Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 24/12/2009 04:09:12
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Dark matter is so-named because scientists can't see it or detect it - so what's the evidence that it actually exists?
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The evidence is from observation of galaxies and making assumptions based on observations of light output vs mass for stars in our own galaxy. There is a well accepted theorem (the virial theorem) that relates the kinetic and potential energies of objects moving freely in any closed system. In a system with gravity as the only significant force acting it results in the average kinetic energy being equal in magnitude to 1/2 the average potential energy. The caviat that the system is "closed" is considered reasonable for galaxies even though there is some ejection of mass and also some collection of external mass. In the distribution of mass in galaxies, we see a relationship between light output (related to mass) and doppler shift (related to speed) that does not fit with the virial theory predictions. It suggests there must be more mass than we can see and that the mass is distributed throughout the galaxy - not all in the middle as a giant black hole for example. The same applies to galaxy clusters. There is so much extra matter required, and its distribution such, that it would be surprising if this were made up of objects we are aware of (dust, neutron stars, black holes etc) as we have not got such evidence from local observations.
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Dark matter is so-named because scientists can't see it or detect it - so what's the evidence that it actually exists?
Dark matter was named because it can't be seen through standard astronomical techniques, not that it cannot be detected.
Here are what I think of as the astronomical techniques to detect it (not comprehensive):
1. Rotation curves of galaxies: We can look at how fast a galaxy is rotating and use that to calculate how much matter is in the galaxy (and roughly its distribution). We can also look at how much light is coming from the galaxy and use that to tell how much matter is there (and roughly its distribution). These two do not match up.
2. Rotation curves of galaxy clusters: We can look at the rotation of and the light from groups of galaxies just like we can look at galaxies.
3. Gravitational lensing: We can look at the way light bends around galaxies and galaxy clusters to see how much matter is there and compare this to how much we can estimate from the light that we see.
4. Distributions of collided galaxy clusters: We can look at how two galaxy clusters that ran into each other are now distributed. The only example I know of this is the Bullet Cluster. It formed from two clusters that came together, and we can tell the overall distribution of matter in the cluster from various motions and gravitational lensing. There is more matter than we can see in the form of stars and in different places.
Here are what I think of as the cosmological techniques to detect it (not comprehensive):
1. Looking at distant (type Ia) supernovae gives us a means to measure the amount of mass in the universe. There is a ton more than we can estimate given how much light we can see,
2. Looking at the background radiation gives us a measurement of how much mass is in the universe and how much of that mass is in the form of standard matter. There is a ton more matter than there is standard matter.
3. Looking at the relative abundance of the lightest elements in the universe gives us a measurement of how much standard matter there is. There isn't very much compared to our overall measurements of how much matter there is.
4. Looking at how galaxy clusters form gives us a measurement of how much matter there is overall. This is a ton more than the standard matter we can measure.
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I wonder why the possibility that one of our tenants might not apply equally at large distances never occurs to us? For instance, If F = ma was only valid locally that would explain all of your proofs.
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I wonder why the possibility that one of our tenants might not apply equally at large distances never occurs to us? For instance, If F = ma was only valid locally that would explain all of your proofs.
I tend to believe that we should believe our observations a bit more and be less convinced that gravity behaves exactly as we currently think it does (I'll probably get some heat for making that statement!)
For example, it is commonly accepted that gravity is the result of matter distorting spacetime. We observe gravitational effects in our "near space" where there is, relatively speaking, a lot of matter, and, therefore, our "near space" is considerably distorted. Are we sure that gravitational effects are the same in volumes of space that contain relatively little matter and are, therefore much less distorted?
In otherwords, could the presence of matter in space attenuate the gravitational effect? I'm sure there must be some analogies for this, but I just can't think of any right now!
The amount of dark matter required to make our observations fit our current theory of gravity is truly colossal. Does that not suggest we could be forcing nature to fit our model?
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I wonder why the possibility that one of our tenants might not apply equally at large distances never occurs to us? For instance, If F = ma was only valid locally that would explain all of your proofs.
So far, the most well-developed alternative to standard gravitation theory, MOND, can only account for what I call astronomical evidence 1 & 2. If fails on astronomical tests 3 and 4 and on all the cosmological tests. The newer TeVes can account for astronomical tests 3, but not 4.
We can always say, "Hey, maybe physics in far away places is just crazy!" But this is just to give up. And it's not really that likely an answer, since assuming that physics in far away places isn't crazy actually gives us good results.
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In other words, could the presence of matter in space attenuate the gravitational effect? I'm sure there must be some analogies for this, but I just can't think of any right now!
Yes; there is a simple logical proof of this. The proof is this: Gravity affects time. Acceleration due to gravity has time as an element. Therefore acceleration due to gravity is affected by gravity.
Now, what is the effect of gravity if not acceleration of objects.
So gravity provides negative feedback for gravity.
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So gravity provides negative feedback for gravity.
Ooooooo! I like negative feedback. I might actually be able to understand that [;D]
We can always say, "Hey, maybe physics in far away places is just crazy!" But this is just to give up. And it's not really that likely an answer, since assuming that physics in far away places isn't crazy actually gives us good results.
Completely agree with that. I think any theory has to be consistent throughout the Universe. The "attenuation of gravity by matter" would simply mean that gravity exerted a greater force in volumes of space that contained very little matter and less force in volumes of space that contained more matter. Our direct observations of gravity are largely based on the latter situation.
Matter does seem to "stress" space in some manner, so the effectiveness of gravity might be an inverse function of the amount of stress within some unit volume of space. I really should read the alternative theories. Perhaps they incorporate this or a similar idea.
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The Pioneer anomaly is the perfect example of something being slightly wrong with F = ma.
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For example, it is commonly accepted that gravity is the result of matter distorting spacetime. We observe gravitational effects in our "near space" where there is, relatively speaking, a lot of matter, and, therefore, our "near space" is considerably distorted. Are we sure that gravitational effects are the same in volumes of space that contain relatively little matter and are, therefore much less distorted?
In otherwords, could the presence of matter in space attenuate the gravitational effect? I'm sure there must be some analogies for this, but I just can't think of any right now!
Do you mean that, for example, gravity constant G is stronger if the density of matter is lower? Then we could invoke void polarization and negative energy: a great density of matter would generate greater negative energy and so lower gravitational interaction.
Dark matter and dark energy explained together!
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Do you mean that, for example, gravity constant G is stronger if the density of matter is lower? Then we could invoke void polarization and negative energy: a great density of matter would generate greater negative energy and so lower gravitational interaction.
Dark matter and dark energy explained together!
I'm not sure I'd use the term "density" exactly although, technically, that is correct in that the total amount of matter in a unit volume of space does determine the density of that volume. It may be a ridiculous theory, but I'm not sure it is any more ridiculous than assuming the existence of about nine (?) times more matter than we can currently account for.
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The Pioneer anomaly is the perfect example of something being slightly wrong with F = ma.
Or a perfect example of some kind of calculation error. Or a perfect example of some kind of systematic distribution of unknown material bodies in the solar system.
It could be many things. Science is not about throwing out everything because of anomalies. If these anomalies can be used to measure some effect and this can be related to almost everything else that we measure, then we may get a scientific advance. So far, the theories based on the Pioneer anomaly have lead to absolutely nothing.
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Dark matter could be explained by that elusive field, those 'bosons' you know. Higgs bosons, was it?
http://www.mpia.de/homes/rix/hwr_obsdarkmatt.ppt
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And http://en.wikipedia.org/wiki/Higgs_boson
Ah well, egg toddy is dangerous stuff, but I'm brave...
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Get me another :)
And make sure it contains single malt and no eggs.
And you can stop cackling...
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So all the evidence would indicate that the standard model is at best 10% correct as we are unable to account for 90% of the universe using current mass attraction theory.
But what if there is a magnetic binding force which turned out to be nine times stronger than gravity? Google magnoflux for an alternative electro-magnetic view of the universe.
Happy New Year
Clive
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So all the evidence would indicate that the standard model is at best 10% correct as we are unable to account for 90% of the universe using current mass attraction theory.
What do you mean by "standard model"?
If you mean the standard model of particle physics, then the evidence is that the standard model is incredibly accurate, but that there are yet unknown particles that have mass and are more numerous than baryons.
If you mean the standard model of cosmology, then the evidence is that the standard model is incredibly well supported, as this model takes into account dark matter.
But what if there is a magnetic binding force which turned out to be nine times stronger than gravity? Google magnoflux for an alternative electro-magnetic view of the universe.
This would be a theory supported by effectively no evidence.
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If you mean the standard model of cosmology, then the evidence is that the standard model is incredibly well supported, as this model takes into account dark matter.
That is a valid perspective. An alternative view suggests that the standard model of cosmology requires the existence of dark matter.
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Both the standard models have been conceived assuming that electricity in space can be neglected. This could be accepted before the solar wind was identified but not now.
In fact voyager has just reported flying through a magnetic fluff cloud that should not be there; far out in the solar system. But magnetism is there??? and it has an effect!!
Regards
CliveS
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Both the standard models have been conceived assuming that electricity in space can be neglected. This could be accepted before the solar wind was identified but not now.
In fact voyager has just reported flying through a magnetic fluff cloud that should not be there; far out in the solar system. But magnetism is there??? and it has an effect!!
Regards
CliveS
The electric universe idea is a crank idea with a very long pedigree. It cannot show how we should take electricity into account in a way that it can reproduce the very basic observations that found cosmological research.
It's like saying that we can't trust the science of electricity because the standard model of electricity does not take neutrinos into account.
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Clive, why not create a thread in 'new theories' where you link up your ideas and substance them with thing like voyagers meeting magnetic fields?
Voyager spacecraft discover how Solar System stays together (http://www.itwire.com/index.php?option=com_content&task=view&id=30244&)
Voyager (http://vgrmag.gsfc.nasa.gov/helio-pubs.html)
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dark matter is just a imaginary that we cant able to see but we observe during studying about space like black hole, may be it is responsible for big bang as it has a mass that can displace by any object in the universe,
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But what if there is a magnetic binding force which turned out to be nine times stronger than gravity?
Gravity bends, crushes, conforms time. That is not so weak. If the electromagnetic forces, or magnetic binding forces, transform time locally nine times more, time could be seen as changing in local uses. A big toaster could make one late for work ( a really big toaster ).
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Another thing to consider is the term "dark matter" just applies as it has no interactions with light. Dark matter does not necessarily need to be comprised of WIMPS alone other things may be out there that would also account like macho's (I believe that's the right term) such as brown dwarfs fair enough all dark matter does not need to be brown dwarfs but evidence points to a lot of them out there and this is just ordinary matter.
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Murchie85, nobody has been able to construct any viable models of galaxies, as far as I know, with just ordinary matter. Most galactic models that work, i.e. that fit well with observation, have dark matter distributed in halos that, if not exactly spherical, extend well outside the galactic disks. Although galaxy formation is not wholly understood, it is generally accepted that disk formation is a result of a purging of matter from other potential orbits via interaction (mainly collisions) with disk matter over a long period. If the dark matter were made of normal matter (e.g. brown dwarves) then such purging would have occurred. This would not then fit with the current models.
In models of galaxy clusters the distribution of matter, again, is not the same as the visible matter. I don't think the concept of Dark Matter being somewhat exotic would have been arrived at if it was so easy to justify using more conventional matter. So whilst it is possible, the evidence suggests that this phenomena is not so easily explained with ordinary matter and in fact the concept of Dark Matter being non-interactive with ordinary matter (not just with photons) is actually the simplest explanation.
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I totally agree, but what I was getting at was the so called 90% may not actually be 90% exotic matter. Brown dwarfs are quite unique in that they don't have to be formed on the disc, for example in the halo most of the stars are part of globular clusters low mass and old but some could be failed stars. As you rightly say we are still in the process of mapping our own galaxy let alone others but just as we have made ball park figures for the number of galaxies in the universe and distribution of certain stars, distribution calculations have also been made for brown dwarf which of course is far below 90%. Although they still indicate towards there being a lot and this would definitely affect the overall figure of 90% exotic matter and therefore should (and already is) considered when mapping dark matter distribution in the universe.
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I totally agree, but what I was getting at was the so called 90% may not actually be 90% exotic matter. Brown dwarfs are quite unique in that they don't have to be formed on the disc, for example in the halo most of the stars are part of globular clusters low mass and old but some could be failed stars. As you rightly say we are still in the process of mapping our own galaxy let alone others but just as we have made ball park figures for the number of galaxies in the universe and distribution of certain stars, distribution calculations have also been made for brown dwarf which of course is far below 90%. Although they still indicate towards there being a lot and this would definitely affect the overall figure of 90% exotic matter and therefore should (and already is) considered when mapping dark matter distribution in the universe.
Part of the determination of the density of exotic dark matter comes from the nature of how matter clumps together in the early universe and in the formation of galaxies. If more than a small amount of matter was in the form of ordinary matter (baryons), then we wouldn't see small deviations in the background radiation that we do and we wouldn't see the kind of galaxy clusters and super-clusters that we do.
The WMAP project, http://map.gsfc.nasa.gov/ , and the SDSS project, http://www.sdss.org/ , cover these two sources of dark matter measurement.
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Hmm I am not so sure I agree that having say a 80% -20% or 70%-30% distribution would effect the deviations so much that they would not be observable. Dark matter is still unknown and the sloan and wmap rely on detecting the effects of gravity from dark matter, in fact all matter has a relationship with gravity.
What I am getting at is until we have conclusive evidence that DM is completely made of exotic matter lets not rule it out. The universe is big, very big and the data obtained by the satelights rely on such small variations or tiny clues that I would not at all be suprised that if some of the current estimates turn out to be wrong.
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Hmm I am not so sure I agree that having say a 80% -20% or 70%-30% distribution would effect the deviations so much that they would not be observable. Dark matter is still unknown and the sloan and wmap rely on detecting the effects of gravity from dark matter, in fact all matter has a relationship with gravity.
It makes a huge difference, because WMAP and SDSS depend not simply on the relationship between matter and gravity, but also on the relationship between light and baryons. Because of the tight relationship between baryons and photons in the early universe, the universe as we see it today simply could not exist unless there is a substantial non-baryonic component to the matter density of the universe. The distribution of ordinary matter in the early universe was too uniform to grow into the galaxies that we see today unless there were pockets of dark matter that acted essentially as seeds of structure. (The alternative is to throw out almost everything that we know about large-scale structure formation, and nobody seems willing to do that.)
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Can I point out that a non-uniform energy distribution causes gravity. Matter only causes gravity because of its energy content. This is important, because space has its vacuum energy or energy density, and this has a mass-equivalence by virtue of E=mc². Should the energy density vary, it's going to cause gravity.
Check out Einstein's Leyden Address (http://www.zionism-israel.com/Albert_Einstein/Albert_Einstein_Ether_Relativity.htm) where he talks about a gravitational field, saying space "is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν)". Also check out The Foundation of the General Theory of Relativity where on page 185 of Doc 30, he says "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".
This is saying that a gravitational field is non-homogeneous space, where the energy density is non-uniform, causing an extra gravity component. But we don't expect to find dark-matter particles in a gravitational field. Hence we shouldn't be surprised when we can't find dark matter particles at large. The cause might simply be inhomogeneous space. After all, space is dark, and it has that mass equivalence. The evidence for gravitational anomalies is rock solid, but dark matter is merely one hypothesis that attempts to explain it, and there is no actual evidence for dark matter per se. Large-scale structure formation does not support the case for dark matter in the form of WIMPs or any other particles, merely for variations in energy density, whatever the underlying cause.
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1. Rotation curves of galaxies: We can look at how fast a galaxy is rotating and use that to calculate how much matter is in the galaxy (and roughly its distribution). We can also look at how much light is coming from the galaxy and use that to tell how much matter is there (and roughly its distribution). These two do not match up.
In a rotation curve of stars like http://astro.berkeley.edu/~mwhite/darkmatter/rotcurve.html (http://astro.berkeley.edu/~mwhite/darkmatter/rotcurve.html),
star speeds are about 150 km/s. That means they will travel 4,7 x 1011km or 0,000016 kparsec after 100 years. If we suppose they travel in a circle around the galaxy center, that result in 0,00009 degrees of arc for a radius of 10 kpc as shown in the graphic.
My doubt is: astronomers have instruments precise enough to say that after some years of monitoring, the stars are orbiting the galaxy center? Instead the galaxy could be expanding. In that case there is no contradiction between the graphic and the gravitation theory. Stars distant to the center are travelling faster than expected to be orbiting, but they would be just escaping from the orbit and that expansion is too small to be detected in a reasonably scale of time.