Astronomers claim dark matter breakthrough

The identity of the Universe's dark matter may finally have been discovered. In what seems to be the most convincing claim for dark matter so far, researchers in England and France say gamma rays coming from the centre of our galaxy show hallmarks of these ghostly particles.

The research has only just been made public, so the team is still waiting for a response from other dark matter experts. But though the researchers are cautious, there is no hiding their excitement. "I've dropped everything else to work on this," says Dan Hooper of the University of Oxford. "We're really excited," adds his colleague Céline Boehm, also of Oxford. "I'm cautious but it's surprising everything fits so well."

The identity of the Universe's dark matter, which outweighs the visible stuff by at least a factor of seven, is the outstanding mystery of modern astronomy. Scientists think it must exist because its gravity affects the way galaxies hold together. But the particles do not emit any electromagnetic radiation so they have never been detected directly. No one knows what the particles are like, or exactly how they are distributed.

However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy. So the team turned their attention to a distinctive pattern of gamma rays coming from the centre of the Milky Way (see graphic). The sharp signal, which has an energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons the antimatter equivalent of electrons.

But where did the electrons and positrons come from? People have speculated that the source is anything from the blast waves of a "hypernova" a super-powerful supernova to a neutron star or black hole. "But none of the explanations have seemed satisfactory," says Hooper.

me:However, remember that we have access to millions of spirals in the universe through optical observations. Not exactly because in order to get correct measurements the galaxy has to be perfectly oriented toward us so there is not that many.

meStatistically, if a spiral galaxy is behaving in some odd way, we would should see it, unless the timescale of the odd behaviour is extremely short.

Actually, It would be the opposite being that the timescales are extremely long. If a spiral arm was expanding, contracting, or there are stellar eddies or whatever else could be happening that we don't/wouldn't know about due to the extremely long timespans we wouldn't be able to percieve it for millions of years.

Not necessarily. If it's a dynamic process, where in the arms contract for X time, and then stay static for Y time, and then expand for Z time, then if you have a sample of N galaxies that are edge on to you (or within 30 degrees), you should see the expansion and contraction of the part of the galaxy closest to you in the form of a doppler shift of the spectral line either blueshifted or redshifted for at least some of the galaxies. Because the process is dynamic (not on all of the time), you won't see it in all of the galaxies you observe, but you should see it in at least some of them.

Spiral Galaxies are very complex and there is nothing anywhere near comparable to them for a reference. So until we can get a really good computer model we don't know. Meanwhile Elipitical Galaxies are more uniform and they behave in an understandable "Kepler" fashion so there is no need for scientist to invent this mysterious Dark Matter with them. Spiral galaxies evolve into elipitical galaxies so where does all this matter go? Are we to believe that 90% of the mass that some scientist claim dark matter makes up in spiral galaxies just goes *poof* and vanishes into the void of space?

This is a good question. I just don't feel that with ellpticals that no one has come out with a paper that puts a dagger in the heart of the CDM model. With ellipticals, we are hampered by what we can measure reliably, and that is the problem.

Also it should be noted that in elipitical galaxies they use planatary nebula instead of Hydrogen clouds to measure rotation. Planatary nebula might be a better choice because also do we really know how exactly a Hydrogen cloud behaves over time?

This is kind of spurious logic. The reason observers use planetary nebulae instead of hydrogen in ellipticals is mainly because most ellipticals have little or no hydrogen gas clouds in them to measure. Elliptical galaxies do not have a young stellar component to them like spirals, because there are very few hydrogen clouds in them to create a young population with them. The current thinking is that ellipticals are formed from the merger process of many spiral galaxies. The mergers accelerate star formation, as well as send some of the gas out of the resulting merged elliptical. The resulting leftover galaxy usually has nothing left to create stars with after the merger, so the young stars all die off, leaving the last population of stars as the only stellar population.

Also, planetary nebulae are a very BAD source of data compared to a self gravitating cloud of hydrogen gas(or molecular cloud for that matter). Planetary nebulae is the end product of a stellar lifetime, when a star dies therefore it only has a mass of a solar mass or two, while a cloud of hydrogen can have anywhere from tens to thousands of solar masses. For the most part, only another hydrogen cloud or larger can perturb its motion through many orbits. However, since the planetary nebulae is the end product of a star, every passing by of a nearby star during the stars life can perturb its motion. At the planetary nebular stage, its motion is nothing like the motion of the star at birth.

Using Occam's Razor which is more plausible.

A) There is this mysterious, invisable, undetectable Dark Non-Periodic table matter with multiple properties in spiral galaxies that magically vanishes when the galaxy evolves into an eliptical one.

Are you referring to this paper? Remember that the paper in Science announced its result after only three galaxies. Ellptical galaxies are notoriously hard to measure, because of the things I mentioned above: Little or no gas and dust clouds, and the only thing you can point at are Planetary Nebulae, which are subject to a stellar lifetime (billions of years) of gravitational perturbations. Because of this limitation, people have been unable to get farther than two scale lengths away from the galaxy core. This may not be far enough to measure the CDM. This paper purports to get to 4-6 scale lengths, which is great, but those errors aren't good out at 4 and 6 scale lengths. Planetary nebulae just aren't a great measurement for this, even though it is the best we have. I wonder if anyone has done this for X-ray binaries yet? I know someone I can ask that question to.

I am also skeptical of its result simply because of the extremely small size of its sample. It looked to me that someone needed to pad their publishing statistics for tenure, instead of waiting for their project to finish.

B) Our measurements of spiral galaxies are wrong

I can't rule it out, but on the other hand, the results for every spiral galaxy is very convincing. My answer to this whole debate is that CDM models are more convincing than non-CDM models.

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	Mystery cloud