Naive Question about Dark Matter

[meBigGuy]

Very roughly put, my limited understanding is that dark matter is postulated because of effects seen in the geometry of space similar to those created by matter.

Is it necessarily true that there is some form of matter causing that spatial distortion? Couldn't it be postulated that there are spatial distortions caused by some unknown phenomena, perhaps a nature of the fabric of the universe we don't understand? (interaction with a parallel universe for a far-fetched example).

I'm not trying to promote any pet theory (far from it), just trying to understand a little better.

I ask this because the public presentation of this idea seems entirely focused on some form of matter permeating space as opposed to understanding a distortion permeating space that might be caused by some form of matter. Expecting to find some new form of matter seems like wishful thinking (but an easy place to start looking).

Am I way off base?

 

[zdroide]

I do know about the calculation,
What I know it there is a lots of matter is missing in that equation.
In theory I suppose that the mass of universe in that calculation is the latest mass calculate.
Does neutrinos are include in black matter?
Expansion speed of universe is take in account.
Speed of inflation always been same?

There is spatial and time distortions anywhere where there is huge amount of gravitational pull.
Hubble did show hidden galaxy. Behind Galaxy that should not had enough to bend that much.
There is a force that may missing in that equation.
On the event horizon of black hole. All that fal may had left more then traces.
That calculation must have include correct weight of each super black holes in there centers.
I am not sure of anything about this calculation, hope one can enlight better.
This go also for dark energy. There is hell lots more of that stuff then dark matter.
I think that some force, like electric shield of planets, galaxy should be includes.
I not see magnetic shield map of galaxy and there influence.
We still not weight gravity particles yet.
Just calculations.
Base on what?

 

[mathman]

Astronomers have been able to ascertain spatial distributions for dark matter around galaxies. These don't appear to be properties of space.

 

[zdroide]

Astronomers have been able to ascertain spatial distributions for dark matter around galaxies. These don't appear to be properties of space.

I seen on NASA site also distribution of magnetic fields of our solar system, witch extend to next star.
Is energy matter?
Same go for our galaxy, it does have bigger then itself magnetic field. Or something that look like it.
Closer to galaxy arm or center, more it becomes instable.
There is so much gamma ray and probably others energy we not known that life have not the time to questions itself.
We live in a peaceful place in our Galaxy.
Just quiet enough
to get evolution last just enough.

 

[meBigGuy]

Astronomers have been able to ascertain spatial distributions for dark matter around galaxies. These don't appear to be properties of space.

A seemingly plausible distribution of matter that explains the observations would make that a rational avenue to pursue.

 

[rootone]

is energy matter?.

Matter is one form of energy according to relativity theories
Mass and energy are equivalent.

 

[Vedward]

Looking at various galaxies, some have spiral arms of stars, others have stars scattered fairly evenly throughout (elliptical galaxies). It becomes obvious that something is holding those stars in place relative to each other (like the Earth stays together as it rotates) instead of farther out stars moving more slowly like the planets in our solar system. The dark matter in the galaxies hold the stars together like one solid mass.

 

[Janus]

Looking at various galaxies, some have spiral arms of stars, others have stars scattered fairly evenly throughout (elliptical galaxies). It becomes obvious that something is holding those stars in place relative to each other (like the Earth stays together as it rotates) instead of farther out stars moving more slowly like the planets in our solar system. The dark matter in the galaxies hold the stars together like one solid mass.

Galaxies do not hold together like a solid mass. Stars at different distances from the center take different time periods to orbit the center (The orbital velocities can be close to equal, but stars further out have to travel in a longer orbit.)

 

[meBigGuy]

I guess my question is an attempt to get some insight into why it is assumed that there is "missing matter" causing the observed effects rather than saying that there is unexplained spatial distortion which might be due to matter. mathman's answer was a reasonable answer. Is that the answer everyone agrees with?

 

[_PJ_]

Based on Einstein's GR and the known properties of all 'normal' matter and energy, from what we are able to directly observe, galaxies should spin faster at points away from their centre and extremeities would be thrown off into space. They are not, therefore, something is providing a gravitational contribution. That something is not observed, it does not seem to have any interactions with anything other than via the gravitational effect.
I think I understand your question, and I agree the designation of Dark 'Matter' is already implicative that the cause of the disparity between calculated/expected results and actual observation was due to some exotic 'matter'.
Because of the gravitational interaction, there is a reasonable expectation for the Dark Matter to have (rest) mass, for it to be 'matter' in a widely accepåted form, although it is not quarklike, nor neutrinolike nor electronlike (including of course, mu and tau) so does not correspond to the matter we are 'familiar with'.

However, I think it's important to consider a definition of matter in terms of stress-energy is not simply that which has 'mass' (rest-mass), but any contribution to the energy of a region of spacetime (which will result in observable gravitational effect), in line with Einstein's mass-energy equivalence, this way, energy such as 'massless' photons can be considered as a form of 'matter'.

Whilst never directly observed, much of the properties and 'nature' of "Dark Matter" is known, we just don't actually know for sure what it is.

Think of trying to track an elusive beast in the jungle. You can see its footprints, which can tell you how large and 'heavy' it can be- how its density is distributed. One can look for the signs of its passing and maybe any waste left behind which reveal the extent of its territory - patterns of its routines etc.

We have analogous knowledge of 'Dark Matter' in that we are aware of its distribution, how it is dispersed around galaxies. Using this, we are able to identify the patterns of its effects to see the 'clumping' of galaxies around threadlike filament structures on huge scales.

There is something causing this that we do not have a definite answer for in terms of our accepted standard models.

 

[BillTre]

I kind of like what meBigGuy (mBG) is saying.
Basically why are people only looking for new particles they have not been able to find. Other explanations are possible.
The observation to be explained is a mapped distribution of matter-like effect on space-time. Acts like matter, we can't see it.

It might be a kind of matter (dark matter) floating around in our universe acting like matter but not interacting with the stuff we interact with (non-dark matter).
Its distribution is similar, but not the same as normal matter.

Other things could explain this. They may not require having someway to not interact with normal matter. They may not require new particles.
mBG mentioned interactions with a parallel universe.
This idea has intrigued me for a while.

I can imagine interactions between numerous parallel universes based on entangled connections between the different possible results of a collapse of an undefined quantum state into several resultant quantum states in parallel universes.
This would of course, happen a lot.
The entanglement could be involve pairs or larger groups of particles, each in a different universe.
Perhaps this could in some way act as matter would on space-time, in some way spreading a particle's gravity through not just its own universe, but also other universes containing particles entangled with it.
In this view, our universe (the stars and other stuff we can see) is one in a cloud of other universes (all varying in some way). This might explain a "dark matter" distribution being similar to, but different from, that of our universe.

 

[meBigGuy]

and I agree the designation of Dark 'Matter' is already implicative that the cause of the disparity between calculated/expected results and actual observation was due to some exotic 'matter'.

However, I think it's important to consider a definition of matter in terms of stress-energy is not simply that which has 'mass' (rest-mass), but any contribution to the energy of a region of spacetime (which will result in observable gravitational effect), in line with Einstein's mass-energy equivalence, this way, energy such as 'massless' photons can be considered as a form of 'matter'.

What would be an example of something with no rest-mass that can result in an observable gravitational effect? Or are you saying that is what we might be looking for?
Can a hypothetical "flock" of photons cause a gravitational effect?

 

[BillTre]

Would not a flock of photons have a mass equivalent and therefore have an effect on space-time?

 

[bahamagreen]

People say that virtual particles are not real, just terms that exist in the perturbation calculations for a while but disappear by the time the calculations finish. If these virtual particle terms are necessary, is it because of their properties, and would these include mass-energy (even if only temporary)? If so, what happens to their gravitational contribution, which would remain even "after they were gone"?

Has anyone asked if the dark problem isn't missing mass, but too much gravity... :)

 

[ohannuks]

There should be at least two major theories; MOND (Modified Newtonian Gravity) and dark matter. The reason dark matter theories are currently regarded as the better theory is because of MOND can not explain e.g. bullet cluster (predictions are wrong), whereas dark matter can. Mind you, dark matter has its own problems, such as the missing satellite problem and core-cusp problem. But generally, the problems with dark matter are less fundamental than those of MOND.

There are a plethora of dark matter candidates, and the current research on dark matter is focused on either:

a) Explaining away the dark matter problems by baryonic physics
b) Explaining away the dark matter problems by particle properties

 

[Chronos]

Ive seen no pet theory here that would account for CMB data from Planck.

 

[_PJ_]

What would be an example of something with no rest-mass that can result in an observable gravitational effect? Or are you saying that is what we might be looking for?
Can a hypothetical "flock" of photons cause a gravitational effect?

Would not a flock of photons have a mass equivalent and therefore have an effect on space-time?

Hey guys, sorry I took a while to respond, been enjoying the Christmas and new year!
To begin with, then, yes - examples of something "with no apparent rest mass but with sufficient gravitational effects" could exhibit the same effects as Dark Matter. But there are a few caveats. Firstly, concentrated very high frequency (ultra high energy gamma rays?) photons can indeed cause such gravitational effect (even Black Holes if concentrated on a small enough volume) but there is no natural process to create this concentration in such a way to create the actual patterns and distribution of Dark Matter, Also, that if this were the case, there must be some of the rays or their more direct effects as Gamma Rays that would be detectable (which could be somewhat disasterous for Earth too).
The leading candidates remain neutrinos, yet even in the incredible numbers produced by the stars, but the required energetic intraction have never been detected. The WIMP proposition for Dark Matter candidacy also coincides with Supersymmetry for which no experimental evidence has yet been realized either.

Even the "best" current theories to explain Dark Matter seem (in my humble opinion) either to be beset by calculational issues (such as only producing results that meet around half or so of the required effect), or an alarming lack of experimental verification.
That said, I think it's fair to say that due to the sensitive nature and complexity of the experiments underway, it's reasonable to expect quite aduration before concrete evidence either way can be relied upon.

People say that virtual particles are not real, just terms that exist in the perturbation calculations for a while but disappear by the time the calculations finish. If these virtual particle terms are necessary, is it because of their properties, and would these include mass-energy (even if only temporary)? If so, what happens to their gravitational contribution, which would remain even "after they were gone"?

Has anyone asked if the dark problem isn't missing mass, but too much gravity... :)

Virtual particles are 'not real' in a sense they have no effect other than to 'pad out' Feynman diagrams by providing an infinity of lower probability adjustments to the history summation and sit prettily with the uncertain quantum nature of spacetime and the various quantum fields. I don't mean to suggest they are merely a construct, though, they do exist and can be experimentally derived by the Casimir effect or Lamb Shift.
The key point here, though, I feel is that outside of Hawking Radiation, the creation and annihilation of virtual particle pairs occurs so quickly and in such tiny regions, that they do not (and mostly cannot) interact with anything else.
As to what happens to the gravitational effect once these pairs annihilate, bear in mind that the overall energy content remains the same - essentially this 'vacuum energy'' is converted to mass briefly, then back to the void, yet it is still present in the universe and as such, the gravitational influence of this energy (which is indescribably miniscule) remains the same overall.
ON this line of thought, as it may briefly spread out slightly as the particle pair move, but then come back together, then a really tiny gravitational wave would be generated, this wave would propagate throughout the entire universe at the speed of light. However, considering the plethora of multiple such virtual particle annilations occurring simultaneously, I personally would feel confident that such waves would both a) never be noticeable amidst the noise and b) overall cancel each other out.

 

[Chronos]

A good summary of the status of dark matter studies can be found here; http://arxiv.org/abs/1201.3942, Dark Matter: A Brief Review. Hopefully, this will aid in understanding the known and theoretical properties of dark matter.

 

[ohwilleke]

To answer the original post a bit more directly, dark matter is one of two basic possibilities to explain the phenomena that are observed. The other is that general relativity is close to the right theory of gravity but needs some tweaking in the very weak field limit.

No other possibility is really plausible because dark matter effects have a very well defined correspondence with distributions of ordinary matter that we observe in the universe. For example, phenomena not attributable to ordinary GR seen in spiral galaxies of a given size is very predictable, as is the magnitude of such effects in elliptical galaxies, dwarf galaxies and galactic clusters. This could be caused by feedback effects between clumps of ordinary matter and clumps of dark matter, or it could be caused by tweaks in the gravitational weak field by clumps of ordinary matter without dark matter.

The trouble is that while it isn't that hard to devise a dark matter model or modified gravity model that is consistent with a lot of observations made by astronomers, devising one that is consistent with all of the observations turns out to be devilishly hard. Some of the early, very simple version of each model, such as a model with a single kind of dark matter particle with a particular mass in the 100s of GeV (100 times the mass of a proton more or less), or an early toy model with a very simple modification of gravity, don't fit the data. The former generates the wrong shaped dark matter halos and has other problems at the galactic scale, even though it does a great job at the cosmological scale of the entire universe's structure. The latter works fine at galactic scales, but fails in galactic clusters where it predicts effects that are too small (among other things).

Lots of astronomers are actively working on developing better models of both types. One leading dark matter model called warm dark matter, assumes objects intermediate in mass between an electron and a quark. Another leading dark matter model assumes that a force roughly similar in strength to electro-magnetism carried by a massive "dark photon" causes dark matter particles to interact with each other to some extent. There are also several much more sophisticated modified gravity models that fit data over a much wider range of circumstances, some of which are purely empirical and others of which have some kind of plausible theoretical basis.

Ultimately the winner will be determined by more astronomy data. For example, one key observation right now involved the movement of stars that above or below the galactic disk of the Milky Way that are called RAVE stars which are sensitive to differences between models that other kinds of data can't distinguish. It isn't unreasonable to think that we may even be able to solve this problem in our lifetimes, because the amount of astronomy data and the computational power needed to analyze it, are both becoming far more available.

But, it could not be caused, for example, by something happening in another dimension, because otherwise the correspondence between ordinary matter distributions and inferred dark matter distributions wouldn't be so strong.

 

[serp777]

To answer the original post a bit more directly, dark matter is one of two basic possibilities to explain the phenomena that are observed. The other is that general relativity is close to the right theory of gravity but needs some tweaking in the very weak field limit.

No other possibility is really plausible because dark matter effects have a very well defined correspondence with distributions of ordinary matter that we observe in the universe. For example, phenomena not attributable to ordinary GR seen in spiral galaxies of a given size is very predictable, as is the magnitude of such effects in elliptical galaxies, dwarf galaxies and galactic clusters. This could be caused by feedback effects between clumps of ordinary matter and clumps of dark matter, or it could be caused by tweaks in the gravitational weak field by clumps of ordinary matter without dark matter.

The trouble is that while it isn't that hard to devise a dark matter model or modified gravity model that is consistent with a lot of observations made by astronomers, devising one that is consistent with all of the observations turns out to be devilishly hard. Some of the early, very simple version of each model, such as a model with a single kind of dark matter particle with a particular mass in the 100s of GeV (100 times the mass of a proton more or less), or an early toy model with a very simple modification of gravity, don't fit the data. The former generates the wrong shaped dark matter halos and has other problems at the galactic scale, even though it does a great job at the cosmological scale of the entire universe's structure. The latter works fine at galactic scales, but fails in galactic clusters where it predicts effects that are too small (among other things).

Lots of astronomers are actively working on developing better models of both types. One leading dark matter model called warm dark matter, assumes objects intermediate in mass between an electron and a quark. Another leading dark matter model assumes that a force roughly similar in strength to electro-magnetism carried by a massive "dark photon" causes dark matter particles to interact with each other to some extent. There are also several much more sophisticated modified gravity models that fit data over a much wider range of circumstances, some of which are purely empirical and others of which have some kind of plausible theoretical basis. Ultimately the winner will be determined by more astronomy data. For example, one key observation right now involved the movement of stars that above or below the galactic disk of the Milky Way that are called RAVE stars which are sensitive to differences between models that other kinds of data can't distinguish.

But, it could not be caused, for example, by something happening in another dimension, because otherwise the correspondence between ordinary matter distributions and inferred dark matter distributions wouldn't be so strong.

I doubt its that general relativity needs to be modified. Cosmologists and astrophysicsts have shown that the distribution of dark matter isn't even, and you would expect the effect of dark matter to be directly proportional to the galaxy's mass if it was a problem with general relativity. Instead the effect of dark matter need not necessarily depend on the mass of the galaxy it affects.

 

[ohwilleke]

The relationship between a galaxy's mass and the inferred dark matter effects is non-linear. But, actually, one of the strongest arguments against a dark matter hypothesis is that the magnitude of the inferred dark matter effects and the ordinary matter distribution are linked far more tightly than a naive dark matter hypothesis would suggest. In spiral galaxies, this relationship is called the Tully-Fisher relation, and there are similar power law relationships for other kinds of galaxies and for galactic clusters.

In a dark matter scenario you would expect these relationships to hold true approximately, but there is far less scatter in the relationship than there should be in a typical dark matter model. This could be due to ill understood feedback relationships between dark matter and ordinary matter in the process of galaxy and cluster formation, but for reasons that aren't easily summarized in a single comment (such as the surprisingly high percentage of spiral galaxies that are bulge-less) the existence of such a tight feedback relationship is pretty hard to model in a plausible way.