Dark Matter

[Time_Travel]

Dark matter can not be seen,heard,touched or tasted or measured, it's has such a mysterious property that it flows right through any matter whether be it an animal or the entire earth.

How do scientists no it exists?

Because the stars at edge of galaxies are moving at same speed as that of center of galaxy which violates Newton law they introduce the term Dark matter and accordingly calculate the amount of unseen matter required to keep the stars at the edge of the galaxy moving at the same speed as that of stars at center of galaxy.

If it can't be seen or measured using any machines made of matter(since Dark matter passes right through normal matter according to its property) why introduce this dark matter to fit to the equations. Can it be possible that something is not correct?

[sanctus]

It interacts gravitationally, that is a way you can measure its presence (just like the example you brought with the the rotation speed of stars at the edge of the galaxy, although they don't have the same like at the center as you claimed, but just too quick to be hold in orbit by the visible matter inside). Also some models (or all?) of dark matter consider the dark matter particle interacting weakly (see WIMP for instance, stands for: Weak Intercating Massive Particle)

[Time_Travel]

I am not a cosmologist, just a small trader and small businessman with immense interest in knowing "how the universe works, and why it came to existence(instead of asking why humanity exists it makes a grand scale sense of why Universe exists)".

 

 

..... (just like the example you brought with the the rotation speed of stars at the edge of the galaxy, although they don't have the same like at the center as you claimed, but just too quick to be hold in orbit by the visible matter inside).......

 

 

But i didn't made it myself or claim about the Dark Matter wrt the edge of galaxy speed wrt its center.

Here is the link why i said so Galactic rotation curves. Especially the diagram on RHS says Velocity Curve should be falling as the distance increases, but it is flat.

 

Anyway my main question is whenever we discover something the standard models are tweaked and adjusted accordingly to fit to the observed data, why couldn't the so called standard models predict everything before the observed data is published?

[JMJones0424]

Anyway my main question is whenever we discover something the standard models are tweaked and adjusted accordingly to fit to the observed data, why couldn't the so called standard models predict everything before the observed data is published?

 

Because our knowledge of the universe is incomplete, and the standard model is the our best attempt to explain what we observe. When an observation is made that contradicts the model, then the model is adjusted to reflect those new observations.

 

It could very well be that our idea of dark matter is simply a representation of the inaccuracy of our understanding of gravitational effects at galactic scales. Or, it could very well be that dark matter as we imagine it does exist. Since our only way of detecting it right now is through its gravitational effect on visible matter, it is difficult to distinguish between the two options.

 

In a similar way, the long search for "Planet X" started as an attempt to explain oddities about the orbit of Uranus. Neptune was discovered, but then there was still a small amount of discrepency to be explained, and once Pluto was discovered, everything seemed to make sense. But then as more observations were made, it didn't seem that Pluto had enough mass to give the effect that was seen on Uranus and Neptune, and once Voyager 2 data was used to revise the mass of Neptune, all need for a massive "Planet X" went away.

 

All we have is our observations, and we build models based on that. As better observations come in, we adjust the models to compensate. The previous model would have stars at the edge of the galaxy orbiting the center at a much lower velocity, but that is not what we observe. Dark matter is the current best guess to bring our model in line with our observations. Unlike a theory, which seeks to explain WHY we make the observations we do, and can be used to predict future observations, a model just helps us to put everything together into a current picture of the observations we make.

 

That is my layman's take on the situation, at least.

 

It seems, though, according to the source you provided, that we do have more direct evidence of the existence of non-baryonic matter.

The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster. In most regions of the universe, dark matter and visible material are found together,[29] as expected because of their mutual gravitational attraction. In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter. X-ray observations show that much of the baryonic matter (in the form of 107–108 Kelvin[30] gas, or plasma) in the system is concentrated in the center of the system. Electromagnetic interactions between passing gas particles caused them to slow down and settle near the point of impact. However, weak gravitational lensing observations of the same system show that much of the mass resides outside of the central region of baryonic gas. Because dark matter does not interact by electromagnetic forces, it would not have been slowed in the same way as the X-ray visible gas, so the dark matter components of the two clusters passed through each other without slowing down substantially. This accounts for the separation. Unlike the galactic rotation curves, this evidence for dark matter is independent of the details of Newtonian gravity, so it is held as direct evidence of the existence of dark matter.[30] Another galaxy cluster, known as the Train Wreck Cluster/Abell 520, seems to have its dark matter completely separated from both the galaxies and the gas in that cluster, which presents some problems for theoretical models.[31]

 

[Time_Travel]

.................

 

It could very well be that our idea of dark matter is simply a representation of the inaccuracy of our understanding of gravitational effects at galactic scales................

 

 

 

This is what i believe at the largest level of the structures is Galaxy and the smallest level is well the pre Big bang the smallest state known yet.

At these levels standard models failed to predict(in case of Dark matter at galactic levels) or no longer apply(in case of BB).

[Jay-qu]

The existence of dark matter is well established beyond just galactic rotation curves. See, for example, the Bullet cluster merger: http://apod.nasa.gov/apod/ap060824.html, which clearly shows that the majority of mass in this system is non-baryonic.

 

There are theories that attempt to reconcile dark matter by modifying gravity, given the name MOND. All I have seen of these theories so far are in-elegant, adhoc solutions that cant cohesively provide a single hypothesis to cover all dark matter observations.

 

If you are still unconvinced have a read of this: http://www.sdss.org/news/releases/20031028.powerspectrum.html

[sanctus]

 

 

But i didn't made it myself or claim about the Dark Matter wrt the edge of galaxy speed wrt its center.

Here is the link why i said so Galactic rotation curves. Especially the diagram on RHS says Velocity Curve should be falling as the distance increases, but it is flat.

 

 

I know you didn't come up with this ;-).

 

You just read the plot a bit wrong, it's starts from very low values quickly increases and then is flat instead of falling off. So you said the ones at the center have the same rotation speed as the ones at the edge, and that is what I said is wrong. The ones at the edge have (about) the same rotation speed as the ones further in, but are quicker than the ones really at the center.

I might just be a little pedantic, I know:-).

[JMJones0424]

Questions I've always had about dark matter but have never asked:

 

1) If dark matter makes up such a large majority of matter in the universe, why isn't there any in our solar system? Or is it possible that there is dark matter in the solar system, but like regular matter, the vast majority of it is contained in the sun and the planets? Would dark matter at the center of the Earth be detectable through seismic waves due to regions of increased density, or does the density of dark matter have no effect on sound waves?

 

2) If dark matter interacts with baryonic matter only through gravitation, what would happen to a star that had a portion of its mass as dark matter? I assume this mass could not be used for fusion like the rest of the baryonic matter, and this would therefore shorten the expected lifetime of the star, as there would be less fuel for fusion relative to mass when compared to another star that consisted solely of baryonic matter. If this assumption is correct, a.) Does the possibility of dark matter making up a significant portion of a star's mass throw away the presumed relationship between a star's mass and a star's lifespan? b.) In a very massive star which contains a large portion of dark matter, if fusion energy is not able to counter-balance the gravitational force from dark matter, is it possible for dark matter to cause a star to turn into a blackhole "prematurely"?

[Jay-qu]

These are good questions JMJones, I will do my best to give you some answers:

 

1. Quick answer, yes there is dark matter in our solar system, but not much. Since dark matter interacts mostly through gravitation (it is assumed to interact other ways too, albeit weakly) it acts like a pressure-less gas. The effect of this is that it does not 'clump' as much as normal matter. Data suggests the Milky Way galaxy may have ~90% of its mass in a huge dark matter halo that extends far beyond the visible reaches of the galaxy. At the point of our solar system in the galaxy the density of dark matter is expected to be ~0.3GeV/cm^3, for a typical dark matter particle mass this equates to approximately one particle in a 300cm^3 volume. This may not seem like much, but the density is pretty constant throughout the solar system and the vast interstellar voids in the galaxy - so overall it adds up!

It is expected that dark matter can get trapped by the Earth and Sun's gravity, it probably will not have a sizable impact on the Earth, but it may produce observable effects on the star - this is an ongoing area of research. An interesting and semi-related paper: http://arxiv.org/abs/1103.5086 , essentially Dan Hooper posits that it may be possible that enough dark matter clumps on the interior of a planet to keep it warm (enough for liquid water in the absence of a star) through annihilations of the dark matter.

 

2. As above, you are correct, stars will have a portion of their mass in dark matter - but this portion is probably very low, so low that it will not greatly effect the age of the star. As dark matter will mostly ignore interactions with the stellar material, if their was a large portion of dark matter clumped around a star, it would not do so densely and thus would not be conducive to forming a black hole.

[Time_Travel]

it's starts from very low values quickly increases and then is flat instead of falling off. So you said the ones at the center have the same rotation speed as the ones at the edge, and that is what I said is wrong. The ones at the edge have (about) the same rotation speed as the ones further in, but are quicker than the ones really at the center.

 

If i understand correctly interpretation of the curve is " initially the curve starts from a low value and then rises steeply and flattens out, it means as we move away from center of galaxy velocity of stars increases to certain distance away from center and then becomes constant. The actual line expected was the dashed blue line curve in the plot, where the the velocity of stars as we move away from center should decease with increase in distance."

 

 

This correct interpretation of curve raises a few more logical questions.

1)Why does Dark matter speed up velocity of stars at edge not at center?

2)Is Gravity law a failure at large structures?( for example Mercury's(almost at center of solar system) Orbital speed is 47.87 KM/s causing a faster rotation around sun, and Neptune's Orbital rotation is 5.43KM/s causing it slower rotation around sun.)

[sanctus]

Time_travel as to 1). DM does not speed up the stars at the edge, it just makes it possible to keep them in the galaxy. I imagine it like this: when the galaxy formed the stars/gas at the edge who were even quicker could not been held even by the DM, but the ones who where slow enough (but still too quick for if there were only visible matter) have been kept in orbit around the galactic center. So in my understanding the speed comes from the formation process of the galaxy and not from DM, DM just makes it possible...

[cruel2Bkind]

Apparently galaxy rotation curves and large galactic velocities within clusters can be accommodated by GR without any need to invoke exotic fantasies. DM was hypothesized to explain the failure of the Newtonian weak field approximation to galaxies and clusters of galaxies. Cooperstock, Tieu & Carrick show that although the Newtonian weak field approximation may be appropriate for planetary systems around massive objects, it is invalid for distributed systems where most of the mass is not centrally located.

• F. I. Cooperstock and S. Tieu:
  
  • General Relativity Resolves Galactic Rotation Without Exotic Dark Matter (arxiv:astro-ph/0507619)
    Abstract:
    A galaxy is modeled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one nonlinear equation relating the angular velocity to the fluid density. It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks. Thus the need for a massive halo of exotic dark matter is removed. For these galaxies we determine the mass density for the luminous threshold as 10^{-21.75} kg.m$^{-3}.
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  • Perspectives on Galactic Dynamics via General Relativity (arxiv:astro-ph/0512048)
    Abstract:
    Responses to questions, comments and criticism of our recent paper "General Relativity Resolves.." are provided. It is emphasized that our model is entirely natural to describe the dynamics of an axially symmetric galaxy and that our solution, albeit idealized, contains the essence of the problem. The discontinuity of the metric derivative on the symmetry plane is necessarily interpreted as the effect of the mathematically idealized discontinuity of the gradient of the density and is shown to be naturally connected to the distributed volume density via the Gauss divergence theorem. We present arguments to the effect that for our approximate weak field model, we can choose the physically satisfactory mass distribution without an accompanying singular mass surface layer. To support this contention, we modify our solution slightly by removing the discontinuity with a region of continuous density gradient overlapping the $z=0$ plane. The alternative of invoking a surface layer leads to the presence of a negative mass surface layer approaching the numerical value of the positive mass continuous region. This is in contradiction with the assumed stationarity of the model. We find that a test particle behaves normally as it approaches the $z=0$ plane, the acceleration being towards the direction of this plane. This is in contradiction to the negative mass layer hypothesis as negative mass would repel the test particle. Thus, further support is added to the integrity of our original model.
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  • Galactic Dynamics via General Relativity: A Compilation and New Developments (arxiv:astro-ph/0610370)
    Abstract:
    We consider the consequences of applying general relativity to the description of the dynamics of a galaxy, given the observed flattened rotation curves. The galaxy is modeled as a stationary axially symmetric pressure-free fluid. In spite of the weak gravitational field and the non-relativistic source velocities, the mathematical system is still seen to be non-linear. It is shown that the rotation curves for various galaxies as examples are consistent with the mass density distributions of the visible matter within essentially flattened disks. This obviates the need for a massive halo of exotic dark matter. We determine that the mass density for the luminous threshold as tracked in the radial direction is $10^{-21.75}$ kg$\cdot$m$^{-3}$ for these galaxies and conjecture that this will be the case for other galaxies yet to be analyzed. We present a velocity dispersion test to determine the extent, if of any significance, of matter that may lie beyond the visible/HI region. Various comments and criticisms from colleagues are addressed.
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  • General relativistic velocity: the alternative to dark matter (arxiv:0712.0019)
    Abstract:
    We consider the gravitational collapse of a spherically symmetric ball of dust in the general relativistic weak gravity regime. The velocity of the matter as viewed by external observers is compared to the velocity gauged by local observers. While the comparison in the case of very strong gravity is seen to follow the pattern familiar from studies of test particles falling towards a concentrated mass, the case of weak gravity is very different. The velocity of the dust that is witnessed by external observers is derived for the critically open case and is seen to differ markedly from the expectations based upon Newtonian gravity theory. Viewed as an idealized model for a cluster of galaxies, we find that with the general relativistic velocity expression, the higher-than-expected constituent velocities observed can be readily correlated with the solely baryonic measure of the mass, obviating the need to introduce extraneous dark matter. Hitherto unexplained and subject-to-reinterpretation astrophysical phenomena could also be considered within this context. It is suggested that an attempt be made to formulate an experimental design at smaller scales simulating or realizing a collapse with the aim of implementing a new test of general relativity.
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  [*]J. D. Carrick and F. I. Cooperstock:

  • General relativistic dynamics applied to the rotation curves of galaxies (arxiv:1101.3224)
    Abstract:
    We extend our general relativistic analysis of galactic rotation curves with galaxies NGC 2841, NGC 2903 and NGC 5033. As before, we employ the solution of the Einstein field equations of general relativity with an expansion in Bessel functions. As in our earlier studies, the fits to the data are found to be very precise and the calculated baryonic masses are lower than those based upon Newtonian gravity. Also as in our previous studies, the galactic radii at which the optical luminosities terminate are seen to correlate with densities near $10^{-21.75}$ kg$\cdot$m$^{-3}$. This concordance lends further support to the correctness of the procedure as well as providing a potentially valuable piece of information in the understanding of galactic evolution.
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[sanctus]

I read through this article (Galactic Dynamics via General Relativity: A Compilation and New Developments (arxiv:astro-ph/0610370)) and was very surprised, it is very interesting, and the argument that to the difference to the solar system, where the planets are "test particles", while in the the galaxy the stars contribute to the gravity for sdaying that there the newtonian approximation does not work so well, sounds logic.

 

But it was from 2006 and the last one you linked is from 2007. Any idea of the follow ups?

[Time_Travel]

Time_travel as to 1). DM does not speed up the stars at the edge, it just makes it possible to keep them in the galaxy. I imagine it like this: when the galaxy formed the stars/gas at the edge who were even quicker could not been held even by the DM, but the ones who where slow enough (but still too quick for if there were only visible matter) have been kept in orbit around the galactic center. So in my understanding the speed comes from the formation process of the galaxy and not from DM, DM just makes it possible...

 

 

From what i understand of why Dark Matter theory was invented was because of::

 

Stars at the center of galaxy would be expected to move quickly than stars at the edge of the galaxy according to law of gravity because there is more visible matter at the center of galaxy than at the outward edges of galaxy.

But according to data, stars at edge of galaxy move just as quickly as those closer to the center.Something must be causing them to move quickly , Experts call this as Dark Matter.

Based on this it was calculated how much of Dark matter was needed for the stars at the edge to move as quickly as those of center. It was found that they needed 5 times more dark matter than normal matter.

This is my understanding of how or why Dark Matter theory was invented.

Either dark matter exists or a total failure of Newtonian Gravity at galactic levels. Which one is correct?

[Jay-qu]

I would take those papers listed with a grain of salt. Only one has been accepted in a peer-reviewed journal and even then it was a pretty low impact journal (IF = 1).

[sanctus]

Time_travel, just repeating what i tried saying 2 posts ago. DM does NOT cause the stars at the edge to move as quickly as the ones closer to the center. DM makes it possible that the stars traveling at the edge are so quick but still gravitationally bound to the galaxy.

 

But you are right in saying that DM-theory came up for explaining howcome the stars at the edge move so quick AND are still bound.

 

As to what causes them to move so quick in the beginning one would have to look in galaxy-formation theory.

 

This is how I understand it at least.

[cruel2Bkind]

I would take those papers listed with a grain of salt. Only one has been accepted in a peer-reviewed journal and even then it was a pretty low impact journal (IF = 1).

The merit of those papers is determined by the veracity of their mathematics and empirical analysis, not by whether they are acceptable to a community dominated by the Dark Unseeable Matter Bunch and Milgrom's Mondiots. The essential point raised by those papers is that the Newtonian weak field approximation is the wrong approach to analysis of stellar velocities within galaxies and galactic velocities within clusters. The onus is now on those who subscribe to DM, or to MOND, or indeed to any other patch-up, to prove that the Newtonian weak field approximation is valid in those aforementioned cases. If they cannot do so then perhaps they should accept that Cooperstock et al have shown Einstein's GR has another piece of supportive evidence, viz the correct prediction of galactic rotation curves and galactic velocities within clusters.