Here's NASA's take, what's yours?

http://nasascience.nasa.gov/astrophysics/what-is-dark-energy

Feel free to speculate wildly 

I think it's just energy that is dark. lol

No really; I don't actually know!

Here's the Wikipedia take:

http://en.wikipedia.org/wiki/Dark_energy

This really got going in 1998 when the data from the High Z Supernova Research team showed that expansion of the universe appears to be accelerating when virtually everybody in cosmology was expecting the reverse.

In some ways it reminds me of the situation at the end of the previous century when the Michaelson-Morley experiment showed that the speed of light is the same in all directions i.e. that the earth's motion through the universe made no difference in the measurement.   That experimental result required a huge paradigm shift to accommodate -- i.e. special relativity.

The supernova data may ultimately require a paradigm shift of equal magnitude.  That's what makes this so exciting, at least to me.

I think dark energy is one of the worst excuses for a theory I have ever hear of. It only exists as a theory to substantiate an earlier theory which is almost as bad a theory. There is probably more evidence for the earth being flat or the Ptolemaic model of the universe than for dark energy/matter. How about postulating the emerging suspicion that dark energy/matter cannot be found because it doesn't exist?

Cool.  Here's some evidence in favor of that postulate.

http://www.astronomy.com/asy/default.aspx?c=a&id=2102

Thanks. Not a bad article from Astronomy -- a pretty good astro rag. Dark matter is getting more a more bizarre as time goes on. It is all based on the assumption that the universe is expanding. Maybe there might be a little bit of an adjustment necessary to that assumption and other explanations can be found to related phenomena.

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Dark matter and dark energy are two very different notions.  Did you mean to say dark energy above?   (We could do a whole 'nother thread on dark matter  )

Dark matter is needed to explain things like the observed rotation rate of galaxies which clearly indicates a LOT more mass in the galaxies than the visible matter accounts for.

Dark energy is invoked to explain the Supernova data that indicate not only that the universe is expanding, but that the rate of expansion is actually increasing.  If the rate of expansion is decreasing there is no need for dark energy.

Question is, how would  you explain the data?

Good questions. The rate of rotation of galactic arms has probably something to do with the fact that the arms rotate at a different speed than the stars within the arms. You don't need dark matter to explain that. The dark energy of nova data depends on the distance to measure the luminosity. The distance depends on the rate of "expansion." In other words, dark energy is based on an expanding universe that now needs a repulsive form of gravity or pseudo force in order to make any sense, which it doesn't. The question is not about dark matter/energy. I highly doubt the existence of either. The question is whether or not the galactic red shift is dopplarian in origin. So far it is just an assumption.

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Aren't the spectra that indicate the rotation speed of the arms coming from the stars in the arms?  

What is the alternate explanation for red shift?

You might want to read this article by A. Deur.

"Implications of Graviton-Graviton Interactions to Dark Matter"

 

Abstract:

  Our present understanding of the universe requires the existence of dark matter and dark energy. We describe here a natural mechanism that could make exotic dark matter and possibly dark energy unnecessary. Graviton-graviton interactions increase the gravitational binding of matter. This increase, for large massive systems such as galaxies, may be large enough to make exotic dark matter superfluous. Within a weak field approximation we compute the effect on the rotation curves of galaxies and find the correct magnitude and distribution without need for arbitrary parameters or additional exotic particles. The Tully-Fisher relation also emerges naturally from this framework. The computations are further applied to galaxy clusters.

http://arxiv.org/abs/0901.4005

Great!  Thanks very much.  

Any comments by the non-interacting graviton crowd?

Hmmm.  It appears that the graviton skeptics are not clamoring for the podium. 

First, for the casual reader, a word about gravitons....

http://en.wikipedia.org/wiki/Graviton

The key word in the first sentence is 'hypothetical'.  Now I'm rooting for gravitons as much as anybody.   I can't help it -- it's the Trekkie in me.  Gravitons are so...23rd century.

Gravitons, if they exist, are the force carriers of gravity and are analogous to photons, which are the force carriers of electro-magnetism, but there are some important differences between gravitons and photons that should be pointed out before you start arguing cosmology based on gravitons.

First, photons are ridiculously easy to detect (CCD's, film, eyes) while gravitons have never been detected and appear to be virtually undetectable by any foreseeable technology.  (Gravity waves, maybe, but not individual gravitons -- whereas a dark adapted human retina can detect an individual photon)

Second, and more important, photons belong to a quantum theory -- QED, quantum electrodynamics -- that WORKS.  Its equations spit out numbers that make sense.  Gravitons belong to a quantum theory -- quantum gravity -- that DOESN'T work.  The equations produce infinities that cannot be fixed 'renormalized', at least not yet.  

The jury is out, but the math appears to be telling us that quantum gravity is a bad idea -- ergo, no such thing as gravitons.

Of course at this point we should be hearing from the smug string theorists, who will proclaim that one of the great triumphs of string theory is that it produces gravitons as effortlessly as I produce blunders in the King's Gambit.  so you can have gravitons even if you don't have quantum gravity.

Could be.  But since String Theory (as we all know ) proves everything it proves nothing, so maybe we're back where we started.

Just trying to get the thread moving again...

 

Actually, in support of gravitons, (we used to call them gravitinos but that was a very long time ago) they are tensor wave bundles, similar to vector wave bundles of photon fame, which travel at the speed of light. Because electrical forces are so much more powerful than gravitational forces, photons can be quite small and easily detected. Gravity, however, requires large masses to detect alterations in the space-time continuum. Therefore, gravitons are much larger and more difficult to detect. They definitely exist; we just haven't detected them yet, probably because we are using particle accelerators to find them and we should probably be using telescopes. I used to think gravitons didn't exist, but looking at the complete field equations with Faraday tensors properly included, they definitely do. 

Regarding your question on the spiral arms and rotational speed of the stars which make them up, the answer is no, they are not the same.

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Describing something mathematically doesn’t prove that it exists.  There are precise mathematical descriptions for all sorts of things that don’t really exist – hyper cubes, for example, or epicycles.

Having the wave equation for a graviton doesn’t make it any more real than an epicycle, despite the endorsement of graviton believers across the galaxy.

Mathematical elegance can be very seductive, just ask the string theorists who may be wasting their careers chasing 11 dimensional rainbows.  Copernicus was nearly derailed by the mathemataical elegance of the circle and his assumption that planetary orbits had to be circles caused a lot of needless problems for heliocentric astronomy. 

Better observational data settled the fate of epicycles and it will take better observational data to settle the fate of gravitons. 

“Regarding your question on the spiral arms and rotational speed of the stars which make them up, the answer is no, they are not the same.”

Um, no, that wasn’t actually my question.  My question was: Aren't the spectra that indicate the rotation speed of the arms coming from the stars in the arms?  

I obviously phrased that poorly.  Apologies  for that. My bad. 

The point of the question is that it doesn’t matter whether the arms that we see when we look at galaxies are visual artifacts caused by density waves in the galaxy or anything else.  The measured rotational velocity  of the galaxy doesn’t depend on that.  The rotational velocity  of the galaxy is obtained by the taking the spectra of stars across the galaxy and comparing the redshift/blueshift.  This data shows that (1) The stars away from the center are moving faster than they ‘should’ be, based on the visible mass and (2) the relatively constant velocities moving away from the center  indicate that a large amount of invisible mass is distributed throughout the galaxy in a halo rather than being concentrated in the center. 

Now, how about that alternative explanation for red shift?  Really looking forward to that.  That should juice the thread like nobody's business.  

To support some of what you say, math should follow physics rather than the other way around. The field equations show detectable gravitons should be quite large rather than small. And as a very old prof of mine once said, "No one has ever seen an electron. An electron doesn't exist. There are no such things at atoms. These are just stories to explain what we have observed." Personally, I think he was totally off, but what the heck. The spiral arms all have very young stars at the leading edge and the stars are older farther behind the leading edge of the arms. This would indicate the arms have something to do with stellar evolution and noting to do with dark matter. Please show the math regarding dark matter in the halo.

There are different ways of red shifting light.

1. have the source move away from you very rapidly.

2. get the light to go through something, like a more optically dense media.

3. get the light (photon) to bounce off of something.

If the source is not moving away from you, then there is the possibility that the light is moving through an optically dense media, something like a lens or prism, or it is bouncing off of something, or many things, by the time it gets to you.

What is the media? Does it become optically dense in certain regions? Is light bounding off of anything by the time it gets here? Does the light change direction at any time in its journey?

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Here you go.  I don't think you need anything fancier than Newton.

Rotation Curves

Evidence of dark matter has been confirmed through the study of rotation curves. To make a rotation curve one calculates the rotational velocity of e.g. stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the center.

The rotation curve for the galaxy NGC3198 from Begeman 1989 

 

 

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Stellar Motions

Dutch astronomer Jan Oort first discovered the presence of dark matter in the 1930's when studying stellar motions in the local galactic neighborhood. By observing the Doppler shifts of stars moving near the galactic plane, Oort was able to calculate how fast the stars were moving. Since he observed that the galaxy was not flying apart he reasoned that there must be enough matter around that the gravitational pull kept the stars from escaping, much as the sun's gravitational pull keeps the planets in the solar system in orbit. He was able to determine that there must be three times as much mass as is readily observed in the form of visible light. Hence, Oort's calculations yielded an M/L ratio of 3 for the region of the immediate galactic neighborhood. The M/L ratio increases by several orders of magnitude as larger astro-physical phenomena come under similar scrutiny.

 

 

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Galactic Rotation Curves

When studying other galaxies it is invariably found that the stellar rotational velocity remains constant, or "flat", with increasing distance away from the galactic center. This result is highly counterintuitive since, based on Newton's law of gravity, the rotational velocity would steadily decrease for stars further away from the galactic center. Analogously, inner planets within the Solar System travel more quickly about the Sun than do the outer planets (e.g. the Earth travels around the sun at about 100,000 km/hr while Saturn, which is further out, travels at only one third this speed). One way to speed up the outer planets would be to add more mass to the solar system, between the planets. By the same argument the flat galactic rotation curves seem to suggest that each galaxy is surrounded by significant amounts of dark matter. It has been postulated,and generally accepted, that the dark matter would have to be located in a massive, roughly spherical halo enshrouding each galaxy.

The first real surprise in the study of dark matter lay in the outermost parts of galaxies, known as galaxy halos. Here there is negligible luminosity, yet there are occasional orbiting gas clouds which allow one to measure rotation speeds and distances. The rotation speed is found not to decrease with increasing distance from the galactic center, implying that the mass distribution of the galaxy cannot be concentrated, like the light distribution. The mass must continue to increase: since the rotation speed satisfies v^2=GM/r, where M is the mass within radius r, we infer that M increases proportionally to r. This rise appears to stop at about 50kpc, where halos appear to be truncated. We infer that the mass-to-luminosity ratio of the galaxy, including its disk halo, is about 5 times larger than estimated for the luminous inner region, or equal to about 50.

http://astro.berkeley.edu/~mwhite/darkmatter/rotcurve.html

 

Couple of assumptions that have to be considered. One is that the stars are in a cirrcular orbit and the other is the stellar density or M is proportional to r. These are fair assumptions but error bars resulting from these assumptions would be helpful. Do we know the eccentricity of stellar orbits as a function of distance from the core? Also, what is the stellar density function of a run-of-the-mill galaxy? I don't happen to know the answers to these questions. Maybe someone could chip in? Also, why do older stars form the regions trailing behind the arms while new stars occupy the leading edges?

Anybody want to chip in?  

C'mon gang.  Cosmology is fun  

Looking at your graph, the velocity becomes constant after about 10 kpc. Assuming circular orbits for stars in the arms about the centre of the galaxy, if we have v^2 => m/r and the density of the arms outside the nucleus is m => r, then it makes sense that the velocity would be constant in that region. Within 10 kpc the velocity is linear wrt radial distance. If m=> r^2 in the nucleus, then the graph makes sense and it seems Newton is doing just fine. However, we do have to take into account that the speed of light is constant and it takes time for the effect of gravity to reach each star when we consider very large distances, like kpc for example. Therefore, would you please show the math with corrections taking into account relativistic effects on a gravitationally bound n-body problem? 

I'm not bringing stuff up just for the sake of defending a position. I hope this discussion leads us closer to the truth, whatever it is. When we have something we cannot detect that has a huge gravitational effect, I first suspect the presence of a black hole, or perhaps, many black holes. These are just ideas and It would be nice to hear others.

In the words of my old professor (and probably yours as well) "I leave that to the student for an exercise"