The Bullet Cluster is a much more severe problem for MOND than dark matter; you can't even qualitatively explain the divergence between the galaxy and the lensing with MOND. The Brownstein and Moffat paper you're alluding do doesn't even attempt to address that elephant in the room, which is the problem with MOND and the Bullet Cluster by far. I've also read comments on Cosmic Variance that they assume some coincidental alignments of the galaxies and the result barely reaches statistical significance, but I'm no expert.

Then again, there are the other problems with getting MOND to hold on a number of other scales, which myself and others have discussed elsewhere in these comments.

I actually liken MOND to Keplers laws. It shows how the matter moves. If GR cannot reproduce it, then there is a need for a new theory.

Er, no. MOND doesn't show anything about how matter moves. We observe how matter moves and theorize explanations of it (dark matter, MOND, etc.) There is more than one way to explain observations. And I don't see this as even remotely a thorn in the side of GR, or even much so for dark matter. Galaxies are messy. In the case of the Bullet Cluster, it happens to have a geometry which is a smoking gun against MOND (or at least MOND with no dark matter), but it's much less conclusive about dark matter.

The other problem is that GR and QM don't work well.

That almost certainly doesn't have anything to do with MOND, given the scales at which quantum gravity is relevant (which is not astrophysical!). It certainly doesn't imply something like MOND should be true.

It could be that both MOND and DM are correct.

That would be the most scientifically interesting outcome. But I don't think it's very likely. Even if GR needed to be modified on astrophysical scales, I highly doubt anything like MOND would be it.

DM may exist in significant quantities in Clusters while MOND is sufficient at the Galactic levels.

It's really hard to construct a hybrid theory like that, because if you reduce the amount of dark matter in galaxies by introducing MOND, you completely screw up early universe structure formation. Then you have to explain why all that dark matter in clusters didn't clump in galaxies. You can turn back to hot dark matter, but I think that's going to screw up cosmology. The reason why dark matter has become the dominant theory is because it consistently explains a lot of otherwise unrelated phenomena. It's hard to change one part of it without making things worse.

Re:Where's the evidence?

[Ambitwistor]

Since MOND does revert to GR in high Gravity situations so it will support frame dragging.

Don't confuse MOND and TeVeS. MOND is MOdified Newtonian Dynamics. It doesn't revert to GR or any relativistic theory in high gravity situations; in fact, it grows more different from those theories in strong gravity. TeVeS is a relativistic theory which has MOND as a particular limit (and Newtonian dynamics as another). It probably approaches GR in some other limit (decoupling of the scalar and vector fields?), but whether it agrees with all observations in strong gravity situations is, as you say, unknown. It doesn't automatically agree with GR, so unless someone has performed the frame dragging calculation specifically, we can't say how much frame dragging it may predict.

Have we found them?

[i_liek_turtles]

I've always wanted a pet Nibblonian...

Slightly off topic: Dyson Spheres

[Abeydoun]

I've been curious... if there was an incredibly advanced civilization that was capable of building near perfect dyson spheres around large expanses of space absorbing essentially all the radiation of the stars within it, wouldn't that look like "dark matter"?

Re:Slightly off topic: Dyson Spheres

[Anpheus]

That is a "MACHO," which we have looked for and not seen enough of. MACHOs are unique in that they affect the light behind them (they are opaque and gravitationally lensing) and so while they could account for a lot of matter, we aren't seeing enough lensing and enough holes in the spectrum from "dark stars", areas where the sky is darker from an object blocking light behind it.

And back of the envelope calculations say a dyson sphere wouldn't be anywhere near a black hole's mass, which is what we really would need to find quite a few of in order to find the missing mass.

And this ignores any technical difficulties with actually constructing a dyson sphere.

Re:Slightly off topic: Dyson Spheres

[jabithew]

The sphere itself would still be visible, as it would most likely have a temperature above the cosmic microwave background and would hence give of visible light. This is unless they've found a way to convert disparate, low quality energy into matter or something even more outlandish, like finding a way to destroy energy.

Re:Slightly off topic: Dyson Spheres

[jabithew]

Sorry, I just realised that when I said visible light I meant detectable electromagnetic radiation, not actual visible light.

Re:Slightly off topic: Dyson Spheres

[Jugalator]

Not really, dark matter isn't black matter hiding light. It's only called "dark" in a metaphorical sense.

Signs of it has been seen as being completely transparent and the only real visible sign of it being there thanks to it bending light due to gravitational lensing.

Re:Slightly off topic: Dyson Spheres

[dissy]

I've been curious... if there was an incredibly advanced civilization that was capable of building near perfect dyson spheres around large expanses of space absorbing essentially all the radiation of the stars within it, wouldn't that look like "dark matter"?

Or perhaps even that incredibly advanced civilization consists of one or more Matrioshka Brains which are living thinking entities with very similar characteristics as Dyson spheres have, as well as answer a number of unanswered questions about cosmology (assuming they exist, which statistics say not only should exist -somewhere- in the universe, but are actually likely, and perhaps even inevitable.
Ok, that last one was my own wish more than anything else, perhaps the hopeless romantic in me, perhaps some silly child type want for such things to exist, but read the link above and decide for yourself.
It is an interesting read no matter how you take it.

To answer your question though, no I don't believe so (for either one.)
I'm fairly sure most of the people trying to prove dark matter have narrowed the list down to stuff which can't _block_ light/EM at all, just what affects energy and matter gravitationally.

Re:Slightly off topic: Dyson Spheres

[DMUTPeregrine]

Actually, a Dyson Sphere would have the same mass as a black hole around an equivalent star, as it encloses the star. Black hole mass = mass of solar system. Sphere mass = Mass of sun + mass of sphere = Mass of solar system.

Re:Slightly off topic: Dyson Spheres

[Anpheus]

I wasn't being particular enough, but there's a lack of "intermediate" size black holes from what we can see. We only find really stinking huge ones and then smaller ones, but still many times the mass of our solar sun.

But what we need to find are -ridiculous- numbers of these smaller black holes, or still a ridiculous number of intermediate or supermassive black holes. We don't though. They apparently aren't there.

And for the Sun to be a black hole, it would have to become a lot more massive. There's no way our solar system could ever become a black hole without interacting with another star.

Re:Slightly off topic: Dyson Spheres

[AnObfuscator]

I've been curious... if there was an incredibly advanced civilization that was capable of building near perfect dyson spheres around large expanses of space absorbing essentially all the radiation of the stars within it, wouldn't that look like "dark matter"?

Nice, that's one of the more creative ideas I've heard in a while.

However, if the sphere absorbs the radiation of the stars inside of it, then the sphere will itself become heated and radiate back the energy it absorbs. It has been a while since I've taken thermal physics, but I'm fairly confident that an object that absorbs that much energy would be quite radiant, itself -- just in different wavelengths.

Kahn was right!

[richie_the_toolman]

It is very cold in space!

Dark matter = Dark aliens

[Anonymous Coward]

If > 1/2 of the universe is dark, it reasons that there is dark life as well. It probably wouldn't have to be that far away to remain undetected by us. We tasted Mars for the first time a week ago. That's right next door.

Maybe it's time we act like we might be observed.

Re:Dark matter = Dark aliens

[jabithew]

That doesn't follow. There *might* be dark life *if* dark matter is capable of complex chemistry(-like?) interactions.

Re:Dark matter = Dark aliens

[dreamchaser]

That doesn't reason at all unless you find that dark matter can undertake complex interactions with other dark matter. It's more likely that it can't interact with itself much at all except through gravitation.

ha

[Johnny Chinpo]

More cruft to add to the bogus hypothesis that is dark matter.

I am a physicist.

Re:ha

[Ian Alexander]

I'm not a physicist and I'm not terribly well-informed on the whole dark matter thing. Are there any other hypotheses out there to do the same explanatory work as dark matter that's also less bogus?

Re:ha

[BrentH]

As a scientist you should know that there only are hypotheses. All you can do is find evidence or create tests that agree of disagree with a hyptothesis. As a bachelor student in a world renowned astrophysical institution, I started out as a sceptic (and still am). There are two branches of physics that sort of require the existence of dark matter. One is the theoretical side of astrophysics: the universe appears to require extra matter (and energy, which may well be vacuumenergy from particle physics) to hold the universe together. The other is observational evidence of the sort that there are clusters of stars which could be bound (=exist) with the visible matter we detect). Especially the Bulletcluster (use your google-fu) is a very convincing example of where dark matter would fill the gaps very neatly. It's certainly true that there are other hypotheses (the so called Dark Matter be very well be large number of small and very faint ordinary star systems, which is a hypothesis which I personaly suspect can bring us a long way to solving these lack-of-mass problems). The book "Galactic Dynamics" by Binney and Tremaine offers far more and better explanations of the reasons for Dark Matter.

Re:ha

[khallow]

Do you have anything to say about the nonbogus dark matter hypotheses too?

Re:ha

[Abcd1234]

Well, either you're lying, or you're poorly educated on the topic. The Bullet Cluster results have all but demonstrated the existence of dark matter.

Re:ha

[Ambitwistor]

I'm a physicist too. Please stop embarrassing us. Your personal dislike for the theory is sadly irrelevant to the evidence in its favor.

Found one!

[Tablizer]

One such clump was detected in the headquarters of the SCO corporation.

Re:Found one!

[Anonymous Coward]

One such clump was detected in the headquarters of the SCO corporation.

They hoard AOL disks too? That splains it.
       

Cosmic Smog?

[Tablizer]

It would certainly be interesting if dark matter turned out to be some kind of pollution from advanced civilizations.
     

Writing good code.

[dimethylxanthine]

The clumps of dark matter in the simulation have densities that are remarkably similar to densities that a University of California, Irvine research group found when simulating the formation of the Milky Way

They must have learned something about code reusability in university.

Dark Matter....

[Anonymous Coward]

..... aka the new aether

Re:Dark Matter....

[Nazlfrag]

Perhaps both dark matter and an aether exist due to quantum foam, isn't that one of the hypotheses?

Re:Dark Matter....

[Urkki]

If only you could do the equivalent of Michelson-Morley_experiment , and save everybody a lot of trouble... Now we're stuck doing these other simulations and experiments trying to find something you know will not be found.

Re:Dark Matter....

[Abcd1234]

Now we're stuck doing these other simulations and experiments trying to find something you know will not be found.

Actually, it has been found. We still don't know what it is, but it's there, whether you like it or not.

Re:Dark Matter....

[Urkki]

Just to clarify, I am quite fine with both dark matter and dark energy, I was just giving suggestions to the AC poster comparing dark matter to the aether.

Gravity from "elsewhere"?

[YttriumOxide]

For some time, I've been making random notes from articles relating to dark matter, string theory and quantum entanglement. I've been trying to form a hypothesis of dark matter with information from all of these three.

Interestingly, this article has now given a lot more focus to one part of the idea that was forming that was a bit "wishy-washy" before (okay, it's still very wishy-washy, but less so now).

The overall concept is basically along the lines of quantum entanglement being a property of the fact that two entangled particles are in fact just ONE string that's being bent through space in some rather unconventional ways (extra dimensions neither being "large and flat" nor "very very small and coiled", but rather "hideously complex monstrous things").
This, combined with gravity ("graviton"strings) being freely able to travel through those dimensions rather than tied to an endpoint (hence appearing much weaker than the other forces, even though ALL forces have absolutely identical strength (another wild-ass guess, just because it would be "nice")) would lead us to an elegant idea about dark matter actually being gravity from perfectly normal matter that happens to be showing up in unexpected places.

The fact that there are clumps of it definitely does not blow my ideas out of the water, but it does mean I need to re-work my idea of "hideously complex monstrous things" for the extra dimensions as I was assuming dark matter showed up "generally" in areas with other matter rather than specific clumps as "normal" does. It needs to be more structured than I had been thinking for a clump of matter in one place to form a "clump of dark matter" (e.g. the gravitational effect seen) elsewhere. That's actually a good thing though, because any structure lent to the process makes it closer to a testable hypothesis (anything completely unstructured could never become one, and having "no real clue" about the structure as I was, made it far too vague.)

Note that this is still a very early infancy idea, and is somewhat around the "wild guess" point rather than even "hypothesis", so I'd be quite happy for people to comment on this - can anyone blow me completely out of the water on this line of thinking? Or can anyone offer ideas that support it? Or even just expand it a little? Does anyone know of any other research along these lines that I could read?

Re:Gravity from "elsewhere"?

[Tangent128]

So, are you suggesting that dark matter gravity is actually gravity from matter in a parallel universe or from this universe folded in extra dimensions? Sounds fun, even if false.

Re:Gravity from "elsewhere"?

[YttriumOxide]

My beginnings of a hypothesis are that it's gravity from THIS universe from folded dimensions. "Parallel Universe", while possible, seems to require too many further assumptions.

Fun, yes. False, almost certainly, but not necessarily - that's the point of trying to formulate a hypothesis.

Re:Gravity from "elsewhere"?

[Ambitwistor]

The main problem with your theory is that entanglement just isn't that relevant on cosmological scales. On large scales, physics is pretty classical and entanglement irrelevant. That can change at small scales, so you might argue that it used to be relevant (near the Big Bang). But then temperature screws everything up: thermal excitation screws up the entanglement. Maybe it could have been relevant right at the Big Bang itself, due to poorly understood quantum gravity, but it has little to do with any macroscopic physics going on today. That's why the world around us looks classical and it took so long to discover quantum mechanics.

Re:Gravity from "elsewhere"?

[YttriumOxide]

On large scales, physics is pretty classical

Other than the large amount of seemingly "dark matter"...

entanglement just isn't that relevant on cosmological scales

Certainly true, but I wasn't really implying it needs to be. If we have a pair of entangled particles, they can THEN be separated by (apparently) quite any distance in classical space and nevertheless remain entangled. We haven't found a good way to use this as "faster than light communication", and there are very strong arguments that it can't be, however it seems to remain fact that these particles are somehow "connected" despite being physically quite separate ("spooky action at a distance"). This is why my hypothesis considers that they may actually not be physically separate at all - they are "right beside each other" through twisted "extra" dimensions, just in some way that we currently do not have the ability to measure or understand particularly well. This leads on to the idea of "dark matter" being gravity from "nearby" objects that are classically quite distant, but in reality quite "close".

Note, I don't actually disagree with you, but clearly there's something going on that we really don't understand very well yet.
Also note that I'm probably wrong - my idea is just that, an idea - I don't necessarily believe it!

Re:Gravity from "elsewhere"?

[Ambitwistor]

Other than the large amount of seemingly "dark matter"...

There's nothing non-classical (i.e., quantum) about the behavior of dark matter. At least nothing we've observed so far. It's just matter, maybe like a neutrino but heavier. You don't need to appeal to quantum entanglement or anything exotic to explain it.

If we have a pair of entangled particles, they can THEN be separated by (apparently) quite any distance in classical space and nevertheless remain entangled.

In theory, yes. In practice, they need to be extremely isolated from everything else in the universe to remain entangled. That's why it's so difficult to maintain long-range entanglement for purposes like quantum encryption. You may think space is "empty", but it just takes a single photon to ruin the entanglement.

This is why my hypothesis considers that they may actually not be physically separate at all - they are "right beside each other" through twisted "extra" dimensions, just in some way that we currently do not have the ability to measure or understand particularly well.

Now you're mixing up two different ideas, quantum mechanics and extra dimensions. If they're interacting through other dimensions, fine, but you don't need entanglement to explain that either.

This leads on to the idea of "dark matter" being gravity from "nearby" objects that are classically quite distant, but in reality quite "close".

All the above problems notwithstanding, it's easy to come up with more. Even if you ignore the fact that these particles can't plausibly remain entangled, you have to explain how all these distant particles got entangled with each other in the first place. (If you're tempted to say "the Big Bang", you really can't ignore the previous fact given the extreme temperatures involved. Maintaining entanglement requires no outside interaction with other particles at any time over the intervening 14 billion years.) Furthermore, entangled particles interact with each other, but that interaction in general doesn't look anything like the vector or tensor field theories that give rise to what we think of as "forces" such as gravity or electromagnetism. (Why should this purported "entanglement force" act like gravity anyway?) And there's no explanation for the astrophysical observations like how entanglement with distant particles can simulate the effects of a spherical cloud of dark matter enclosing a disc galaxy.

Basically, it seems like a very bizarre hypotheses that, all the physical evidence against it aside, doesn't even seem to have much to commend it over the alternatives. It's not like the idea of there being weakly interacting particles out there is so crazy; plenty of particle theories predict them for completely independent reasons including possibly the Standard Model, and there are other weakly interacting (though less massive) particles out there already, like neutrinos.

Re:Gravity from "elsewhere"?

[hypomorph]

So you're saying that gravity is observed to be so weak compared to other forces because it is somehow "diffused" throughout these extra dimensions? If the electro-magentic force is not present in these other dimensions, might that not explain why dark matter is dark?!

Re:Gravity from "elsewhere"?

[YttriumOxide]

Yes... that's the basic idea of it! Gravity can "flow" through these other dimensions, but the other forces can not, so even though they're of the same strength, they LOCALLY appear to be very different. Then, the gravity "pops up" somewhere else, and we have this unexplained EXTRA gravity there that we can't account for. We call it "dark matter", but actually can not detect anything other than the gravity. So, my idea is that there actually is NOT anything other than the gravity, and the source of that gravity is perfectly normal matter that happens to be somewhere else in space.

What would be a really nice find would be to see some objects being attracted to a point that quite clearly doesn't have anything else in it - a dust storm condensing in to a planetoid for example. That would show there's "gravity without matter" at that location, which could easily be explained by my idea. What would also be interesting is that the planetoid in question would then appear to have more mass than it actually does, as it would have its own gravity in addition to the extra that pulled it together to begin with.

Re:Gravity from "elsewhere"?

[YttriumOxide]

Hmmm... I think you're latching on to my mention of entanglement a bit much and that's causing a misunderstanding - looking back at my last posts, I can see how I gave that impression... sorry. I'm not saying that there's any relationship between entangled particles and my ideas about dark matter directly - I'm not positing an "entanglement force" of any kind. I was simply giving two somewhat separate ideas with a related core idea - that of the twisted dimensions allowing for distant objects to be in reality "right beside each other" (itself not a new idea really, just something I'm expanding on in a different way).

The mention of entangled particles is one idea that comes from it - you can have two particles that appear distant, but are not, and so the fact that they can influence each other directly does not violate any laws (no faster than light comms between them).
The mention of dark matter being gravity from known objects has nothing to do with these entangled particles other than that the same mechanism of "twisted dimensions" underlies it. We all know that gravity appears much weaker than the other forces, and this gives us a relatively "neat" out for that not only by saying it is actually the same strength and the rest is just "going somewhere" (as has been put forward before in the realm of string theory), but ALSO that the "somewhere" it's going is somewhere that we can actually see and detect (the gravity we attribute to "dark matter")

The key to the idea is to find if there's any correlation between the amount of unaccounted for gravitational influence (dark matter) and the amount of gravity that non-dark-matter would put out if gravity were the same strength as the other forces (just only LOCALLY weaker, with the remainder going elsewhere). If they are approximately similar, it would be a great thing for my idea.
Basically, gravity appears to be about 10^36 times weaker than the other forces. Currently, that's a completely different order of magnitude than the amount of matter vs "dark matter", so it does throw a big spanner in the works. Of course, plenty of the gravity could be popping up in places where it's not obviously affecting anything else (there's a LOT of (almost) empty space between the galaxies and we don't spend much time looking at it!)

Re:Gravity from "elsewhere"?

[Ambitwistor]

I was simply giving two somewhat separate ideas with a related core idea - that of the twisted dimensions allowing for distant objects to be in reality "right beside each other"

Okay, ignore for the moment quantum mechanics and just talk extra dimensions. It's still really hard to explain dark matter by appealing to distant normal matter being nearby in extra dimensions. Think about how these dimensions need to be connected. The standard scenario has them curled up small, which means that matter only interacts with itself through them, not with distant matter. Or you could postulate that they hook up to distant parts of the universe, but doesn't explain how galaxies consistently get connected to other collections of matter. Even if they did, that doesn't explain a gravitational force that acts like dark matter does; it would probably be more like putting two galaxies on top of each other. Or you could also postulate that matter is connected up randomly with other matter, but with that it would hard to explain any consistent attractive effective force, let alone one that is attractive in the manner required. And then I suspect that you would see huge apparent violations in conservation of energy if there are significant interactions going on in unseen extra dimensions. Finally, I can't think of any actual theory within which you could construct any of these scenarios with any plausibility.

The usual extra dimensional gravity scenarios are (a) gravitational interactions between matter and itself in extra dimensions ("large extra dimensions"), and (b) interactions beteween matter in our universe and matter in a "parallel universe next to ours" (e.g., "braneworlds"). I think dark matter scenarios have been proposed in both, but I think they both have problems and they're even more speculative than dark matter is.

Re:Gravity from "elsewhere"?

[Artifakt]

Are the extra dimensions actually limited to those of one of the current models (i.e. the most common 10 dimensional string theory model or one of its alternates?)
(In other words, are you assuming the topology is the only thing that really matters, or just that you could find an interesting topology that shows some predictive power, and then refine both it and how many dimensions it applies to, and the tweaks needed would be easier to test and/or simpler and/or more beautiful than starting from the other end?)
Are any of the extra dimensions supposed to be time-like, or at least significantly partially time-like?
Would the number of time-like dimensions involved impose significant additional limitations on testability, and would the difficulty of testing in general increase (probably exponentially or by some similarly steep function) if the number of time-like dimensions required increases?

Re:Gravity from "elsewhere"?

[YttriumOxide]

Are the extra dimensions actually limited to those of one of the current models (i.e. the most common 10 dimensional string theory model or one of its alternates?)
(In other words, are you assuming the topology is the only thing that really matters, or just that you could find an interesting topology that shows some predictive power, and then refine both it and how many dimensions it applies to, and the tweaks needed would be easier to test and/or simpler and/or more beautiful than starting from the other end?)

Basically, I'm not YET making any assumptions about the topology, but it IS the most important factor in the idea itself. Once I think I have enough of an idea about what I need to be looking for, the first step will be to look at existing models (such as the more common string theory models) and see if they fit (or can be reasonably altered to fit without breaking other things), but if nothing does, I'll try to see if a topology could be described in any way that fits, whether it's a pre-existing idea or not.

Are any of the extra dimensions supposed to be time-like, or at least significantly partially time-like?
Would the number of time-like dimensions involved impose significant additional limitations on testability, and would the difficulty of testing in general increase (probably exponentially or by some similarly steep function) if the number of time-like dimensions required increases?

Time or time-like dimensions could DEFINITELY be a problem for testability - if gravity can "leak" in time as well as more traditional space, then there's no definitive way to measure the amount observed with any real meaning (perhaps there's more leaking to a point 2 billion years from now, and we'll suddenly see a huge spike of "dark matter" around that time, or perhaps quite a lot leaked in to the past - although at least that would let us see that things happened that we couldn't reasonably account for without some extra gravity suddenly appearing). My idea hasn't really considered time-like dimensions yet, although if time really can simply be described 100% as a dimension that is no different to the others, then of course it has to be a consideration. Initially, I will try to consider cases that do NOT have a time-like function to them (completely "static" universe for the sake of simplicity), but I do think it may become unavoidable.

All of this aside, I'd like to re-iterate that after making several posts on this subject, it is still just an "idea" rather than a "hypothesis" (let alone a "theory"), and I haven't considered a lot of things, so there are still MANY things that could trip me up and render this whole line of reasoning dead before I get to the grittier parts such as the possibility of time-like dimensions.

Re:Gravity from "elsewhere"?

[YttriumOxide]

that doesn't explain a gravitational force that acts like dark matter does; it would probably be more like putting two galaxies on top of each other

Yes, I've had to really think about this - it's simply "not enough" to postulate that all of the "leftover" gravity from one relatively large place is displaced somewhere else - that just wouldn't work - a galaxy that had an extra galaxy's worth of gravity (or MUCH MUCH more if it's all the "leftover" gravity from a gravity force that's as strong as the other forces) would simply collapse immediately and we'd very much notice it.

Or you could also postulate that matter is connected up randomly with other matter, but with that it would hard to explain any consistent attractive effective force, let alone one that is attractive in the manner required.

I don't see that's necessarily the case. Rather than saying "random", I prefer to say "not obvious" - that is, the "extra" gravity from Earth (just as an example) isn't necessarily projecting an Earth sized blob of gravity (or Earth * 10^36 sized blob) at one other point in space. More that much smaller parts of Earth are projecting much smaller blobs of gravity at various locations. In places where it happens that lots of "end points" for the gravity are, we see these apparent clumps of dark matter.

And then I suspect that you would see huge apparent violations in conservation of energy if there are significant interactions going on in unseen extra dimensions.

Who's to say we'd see "huge" apparent violations? If our local zone is relatively free of these clumps of endpoints, we'd likely only see VERY minor apparent violations, which would go overlooked unless specifically looked for. The current version of my idea has the end-points spread out all over space, with some places having more than others (the clumps). So, if it were to be correct, we could theoretically test for it by looking for minute amounts of extra gravity in places that shouldn't have them. Unfortunately, stuck at the bottom of a gravity well such as Earth, that's extra-ordinarily hard to do - such an experiment should be conducted in a place with far less local gravity.

The universe is a big place and there's not that much matter in it - if the "extra" gravity was mostly spread evenly, it'd be pretty thin in most places (to the point of being REALLY hard to notice), and only obvious in places where there's more of it, or in very large structures (galaxies). This is pretty consistent with what we see with regard to dark matter.

The usual extra dimensional gravity scenarios are (a) gravitational interactions between matter and itself in extra dimensions ("large extra dimensions"), and (b) interactions beteween matter in our universe and matter in a "parallel universe next to ours" (e.g., "braneworlds"). I think dark matter scenarios have been proposed in both, but I think they both have problems and they're even more speculative than dark matter is.

I don't like the braneworlds idea - I actually think it's MORE speculative than my idea because it requires more assumptions. Mine really only requires one, despite that it's a VERY VERY big one. It is theoretically testable though (once/if I have a hypothesis worth testing), so it should be possible (although likely not very easy) to either confirm or deny it fairly conclusively.

Re:Gravity from "elsewhere"?

[Ambitwistor]

Rather than saying "random", I prefer to say "not obvious" - that is, the "extra" gravity from Earth (just as an example) isn't necessarily projecting an Earth sized blob of gravity (or Earth * 10^36 sized blob) at one other point in space. More that much smaller parts of Earth are projecting much smaller blobs of gravity at various locations.

I don't see how you're going to get an effective central force out of that without a lot of fine tuning. And I don't know what mechanism is supposed to hook local portions of space up to distant locations in any manner other than random. At least in quantum cosmology you could imagine some kind of chaotic spacetime foam.

Who's to say we'd see "huge" apparent violations?

If dark matter is affecting galactic dynamics so strongly that we have to postulate that 90% of the matter out there is dark matter, I can't see how you're going to avoid strong violations.

It is theoretically testable though

I don't know. It's impossible to prove vague theories wrong. To make predictions that are really testable, you need to embed it in a quantitative framework, like a gravitational field theory.

Re:Gravity from "elsewhere"?

[YttriumOxide]

I don't see how you're going to get an effective central force out of that without a lot of fine tuning. And I don't know what mechanism is supposed to hook local portions of space up to distant locations in any manner other than random. At least in quantum cosmology you could imagine some kind of chaotic spacetime foam.

"Chaotic" != "Random"... But if you prefer to call it random, then okay, let's work with that definition. The mechanism is the main thrust of my idea - "twisted" dimensions.

If dark matter is affecting galactic dynamics so strongly that we have to postulate that 90% of the matter out there is dark matter, I can't see how you're going to avoid strong violations.

Work with the word "random" - Quite "at random", our local area doesn't have any large clumps, and it's distributed so finely that we don't see it. This is also the case for most of the universe in general, so we're nowhere special in that regard. The areas that ARE special are those where it's "at random" clumping up the endpoints to a noticeable amount of gravity. The effect that is seen with galaxies is also nothing special - if the "empty space" contains fairly evenly distributed end-points for gravity, it's possible to be weak enough on a small scale, while being pretty strong on a large scale, giving the effect we see.

To make predictions that are really testable, you need to embed it in a quantitative framework, like a gravitational field theory.

Which is why I'm still calling this an "idea", which may become a "hypothesis", which will hopefully be testable. At this point, I don't make any claims whatsoever other than that it's an interesting idea which might be plausible enough to warrant further investigation.

Compression of matter

[Anonymous Coward]

Let's get this right. You get a blob of uranium and compress it into a small area and it becomes really hot. Then you get some so-called "dark matter" and let it attract itself together into a blob and it gets hot. Am I missing something here? Has science really lost the plot?

Nigger Matter

[Anonymous Coward]

Just like in society, our "dark matter" tends to clump together in the ghetto. Aryan matter doesn't want to be anywhere near nigger matter because nigger matter is known for its high rate of crime, low IQ, and massive poverty that brings down property values.

-Aryan Scientist

I've got some dark matter

[Anonymous Coward]

In my pants.

How can they similate the unknown ?

[Alain Williams]

No one has much clue as the nature of dark matter in the way that we do about standard matter: What happens when it clumps together, does it get hot ? Does that heat help to counteract gravity ? Without knowing fundemental things like this - how is it possible to do detailed simulations ?

Re:How can they similate the unknown ?

[boot_img]

No one has much clue as the nature of dark matter in the way that we do about standard matter:
What happens when it clumps together, does it get hot ? Does that heat help to counteract gravity ?
Without knowing fundemental things like this - how is it possible to do detailed simulations ?

Dark matter is postulated to have very simple properties - it interacts only weakly (like neutrinos) which means that collisions between a dark matter particle and any other particles are so rare that they can be ignored.

Consequently, it is actually very easy to put into simulations because the only force that is important is gravity.

So to answer your questions: yes when it clumps together (through gravity) it does get hot, where "heat" means the average velocity of particles with respect to the mean velocity of the clump. This isa consequence of the virial theorem: kinetic energy (heat) balances gravitational potential energy (clumping).

Re:How can they similate the unknown ?

[hypomorph]

Even so, the initial conditions of a simulation govern all. Since we have no clue where dark matter comes from, how do they input anything reliable concerning the initial distribution of dark matter? From observed distant galaxies still in their infancy? But, I was under the impression that we only had detailed knowledge of dark matter from closer middle aged galaxies. We already know more about these older types of galaxies, which lets us make the inferences that lead to dark matter. Understanding of galactic genesis is still pretty speculative, and is a big part of ongoing research.

Re:How can they similate the unknown ?

[boot_img]

The observed temperature fluctuations in the Cosmic Microwave Background allow one to determine the statistics of dark matter fluctuations at that time (a few hundred thousand years after the Big Bang) albeit on slightly larger scales, which then become the initial conditions for these simulations.

Earth collisions?

[Jon Abbott]

So now do we need to worry about dark matter colliding with Earth? Can't this stuff be detected by looking at areas where the cosmic background radiation is lower than usual? I suppose if it were moving it would have some sort of heat signature to it...

Re:Earth collisions?

[Ambitwistor]

Dark matter probably collides with the Earth all the time and we don't notice since it interacts so weakly with us. It's the same with neutrinos. I don't know why dark matter would make the cosmic background cooler than usual, but people have looked for its imprint in the fluctuations found in the cosmic background (places where the fluctuations are denser than usual due to its clumping). That's one of the lines of evidence in its favor, actually.

My God, it's full of -- dark matter!

[flajann]

I am not convinced that MOND or the related TeVeS are the proper approaches to take. The evidence seems to weigh more in the camp for dark matter, but there could be even more exotic explanations that could exlain the gravitational lensing such as what is seen in the Bullet Cluster. Perhaps space-time itself gets a bit "wrinkly" in some parts of space, or perhaps there's a parallel universe that only interacts with ours through gravitation. Of course, verification of this would be difficult at best, so there goes the science.

I strongly hold that there are still some big surprises awaiting our discovery about the true nature of space-time. Alas, I also hold that we humans may never figure them out in time, or probably would lack the intellectual space to understand them. :-)

dark megamass

[UncleBen405]

After the initial formation of the Galaxies, matter begins returning to energy through nuclear fission. In the pre-universe, we went from energy to matter, and since the creation, we are going from matter to energy. In the beginning of the Universe, we started with all the elements, and through nuclear fission, we are breaking them down, using them up. Let's jump ahead to the end of the initial Galactic formation. After as much energy as can be extracted from a Galaxy has been extracted. The remaining heavy elements, made up of spent Stars and Planets and Galactic debris come together through the properties of magnetism and gravity. Polar opposites begin to attach the celestial debris, to each other. Eventually, all or most of the remaining heavy elements from a Galaxy are stuck together by magnetism and gravity, creating a dark megamass at the center of Galaxies. When the magnetic poles start to flip on some of the inner pieces of galactic debris, some matter starts to repel the matter that it was originally attached to. This creates incredible heat and pressure. Eventually, the internal pressure will become greater than the exterior shell of the mass can withstand. Then, a super detonation, an intensely violent explosion, occurs. The formation of a new Galaxy from the debris of an old Galaxy, a Phoenix. Depending on the breach, matter can be blown in all directions, creating a round Galaxy, or a single beam, spiraling out in a disk formation, creating a spiral Galaxy, or any portion thereof creating irregular shaped Galaxies by Quasar. In this immense explosion, all the elements are re-created from the heavy elements that were left from the initial Galactic formation. Soon after the explosion, the heavy molten elements start to accrue through magnetism and gravity with antigravity forces spinning the masses. This in turn allows gravity to start the accretion of gases forming atmospheres. Then, when a sufficient amount of heavy elements and gas have been collected, and critical mass is achieved, a Star is born. The amount of heavy elements and gas collected by a celestial body determines whether it will be a Planet, Star or other Galactic body. The ability of magnetism to repel as well as attract, stabilizes Solar Systems within the Galaxies. A Galaxy expands from the point of the detonation, the shockwave nudging other Galaxies away. Then reaching its maximum expansion, the Galaxy starts to slowly contract due to magnetism and gravity. All the while, converting matter to energy, through fission, until the remaining heavy elements come together again forming another dark megamass, at the center of the Galaxy, and the sequence of events begins/continues. Over the course of time, most of a Galaxy will be transformed from matter to energy until there is not enough matter in the Galaxy to reform. This happens throughout the Universe, and eventually the entire Universe will die. This series of events, scaled down, creates/re-creates Solar Systems, by Supernova. Just so you know.

"Predictions are very hard to do, especially

[ancient_kings]

of future events." -Bohr - F .ne. ma - F .eq. ma+(1/2)ma^2+(1/4)ma^3+....

Re:Where's the evidence? (emission, too!)

[pterandon]

I think gravity is safe. Speaking as a layman, do we really know THAT much about the emissivity of the ordinary matter around us-- on galactic scales? I cannot help but think our theories of the emissivity of objects is not so robust that we have to come up with a new class of matter when this decade's calculations of gravity vs. emission don't add up. Seems like the simpler solution is to refit the emission theories. Q: And aren't there a whole bunch of reasons why emitted photons won't reach terrestrial eyeballs? What if there were really big objects out there? Wouldn't the centers be practically invisible to us?

How is this even REMOTELY valid?

[AbsoluteXyro]

Research like this makes a number of assumptions about "Dark Matter," for instance that it exists at all (at least in the conventional sense), and I think the biggest assumption is that "Dark Matter" is actually some kind of particle. The fact of the matter is we know Jack and Shit about "Dark Matter," and Jack just left town. All we have seen is gravitational effects on observable materials in our universe with no obvious cause... to assume that there's some magical particle out there doing this is just as valid as assuming God farted on a galaxy and made it move that way.

Re:How is this even REMOTELY valid?

[Ambitwistor]

Research like this makes a number of assumptions about "Dark Matter," for instance that it exists at all

Yeah, duh. That's how you do science. Propose a hypothesis, see what it predicts if you assume it's true, and see if that prediction holds in the real world.

I think the biggest assumption is that "Dark Matter" is actually some kind of particle.

How is that a big assumption? It explains the observations easily, most leading theories of particle physics predict such a particle for independent reasons, similar particles (though lighter) are known to exist already, and competing assumptions (brown dwarfs or other compact bodies, modified gravity, etc.) fail to explain the observations.

Re:How is this even REMOTELY valid?

[AbsoluteXyro]

Well I assume the Flying Spaghetti Monster did it all then. That explains the observations easily. I guess it must be true!

Re:How is this even REMOTELY valid?

[Ambitwistor]

You really have no clue how science works, do you?

Dark matter is not some ad hoc "explanation". There are good reasons to believe it exists entirely independent of astrophysics, as I mentioned. Beyond that, it explains a number of very different astrophysical phenomena in a consistent way. Most other explanations fall short; they can explain one gravitational anomaly but are silent on or often inconsistent with others. Furthermore, dark matter is not a unfalsifiable band-aid that can explain anything; it makes very specific predictions about what we should see, and indeed several specific dark matter theories (such as massive compact halo objects) have later failed new tests. Weakly interacting massive particles passed them, and are the only theory to have passed all of them.

Re:How is this even REMOTELY valid?

[AbsoluteXyro]

And the Flying Spaghetti Monster theory passes all of them with flying colors as well. All Dark Matter theories boil down to "Something must be causing this, lets say Dark Matter is!" when Dark Matter has never, ever been observed and there's no more validity to its existence than the FSM (which also has never been observed but in theory fill all the same roles as Dark Matter). It's silly science to be saying "Dark Matter does it!" when there is literally no evidence that this nebulous "Dark Matter" even exists. These "very specific predictions" are manufactured by scientists trying to explain the unexplainable, it's an extremely long way from being fact and I'm tired of seeing it touted as such.

Re:How is this even REMOTELY valid?

[Ambitwistor]

And the Flying Spaghetti Monster theory passes all of them with flying colors as well.

So, no, you don't understand how science works.

The FSM isn't falsifiable, it doesn't make any specific quantitative predictions, its existence is not a natural consequence of other physical theories.

It's silly science to be saying "Dark Matter does it!" when there is literally no evidence that this nebulous "Dark Matter" even exists.

Think for a minute.

If a weakly interacting massive particle existed, what evidence would we see that it exists?

Gravitational anomalies of a very specific quantitative character on scales from galaxies to clusters to cosmology. These anomalies would not be totally independent but would relate to each other in a specific quantitative way linked by the mass of the particle. This is what we actually do see. Furthermore, the existence of these particles has been independently predicted to explain non-gravitational phenomena in elementary particle physics.

These "very specific predictions" are manufactured by scientists trying to explain the unexplainable,

Yes, we get it, you don't understand and don't like science. The fact remains is that dark matter has passed every observational test we've thrown at it, it's extremely unlikely that a random theory would pass all those tests by coincidence, the alternative theories have failed, and we have good independent reasons to believe such particles exist even if we'd never turned a telescope to the sky.

Re:How is this even REMOTELY valid?

[Urkki]

These "very specific predictions" are manufactured by scientists trying to explain the unexplainable

And how is that different from, say, General Relativity?

Re:How is this even REMOTELY valid?

[Anonymous Coward]

The FSM isn't falsifiable

For purposes of comparison, how do you propose to falsify "dark matter"?

Re:How is this even REMOTELY valid?

[Ambitwistor]

Plenty of dark matter theories have already been falsified. The majority of dark matter isn't brown dwarves, black holes, or neutrinos. The whole "hot" vs. "warm" vs. "cold" dark matter debate ended up ruling out slews of candidate particles. Many of the proposed candidates have the wrong mass. Others interact with other particles in the early universe and screw up structure formation. Some are excluded by direct searches in dark matter detectors. And so on.